Complete Color FAQ

As a painter and professor of art - I constantly come across references that optical colour mixing, as in the art movement "pointillism" as well as ink jet printing involves both subtractive and additive mixing in the eye of the viewer. I completely disagree that two dots of colour, at sufficient distance, can 'additively' mix in the eye. I am not a colour scientist, my knowledge of this is entirely empirical, but I would be interested to hear your thoughts on the matter? Is it possible and if so, how?   (answer)

Is there any known research on trying to find colors outside of the color spectrum?   (answer)

How do one estimate color of an object by just looking at their reflectance data? What is the relationship?   (answer)

The difference between "saturation", "chroma", and "colorfulness" can be very subtle. Can you explain? Similarly, is there any difference between the related terms "value", "lightness", and "brightness"? What do HSV and HSL describe?   (answer)

How come it is easier to see a certain color under low light condition while other colors are hard to see?   (answer)

I project two spot lights on a white screen, RGB=(255,255,0) and RGB=(255,0,255), when both lights combine additively, what "color" I will see? Could I predict the mixture in the RGB Color Space?   (answer)

I am confused by the range value of the Y-tristimulus value. For emissive things, like screens, it is supposed to be the absolute luminance in foot-lamberts (so the range could be 0 to infinity in theory).

If want to compare two colors (via deltaE - which deltaE is the 'standard' nowadays), one from a emissive source and one from a reflective source, can I? It seems like a reflectance number couldn't quite compare to a direct luminance measure, but then how can I compare these two things in a color space?   (answer)

What percent of light is reflected from Brown beer bottle glass? Green beer bottle glass? Clear beer bottle glass?   (answer)

Can females see more colors than males?   (answer)

Artists often find it difficult to identify subtle colors. Eg: is a warmer or cooler than another? Shadow and reflection are very hard. The more you stare the easier it is to convince yourself it is almost any color. I have heard that both saccadic eye movements and bending over so as to look at the subject upside-down can be helpful. Do you agree and do you know of any better techniques?   (answer)

Can you explain the science behind gloss levels on the paintwork of a car. I own an auto reconditioning business and I know using different techniques I try to achieve maximum gloss, even at a microscopic level. Let's say you have the spiderwebbing swirls in your clear coat. Scientifically , what is happening to the light when it hits the swirls as opposed to hitting perfect paint? Is it the law of Refraction or should it be explained another way.   (answer)

Why is light source data normalized to 100 at 560 nm? Is this the source´s relative spectral power distribution or is that something else? Doesn't the normalization affect the computed XYZ or L*a*b* values?   (answer)

I am studying the lightness differences in two types of dental crowns. Is there a minimum ΔL that is not perceived by humans?   (answer)

Does color have an objective existence?   (answer)

If the inability to differentiate blue and black and brown is caused by the gradual yellowing of the lens in our eyes, would wearing another color lens make it possible to see the colors?   (answer)

I'm interested in skin colour and I normally reckon it should lie between 0° and 30° in a HSB colour space. In the Munsell space the Hue shifts for less saturated colors. Are there recognized skin values in the Munsell notation?   (answer)

What is the most visible color for a motor cycle helmet?   (answer)

What education and degree is required to become a color scientist?   (answer)

Why does the S-cone see blue-red (i.e. violet) instead of pure blue? Where does the red come from without the L-cone?   (answer)

Why there is no spectral (pure) colors that can be reproduced by mixtures of other colors? It is just a coincidence of human color vision?   (answer)

I have a very old FM 100 Hue Test Kit (in a wooden box) but do not have a score sheet. I see all of the new kits have software. What came with this kit as it's obviously pre-PC.   (answer)

Is there any way to predict what color an object will appear if I change the color of the light source?   (answer)

Why is it so difficulty to follow the process of saying the color of the word as opposed to saying the word itself?   (answer)

I am a Colorist, want to reduce validation work in laboratory. If I add a quantity of pigment and Tio2 paint, and then measure the shade, we get a certain CIELAB value. From that can I predict CIELAB values given a different quantity of pigment and TiO2?   (answer)

Which would yield the least visual discrepancies, a more chromatic color or less chromatic color?   (answer)

I read that Ansel Adams painted the walls of his home studio a 20 percent gray color to be the best background color to display his photos. Do you have any more specific information about this color? Is it warm or cool gray? Is there an actual paint recipe for this color?   (answer)

I bought a charcoal gray couch - love the color in the store. In my living room it looks dark blue! What color should I use on the walls and accessories to make it look gray again?   (answer)

Can fluorescent colors be printed with cymk printing system? I have Roland printer that uses eco-solmax inks. Are there any RGB or CMYK codes that will work?   (answer)

In textile color matching, Is it possible to match target color in all light source without metamerism. If so what will be the possibility percentage?   (answer)

What does a white balance actually do?   (answer)

What makes a color bright?   (answer)

I study the topographic influence on snow. When an individual looks at snow in shadow, they are actually seeing blue light, yet they state they see white. Is this based on learned experience of color? What effect does the question have, i.e. "What color do you see", vs. "What is the color of the snow?"   (answer)

How does a flower´s color help it survive?   (answer)

Many auto accidents happen in early morning and late in the evening when it is dark outside. Does the vehicle color have anything to do with this? What automotive paint color is easiest for the eye to see when it is dark outside? How about in daylight?   (answer)

Although our sun is often depicted in artwork as "yellow", I have heard that it is actually "white", and that is why a white piece of paper appears white. Is this true? If so and we lived in a solar system with a different type of star, such as a red dwarf or a blue giant, would any objects appear "white", or would they take on the cast of their light source?   (answer)

Imagine driving down a street toward a traffic light. How fast would you have to go so that the red light (700 nm) would appear green (500nm)?   (answer)

Is there a way to measure gloss with a spectrophotometer or is it necessary to have a glossmeter? Is there a conversion between CIELAB values and gloss?   (answer)

How can I avoid metamerism?   (answer)

What are L* a* and b*? Are they constants? If not, how can I find those values?   (answer)

Where can I find calculator or software for LRV?   (answer)

We are told that Red, Yellow, and Blue are the "primary" colors, and that you can make all other colors from them, but those 3 can´t be made by combining the others. In additive systems, it is red and green that combine to make yellow. Is there something inherently special about red, yellow, and blue, or are they considered "primary" only in the sense that the most basic medium of color manipulation is a subtractive system?   (answer)

When we compare luminosity of different hues, we find that blue is a relatively dark color, and yellow is a relatively light color. Do hues have an inherent luminosity independent of human perception?   (answer)

I am trying to emulate a smaller-gamut RGB color space on an LCD display using a 3x3 matrix, and I can not seem to figure out the general-purpose way of doing this. You reference Poynton´s book in a similar FAQ but he skips over the process and just spits out the answer for some important cases.   (answer)

Is there is a device that can measure liquid gravure ink spectral transmittance? I need a method to confirm ink recipe components in a liquid ink sample. I could use a faster option than the traditional draw-down proofing method.   (answer)

How many lights are metameric to white light reflected from a paper?   (answer)

I use the Munsell Color Books to develop color harmonies for clients to use on short term projects. Is there an inexpensive way to get swatches or chips that match the Munsell notation? Can I print them?   (answer)

Are there studies of floral arrangement preferences of dichromats?   (answer)

How did the CIE 1931 color matching experiments evaluate the whole gamut when we know that the RGB color gamut is a subset of that of the human eye? In other words, we know there are colors that cannot be matched by the RGB lights. How did they get around this?   (answer)

The sensitivity of the human eye to color difference is greatest at approximately 550 nm. However, in interpreting the MacAdam ellipses , it would appear that humans are least sensitive to differences in color in this region. Why?   (answer)

I measure timber colour using a Minolta Chromameter CR-400. I want to compare my values with those of other woods whose colors are listed in percent Luminance, Dominant wavelength (λ)and percent purity. Can I convert these values over to CIELAB?   (answer)

Is it possible to scientifically determine the original colors in an old black and white stereoview photograph if some of the colors are known?   (answer)

How does aging affect color perception? Are older people able to discriminate brighter colors or softer colors better?   (answer)

What wavelength(s) are used in the calculation of Gardner color?   (answer)

Is it possible to explain color to a blind person if thay have never experienced it? Can the brain of a blind person assign a color to something if they know the shape but not the color?   (answer)

I need to predict CIELAB of a color at different thickness. The color is transparent and .01 inches. How do I predict LAB values at thicknessof .121?   (answer)

Why do different pure hues have differing lightness values?   (answer)

In his 1943 OSA paper "Specification of Small Chromaticity Differences" MacAdam describes constants g11, 2g12, g22 and gives values for them in graphical form. Is there a source for this data in conveniently tabulated form?   (answer)

I need a device that will give me color readings at an observer angle of around 45 degrees or so that will pick up almost 100% specular reflection.   (answer)

Why is it not possible to get an accurate ΔE colour measurement on a metallic powder coating?   (answer)

At what luminance (Candela per meter squared) in the ambient environmental light does scotopic vision turn to mesopic vision, or at what point can we start to see color? And at what point can we see the majority of colors?   (answer)

What is the most common first color that children recognize? The second?   (answer)

We have a WWI aircraft we are recovering with fabric that requires a dark green paint for the wings. This aircraft is flown a lot and displayed to the public outside quite a bit. The problem is that this paint absorbs sunlight heat dramatically which reduces the life of the doped fabric, causing it to shrink prematurely and it is fairly expensive to replace. Would a primer coat of silver dope or white help reduce the absorption of the heat into the fabric under the dark green paint?   (answer)

Do you know the origin of a measurement called the "flop index", which is defined as flop index=2.69(L15�-L110�)^1.11/((L45�)^0.86). My Xrite M68II reports this metric, however, X-Rite does not explain the source of this equation. Do you know where I might find the source of this equation, or another good way to mathematically explain the color difference at several angles?   (answer)

I am turning my formal dining room into an art gallery. We painted today and it is too bright. The way it looks now all you will see is the paint color and not the art. What are the best colors to paint a gallery, given that all the pictures are of different color schemes?   (answer)

Scientifically speaking, is black a color? Is white a color?   (answer)

In comparing light bulbs based on their color temperature, how much difference in color temperature is needed before an average person can notice that difference?   (answer)

Are there any specific details in 45:0c reflectance measurement, such as the effect of distance between detector and sample or distance between light source and sample? Are there restrictions on the circumstances, like the color of the floor on which we put our sample?   (answer)

Is there any approximation to convert between Hunter Lab (illuminant C, 10 degree observer) and CIELAB (illuminant D65, 10 degree observer)?   (answer)

I received data from two sources. The first measured Hunter Lab on light source C under 10 degrees, the second measured CIELAB on light source D65 under 10 degrees. I don't have the spectra. Is there any approximation with which I can convert between them?   (answer)

What observer for XYZ values can be used in CAM02, 2 degree or 10 degree? Some articles use 2-degree observer, the others use 10-degree observer.   (answer)

I am looking for the differences between red, blue, green, and yellow dye. Can you tell me anything different about them, other than they are different colors?   (answer)

What part of the spectrum helps grass grow?   (answer)

Why does the color of a crayon or colored pencil look a different color on paper? For example, some purples show up blue on paper.   (answer)

Would a spectrophotometer be able to accurately read the color differences between chrome plated surfaces? (ie. mirror-like finish) What settings or type of instrument would best achieve this?   (answer)

Is it possible to quantitatively describe a color just by its reflectance or radiative spectrum?   (answer)

How a mixture of R, G & B single wavelength sources (such as RGB laser) can produce white light even though they do not cover the whole spectrum?   (answer)

Is there a formula for calculating Delta E in Munsell?   (answer)

I would like to convert from CIELAB coordinates into the approximate spectral red green and blue component curves. Is this possible?   (answer)

Why do we see colors when we spin a disc that contains figures in black & white?   (answer)

When job requirements demand color discrimination, how often should employees be tested on their color perception?   (answer)

Is there a color model which would allow me to do the following? Given L*a*b*/XYZ/whatever coordinates of a color stimulus, given specified viewing conditions and color, what is the shape/size of the JND "ellipsoid" around the color, and what are the directions of the hue/chroma/lightness axes?   (answer)

Is there still a copyright on the Munsell color system? If so, who owns it?   (answer)

Which light colors are absorbed by a yellow tulip?   (answer)

A one dimensional halftone cell has 50% coverage with reflection density 2.0 and 50 % coverage with reflection density 1.0. How is L* calculated for this cell?   (answer)

Are you aware of any software packages or even just charts, that list Munsell colors by both code and name?   (answer)

What colors (in order) would best absorb a green 529nm laser light?   (answer)

Could you please tell me if there is any function that computes the chromatic adaptation and the color appearance for the various types of color deficiencies?   (answer)

How could I provide (or display) the approximate color of a star given it's Kelvin temperature?   (answer)

I am interested to convert a hex html or decimal rgb value to cie 1931. How can I do this?   (answer)

Is 0.05ppm a suitable tolerance for measuring absorption on a spectrometer for beverages? When is it no longer a visible difference to the naked eye 1ppm, 0.1ppm, 0.01ppm, etc?   (answer)

What is the best color to paint behind a High Defintion tv mounted on the wall?   (answer)

What dyes are mixed to get Dark Shell Gray?   (answer)

I am a retinal surgeon. We sometimes stain the retina with trypan blue in order to highlight superficial membranes that need to be peeled (over the retina). Is there a colored contact lens that I could wear that would highlight the blue spectrum more so that it would be easy to see faint staining on trypan blue on the retina?   (answer)

I would like to come up with a color scheme that assigns an absolute color value to common color words. In this scheme I would want scarlet to have a distinct and absolute value different than crimson or puce. Is there any site or program that has assigned absolute color values to the English names of colors?   (answer)

We measure several prints for our printer characterizations. Two questions: (1) what is best space to average - spectral reflectance, LAB or XYZ and (2) if we wanted to do a weighted average to reduce the weight of data which tended to be "inconsistent", how would we best do that?   (answer)

In the CIELAB color space, what is the approximate difference for each of L*, a*, b*, and ∇E that most people would not percieve as a different color.   (answer)

Are there guidelines or rules of thumb for how colors are perceived as "dominant" in an image? Are there differences in the perception of color dominance among different colors? If so, how is this described in the CIELAB color space?   (answer)

Where can I find a chart showing color names, the colors themselves and CIELAB color coordinates? Ideally, this would also show the RGB color coordinates. I have found many RGB charts like this but none with the CIELAB coordinates.   (answer)

Are there established color values for the color of skin for various ethnic groups (like Lab or Munsell for example)? This is for a makeup manufacturer who wants to design packing for specific groups of people.   (answer)

I dyed 100% cotton red, yellow and blue and placed them in direct sunlight for a week. The red appeared to fade the most, but I can't explain why. Can you help?   (answer)

Why does red disappear when viewed behind red glass?   (answer)

Are grey shades actually constant, because they are nearly achromatic, or is there something about eye biomechanics that create a visual perception not in agreement with spectral readings?   (answer)

I need to know the Munsell color for 'school bus yellow.' The unsourced quote I have says it is 9.4YR, 7.5/9.2.   (answer)

What does LCC stand for in the color science world?   (answer)

Where can I purchase a poster size print of the CIE 1931 Chromaticty Diagram? I realize that the diagram is only a 2D projection of 3D matching data however I still find it useful to illustrate some of the other attributes of the data depicted (color mixing, color temperature, color gamuts)?   (answer)

At what distance, target size and viewing conditions (illumination level/type) does the HVS's ability to resolve color breakdown? Put another way, what is the "chromatic" resolving power of the HVS and how is that defined in terms of distance, size, viewing conditions?   (answer)

Is black a natural color or is it just a very dark brown that people just see as black?   (answer)

We would like to know the complete Reflectance Measurement principle and formula for the dual beam color spectrophotometer including Light trap, PTFE standard and sample.   (answer)

Is there a relationship between visual lightness (when we judge lightness of color) and Light Reflectance Value (a measurement that is expressed mathematically?   (answer)

I am a massage therapist. After a massage a customer said that when I had massaged her on a certain part of her body she could see a certain color, for ex. red, & at another spot she saw the color blue. Is there a reason to this?   (answer)

How do you find a RAL equivalent of an NCS colour reference?   (answer)

Why can't humans see ultraviolet or infrared energy?   (answer)

What color is chroma key paint for green screen back drops?   (answer)

Is it possible to determine the ink densities (or transmissivities) in a small area (~20 mm^2) of a four color image (CMYK) from its reflectance spectrum? The relative areas of coverage and overprint of the inks are not known. Illuminant and substrate spectra are available.   (answer)

How would I measure the yellowness of the sclera (whites) of the eye? Also, what instrument would be an appropriate measure haziness of the cornea?   (answer)

What is the color temperature that makes the road most clear during driving, even under unconditional weather?   (answer)

How do I match the actual fabric color in PhotoshopTM? Images are digitally shot, and the final output is four color offset printing.   (answer)

What is the theoretical principals of the color change test in chemistry?   (answer)

I am interested to convert a hex html or decimal RGB value to CIE 1931. How can I do this?   (answer)

Is 0.05ppm a suitable tolerance for measuring absorption on a spectrometer for beverages? At what concentration is the difference not perceptible?   (answer)

What would an acceptable max display luminance setting be for a display that does not see direct sunight or much ambient light so that colors and text are easily readable?   (answer)

Using the color bar on Powerpoint 2003 I drag the brightness bar on the Device RBG mode, I get a darker color of apparently the exact hue. Is the hue is truly staying the same? Also, if I drag from R 200, to R150 to R50, is it decreasing in the amount of red equally in each interval?   (answer)

"The Desktop Color Handbook" mentions using a "light source color rendering card" which has patches that look the same under light of the appropriate CCT, but different under other lights. Do you know who makes such cards, or how one could create one?   (answer)

I am a Color Technician for a woman's retailer which specializes in black, white, and neutral only clothing. We test our associates with the Munsell 100 Hue test. Is there a similar test to evaluate for neutrals only?   (answer)

Is there an accepted universal standard for the calibration of color Spectrophotometer? Are there white standards available with certified reflectance values?   (answer)

Are there maximum/minimum values for a* and b* like the min and max for L*?   (answer)

Is color a quality in physics?   (answer)

I have noticed a couple of different species of early-flowering trees have pink blossoms which turn white literally overnight. It is the speed of the process which is intriguing. Do you know how it works?   (answer)

Do you know where I can purchase Hess Ives Color standards, both physical standards and papers publications?   (answer)

How can tulips be so many different colors when they are the same plant?   (answer)

Is there such an instrument as a hand-held spectrometer for measuring the color temperature of light sources? If so, how much do they cost?   (answer)

Densitometers can be purchased with or without polarizing filters. Why are both types used? Which type provides the best correlation to perception?   (answer)

How do you convert ΔL ΔC ΔH values for colour differences in CIELCh to the equivalent ΔL Δa Δb colour difference values in CIELab, when you do not have access to the absolute values for the original measurements?   (answer)

Are there some basic averages for the LRV of human skin?   (answer)

At what age do children develop good color perception?   (answer)

Can you define what is Multiflux math model used in color mathing?   (answer)

I have used three online color space conversion calculators and I get three different sets of numbers when I convert from XYZ to RGB. What's happening? Who's right?   (answer)

Can light be a "Brown" color? If not, why not?   (answer)

What is the RAL number for PMS 268 plum? I've attempted to find a cross-reference tool and have not been successful!   (answer)

When purchasing an encapsulated Haz-mat suit, Level A, what colors have the best visibility for the different environments in which they might be used, i.e, sandstorms, snow, heavy rain, thick smoke, etc.?   (answer)

Roughly how large is the overall remaining perceptual non-uniformity of color tolerances in CIELab space, when using the ΔE94 and ΔE2000 color distance formulas? (as compared to the 6:1 non-uniformity of simple ΔE*ab)?   (answer)

How can I have someone create a paint for me using a theoretical spectral reflectance curve?   (answer)

How does chameleon paint (i.e. paint which color depends on viewing angle) work? What is its composition?   (answer)

How do I calculate the dominant wavelength of a color sample with X,Y,Z or x,y? I do not want to draw it in the colorspace.   (answer)

How to convert CIELAB data to Yellowness and Whiteness index?   (answer)

Which minimizes solar heat absorption better, polished aluminium or white paint on it?   (answer)

How does the eye determine the various colors of visible light?   (answer)

Could you please tell me, what color would 1.2Y 6/11 be?   (answer)

Why do black clothes appear blue in the artificial light of a clothing shop? Then when you take the item into natural light it is black? Why is that?   (answer)

What type of Pantone Guide shall we refer to & which instrument is suitable to measure the values of colour on aluminum foil with a flexo printing process? Since Aluminum has glossy as well as the matt finish then is there any need to change the settings of the instrument?   (answer)

We print on Aluminum foil with a flexo process. What type of Pantone Guide shall we refer to & which instrument is suitable to measure the values of colour? Since Aluminum has glossy as well as the matt finish then is there any need to change the settings of the instrument?   (answer)

How do I convert from spectral reflectance (from a hyperspectral imager) to CIELAB colorspace?   (answer)

Can a consumer grade flat bed scanner be used as a colorimeter?   (answer)

The lighting goal for our new indoor shooting range is to have 100 footcandles of light on each target. Our question is this: should the walls and ceiling around the target be light in color or dark in color?   (answer)

Can you suggest an ideal color scale for a digital elevation map to be printed in a scientific paper? Is it possible to have a color scale that looks good in color and in black and white?   (answer)

Suppose I introduce the same amount of noise in individual channels of a YUV image of 420 format (chroma has been downsampled). Which image is the most noticeable, the one with noisy Y, U, or V?   (answer)

Can you point me to the actual formula the Photoshop Hue/Sat tool is using to change given colors in an image?Can you point me to the actual formula the Photoshop (tm) Hue/Sat tool is using to change given colors in an image?   (answer)

What are the colors of a complete spectrum? (Starting with the color with the longest wavelength.)   (answer)

What color measurement system would be best to define colors for a liquid, to be described in a US patent?   (answer)

Why is blue ink most commonly used for writing?   (answer)

What is the difference between the L*a*b* values and the L a b values when using a Minolta colorimeter?   (answer)

Do you do research on ink jet printer color consistency? We are interested in printing solid colors for visual evaluations. We have a high-end ink jet and use photo paper, but we see color differences of up to 0.5 Units ΔE* between prints using the same printer, inks, profile and paper.   (answer)

My spectrodensitometer states its measuring geometry is 0/45, and its standard observers are 2 and 10. Are these angles related? Which should I use?   (answer)

Is there a way to determine what a symmetrical circle 1% dot, 120 line screen should measure? I am looking for a published table or a equation as I need to determine this on several different screen rulings.   (answer)

I am working on a plastic matched automotive black (GM 848) where the color is visually darker, but it measures LIGHTER than the standard. Can you please explain why?   (answer)

What is the mathematical equation to convert Lab or XYZ to Density?   (answer)

I sometimes catnap at my computer or while reading and when I wake everything looks green tinged for about 20 seconds. Why?   (answer)

Does the color of a laser affect the velocity of the light wave projected by the laser?   (answer)

What is the physics behind a blue-yellow polarizer such as the Singh-Ray Gold-N-Blue filter which photographers use to add yellow or blue highlights to polarized parts of a scene?   (answer)

Is is possible to convert R, G, B values of an image captured using a flatbed scanner to densities? I can convert R, G, B to CIELab using the ICC profile of the scanner. Is there a way to convert CIELab to Densities?   (answer)

Where could an art reproduction facility obtain an optical device that used spectral-based imaging?   (answer)

We know density is derived from the transmitted spectral power distribution. Can you suggest a reference that provides the responsivity functions for Status A and Status M densities, as well as the specific function for the calculation of density? Also, what is the proper illuminant to use?   (answer)

What is "camera gamma" and its relationship to NTSC/HDTV video?   (answer)

How does coloured text affect accommodation?   (answer)

How can I meaure the color of clear solutions? We want an objective measurement (wavelength) of colors of various juices and wines.   (answer)

When choosing an umbrella, what color is best to protect yourself from UV light?   (answer)

I am teaching color theory to our press operators and I am wondering if there is a full color spherical L*a*b model illustration that I can download, and if necessary purchase.   (answer)

I would like to know if the color of the water (by adding food color dyes) will affect the absorption rate of water and thus the color of the white flower over time.   (answer)

What is the Kubelka Munk Theory? What its use in textile dyeing process? Would you please provide me any literature on the Kubelka Munk theory?   (answer)

Is there an existing paint industry standard system for expressing (and thus potentially matching) all the visual characteristics of a metallic or metallic flake paint, whether one-coat or two-coat (colored transparent over metallic or flake)?   (answer)

I am trying to calculate the colour of a coating. I have a graph of wavelenth versus reflectivity. Is there a simple way to convert this graph into RGB values or some similar standard?   (answer)

Since a mirror reflects most of the light that falls on it, as does any white body, what is the difference between the two?   (answer)

I know that the color of a body depends upon the color that it reflects. What causes some bodies to be transparent, others translucent still others to be opaque?   (answer)

I found a source that says an RGB of 194, 32, 11 for Calder Red. Can you tell me what proportions of red, yellow, and blue paint should be mixed to get that RGB?   (answer)

I want to make a slate grey color from white exterior latex paint. What additional colors and quantities of color will I need to mix?   (answer)

This pertains to colors being used for home theater projection screens. Neutal grays have been suggested as the best color since they should not shift the color of the reflected image. Munsell grays are reported as neutral for illuminant C. What would the RGB value be for Munsell N8 and N9 under D65? One source lists N8 as 202 202 202 (L*ab- 81.3256 0.0044 -0.0087, Yxy as 59.0619 0.31272 0.32900). Is this correct?   (answer)

I am an artist interested in painting with photochromic inks or paints. Do you know of a supplier?   (answer)

We have two items painted with Munsell N7 which are significantly different in shade. Is this normal? How can I measure if the differences are within specification?   (answer)

I am building a pit solar greenhouse and need to decide what color to paint the non-glazed walls. White would be the best for reflection onto the plants but I am also looking for more pleasing colors. What is the best color for growing plants?   (answer)

Why isn't E point (x=0.3333 - y=0.3333) used as a standard illuminant for color matching and ΔE?   (answer)

I am working on honeybee color vision. Using a honeybee specific color diagram, we know how to calculate dominant wavelength. How do I calculate the dominant wavelength of a color sample which resides in my color diagram in the "magenta" (or purple) zone.   (answer)

Why are the colors of Christmas typically red and green?   (answer)

I work for an automotive finishing company and use an X-rite MA68II for measuring color. Which should I use, L*a*b*, or L*C*h�? Everyone seems to use L*a*b*, but I'm not sure why.   (answer)

How long does it take for your brain to recognize color?   (answer)

Can you provide a mathematical formula to convert a RGB color value in CMYK color value?   (answer)

How do mood rings work?   (answer)

At what illuminant temperature are Munsell neutrals (N6, N7, etc.) designed to be neutral?   (answer)

Obviously having a single colour on two different stimuli (each encompassing a different meaning) on a workstation will cause confusion. What are these problems from a psychological, cognitive, or ergonomic perspective?   (answer)

Are the colors that we see during the day, present at night - only we cannot see them? Or are the colors that we see during the day not present, or altered, because of the low light?   (answer)

What are the names of pigment suppliers who mfg or distribute metameric pigments?   (answer)

I am developing paint color regulations to help buildings constructed on mountainsides blend into their surroundings to minimize their visual impact. What maximum LRV do you recommend for an environment typically consisting of darker browns, grays, and greens as the "base" colors in order to keep buildings from "standing out"? What would be the most appropriate instrument for measuring the LRVs of paint/stain for enforcement purposes?   (answer)

How does the color of a room affect people's moods?   (answer)

What are the equations for McAdams ellipses? Are they location dependent in the 1931 chromiticity diagram?   (answer)

It makes sense that I should want window glass with the highest Color Rendering Index available in the performance category that I need; but how do I judge the relative CRI when all options are between 87 and 95?   (answer)

What mixture of acrylic painiting do I need to achieve a copper tone?   (answer)

Can CAM02 to predict Appearance Correlates of a color on different colored backgrounds?   (answer)

Is it proper to state a Munsell notation for a metallic or pearlescent color, or is Munsell a solid color only notation system?   (answer)

Is it possible to draw plankian line in CIE diagram? How?   (answer)

Is there any source of spectral reflectance data for samples in the Munsell Book of Color?   (answer)

Does the Munsell system account for the Helmholtz-Kohlrausch effect?   (answer)

How closely do the Munsell Renotation data reflect the actual tristimulus values of samples in the Munsell Book of Color as viewed under Standard Illuminant C?   (answer)

Why does the sheen of a finish affect the appearance of a color? Do the wavelengths of color get magnified, refracted, or intensified as the pass through the layers of a paint film?   (answer)

Can you explain the difference between value and luminance? Is Light Reflectance Value different than luminance?   (answer)

I understand that the visible spectrum is comprised of the wavelengths red, orange, yellow, green, blue, indigo and violet. Combined they make up white light. So does that mean that white is then a wavelength too? Do all the other colors/wavelengths add up to something that we recognize as white?   (answer)

What's a good reference for information on the Munsell system?   (answer)

To measure process color is it better to use a spectrometery or density, hue, and gray?   (answer)

Where can I find absorption or reflectance data for inorganic chemicals or pigments?   (answer)

Is there an index for the degree of yellowing? I want to characterize the degree of yellowing in trees as an indicator of poor health from aerial RGB photographs.   (answer)

Please give an example of how to begin with a tertiary color,subtract one color and then subtract another color but don't end up with a primary color.   (answer)

Where can I buy the Munsell color chart that has the 14 samples used for CRI tests?   (answer)

Where do the beautiful colors of fireworks came from?   (answer)

Why does the greenery of parks and gardens look beautiful to our eye? Please elaborate with regard to perception and function of the eye.   (answer)

Do you a reference for this equation used to get color decimal value from RGB values: "color_code = (r*65536) + (g*256) + b"   (answer)

What makes colors fade in plastics? What part or parts of the light spectum?   (answer)

Are there any instruments or methods which measure the intensity of gray color?   (answer)

What the physical meaning of sharper cone reponse in CIECAM02?   (answer)

How do paint colors affect the room temperature?   (answer)

How can I mathematically find the best match for a CMYK color from an array of CMYK colors?   (answer)

I have some images taken with a digital SLR at the wrong white-balance setting (tungsten instead of daylight). Is there a fixed set of RGB correction factors that I can apply to the entire batch?   (answer)

Why do boys choose blues, grays, black, and browns when coloring and girls choose more brighter colors like greens, reds, oranges when coloring?   (answer)

Do some people, through genetics, have dramatically different "rods and cones" in their eyes and see colors totally differently than "most people". This does not include colorblindness, but that some people might see red as yellow and blue as orange, etc. Is this true?   (answer)

I am trying to establish a color system criteria for silicon parts which are white. What do you suggest in setting up a criteria for the color white?   (answer)

What is meant by plankian locus? What is its importance?   (answer)

Are there any estimates of the probability that a photon impinging upon a cone photoreceptor will be absorbed by the visual pigment within the receptor? How does this probability change as the radiance is increased so that more of the pigment is in the bleached state?   (answer)

Can a person with a color deficiency learn colors and shades and be able to distinguish differences even if they are not able to see the exact color that a normal vision person might see?   (answer)

Would it be possible to correct color blindness (for computer users) by measuring a user's color sensitivity at various wavelengths, then adjusting the output of the monitor to compensate for it?   (answer)

How do I calculate whiteness index (WI by ASTM standards) if I don't have a spectrophotometer but am in possession of reflectance data for materials that have <95% reflectance @ 600nm?   (answer)

I am a massage therapist trying to do research on what color is best to have in a massage room and how it affects the client, such as improves relaxation, promotes healing. Any guidance?   (answer)

Does a sheet of colored glass reflect, refract or absorb light? Or all 3?   (answer)

Could you possibly tell me the formulas for the harmonic equivalents (tones) of red, orange, yellow, green, blue, violet?   (answer)

I need a fairly fool-proof way for a moderately skilled technician to measure a large number of samples and present the data using the CIECAM02 coordinates. I am willing to buy a new instrument/software. Any thoughts?   (answer)

I am studying the colour of abalone shells. How can I statistically differentiate cbetween two shells? Eg: this shell is bluer than that one. I plan to use digital photos, D65K light with the camera at the normal angle.   (answer)

Does the color of one's eyes (light colored vs dark color) have any effect on vision?   (answer)

Can the color of the bedroom walls prevent proper sleep? My husband sleeps fine in a blue room, but has trouble in a red room (same bed!).   (answer)

Are car color choices determined by age, gender or ethnic group?   (answer)

What is linear tiff data. What does it record?   (answer)

What are the differences in perception of color by males and females?   (answer)

Does the color of glass effect the index of refraction?   (answer)

Why do colors abosorb and/or reflect heat?   (answer)

What trends are typical when Munsell testing a group of people? Are there effects due to age, gender, education, or ethnicity?   (answer)

Why doesn't Pointer's gamut (CR&A, 1980) fully cover all real points in Munsell system as defined by real.dat?   (answer)

I heard that HDMI for HDTV's can have billions of colors but I also heard that our eyes can only see around 10 million colors. Does this mean all that extra color doesnt matter or will it seem clearer in some way?   (answer)

Is there any standard by which Munsell colors are called "light" or "dark" or are those terms that are too subjective so that they don't get used?   (answer)

Do you know of any freely available color matching software?   (answer)

sRGB has a gamma of 2.2. Why do the equations have an exponent of 2.4?   (answer)

Can you comment on how does the following colors affect human psychology when considered in relation to fashion designing: red, yellow, blue, green, orange, violet, brown, pink, white, black, grey?   (answer)

Are there any filters that can be placed over the eyes to make a substance, which is invisible to the naked eye, visible through the filter? Can you give an example?   (answer)

Can you provide me with a formula for RGB values given Munsell coordinates?   (answer)

Is there a range of Munsell or L*C*h coordinates that would describe browns?   (answer)

With all things being equal, is there any reason to see an increase in black dot gain simply by changing the print sequence from CMYK to KCMY?   (answer)

What is the ideal working space for digital painting in Photoshop and painter is. I tried using Adobe RGB but my reference images look so saturated.   (answer)

What color is best to paint a room with hardwood floors to enhance productiivity?   (answer)

Is there a guideline, perhaps application-based, for the number of patches needed to make adequate ICC profiles for printer output? Is there an upper limit on the number of patches beyond which more patches do not necessarily improve the ICC profile?   (answer)

I am looking for products to protect my eyes from UV-C light (254 nm). What substances are opaque or reflect to UV-C light? How harmful is UV-C light to human skin and eyes? Can you suggest any protective measures I should take?   (answer)

hat is the correlation between Hunter whiteness (L, a , b) and CIE whiteness (L*, a*, b*)? Can I relate them mathematically?   (answer)

I have a Minolta 2600d portable spectrophotometer. Can you tell me the best way to measure the fluorescence of teeth in vivo? Also, can I use the 2600d to measure the opalescence of both natural teeth and dental materials?   (answer)

Is there a certain color that has been proven to be least tiring to the eye on web page backgrounds?   (answer)

What are the calculations used to convert spectrophotometric spectral data to density, CIE XYZ, and RGB variables/values?   (answer)

Why can't humans see colors that fall in the UV or IR part of the electromagnetic spectrum?   (answer)

Could you please tell me the equivalent Federal No. (Std 595) for Munsell No. 2.1 YR 5.6/16 (Orange)?   (answer)

How can I convert NCS color mode to RGB mode in Photoshop?   (answer)

Do HSV and HSI refer to the same model? If not, what is the difference?   (answer)

How often should color vision testing be assessed if the ability to see colors is important in one's occupation?   (answer)

I would like to make a children's computer program emulating colour mixing of paints. Since coputers generally use the RGB colour model, I don�t know how to achieve this. Can you help?   (answer)

Can chromatic discrimination change over time or can it improve through practice?   (answer)

Any correlation between age and chromatic discrimination ability?   (answer)

Do any illnesses/physical characteristics/disabilities impact one's chromatic discrimination ability?   (answer)

How can a color display with three monochromatic lasers produce a finely rendered colorspace, with what seem like the appropriate pastels and everything?   (answer)

What are the different textures of colour?   (answer)

How can I convert CIELCH to CIE 1931 x,y coordinates?   (answer)

What is the utility of converting tristimulus values to L*a*b* values?   (answer)

Can my company's color references be accurately reproduced 10 years from now based on the tristimulus values we maintain ?   (answer)

How do the eye and the brain work together to create the chromatic adaptation 'illusion' effect with colours? Also, why does looking at certain stimulus for certain lengths of time effect the time span of the adaptation?   (answer)

What makes wet grass greener than dry grass? A scientific explanation of more than just refractive index would be much appreciated.   (answer)

What does the ratio b/a give or mean? Here a is negative and b positive.   (answer)

Can you provide list of non toxic pigments that are insoluble in water measuring about 1 micron particle diameter?   (answer)

I am preparing a 3 band fluorecent lamp powder recipe for color point x=0.312, y=0.336, Ra= 82,SDCM<3. Can you provide any useful recipe development procedures, methods or any useful website for me about the color coordinates adjustment?   (answer)

Can you help me transfer color temperature to 1931 CIE xy-coordinates?   (answer)

I am shopping for a new motorcycle helmet and want to know the color's that would keep my head cooler in the sun. Where do colors such as grey or silver fit in as far as reflecting light/heat and staying cooler?   (answer)

Is there any research on changing the colors of a light to improve a surgeons abilility to see in different surgical enviroments? Can you improve their contrast sensitivity like skiers do with colored lenses.   (answer)

How do I assign a RAL number to a standard color used by my company?   (answer)

I'm measuring several 3000K white lamps. I'd like to know the color variation of these lamps. Is there any other way to measure color variation of a population of lamps, aside from standard deviation and MacAdam ellipse?   (answer)

I require formulation for a Paint that absorbs magnetic rays or RF energy used by RADAR and do not reflect back.   (answer)

Is it possible for a liquid subtance to appear clear or colorless when placed on a white piece of paper with the naked eye, but will appear a unique color when seen through a filter?   (answer)

I am interested in obtaining a very accurate spectral match of my color sample. Some of the color matching systems I have used in various paint stores do a pretty good job of matching the color but not necessarily the spectrum of my sample.   (answer)

Can you explain which coloured dyes change colour or fade and why when used as a gamma radiation dose indicator?   (answer)

What is the problem if a person sees both gold and silver as silver? This person also has red-green defects.   (answer)

What is the top that is black and white and makes pale colors when spun?   (answer)

I would like to perform the Farnsworth-Munsell 100 hue test on my monitor. Will this work? Can I have the values of each colour caps to form the colours?   (answer)

I am creating a colorant database for my matching software. How do I reduce the bronzing of Pth Blue at high concentrations?   (answer)

Which colored paper reflects more light?   (answer)

The color of an object depends on the reflection and the absorbtion of light. How?   (answer)

What are good books to get detailed information on both the Munsell and CIE systems? I'm an artist and have a general education of both systems but would like to know indepth knowledge of each, including the math and reasoning behind the constructions of each system.   (answer)

I am building a fiber glass airplane. The kit mfg says paint it any color you want as long as its white due to heat build up in the sun. Any idea of how far off-white I can get? How can I calculate the increase in heat absorbtion if I go a little blue or yellow off of white?   (answer)

Is there a specific color that could be called Munsell green? What is its Munsell designation? I understand this color is commonly used at places like university campuses for exterior electrical equipment enclosures.   (answer)

How is the location of an illuminant on the Planckian Locus and R-G Axes related to colour vision tests?   (answer)

What is the "straight line rule" when referring to the CIE 1931 Chromaticity Diagram?   (answer)

Which RGB values or others make color that is visible only under ultraviolet ray on the white paper?   (answer)

If one measures same color on various spectrophotometers the spectral profile is the same,but there are differences of 2-3 % in reflectance. Can you explain why? Which is the most correct reading?   (answer)

Can I use BCRA standard tile for calibration of spectrometer? Where can I buy it?   (answer)

Can you please define "dominant wavelength" and "wavelength of maximum absorbance"? Which should be used in the calculation of strength?   (answer)

I am writing a story about colour for children. Can you help explain (in simple terms) how a rainbow works and how the mixing of just a few colours in printing pictures in papers works?   (answer)

Would you recommend a source for standards tile for calibration of spectometer?   (answer)

How long does it take to perceive color? Are some colors perceived more quickly than others?   (answer)

I have an RGB digital SLR camera. Is it possible to find relationship between the camera output and the spectral reflectance of objects?   (answer)

How can I determine color names corresponding to hex color values?   (answer)

What is the meaning of NET profiler of a color spectrophotometer?   (answer)

When I measure delta L on low-gloss (matte finish) parts, I notice more variation between measurments than with higher gloss parts. Is there a scientific reason for this? Does the gloss play a roll?   (answer)

How can I extract a specified color from an RGB image using Matlab?   (answer)

How does one go about using use color measurements to determine the concentrations of colorants/dyes in aqueous solutions?   (answer)

I evaluate llighting systems by measuring u'v' with a spectroradiometer. How can I calculate deltaE color difference from this data?   (answer)

How is the Light Reflectivity Value (LRV) measured? Why do building codes require a certain percentage of LRV in the materials used?   (answer)

Do people with light colored eyes have better night vision than people with dark colored eyes?   (answer)

Do you know of a color/temperature chart showing various colors and their related heat reflection/absorption properties?   (answer)

What sorts of jobs would a color scientist do?   (answer)

What is the relationship between the CIELAB a*-b* opponent axes and the perceptual unique hues?   (answer)

When the 3 hands on a spinning toy (which are 3 different sizes, long, medium and short) spin around really fast, they leave 3 "trails" of light in concentric circles. Why do we see 3 complete circles of light all the time?   (answer)

Why do colors fade in the sun? is it the heat or the light?   (answer)

Do humans have a natural affinity for certain sorts of colors more than others as compared to other creatures?   (answer)

Do insects that have UV sensitivity in their visual system see the UV portion of the spectrum as "color"?   (answer)

Is color limited to the interaction of light with an object and our eyes?   (answer)

If no light is falling on an object, does it still technically have a "color"?   (answer)

How does a color TVthere is a great website describing how TV works at . work?   (answer)

How can I know the spectral sensitivity of a common CCD camera?   (answer)

Does the color green inspire thought?   (answer)

Do objects that are the same color have the same chemicals (elements) present? For example if I looked at the chemicals in red paint would I find the same ones in a red flower?   (answer)

How I can mathematically convert from CIELAB (D65,10�) to XYZ (D65,2�) or RGB?   (answer)

How do the colors you wear affect your appearance? What colors make you look certain ways?   (answer)

What is an easy way to create a color scheme using an elliptical path and a Munsell conversion program?   (answer)

Is the temperature the same in shadow under a dark colored fabric awning as under a a light colored awning?   (answer)

Sailors know from experience that darker colored fabrics last longer in full-sun exposure than white or very light colored fabrics. I am specifically referring to Sunbrella bimini and sail cover fabrics. Why?   (answer)

How do grey scale images differ from color (RGB) images?   (answer)

How can I match a paint color to Munsell N8?   (answer)

I'm looking for a Circular Spherical Vision Spectro Colorimeter for analyzing luminance and other parameters (axis etc...) of reflective materials in different colors for road construction. Can you help?   (answer)

Is there a source for a neutral gray paint designed to be used on floors and/or walls in a TV broadcast studio?   (answer)

I would like to find pigments with relatively narrow spectral aborption bands. The goal is to create paint which offsets the peak emission lines of different artifical light sources. Can you help?   (answer)

As an Interior Design student I know that paint manufacturers, geologists, anthropologists, and color designers use the Munsell system of color notation. What other industries currently his system to code their color? Automobile industry? Textile Industry?   (answer)

Why do we recognize uniqueness only for four colors, blue, yellow, green, and red? Is there any color map of unique hues in a visual system?   (answer)

How do we see metallic colors like gold and silver?   (answer)

Why is a red Popsicle red?   (answer)

What is the difference between value and chroma? If I add black or white to a color I am changing its value. How do I change its chroma?   (answer)

I measure pigments from bird egg shells with a spectrophotometer CM-2600d, should I work with 100%UV or 0%UV? What about pigments from bills and legs of birds?   (answer)

Out of blue, green or peach, which color is more likely to attract flies or insects and why?   (answer)

What effect does light have on white paper: does it reflect or transmit?   (answer)

What is chromatic textile? What properties do this textile have?   (answer)

How do I compute the correlated color temperature of an Lab/Luv color coordinate?   (answer)

I'm trying to understand whether there is an optical/physical basis for this "chameleon effect" (the ability of the materials to blend with surrounding tooth structures to create naturally appearing restorations) and whether there may be way to measure it.   (answer)

I'm looking for the paint formulation for the official colors of West Virginia University. The Pantone colors are either: A) PMS 873 metallic gold and PMS 295 Navy Blue or B) PMS 124 Gold and PMS 286 Blue. Can you help?   (answer)

Can you help me calculate the color difference between my Macbeth Colorchecker and its image after capture and display on an LCD monitor?   (answer)

I would like to purchase a grey scale color variation kit for comparing the change in color wash for Denims and Textiles. Can you recommend a source for this kit?   (answer)

Why does eye (iris) color appear to fade with old age? Is it because of the yellowing of the lens?   (answer)

Where can I find a tabulation of all pairs of distict spectral monochromatic lights that are perceived as achromatic?   (answer)

At our paint-supply company we need to match a color to existing color on a clients' wall. Is there a portable instrument we can use to determine the required mix to match the exisitng paint?   (answer)

How can I know the tricolor chromaticity coordinate of a common CCD camera?   (answer)

How does color affect our vision?   (answer)

How do I calculate the chromaticity coordinates of a single wavelength of light?   (answer)

What factors affect the intensity of color?   (answer)

What do I tell the paint store to mix in order to get a color that best matches a wavelength (5040 Angstroms)?   (answer)

Do different people see the same color in different shades? Why?   (answer)

How does one resolve the concepts of the 'scientific' (linear) color frequency model and the 'artistic' (circular) color wheel model in regards to color perception?   (answer)

I have a thin PET film that is colored with dyes. I am suprised by the amount of dye required to match the opacity of the current film. Would you say that dyes are more linear than conventional pigments are when referring to thickness and saturation?   (answer)

Do you have a table of NCS color code corresponding to RAL color code?   (answer)

What does RAL stand for?   (answer)

I am a member of an internet forum dedicated to the Rubik's cube. What is the set of 6 colors (including black/white) that human eye is best able to discriminate?   (answer)

What is the equation for plot a MacAdam ellipse? Also, what steps are the Macadam ellipses on the standard x,y chart and what do these "steps" mean?   (answer)

I am a competitive rifle shooter. Are there any colored filters to place in my rifle sight that would help to reduce eye fatigue or even simply aid in aligning things?   (answer)

What is the best method to measure the opaque black color of a plasic material?   (answer)

What distance can humans still distinguish between different colors?   (answer)

What wavelength out of red, green, blue and full spectrum would purple cabbage reflect, and what would it absorb?   (answer)

Is black paint recommend for a boy's bedroom?   (answer)

What's the RGB composition of RAL 7005 & RAL 1021?   (answer)

How can I measure the LRV value of something. Or can I convert CIELAB data into an LRV value?   (answer)

What material has the most uniform reflectance spectra, including visible, IR, and UV?   (answer)

Why are common soccer balls black and white?   (answer)

Can you help me convert between Pantone and RAL colors?   (answer)

Do clear objects reflect or absorb heat?   (answer)

Is there a simple formula to take numbers obtained by matching paint samples to the 1929 Munsell book and convert them to the numbers that would be obtained using the current Munsell book?   (answer)

Is it possible to make a color that is not visible to the human eye?   (answer)

Do age and gender matter when seeing color?   (answer)

I need a color swatch for 10YR 9/1, do you have this?   (answer)

We are trying to measure color reproduction for digital cameras using the Macbeth ColorChecker. We image the Color Checker under various illuminants. Is there a better metric to calculate the color reproduction than ΔE?   (answer)

I am trying to image a white fabric bag using a light source inside of the bag. Are there any wavelengths of light that would be most appropriate? That is, which wavelengths might be absorbed by a white acrylic fabric?   (answer)

What colour paint dries the fastest?   (answer)

What hue can the human eye dicern the most shades of?   (answer)

What does the color blue mean?   (answer)

When reading a violet, what does the Δa and Δb values represent? Example is the Δa dirty/clean or red/blue?   (answer)

In literature about discoloration of wood (furniture or parquet floors), UV-light is mentioned as the cause for the discoloration. But doesn't the glass window act as a UV-filter? So is the discoloration caused by visible light instead of UV-light?   (answer)

How I can find the formulation of paint after coating?   (answer)

I am wanting to know how broad the primaries were for the 1931 color matching experiment. Were they monochromatic, as in a laser? If not, what was the spectral half width at half max for each of the colors?   (answer)

Does the color of an ice cube affect the rate at which the ice cube will melt ?   (answer)

Can you tell me please what the CIELAB reading is of matte Munsell gray to be used inside color viewing cabinets for the print industry?   (answer)

I would like to measure the color intensity of various parts of birds. What instruments will I use?   (answer)

Why does some plastic yellow over time?   (answer)

Can you help me convert between candella and power wattage as measured by photodiode?   (answer)

I am trying to characterize my computer display. I have a failure of additivity. That is, the sum of the individual R, G, and B XYZ values does not equal the XYZ of white. Can you help?   (answer)

Can the brain take a 2D image (eg: a picture) and convert it to a 3D image?   (answer)

Assuming a monitor calibrated to the sRGB standard, is there a combination of available adjustments (gamma/contrast and/or saturation) that can be calculated and used to provide a better 'ballpark' display of an NTSC signal on an sRGB monitor?   (answer)

Can you tell me what color eye shadow would look the best for a given complexion?   (answer)

Can you tell me what the cmyk formula for silver is?   (answer)

Can one measure color through heat frequencies?   (answer)

What are the colour combinations or chemical substances that can be used to obtain jet black colour in a cotton flannel material?   (answer)

What is the real meaning of color blind?   (answer)

Did humans use chromatic adaptation long ago, for example in hunting?   (answer)

Does the color of your eyes affect your vision? Do people with blue eyes have better vision than those with green?   (answer)

What is the minimum ΔE that a human eye can detect? Which &Delta E"   (answer)

Do you know of any table of rgb value pairs that make up visual complementaries (colors that are afterimage of each other)?   (answer)

Why is blue light plus yellow light equal white light, but blue and yellow paints mix to green? Why do the color wheels from various systems use different primaries?   (answer)

When refering to "pure" colors (red, green, blue, orange) what is the corresponding notation in the Munsell system?   (answer)

What's method that appropriate for measure color of gemstone?   (answer)

How does color affect our mood?   (answer)

Which of these Munsell soil designations is the reddest: 5R 6/4, 5R 5/2, 5R 7/3, 5R 6/3?   (answer)

Can you help me explain (in simple terms for a 5 year old child) why some colors of popsicles melt faster than others?   (answer)

Can you provide the pantone equivalent to GE plastic FXM171R-GY2A360M?   (answer)

Can you help me find a cost-effective way to measure black-gray-white and all shades in between. My son is doing an science experiment to determine how well erasers work.   (answer)

Can you help me create the color green that is exactly in the middle of the green color spectrum using the RGB values of my display?   (answer)

I need an example or a graph help to explain why humans cannot see colors that fall in the ultraviolet or infrared part of the electromagnetic spectrum.   (answer)

Specifications on the web given by SAE, ECE eg, mention that the colour should fall within the given boundary:
y = 0.335 (yellow boundary)
y = 0.980 - x (purple boundary)

Is there anyone who I can pay to convert 12 colors with LCh values into Lab or Munsell values?   (answer)

In my prepress company we have to convert fabric samples by eye in to CMYK values. This is time consuming. Is there a software hardware combo that can read a piece of fabric and give the best cmyk values in a specific profile?   (answer)

How does the eye see colors that are moving very rapidly? (eg: a spinning color wheel)   (answer)

Why does light create color?   (answer)

What is neutral grey colour shade and the pantone number for neutral grey in pantone formula guide solid uncoated and uncoated ?   (answer)

How can I make a colored chromaticity diagram?   (answer)

Could you recommend a good website for the glossary of color imaging terms?   (answer)

Do you know of any research into the quantity of a particular colour eg., yellow that is in a painting and whether that painting is one that gives pleasure?   (answer)

Which colour has the most easily visually discriminable levels of saturation? The literature seems to suggest blue violet as this has the most steps when white light is added in 'just noticeable' amounts - would you agree with this?   (answer)

What do the various CIE illuminant designation mean (D55, D65, etc)? How are they related to each other and to CCT?   (answer)

What colors should I use for my experiment on which colors absorb more heat? I already know I am doing black and white.   (answer)

How do colored objects reflect heat?   (answer)

Can you help me transform Lch values to Lab?   (answer)

Does the first color you see when you wake up effect your mood?   (answer)

Does the color of your car determine the temperature in the car?   (answer)

Do you think people can remember words better from colored paper or white paper?   (answer)

I would like to do a science experiment on "Will male and females chose the color that has been associated with their gender?" For example, pink for girls and blue for boys. Can you help be to create a 5th grade science experiment to test this idea?   (answer)

In literature it is mentioned that the difference between SCE and SCI color measurements gives an indication for the gloss of the surface. Which parameters are used to evaluate this: the difference between the SCI and SCE values for CIE-L, a, b or a combination?   (answer)

What would be the reason to use CIELAB, CIELCH, CMC, CIE94, and CIE2000 (all of the above together) to calculate color difference from sample to standard?   (answer)

What is the best way to test color fading in construction paper? My child is making an experiment on what color will fade the fastest when exposed to sunlight.   (answer)

What chemicals are used to create fluorescent paint?   (answer)

Can you provide the CMYK formula for ANSI Safety Green?   (answer)

Is there direct co-relation between CIE 76 and ANLAB 40 and ANLAB 50 color spaces?   (answer)

What is the primary calibration of a spectrophotometer?   (answer)

Are HSL values that the Paint accessory of Windows system use recognised by CIE ?   (answer)

Is there a mathematical conversion from RGB to HTML color code?   (answer)

Can you derive CIELAB values directly by visually inspecting the reflectance curve?   (answer)

Can you explain the mechanism of UV filter in color spectrophotometer, to get universal Whiteness values on all spectros. Is it a wedge filter or an attenuator filter.   (answer)

Does the color of light affect how much snow is melted?   (answer)

Can human eyes see noise? Or, can our brain recognize what's signal and what's noise?   (answer)

I work in printing. Can you tell me what the effect of dot gain is on delta E? Also, can we get an accurate delta E measurement from a coated sheet to an uncoated sheet?   (answer)

What colors absorb more heat, when a piece of ice is sitting on the piece of paper on a hot and sunny day?   (answer)

How specifically does a magenta-dyed silver halide crystal absorb green energy? I know it has to do with wavelength ... is magenta somehow 'dovetailing' with green light to chemically interact?   (answer)

How do I figure out what color a reflection of another color might be? For example, if I have a yellow car and something red is reflecting off of it, what color will show up as the reflection on the car?   (answer)

What effect renders UV lamp when selecting the colour with use light booth?   (answer)

Which will melt faster, blue Popsicles or red Popsicles?   (answer)

What would be the ideal color for drafting rooms or any rooms used by art students?   (answer)

What is a full-spectrum paint and what is it good for?   (answer)

Is there a concordance listing the ISCC-NBS color name for each chip in the Munsell Book of Color?   (answer)

When visually comparing colors, why is D65 light source better to use than Illuminant C?   (answer)

We are looking at colour and redness of scars on human skin. Which is better for the job, a hand held spectrophotometer or a hand held chromameter? Some literature validates the use of a chromameter CR300 in skin research, but spectrophotometers are apparently more accurate.   (answer)

Can you tell me the name of any software for changing a gray image into color. Is any software to convert a real time gray video to color?   (answer)

I can get x,y,z color coordinates. So how would one relate to the materials response for a particular wavelength?   (answer)

How can I calculate hue angle from CIELAB? What is pi?   (answer)

Is there something like color adaptation? I was wearing a blue light filter over one eye and felt that the differnce in color perception between the right and left eye disappeared after some time. When removing the filter, color perception in the previously uncovered eye felt like wearing a blue light filter for a short period of time.   (answer)

What is the RAL number that should be used in the background of a light cabinet?   (answer)

If someone walked down the street, what color clothing or facial features would that person have for me to easiest remember him/her?   (answer)

Does age and sex determine what color car a person buys?   (answer)

We are a printing company. We print offset (litho), gravure and flexo, both on coated and uncoated paper. What spectrophotometer should we buy spectrometer for Quality Control and Color matching?   (answer)

I am doing a science project on what colors are easiest to see in the dark. Can you help me figure out what experiment I should do?   (answer)

I want to purchase an LCD HD TV. But I can't decide between 2 TVs. One boasts 3.2 billion colors and the other 16.7 million colors. Do I care?   (answer)

Does the color of water effect its heating rate?   (answer)

I capture video camera RGB values through a microscope for image analysis of translucent microfossils. If I repeat this using a different camera I get different RGB readings. Are there any translucent standards available that I can use to calibrate between the two setups?   (answer)

I would like to ask if colour filters use to photograph - the filters we use to put in front of a lens in a camera- is a subtractive method?   (answer)

Where can I purchase a "student model" Munsell Color Tree?   (answer)

I need to measure human skin color directly. What instrument should I use?   (answer)

I am building a color & B/W headshot photo lab. My primary light source is 5000K fluoresent tubes with a CRI rating of approx 93. Since skin tones are critical, what color should I paint the walls?   (answer)

How do yo measure ΔE*cmc, ΔE*94, ΔE*fmc2 from the L*, a*, b*, C*, h* ΔH*, ΔE*?   (answer)

How can people distinguish edges of objects? Is this a function of rods or cones, or some organs else?   (answer)

I have a mapping website. Is there any function or algorithm that produces a collection of discrete colors that all have some minimum level of perceptual contrast between each other?   (answer)

When measuring the color of colored plastic plaques with a spectrophotometer, how thick should the plaques be and why?   (answer)

On what things color depend so that only some colors disappear while others remain?   (answer)

Are there metrics that quantify whether items in an image are discriminable or identifiable?   (answer)

I am doing a problem on image permanence for large format inkjet. Should I use Delta E or Delta E 2000? Why and what is the difference between the two?   (answer)

Could you help me convert the Munsell codes to RAL codes?   (answer)

Will we ever be able to see colours we have never have seen? When I tell other people about my wonder to have different perceptions they think it is bogus. Will genetics and medicine make it possible in the future? When?   (answer)

Why is it more difficult to say the color of the word when it is printed in a different color?   (answer)

We always measure the color difference by using d/8 spectrophotometer with included specular reflectance. But why do we always measure the white with excluded one ?   (answer)

I have an emission spectrum from a sheet of luminescent material. Can you help convert the emission spectrum to an RGB color to display?   (answer)

Can white light be captured inside an opaque container. If not, why? When the source is disrupted it becomes dark. why?   (answer)

What is the best color model for use with colors that have a low saturation? CIELAB is not working for me because when a* and/or b* are close to zero I get poor values for hue.   (answer)

Can you hep me match the PPG paint color "burnt copper" with a CMYK color so I can print material that match the paint on my motorcycle?   (answer)

I have to purchase a system to compare shades of yellow in plastics with rough surfaces. Would you please recommend a company and/or instrument that would have good repeatability and be low maintenance   (answer)

Are RAL 7032 and ANSI 61 are same colours? Where I can I locate a colour cross reference table between RAL & ANSI standards?   (answer)

Why are there chromaticity values of zero or less than zero in the Munsell data on the MCSL web site?   (answer)

How do I measure contrast?   (answer)

Do the tiles in the Farnsworth Munsell 100 Hue test kit require calibration? If so, what can change that would require their recalibration? If they require calibration, what is the frequency?   (answer)

Does there exist a (known) correlation between a Minolta CM3500D and a BYK Gardner or does this depend on the product type?   (answer)

What are "suprathresholds" and "thresholds," with reference to color difference?   (answer)

Can the Munsell 7GY3.29/1.5 Green be converted into a ANSI number?   (answer)

What color will a solution that absorbs all the green, yellow and red light appear?   (answer)

CIE chromaticity diagrams are usually exhibited in a shape of a hoof. Why are the single waves spread on the ellipse side instead of the straight?   (answer)

Do you know the RAL Number for a cream colored paint known as Queen Ann or Magnolia?   (answer)

Of papers colored blue, white, red, green, purple, yellow, and black, which would be easiest to remember things if they all had black letters (except the black paper, which would have white letters)?   (answer)

Why do flat black objects get hotter than lighter colors?   (answer)

Which color model is best suited for tonal changes (i.e. curves) without changing the perceived color (hue, saturation; however defined)?   (answer)

Suppose that we want to measure the difference between the color of a test object and a reference object. How would I choose between using RGB color space and CIE Lab for color difference?   (answer)

Can I determine the color rendering index of this source purely from the light-emission spectrum?   (answer)

We capture colour images printed on paper through a CCD camera. How can we eliminate the scattering effect of the paper and to account for optical dot gain?   (answer)

Is there a chemical compound for invisible uv red ink compound. How we can get some or make our self?   (answer)

I want to create 3D look up table for CMYK data using Matlab. Can you help?   (answer)

Are there any objects that can physically reflect UV light to be able to be seen with the unaided eye?   (answer)

Can you provide an example showing how to calculate ΔE using Δa*, Δb*, ΔL*, ΔC*, Δh for textile pigment?   (answer)

I want to see what make-up shades and colours suit which skin tone, for a school project. Please get back to me with a colour wheel and and some useful information.   (answer)

I'm a wood scientist. Can you help me convert values from NCS to CIELAB?   (answer)

Should ideally conditioned standards (kept humidity controlled, dark envelope, minimum exposure to light, minimum handeling) develop metamerism?   (answer)

I have a Munsell paint specification of 7GY3.29/1.5. My paints use IF numbers. Can you help me transform this request to an IF number?   (answer)

Does the melting rate of a substance vary by color?   (answer)

Why does the spinning Benham top look colored? Is there any simple explanation of that phenomenon?   (answer)

How is a star's color different based on the temperature?   (answer)

What size print can I make with a 8 megapixel camera? If I print in high resolution (300dpi) and low resolution (72dpi). There is any formula to do that transformation from megapixel to inch?   (answer)

Can you help me compare RAL specification paint with ANSI paint for color match?   (answer)

Please can you explain what Macadam limits are (as mentioned in description of the 'real' set of Munsell values on your Munsell Renotation Data page)?   (answer)

I am searching for paint color mixing recipes for liquid (bottle/jar) opaque water based pigment paints. I have found a book of colored paint mixing recipes for paste (tube) bodied paints with a volumetric proportioning system for only paste consistency (tube) paints: Color Mixing Recipes ; by Walter Foster Publishing Staff, (48 pages, Trade Paperback); 2005. Do you have any resources and/or pertinent leads that you could provide me?   (answer)

I am a dental student. As part of my project I need to convert the RBG values obtained from Adobe Photoshop into CIELAB values in order to compare with values obtained with a spectrophotometer?   (answer)

Can you explain how to select the color center (or sample color) which can be used to evaluate color difference formula?   (answer)

I am trying to find a precise, systematic way to describe the colors of inks and metals used in medieval manuscripts. For example, I want to be able to define "red" as a range of similar colors and "scarlet" as a different range.   (answer)

Can you help me to discriminate between color space, color model, and color system? I'm confused!   (answer)

People of my area prefer reddish black on textiles. What kind of black do people in your area prefer on textiles?   (answer)

As we know black colours, for example on textiles, usually have a pale tint. Is there any formula for measuring blackness?   (answer)

How can light affect an interior color? For example: what is the effect of natural light on a red wall, blue wall, etc.   (answer)

Where can one buy an anomaloscope? Does an instrument exist that has a bipartite field like an anomaloscope but allows any combination of one or two monochromatic lights in each half of the field?   (answer)

What is colour? How is it detected?   (answer)

Does color affect the reflection of heat?   (answer)

Would CIECAM02 or CIEDE2000 be better at predicting spot color differences?   (answer)

How many colors does Munsell solid establish?   (answer)

Can mixed paint be measured while it is still wet? In our factory we mix paint, we want to measure it's color before painting parts.   (answer)

I would like to know if there is a way to determine the amount of ultraviolet light absorption by causing a change in the color of a particular substance and what the substance would be.   (answer)

I am trying to describe the colour of my sediment cores and have L*, a*, b*, and C* data from a spectrophotometer. I am looking for some directions on how to 'describe' the colour of my mud! Is there some tool or guide to convert numbers into a colour category?   (answer)

I painted a very pale yellow on one of my bedroom walls. I do not like the contrast with the country blue on the three other walls. Can I paint a brown that complements the blue over the yellow without affecting the brown color?   (answer)

I wish to have a white bedroom, where in the walls ceiling and floor are white. The windows doors and all wooden furniture will be painted white too. Kindly advise me on the suitability of the scheme. Should I be using stark white or would the use of ivory white be better. What should be the finish of the paint?   (answer)

Is it true that only some dreams are in color, while others are in shades of gray?   (answer)

Sign language interpreters have always been taught that they should wear colors that are in contrast to their skin tone to create maximum visibility and minimize eye fatique. Is there a more scientific way to teach these principles? What does research tell us about clothing color and eye fatique?   (answer)

I am trying to get the appearance of gold and silver on my monitor but they always look like yellow and silver. How do you get metallic gold on a monitor?   (answer)

How does one measure color of ink in its raw liquid form? I need to measure the color in that form. What tools are required? Are there standardised methods to sample ink to measure it?   (answer)

Somedays my eyes look green and other days they look more blue. What color of clothing or eye shadow would I wear to make them appear more green? What colors should I NOT wear if I don't want them to look blue?   (answer)

Each of our spectrophotometers has its own calibration standard. If all of the spectros are not calibrated to the same standard, aren't we calibrating in variation between the spectros? We can see anywhere from a .2 to over a .9 difference in our ΔE readings when we read the same areas of a sample on the different spectros. What would happen if we lost one of the standards since they are suppose to be unique to the manufacture of one specific specto?   (answer)

My customers set up the upper limit of ΔE=1. This a very difficult to match, and I've got the information that the human eye can't detect a deviation below ΔE=2. Which would you recommend to set-up? Also, what is the grayscale-measurement for plastic parts?   (answer)

I have read that a digital darkroom should be painted Munsell 8 gray. What should I tell my paint dealer so he can mix a color close to this?   (answer)

I have a paint is specified as Munsell colors and I need the RGB equivalents to display on my computer. Can you tell me the RGB for 1BG 6.3/1.4 and 4.6G 9.0/0.5?   (answer)

What is digital colour? The history of digital colour? Digital colour challenges? Problems encountered with digital colour?   (answer)

I've seen so-called UV energy beads change from white to various colors when exposed to UV light. What type of chemicals are embedded in those plastic beads, and are they toxic?   (answer)

I am testing whether colored (red and blue) backgrounds enhance or detract from a person's ability to assemble puzzles. Have these experiments like this have been done before? Any idea which age groups to test?   (answer)

I need a means to estimate the difference in general light reflection between walls in a windowless room painted matt magnolia (Munsell 10YR9/2), compared to matt brilliant white.   (answer)

What is the Reflectivity Index and what is the index for various colours, particularly silver?   (answer)

Blue colors wouldn't have to have higher UV reflectance than other colors and I am sure exceptions can be found. However, in general blue objects are reflecting highly in the short-wavelength end of the spectrum and that reflectance continues into the UV. Another way to look at it is that the wavelengths they tend to absorb are mainly longer than the the blue and UV wavelengths while other colors are largely absorbing the shorter wavelengths.   (answer)

Why does the color blue have higher UV reflectance properties than other colors?   (answer)

I am an artist, and I would like to display the color of the universe. Can you send me the RGB values?   (answer)

When I transform D65 illuminant values (in CIELAB) to RGB, I do not get R=G=B. Why?   (answer)

Various color systems (NTSC, SMTPE, CIE) use different white points. How are these defined?   (answer)

Can you provide information on Ultralume U5000? Also, what are ΔE GE, ΔE Audi,and ΔE PQ?)   (answer)

Has research been done to determine how closely the color receptors in various species match what one might expect given their evolutionary environment?   (answer)

What are these devices: densitometer, spectrophotometer, and colorimeter?   (answer)

When driving at night, what colors on cars are easier to see and what colors are harder to see?   (answer)

Why was Munsell's colour system the most widely and globally accepted one?   (answer)

I want to scan the Ishihara plates and create the same colors on my monitor. Can you help?   (answer)

What is the equivalent RAL paint code for Munsell 3Y7.8/1.1?   (answer)

I want to write a paper on color measurement. Can you tell me the latest developments on this, and what we can do?   (answer)

I have a severe color vision deficiency. Can you help me modify images so I can see the world the way others do?   (answer)

Does color affect reflection of heat?   (answer)

To what degree is violet light magnetified to change into UV heat radiation?   (answer)

Can you help me separate Yu'v' values into Y, r-g, and b-y opponent channels for fourier analysis?   (answer)

Is there such thing as a perfect hue angle for Cyan and Magenta, in Lch?   (answer)

We design commercial kitchens with lots of stainless steel, white tile, and fluorescent lights. Why do our cooks prefer dark, mat finish tiles?   (answer)

What is the Pantone equivalent to color RAL 3020?   (answer)

I use a colorimeter that returns x,y,Y values. What units do they express?   (answer)

I have a densitometer that measures visual density. How do I convert the density values to CIE L*?   (answer)

Why are plants green. It seems they should be shades of black.   (answer)

I have a white printing on LDPE which is yellowing. How can I determinate the yellowing in a datacolor instrument?   (answer)

What is the differance in reflective properties between white and orange hard hats working in the sun? I know white reflects more but how much more does it reflect?   (answer)

In performing a mechanical task with several different colored backrounds, which colors would bring the most contrasting results in the efficency of which the task is performed?   (answer)

What is the maximum opacity for 1 micron thick white ink? is there a theoretical calculation?   (answer)

Is there a way to express Lch differences, in terms of units? (not referring to ΔL*, ΔC* or ΔH*) Hue for example is expressed in degree?s (eg 5°). What unit do I use to express the difference in lightness and chroma?   (answer)

The LAB values output by the GretagMacbeth Munsell conversion software are different than those I can input in Photoshop. Any solution for this?   (answer)

What is the best test for screening color vision in employees using penetrant fluorescent dyes to inspect manufactured metal items for defects?   (answer)

What is the best color to paint walls in a machine shop to enchance productivity?   (answer)

For color rendering/reproduction purposes, is it better to use a high-end CRT or LCD monitor for accurate color creation? What are the pros and cons of each system, and why have high-end CRT manufacturers discontinued their CRT products in favor of LCD?   (answer)

How can I transfer from Munsell notation to a single number indicator of overall color?   (answer)

I am working on matching a number of colours for plastic production. How do I convert my CIELAB values to colour pigments for production?   (answer)

What are the general norms for pale & pastel colours?Which are the colours (generic names) are considered as pale & pastel colours?   (answer)

If the fovea contains no S cones why don't humans have functional dichromancy in foveal vision?   (answer)

The freeware from gretagmacbeth.com cannot convert RGB values less than (34,34,34), the software tells me that the values are "Out of Munsell Range". Does Munsell support no color below (34,34,34)?   (answer)

How can I measure the color temperature of a image?   (answer)

How does the color pink affect the minds of males and females?   (answer)

Are you aware of an algorithm in the public domain for calculation of NCS notation from reflectance spectra?   (answer)

Can you give me a ballpark magnitude of color difference in ΔEcmc from monitor to monitor (crt) that one might experience?   (answer)

Which is the most appropriate measurement geometry for quantitative evaluation of metallic colors? What are the variables that impact the viewing conditions (size of sample, illuminant, surround, intensity) and is there any research that addresses this questions?   (answer)

Does the color of ice affect its melting rate?   (answer)

How do I get the value of Munsell renotation coordinates?   (answer)

Is there any standard Colorimetric values of this GSC (Gray-Scale Comparison Method) method used for visual assessment was used by Luo and Rigg?   (answer)

I am a geologist working with Munsell colour chips and need to know how accurate and precise manufactured chips are and how long they can be expected to last (i.e. before fading).   (answer)

Please explain to me what the Munsell Color Disc and Munsell Charts are. The discs used are: red (5 R 2.6/13), yellow (2.5 YR 5/12), black (N1), grey (N4). I need to measure the color of tomato paste by this method.   (answer)

Could you recommend a book that would serve as a good introduction to colour science for the lay reader? I am interested in learning the basics of colour science but am not a scientist or mathematician.   (answer)

Where I can find the exact value of Xn, Yn (the luminance factor for the illuminant or perfect white) and Zn?   (answer)

What's the minimum Δfreq or Δwavelength an average person can differentiate so as to recognize two different colors?   (answer)

Where I should look for for the energy distribution of commercially available sources?   (answer)

What is the best method (color space, sci vs. sce, degree observer, Illuminant,etc.) for calculating Light Reflectance Value (LRV) on a spectrophotometer?   (answer)

Why do we have color vision, and what is the evolutionary drive for color vision? It is striking how colorful the man made environment is compared to natural settings. What was the selective pressure that made individuals with color vision survivors-breeders in our natural habitat?   (answer)

Can you tell me the RAL Number which is most closely match with Munsell N9.5?   (answer)

How is color temperature of a monitor set to a particular temperature, say 6300k or 9300k? Is there any specific formula for this? How can I change RGB values of a particular temperature to anothertemperature? Is there is any concept of change in offset of R,G,B?   (answer)

How does color affect heat absorbtion?   (answer)

I am looking for color matching software for matching translucent materials. Are there any commercial packages that employ multiflux theory to aid in prediction and correction of CIELAB values as well as contrast ratio in formulations?   (answer)

When is white not white?   (answer)

Why is yellow the lightest color of the spectrum?   (answer)

How does the yellowing of the lens with age affect: colour vision, scotopic vision and the contrast of tests charts that are spectrally neutral?   (answer)

Where I can find the ultraviolet reflectivity index of various colors? How about the heat absorption index for various colors?   (answer)

In a chromatic adaptation transform, how can we measure the white reference of the incoming video signals?   (answer)

Is there a reference offering a method for the bi-directional calculation of densitometric quantities (particularly status M) and spectral transmittance to be used as weighting coefficients in the integration to solve for XYZ tristimulus values?   (answer)

What is the difference between perceptual colour gamut mapping and categorical colour gamut mapping?   (answer)

What is the basis for 18% reflectance being the center of the lightness scale in many different systems? For example: Munsell, Ansel Adams zone system, and CIELAB. Coincidence? I doubt it.   (answer)

How you can relate the color temperature to RGB values of video signals? Will the color temperature vary frame by frame or will it vary pixel by pixel of the signal? Can you give the mathematical equation by which color temperature and RGB values are linked?   (answer)

I measure animal colouration, in particular contrast, which is dependent on spot size, not just area coverage. I cannot measure the spots on a lizard belly, but I can get a numerical matrix from a digital photo. Can you help me calculate perceived contrast?   (answer)

Would you explain the meaning of ΔH*?   (answer)

Can visual color discrimination be taught? Does color discrimination become better through practice? What affect does the aging of a person's eyes have on color discrimination?   (answer)

What is the scientific termonology for when you fold a piece of fabric and two areas appear to be entirely different colors? I've seen this demonstrated: after folding, both sections of fabric appeared to be different colors.   (answer)

I need to find a way to measure the saturation and brightness of printed colors and pen ink. I need to select hues of equal brightness and saturation in my studies. Can you provide information about machines that can make such readings?   (answer)

I have used a spectrophotometer to analyse colour samples. How do you interprete the curves, for example prussian red, since there are many peaks shown?   (answer)

My technical director asked if I could find an "easy to understand" explanation of "CIE 2000." He may mean the CIE DE2000 mentioned in this publication. Is there a publication or expert that could assist us?   (answer)

Can you provide me a graph of grayscale Values (0-255) verses density? I need it to evaluate a grayscale calibration wafer.   (answer)

How can I specify a color so that manufacturers the world over will use the same color paint?   (answer)

Could you recommend a source for calibration tile standards for a camera-based color measurement system?   (answer)

How do I create a value Intensity scale illustrating three dimensions of colour? Could you define a cool and warm colour and whats a colour scheme? Could you give me examples of complementary, split-complementary colors?   (answer)

Where do I find a definition of the term CIEJab? I found the term CIEJab in the Microsoft Longhorn specification in their document "Gamut Boundary Description and Gamut Shell.doc"   (answer)

Did Munsell ever design and make public an eight-hue color circle? If not, such false facts are circulating on the internet. For example, see mauigateway.   (answer)

Why do painted walls of saturated complementary colors cause eyestrain when placed close to one another?   (answer)

In relation to Kodak absorption filter graphs; What does "Transmittance" & "Diffuse Density" mean? and the difference between them?   (answer)

Regarding the use of Kubelka-Munk theory and reflectance values, if I used the spectral reflectances for the Macbeth Color Checker, how do I determine the K and S values needed for the calculation (if I want to used the two-constant theory)?   (answer)

Outside my building, someone has spray painted the letters USAF with blue paint. The snow under the paint has melted putting the letters in relief. How come?   (answer)

A customer has asked for these colors: Light Green: 0.5G6.25/6.3 and 0.5G5.6/7.0 and 0.5G5.1/7.5, but I don't know how to translate it. He told me it's from Munsell and color is light green. How can I transfer to Pantone or NCS etc. to make a sample for him?   (answer)

Can I increase the opacity of a white Ink (at a given thickness) without affecting the color? I use TiO2. Are there other pigments that will increase opacity without affecting the whiteness index or CIELAB values?   (answer)

Digital image sensors (such as those used in digital cameras)use red, green, blue ink-based color filters to generate color. Do they therefore have a color gamut that limits the range of colors that they can detect?   (answer)

Do you have any training materials to illustrate color that I could use as an educational tool?   (answer)

I have made four boxes out of colored clear plastic and will be putting glass containers inside full of snow inside each one, then leaving them in the sun to see which melts faster. The colors are red, blue, yellow and clear. Can you help me find information on how color will make the snow melt faster?   (answer)

Can you provide spectral reflectance values for the Farnsworth-Munsell 100 Hue test?   (answer)

I need a teaching aid that uses three areas of colored cellophane (R,G,Y; or R,G,B) which is set on top of a background with text of different colors. How can I find the right cellophane and text color combinations that will allow the user to see only one group of text, while that same text is not visible through the other two cellophane colors segments?   (answer)

I am working on a digital conversion for raw files from RGB digital cameras. Can you assist me with a formula to convert floating point data from a digital camera into LCH space and help with transforming that space through a LUT to 4-channel printer separations?   (answer)

Is there an analytical method for calculating relative powers needed to achieve a specific white balance for monochromatic sources (eg: lasers) of known wavelengths?   (answer)

My spectrophotometer is getting old. I need a new instrument and software for evaluation and formulation of color paints and ceramics. What recommendations can you make?   (answer)

I have a reflectance spectrum of a soil sample. Can you help me convert this reflectance to CIE X,Y,Z and then to Munsell HVC?   (answer)

How can I determine the whiteness/blackness of CMYK inks?   (answer)

Can you help me convert from a Munsell color specification to a RAL color specification?   (answer)

I am developing a visual program to pick color harmonies of a color based on RGB values. Are there equations that could help me determine color harmonies?   (answer)

Would you explain what ΔE is and how is it measured?   (answer)

Why might the published measurements for Munsell neutral scale differ? For example, the reflectance values for the 1929 "Munsell Book of Color" are different in the 1933 Munsell et al article as compared to the 1940 Glenn article.   (answer)

What is the method for selecting the best combination of CMYK inks for a cmyk ink jet printer? What is the best way to determine the CMY values in order to get as large color gamut printer as possible?   (answer)

I do not have access to a spectrophotometer to measure paint reflectances. Where can I obtain the reflectance data for determining the Kubelka-Munk coefficients?   (answer)

I want to calibrate my monitor by eye. Are there color charts available for download that can help me do this? I require skin tone photo, color and gray scale patches.   (answer)

What is the established color difference tolerance for the plastics industry?   (answer)

What is a color management system?   (answer)

How should a color lab be designed? What colors should the walls and furniture be, so as not to influence the perception of color?   (answer)

Can you help me convert between RGB or CMYK and Munsell, CIELAB, or CIEXYZ?   (answer)

My two eyes see color differently. What causes this?   (answer)

What are color vision deficiencies? How common are they?   (answer)

How do we see in color?   (answer)

What do we know about research done on colors & their meanings?   (answer)

What does it mean to be color normal?   (answer)

What parts of the eye are important for color vision?   (answer)

How do we select names for colors?   (answer)

How are cones organized in our eyes?   (answer)

What spatial and temporal processing takes place in the eye? the brain?   (answer)

How do we characterize a person’s color vision?   (answer)

How many colors can we see?   (answer)

As a painter and professor of art - I constantly come across references that optical colour mixing, as in the art movement "pointillism" as well as ink jet printing involves both subtractive and additive mixing in the eye of the viewer. I completely disagree that two dots of colour, at sufficient distance, can 'additively' mix in the eye. I am not a colour scientist, my knowledge of this is entirely empirical, but I would be interested to hear your thoughts on the matter? Is it possible and if so, how? (928)
Do you think that a TV screen or computer monitor are accomplishing additive color mixing? They are and they are doing so with small dots that additively mix by spatial blurring in the eye. The simplest proof is that Grassmann's laws of additive color mixing, derived for superimposed lights, also hold perfectly well for adjacent spots (spatial blurring) and those that flicker quickly in time (temporal blurring). I don' t know what more I can say, it's been proven empirically time and time again.  (Back to top)

Is there any known research on trying to find colors outside of the color spectrum? (927)
There's no such thing. The spectral limits of possible colors are defined by the response of the human visual system. It's a bit of a circular definition, but since the limits are defined by our vision, we can't possibly stimulate our visual system with energy beyond those limits.

However their are other creatures who have color vision that responds to types of energy we cannot see (e.g., some types of birds, fish, insects). Perhaps the most referenced of those is the honey bee. Bees can respond to ultraviolet radiation that we can't possibly see. If you google "bee color vision" you will find lots of information. Here's a link to one overview at .  (Back to top)

How do one estimate color of an object by just looking at their reflectance data? What is the relationship? (926)
You cannot estimate the color accuracy from just the reflectance. The color depends on the combination of that reflectance distribution and the visual responses to light. That is why it is possible to have different reflectance spectra produce the same color.  (Back to top)

The difference between "saturation", "chroma", and "colorfulness" can be very subtle. Can you explain? Similarly, is there any difference between the related terms "value", "lightness", and "brightness"? What do HSV and HSL describe? (925)
Starting with brightness and lightness, brightness is your perception of how much light is coming from a stimulus. On a sunny day, everything is brighter than under dim illumination. Lightness is the brightness of an object relative to the brightness of a white object under the same illumination. Thus, lightness is relative brightness and stays more or less constant across changes in illumination level. Value is a term that comes from the Munsell system and is synonymous with lightness.

There is a similar relationship with the chromatic terms. Colorfulness is analogous to brightness and refers to how much "color" (or difference from neutral) is perceived. Objects get more colorful when there is more light falling on them, just like they get brighter. Like lightness, chroma is relative colorfulness. It is the colorfulness of an object relative to the brightness of a white object under the same lighting. And like lightness, chroma is more or less constant across changes in illumination level. Then it gets more confusing because there is a second relative colorfulness term, saturation. Saturation is the colorfulness of an object relative to its own brightness (rather than a white). This might seem redundant with chroma (and it is), but it is something different and one of the reasons it exists is because sometimes colors are see without a white object to relate them to.

All of this has nothing to do with HSV and HSL since they are huge approximations to color appearance and are not in any way calibrated to visual perception. You should think of their usage of color appearance terms as nothing but a rough analogy.  (Back to top)

How come it is easier to see a certain color under low light condition while other colors are hard to see? (924)
There are two possible reasons.

One is that the color of the light might be changing. For example in low light outside the light tends to be more blue since the sun has set. This makes blue objects lighter and easier to see and red objects darker and more difficult to see. Secondly, the sensitivity of the visual system shifts toward blue for lower light levels. This causes the same effect described above.

In most cases, it is a combination of the two effects.  (Back to top)

I project two spot lights on a white screen, RGB=(255,255,0) and RGB=(255,0,255), when both lights combine additively, what "color" I will see? Could I predict the mixture in the RGB Color Space? (923)
Assuming you are viewing the spots in the dark and that they completely overlap (to avoid simultaneous contrast), then you will see a light pink color. You can predict the RGB mixture by adding the values to get 510,255,255. Since a single display can't typically output "510" you would have to scale them down to the max of 255 and the projected color will be dimmer than the mixture you describe. If the display was linear the scaled values are 255, 128, 128. However most displays are not linear so you would need a characterization of that nonlinearity (called gamma) to get the correct scaled down values. For most displays it would be something like 255, 189, 189 in order to get a dimmer version of your color.  (Back to top)

I am confused by the range value of the Y-tristimulus value. For emissive things, like screens, it is supposed to be the absolute luminance in foot-lamberts (so the range could be 0 to infinity in theory).

If want to compare two colors (via deltaE - which deltaE is the 'standard' nowadays), one from a emissive source and one from a reflective source, can I? It seems like a reflectance number couldn't quite compare to a direct luminance measure, but then how can I compare these two things in a color space? (922)

The normal procedure is to select a luminance level that represents white (like the white point of a display) and then normalize the absolute tristimulus values to that point (making it 1 or 100% to match the reflective values). Then the two sets of values will be on the same scale and can be reasonably compared.  (Back to top)

What percent of light is reflected from Brown beer bottle glass? Green beer bottle glass? Clear beer bottle glass? (921)
The important question is what percentages are absorbed and transmitted. Since the glass is transparent, very little light is reflected and that amount is about the same for each color. It is the percent transmitted that gets through to the beer and gives the glass its color. A clear bottle transmits something like 90% of the light and equally at all wavelengths. A green bottle would transmit a fairly high amount of green light (maybe 40-50%) and very little red and blue light (nearly 0%). The brown bottle will transmit nearly 0% of the blue and green light and then a small amount of red (maybe 5-20%) to get its brown color. Also remember that the transmittance will vary from bottle to bottle. The light that is not transmitted is absorbed by the glass.

If your beer is stored in the dark, then it doesn't matter what color the bottles are. If it is stored in a lighted environment, then the dark bottles (brown) will keep the beer fresher for longer.  (Back to top)

Can females see more colors than males? (920)
No. Color discrimination varies from person to person, but females have no specific advantage. That assumes normal color vision. Males, however, are far more likely to have a color vision deficiency (color blindness) with about 8% of the male population having a deficiency and less than 0.5% of the female population having a deficiency. If you are comparing males and females with normal color vision, then there's no difference. If you choose random females and males, you are far more likely to run into a male with a deficiency.  (Back to top)

Artists often find it difficult to identify subtle colors. Eg: is a warmer or cooler than another? Shadow and reflection are very hard. The more you stare the easier it is to convince yourself it is almost any color. I have heard that both saccadic eye movements and bending over so as to look at the subject upside-down can be helpful. Do you agree and do you know of any better techniques? (919)
This is a great question and as you might have guessed, more complicated than it seems.

Our perception of color depends tremendously on context (think about demonstrations of simultaneous contrast or afterimages) and part of what our visual system does is to remove subtle changes in color through mechanisms of chromatic adaptation in order to more easily detect the more significant changes (such as important objects in a scene). This adaptation is one thing that makes it difficult to discern subtle gradations in color, shadows, etc. Some quick eye movements back and forth between areas in a scene will indeed help since it will tend to set your state of adaptation to some intermediate level rather than adapting to, and therefore becoming less sensitive to the color of, a certain area. We also tend to associate colors with known objects (e.g., the same physical yellow might look different on a banana than it would on a car) and if we can isolate the color stimulus from the object we can sometimes better identify the physical color present. Your suggestion of looking at objects upside-down is a step in this direction; it would tend to isolate the color perception from the object perception and make it a bit easier to identify the color. Another way to do this is with a reduction screen or aperture. Cut a small hole in a piece of neutral cardboard and look through it at the object. This will allow you to focus on the edge of the hole and see the color behind in isolation from the object. I'm sure there are other tricks that artists have come up with, but these do indeed help.  (Back to top)

Can you explain the science behind gloss levels on the paintwork of a car. I own an auto reconditioning business and I know using different techniques I try to achieve maximum gloss, even at a microscopic level. Let's say you have the spiderwebbing swirls in your clear coat. Scientifically , what is happening to the light when it hits the swirls as opposed to hitting perfect paint? Is it the law of Refraction or should it be explained another way. (918)
It is probably a combination of things, but ultimately it comes down to scattering caused by diffraction at the edges of scratches and other imperfections. The diffraction redirects the light that would be reflected into a single angle on a perfectly glossy surface. You might also have some irregularity in the surface (such as orange peel) causing it to look less glossy since the gloss reflection will be in different directions for different locations on the surface.  (Back to top)

Why is light source data normalized to 100 at 560 nm? Is this the source´s relative spectral power distribution or is that something else? Doesn't the normalization affect the computed XYZ or L*a*b* values? (917)
The normalization is a tradition. Any wavelength could be chosen, but 560nm was selected since it is the middle of the visual spectrum and approximately the wavelength of maximum visual sensitivity. Yes, these are the relative spectral power distributions and the normalization does not affect the XYZ or CIELAB computations because there is another normalization in the computation of XYZ (the k factor in the equations) that makes this one unimportant.  (Back to top)

I am studying the lightness differences in two types of dental crowns. Is there a minimum ΔL that is not perceived by humans? (916)
The minimum perceptible change in L* depends on the samples and viewing conditions. Often a value of 1.0 is used as a general rule. However, for large uniform samples on a uniform background at a high illumination level, people can see smaller differences. In your case, you have a fairly complex stimulus (the crown) in a very complex and poorly illuminated environment (the mouth), so I would think it is very safe to say that nobody could see a difference less than 1.0. You might even be safe with differences of 2-3 units.  (Back to top)

Does color have an objective existence? (915)
Yes. But perhaps not in the way you are thinking. Color is defined as a human perception and therefore does not exist outside of humans. Objects and light sources do not have a color unless someone is there to observe them. (They do have spectral power properties that provide the stimulus for color, but the actual perceived color depends on those as well as attributes of the human observer.) That said, perceptions can be just as objective as physical objects. In the laboratory, we are able to complete repeatable, objective experiments to measure observers' color perceptions. That makes the perception of color, and therefore color itself, an objective phenomenon ... even though it is "only" a perception.  (Back to top)

If the inability to differentiate blue and black and brown is caused by the gradual yellowing of the lens in our eyes, would wearing another color lens make it possible to see the colors? (914)
The yellowing of our lens with age does contribute to our inability to distinguish black, blue, and brown in dim lighting, but the dim lighting is probably a bigger factor. The yellowing of our lens means that less blue light gets through to our retinas, but dim incandescent bulbs also contain very little blue light. The lack of blue light to respond to is what causes these particular difficulties with dark colors.

Unfortunately it can´t be fixed with a filter. Another filter would just take away more light and, even if that light was of a different color than blue, less light makes it even harder to distinguish colors. The best answer is to look at the colors (typically socks!) under brighter light and, if possible, bluer light such as natural daylight.  (Back to top)

I'm interested in skin colour and I normally reckon it should lie between 0° and 30° in a HSB colour space. In the Munsell space the Hue shifts for less saturated colors. Are there recognized skin values in the Munsell notation? (913)
There isn't a specific standard, but one resource, The "ISCC-NBS Method of Designating Colors and a Dictionary of Color Names" indicates that colors that are named "flesh" and "pale flesh" can be found with Munsell hues ranging from 9RP to 8YR (that's roughly 36 degrees worth of hue). Of course, whether or not a color of those hues looks like skin would also depend on the lightness and chroma. There might be other synonyms for skin color in the ISCC dictionary if you want to search more (it is out of print, but usually can be found in online used book searches like bookfinder.com).  (Back to top)

What is the most visible color for a motor cycle helmet? (912)
I ride a motorcycle and I did a lot of research into this question before I purchased my helmet. White helmets are significantly more visible than darker helmets and the visibility of the rider is a significant factor in motorcycle helmets. Those conclusions also make sense from a color science perspective. My helmet is white and I wouldn't choose any other color. Also, the addition of a bit of reflective tape makes the helmet/rider much more visible at night and twilight.  (Back to top)

What education and degree is required to become a color scientist? (911)
At RIT, someone can get M.S. and Ph.D. degrees in Color Science and either of those are excellent qualifications to become a color scientist. Such degree programs are rare; in fact RIT's is the only program in Color Science in the United States. However, there are plenty of color scientists out there who have degrees in other fields and then learned more about color science through their work or research. The students in our graduate programs also come from these various fields with undergraduate degrees in areas such as physics, chemistry, biology, mathematics, psychology (perceptual or experimental), cognitive science, computer science, etc.  (Back to top)

Why does the S-cone see blue-red (i.e. violet) instead of pure blue? Where does the red come from without the L-cone? (910)
To understand the appearance of short wavelengths of light as violet, you have to look beyond just the cones in the visual system. As you suggested, the S-cones only respond to short-wavelength light and the L-cones to long-wavelength light. At the next conceptual step in visual processing is when the cone signals are combined into opponent-colors signals. These are three signals that respond to light vs. dark, red vs. green, and yellow vs. blue. The light-dark signal is derived from summing all three cone types. The yellow-blue signal is obtained by adding the L and M cones together to get a yellow response and subtracting the S cone signal to get an opposing blue response. The red-green signal is obtained by adding the L and S cones together to get the "red" response and subtracting the M cones to get an opposing green response. Thus, our perception of "redness" depends on signals that originate in both the L and S cones. Of course, this is greatly simplifying what happens in the visual system, but it is the general concept behind the appearance of "violet".  (Back to top)

Why there is no spectral (pure) colors that can be reproduced by mixtures of other colors? It is just a coincidence of human color vision? (909)
Actually, you can match some spectrally pure colors with mixtures of other colors. For example, you can match a yellow-appearing wavelength with mixtures of red- and green-appearing wavelengths.

However, the cases in which this is possible are somewhat limited. In general, the limitations of what colors can be produced by mixing other colors depends on the colors being mixed, the method of mixing (e.g. additive vs. subtractive) and also the properties of the human visual system.  (Back to top)

I have a very old FM 100 Hue Test Kit (in a wooden box) but do not have a score sheet. I see all of the new kits have software. What came with this kit as it's obviously pre-PC. (908)
The old tests came with an instruction manual and pad of score sheets. I am not sure if it is still possible to purchase the score sheets, but I did find a PDF containing both through a Google search. See www.qcqa.com.  (Back to top)

Is there any way to predict what color an object will appear if I change the color of the light source? (907)
Yes. The CIELAB values are computed from the measured spectral reflectance of the object. You could do these computations yourself for the illuminant of your choice (compute CIE XYZ and then CIELAB values from measured reflectance). A text on colorimetry (e.g. Berns' Principles of Color Technology, 3rd Ed.) can provide an overview of all the equations and their application. Another alternative is that your instrument manufacturer might be able to enter your custom illuminants into their software.  (Back to top)

Why is it so difficulty to follow the process of saying the color of the word as opposed to saying the word itself? (906)
This is called the "Stroop Effect". If you do an internet search on that term, you will find many demonstrations and explanations. The basic concept is that our brains more easily identify words than colors so it is easier to say the word than the color.  (Back to top)

I am a Colorist, want to reduce validation work in laboratory. If I add a quantity of pigment and Tio2 paint, and then measure the shade, we get a certain CIELAB value. From that can I predict CIELAB values given a different quantity of pigment and TiO2? (905)
There is no direct way to do this from the CIELAB coordinates. You would need to use a colorant formulation system along with a database to characterize your colorants and paint bases. Such a system would also allow you to predict mixtures of various colorants.  (Back to top)

Which would yield the least visual discrepancies, a more chromatic color or less chromatic color? (904)
This is a difficult question to answer because it all depends on how the color is measured and color scientists work to create color metrics where the answer is that they are equal. However, one point to consider is that there is more potential for significant variations in perception for less chromatic colors, especially grays. For example two people viewing the same near-gray sample might use completely different hue names (e.g., one saying it is bluish and the other reddish) while such name changes would be unlikely to occur for a highly chromatic color (both people more likely to give it the same name).  (Back to top)

I read that Ansel Adams painted the walls of his home studio a 20 percent gray color to be the best background color to display his photos. Do you have any more specific information about this color? Is it warm or cool gray? Is there an actual paint recipe for this color? (903)
I am familiar with Ansel Adams comments on this topic and he wasn't too particular about the color itself, just that it have an overall reflectance of about 20% (perceptually midway between white and black in lightness). That requirement was to best reproduce the perception of contrast in his prints and assure that the dark areas were darker than the surroundings and the light areas lighter. So there is no specific gray or recipe in his recommendation, although a fairly neutral color would probably be most pleasing overall.  (Back to top)

I bought a charcoal gray couch - love the color in the store. In my living room it looks dark blue! What color should I use on the walls and accessories to make it look gray again? (902)
To make something appear less blue (more neutral) you would need to surround it with objects that are even bluer. It is likely that the environment in your home (including the illumination) is more yellow than what you saw in the store and this makes your couch appear a bit more bluish through simultaneous contrast.  (Back to top)

Can fluorescent colors be printed with cymk printing system? I have Roland printer that uses eco-solmax inks. Are there any RGB or CMYK codes that will work? (901)
The only way to print fluorescent colors is to use fluorescent inks. While some CMYK inks might exhibit a bit of fluorescence, it is unlikely they are fluorescent enough to produce colors that we would typically call "fluorescent".  (Back to top)

In textile color matching, Is it possible to match target color in all light source without metamerism. If so what will be the possibility percentage? (900)
It is possible, but only when the spectral reflectance of the match and the target are identical. This requires that the properties of the textile substrate be the same and that the same colorants are used in the target and the match. If those conditions are met, then the probability of a spectral match depends only on the control of the coloring process. If those conditions are not met, then the probability of a spectral match is difficult to predict, but it would be quite low.  (Back to top)

What does a white balance actually do? (899)
It normalizes the RGB responses of a camera system to account for changes in the color of illumination. Ideally a proper white balance would result in R=G=B for neutral white and gray objects regardless of the illumination color.  (Back to top)

What makes a color bright? (898)
A color looks bright when it both reflects a lot of light and is very colorful. A white object under a lot of illumination will look bright, but a very colorful yellow object under a similar amount of illumination might look even brighter because it is colorful in addition to reflecting a lot of light.  (Back to top)

I study the topographic influence on snow. When an individual looks at snow in shadow, they are actually seeing blue light, yet they state they see white. Is this based on learned experience of color? What effect does the question have, i.e. "What color do you see", vs. "What is the color of the snow?" (897)
Great question! First keep in mind that color is a perception and not a physical property of light. There is a relationship of course, but it is not as direct as one might think. If they say the snow is white, then it is. The fact that a stimulus with more energy in the blue region of the spectrum (snow in the shadow) can still look white is due to chromatic adaptation. Our visual system adapts to changes in the color of the illumination and the result is that object colors like the white snow tend to appear relatively constant across the changes in illumination. Some observers can learn to separate the effects. Artists often do this and can distinguish easily between the color of the object (or material) and the color of the light reaching the eye.  (Back to top)

How does a flower´s color help it survive? (896)
The color attracts bees and other insects that pollinate, and therefor propagate, the plant. It also makes the flowers attractive to humans who then care for them and see that they survive to future generations.  (Back to top)

Many auto accidents happen in early morning and late in the evening when it is dark outside. Does the vehicle color have anything to do with this? What automotive paint color is easiest for the eye to see when it is dark outside? How about in daylight? (895)
I have not seen any statistics correlating car color with accident rates. Accidents are not common enough and there are two many other variables involved to get the required statistics. However, there is little doubt that visibility is one important factor in automobile accidents (this has been shown for motorcycle accidents since they are often "not seen" by other drivers). The most visible car color (and motorcycle helmet color for that matter) is white in low light conditions. This is because it reflects the most light and normally contrasts the most with the background (a snowy background would be an exception where a black car might actually be more visible). In general white is also most visible in the daylight because it contrasts most with the typical background.  (Back to top)

Although our sun is often depicted in artwork as "yellow", I have heard that it is actually "white", and that is why a white piece of paper appears white. Is this true? If so and we lived in a solar system with a different type of star, such as a red dwarf or a blue giant, would any objects appear "white", or would they take on the cast of their light source? (894)
Our sun is yellow in comparison with our blue sky. What looks white to us can be produced by a wide range of visual stimuli since our visual system is able to adapt to the color of the illumination. That adaptation is why a white piece of paper looks white under a variety of light sources (outside on a sunny day it is illuminated by the yellowish sun and the bluish sky together). If we lived under a sun of a different color, we would still adapt and objects would appear roughly the same as they do now (at least that white piece of paper would still look white).  (Back to top)

Imagine driving down a street toward a traffic light. How fast would you have to go so that the red light (700 nm) would appear green (500nm)? (893)
It would be approximately 2/5 times the speed of light. Faster than your car could go and stay intact. Sorry.  (Back to top)

Is there a way to measure gloss with a spectrophotometer or is it necessary to have a glossmeter? Is there a conversion between CIELAB values and gloss? (892)
You cannot convert CIELAB values to gloss as they are designed to measure the diffuse color and not the gloss. You can get some measure of gloss with an integrating sphere spectrophotometer by comparing the Y tristimulus values for measurements in specular included and specular excluded geometries. A diffuse sample will have no difference while a very glossy sample will have a difference on the order of 4% or so. It is better to have a gloss meter.  (Back to top)

How can I avoid metamerism? (891)
Metamerism cannot be avoided since it is really a property of the human visual system. Metamerism refers to the fact that two colors do not have to be identical in spectral power distribution in order to match in perceived color. You are probably thinking of illuminant metamerism in which two samples match for one light source (a metameric match in which the spectral reflectances differ) and do not match for another light source. The only way to avoid that is to produce spectral matches. (Sorry.)  (Back to top)

What are L* a* and b*? Are they constants? If not, how can I find those values? (890)
L*, a*, and b* represent the lightness, redness-greenness, and yellowness-blueness dimensions in the CIELAB color space. The values are not a constant, but rather are different for every object and illumination condition. They are found by measuring the spectral reflectance of materials (or spectral radiance of self-luminous stimuli) and doing some colorimetric computations. I would recommend a good introductory text on color measurement like Berns&apost; "Principles of Color Technology, 3rd Ed." to get a good overview of the CIELAB space.  (Back to top)

Where can I find calculator or software for LRV? (889)
I don't know of a specific LRV calculator or software, but it appears that LRV is the same as the CIE Y tristimulus value which is readily obtained from color measurement instrumentation.  (Back to top)

We are told that Red, Yellow, and Blue are the "primary" colors, and that you can make all other colors from them, but those 3 can´t be made by combining the others. In additive systems, it is red and green that combine to make yellow. Is there something inherently special about red, yellow, and blue, or are they considered "primary" only in the sense that the most basic medium of color manipulation is a subtractive system? (888)
They are but one set of subtractive primaries and red, green, and blue are the typical additive primaries. There is nothing special about them. Primaries are simply three lights (additive) or colorants (subtractive) that can make a range of colors and chosen such that none of them can be made by a mixture of the other two.  (Back to top)

When we compare luminosity of different hues, we find that blue is a relatively dark color, and yellow is a relatively light color. Do hues have an inherent luminosity independent of human perception? (887)
No. It is due to the spectral response of the human visual system. Anything you see in a rendered black and white image is caused by the spectral response of the imaging system ... which is often designed to mimic the human visual system.  (Back to top)

I am trying to emulate a smaller-gamut RGB color space on an LCD display using a 3x3 matrix, and I can not seem to figure out the general-purpose way of doing this. You reference Poynton´s book in a similar FAQ but he skips over the process and just spits out the answer for some important cases. (886)
You might try Berns´ "Principles of Color Technology, 3rd Ed." to get a more illuminating explanation. It is hard to explain it all in a short response, but you need to first linearize your RGB values, then convert to XYZ with an appropriate 3x3 matrix, then convert to the new (smaller gamut) RGB values with the appropriate 3x3 (this one will be different from the inverse of the first or you will make no change) and then do any correction for display nonlinearities (e.g. gamma).  (Back to top)

Is there is a device that can measure liquid gravure ink spectral transmittance? I need a method to confirm ink recipe components in a liquid ink sample. I could use a faster option than the traditional draw-down proofing method. (885)
Many spectrophotometers are capable of measuring the transmittance of liquid samples placed in a glass cell. You can inquire with instrument manufacturers to get an instrument with appropriate capabilities (or find out if you already have one). The only question would be whether the inks are transparent enough to allow a measurement. If they are translucent (scatter light), then the measurements might prove difficult.  (Back to top)

How many lights are metameric to white light reflected from a paper? (884)
There is no limit to the number of metameric lights that could be created.  (Back to top)

I use the Munsell Color Books to develop color harmonies for clients to use on short term projects. Is there an inexpensive way to get swatches or chips that match the Munsell notation? Can I print them? (883)
You can get authentic Munsell chips from X-Rite (they sell all Munsell products), but they are not particularly inexpensive. It would be difficult to print them accurately. I would suggest using some inexpensive samples such as the Coloraid papers and then using your Munsell books to visually determine a Munsell designation for the various samples. That would probably be the most cost effective method.  (Back to top)

Are there studies of floral arrangement preferences of dichromats? (882)
Unfortunately I am not aware of any such studies. However, you might want to look at the website vischeck.com. It allows you to upload images and then see what they might look like to people with color vision deficiencies. While the rendered colors might not be accurate, it would at least let you see which colors become indistinguishable. By testing some images of floral arrangements you would be able to see if the various flowers contrast more or less, etc.  (Back to top)

How did the CIE 1931 color matching experiments evaluate the whole gamut when we know that the RGB color gamut is a subset of that of the human eye? In other words, we know there are colors that cannot be matched by the RGB lights. How did they get around this? (881)
It is really quite easy. RGB primaries are/were used and there are certainly many spectral colors (monochromatic lights) that cannot be matched by positive amounts of RGB. (The number depends on the selection of RGB.) The key here is the word positive. What is done is that the light to be matched is desaturated with one of the RGB primaries and then that desaturated light is matched using the other two. For example, 500nm is a saturated cyanish wavelength that is outside almost any RGB gamut. What is done is that it is mixed with a little bit of R and that mixture is matched by a combination of B and G as follows.

500nm + R matches G + B

Grassmann´s laws tell us that additive color matches follow simple algebra (that´s why the whole system works at all) and that allows us to subtract the amount of R from both sides of the match resulting in

500nm matches G + B - R

Thus, we can have negative amounts of light to match every possible color. That is why plots of the color matching functions in terms of RGB go negative at some wavelengths. (Note: For clarity I have left out the distinction between the amounts of RGB and the definition of them ... essentially, I left of the units.)  (Back to top)

The sensitivity of the human eye to color difference is greatest at approximately 550 nm. However, in interpreting the MacAdam ellipses , it would appear that humans are least sensitive to differences in color in this region. Why? (880)
Humans are most sensitive to light at 555nm (when using their cones), but that is a response to seeing, or not seeing, a light stimulus at all, not to distinguishing a color difference. It really is pretty much impossible to say what color people are most sensitive to since any measure would depend on the color space used. MacAdam ellipses in xy are definitely not the way to go. First they represent the results of only a single observer and second they are just showing the non-uniformity of the xy diagram, which really has no direct physiological significance with respect to color discrimination.  (Back to top)

I measure timber colour using a Minolta Chromameter CR-400. I want to compare my values with those of other woods whose colors are listed in percent Luminance, Dominant wavelength (λ)and percent purity. Can I convert these values over to CIELAB? (879)
You are in luck. The computation is possible, but it is a little tricky. First, the % luminance is your CIE Y tristimulus value. Dominant wavelength and purity are directly related to the CIE xy chromaticity coordinates. Dominant wavelength is the wavelength at which a line from the white point (xy) through the sample (xy) intersects the spectrum locus on the chromaticity diagram. Purity is the fractional distance from the white point that the sample lies in the xy diagram. In other words, the dominant wavelength and white point define a line in the chromaticity diagram and the purity defines where on that line the sample falls. That would allow you to get xy chromaticities for your samples. Then together with Y (% luminance) you can convert to XYZ using the following.

X = Y(x/y)
Y = Y
Z = Y((1-x-y)/y)

With XYZ you can compute CIELAB coordinates. I would recommend you refer to a colorimetry text to get all the necessary details. Purity and dominant wavelength are discussed on p.61 of Berns Principles of Color Technology, 3rd Ed. You can also get all the details and a great reference book in Wyszecki & Stiles Color Science.  (Back to top)

Is it possible to scientifically determine the original colors in an old black and white stereoview photograph if some of the colors are known? (878)
Unfortunately, no. You might be able to make some good guesses based on the objects in the photographs, but color is three dimensional and in a black and white photograph it is mapped into one dimension. So, for example, a light red and a dark blue might map to the same gray level in photograph and you have no way to tell which color the object was without some additional information from outside the photograph. And unfortunately, knowing about some objects does not tell you about others.  (Back to top)

How does aging affect color perception? Are older people able to discriminate brighter colors or softer colors better? (877)
As we age, the lens in our eye begins to absorb and scatter more blue light (it becomes yellower). This causes a decrease in visual sensitivity to shorter wavelengths that make us less sensitive to blue colors and less able to discriminate purple and red colors. There also tends to be more of a negative effect of glare as we age. So really, no colors are discriminated better as people get older and whether brighter or softer colors are preferable would probably depend on the particular person and the illumination and viewing conditions.  (Back to top)

What wavelength(s) are used in the calculation of Gardner color? (876)
The Gardner Index is computed from CIE chromaticity coordinates (Yxy) and is therefore based on the entire visible spectrum of wavelengths from approximately 380nm to 760nm.  (Back to top)

Is it possible to explain color to a blind person if thay have never experienced it? Can the brain of a blind person assign a color to something if they know the shape but not the color? (875)
We can not say for sure, but I would think this would be impossible. It is very difficult to describe colors of objects to a seeing person and all of the definitions of color use examples. If one has never experienced the examples, then it seems it would be extremely difficult, if not impossible, to give them an impression of color appearance.  (Back to top)

I need to predict CIELAB of a color at different thickness. The color is transparent and .01 inches. How do I predict LAB values at thicknessof .121? (874)
It can be done if you have the spectral transmittance data for your material. It cannot be done from the CIELAB values alone. You would basically want to use the Bouguer-Beer Law that relates the transmittance of a material to the colorant concentration (fixed in your case) and the thickness. In the simplest terms, for each doubling of thickness, you square the spectral transmittance. However, you should look at the details in a textbook such as Berns Principles of Color Technology, 3rd Ed. (pp. 157-161). This is sometimes referred to as simple subtractive mixing.  (Back to top)

Why do different pure hues have differing lightness values? (873)
The perception of lightness is complicated and depends on many variables. However a simple way to look at this question, that does explain most of what is observed is to think about our relative sensitivity to light of various wavelengths through the visible spectrum. In general we are more sensitive to wavelengths in the middle of the spectrum (approximately green) than to those at the ends (approximately blue and red). A pure green hue will be made up of light that we are very sensitive to and thus generally have a higher lightness than a pure red, blue, or purple (which is made up of red and blue). Of course it is possible to create stimuli that do not follow this trend, but for reflecting objects, or lights with equal amounts of energy, this general rule will hold. Usually highly saturated greens and yellows have a relatively high lightness, while reds, blues, and especially purples have a relatively low lightness.  (Back to top)

In his 1943 OSA paper "Specification of Small Chromaticity Differences" MacAdam describes constants g11, 2g12, g22 and gives values for them in graphical form. Is there a source for this data in conveniently tabulated form? (872)
The data required to compute the ellipses can be found in tabular form in Wyszecki & Stiles' "Color Science" book. They provide orientation angles and axes lengths rather than the gij coefficients, so it might not be exactly what you are looking for, but it is a commonly used source for this information. As you are using the ellipses, be sure to keep in mind that they represent data for only a single observer!  (Back to top)

I need a device that will give me color readings at an observer angle of around 45 degrees or so that will pick up almost 100% specular reflection. (871)
You are correct that most color measurement instruments are designed to measure angles away from the specular reflection. There are gloss meters that measure the specular reflection, but they do not measure color. To do both you need a goniospectrophotometer that measures spectral reflectance at any combination of illumination and view angles. Murakami makes one such instrument and here is a link to some information from the US distributor.  (Back to top)

Why is it not possible to get an accurate ΔE colour measurement on a metallic powder coating? (870)
It is possible, but the result is limited to the geometry of illumination and view present in the spectrophotometer. Metallic materials exhibit geometric dependencies in their color that are not easy to capture and represent with spectrophotometric measurements (and therefore ΔE*). Sometimes such materials are measured at several combinations of illumination and view angle to try to characterize these properties. The other potential limitation is that the metallic highlights have reflectance factors that significantly exceed those of normal reflecting materials and this results in CIELAB coordinates outside the normally well-understood range (e.g. L* can be much greater than 100). While such values can be computed and used in a ΔE* calculation, the meaning of such values remains very uncertain.  (Back to top)

At what luminance (Candela per meter squared) in the ambient environmental light does scotopic vision turn to mesopic vision, or at what point can we start to see color? And at what point can we see the majority of colors? (868)
The specific answer would depend on the individual observer, viewing conditions, and the state of adaptation. However at approximately 1 cd/m^2 the responses of rods and cones are equal, so this is essentially the midpoint of the mesopic region; some color is visible. As a rule of thumb, you could assume that reasonably good color vision (majority of colors) is present at 10 times that luminance (10 cd/m^2) and that no color vision is present at 10% of that luminance (0.1 cd/m^2).  (Back to top)

What is the most common first color that children recognize? The second? (867)
I don't think I can answer any better than this website. Vision works on contrast, not individual colors. The first is light-dark contrast, so you could say that infants can see white, black, and grays first. This is probably followed by red-green discrimination and then yellow-blue discrimination.  (Back to top)

We have a WWI aircraft we are recovering with fabric that requires a dark green paint for the wings. This aircraft is flown a lot and displayed to the public outside quite a bit. The problem is that this paint absorbs sunlight heat dramatically which reduces the life of the doped fabric, causing it to shrink prematurely and it is fairly expensive to replace. Would a primer coat of silver dope or white help reduce the absorption of the heat into the fabric under the dark green paint? (866)
Unfortunately that won't help since the dark green will have already absorbed the light and converted it to heat (a temperature increase). The white or silver layer would only work if it was exposed to the light and then, of course, your plane would be white or silver instead of green.  (Back to top)

Do you know the origin of a measurement called the "flop index", which is defined as flop index=2.69(L15�-L110�)^1.11/((L45�)^0.86). My Xrite M68II reports this metric, however, X-Rite does not explain the source of this equation. Do you know where I might find the source of this equation, or another good way to mathematically explain the color difference at several angles? (865)
Unfortunately I cannot find a source for this, or any, flop index equation. It appears to be some form of empirical fit, so there must have been an experiment done somewhere. I would suggest you contact X-Rite to see if they can provide a source for the equation. This particular equation only depends on L* (measured at various angles), so it is only telling you about lightness differences. If you are interested in full color differences, you can compute normal CIELAB ΔE* color differences across the view angles. You might get some unusual values in comparison with color differences computed within a single geometry, but with care, the computed values could still be useful.  (Back to top)

What is the differance between additive color and subtractive color? (864)
There is no difference in the colors themselves, the difference is in how they are produced. The following answers come from WhyIsColor:

Q: How does additive color mixing work?
A: Additive mixing happens when two or more light sources are superimposed by either illuminating the same spot or by being so close together in space and time that oure visual systems cannot distinguish them. What we perceive is the sum of the energy from each of the lights. This is how computer displays and televisions reproduce color. Red, green, and blue are common additive primaries since it is easy and efficient to makea wide variety of colors with them.

Q: How does subtractive color mixing work?
A: Subtractive mixing happens when two or more dyes or pigments are combined by either layering or mixing them. Each dye or pigment absorbs certain colors (wavelengths) and what we perceive is the light that remains after each colorant has absorbed its part. This is how some printing systems and paint mixtures produce various colors. Cyan, magenta, and yellow are common subtractive primaries since it is easy and efficient to makea wide variety of colors with them.  (Back to top)

I am turning my formal dining room into an art gallery. We painted today and it is too bright. The way it looks now all you will see is the paint color and not the art. What are the best colors to paint a gallery, given that all the pictures are of different color schemes? (863)
The best colors for walls tend to be of medium lightness (perceptually half way between white and black). That way both the light and dark areas of the artwork can contrast with the background. Theoretically, the ideal color is also a neutral gray, but that can become monotonous, especially in a home. The selected color is not nearly as important as the lightness, but it is best to avoid any very saturated colors. Pick a color of medium lightness, that is a hue you like, but not very saturated.  (Back to top)

Scientifically speaking, is black a color? Is white a color? (862)
There is no question that black and white are colors.

The technical definition of color that is internationally agreed upon includes them as colors and, specifically, there is a definition of achromatic colors as follows: "A color lacking hue; white or grey or black".  (Back to top)

In comparing light bulbs based on their color temperature, how much difference in color temperature is needed before an average person can notice that difference? (861)
It would depend on the viewing conditions. For example, if the two bulbs were viewed side-by-side, then a smaller difference would be perceptible than if the bulbs were viewed one at a time. However in general, it is probably reasonable to assume that a difference of about 100K or greater is perceptible. (The actual value will also depend on the initial color temperature as well with the change required to see a difference becoming larger at higher color temperatures.)  (Back to top)

Are there any specific details in 45:0c reflectance measurement, such as the effect of distance between detector and sample or distance between light source and sample? Are there restrictions on the circumstances, like the color of the floor on which we put our sample? (860)
The specifications are in CIE Publication 15:2004. They include restrictions about illumination and view angle and solid angles, but nothing about distances. They also do not restrict the backing of the sample, although you might want to be careful of that if the sample is such that light might pass through.  (Back to top)

Is there any approximation to convert between Hunter Lab (illuminant C, 10 degree observer) and CIELAB (illuminant D65, 10 degree observer)? (859)
There is no unique solution since there are many spectra that can produce a given set of colorimetric coordinates (metamerism). You could make a rough approximation by using the inverse HunterLab equations to convert back to XYZ tristimulus values and then compute the CIELAB values for them (perhaps just using Ill. C since it is close to D65 and this will be only an approximation).  (Back to top)

I received data from two sources. The first measured Hunter Lab on light source C under 10 degrees, the second measured CIELAB on light source D65 under 10 degrees. I don't have the spectra. Is there any approximation with which I can convert between them? (858)
There is no unique solution since there are many spectra that can produce a given set of colorimetric coordinates (metamerism). You could make a rough approximation by using the inverse HunterLab equations to convert back to XYZ tristimulus values and then compute the CIELAB values for them (perhaps just using Ill. C since it is close to D65 and this will be only an approximation).  (Back to top)

What observer for XYZ values can be used in CAM02, 2 degree or 10 degree? Some articles use 2-degree observer, the others use 10-degree observer. (857)
CIECAM02 was derived using the CIE 1931 2-deg. color matching functions, so they are the best choice. However, it will still work well with 10-deg. tristimulus values but the results will not be identical.  (Back to top)

I am looking for the differences between red, blue, green, and yellow dye. Can you tell me anything different about them, other than they are different colors? (856)
The dyes will have difference chemical structures that cause them to interact with light differently. For example, a yellow dye absorbs blue light while a blue dye does not absorb that light, but would absorb green and red light. It all comes down to the chemical structure.  (Back to top)

What part of the spectrum helps grass grow? (855)
Chlorophyll in the plants absorbs light and converts that energy into the forms required for the plant to grow. Since chlorophyll is normally green, it is mainly the blue and red wavelengths in the visible spectrum that would be most strongly absorbed to provide this energy. However, a significant amount of ultraviolet energy is also utilized by the plant and even some of the green light is absorbed.  (Back to top)

Why does the color of a crayon or colored pencil look a different color on paper? For example, some purples show up blue on paper. (854)
Because the colorants in the crayon interact with the paper depending on the amount of wax you put down (how hard you press) and the lighting. It is possible to make a wide variety of colors using a single crayon. As more wax is applied to the paper, the color should begin to look more and more like the crayon itself.  (Back to top)

Would a spectrophotometer be able to accurately read the color differences between chrome plated surfaces? (ie. mirror-like finish) What settings or type of instrument would best achieve this? (853)
Yes, but it would take a special spectrophotometer designed to measure the specular (or regular) reflectance, which is the main component in a chrome surface. You could probably get a reasonable measurement from an total reflectance geometry (integrating sphere with specular component included), however their might be differences in the geometric distribution of the reflected light that would be masked in that measurement so you might also want to include an SCE measurement as a second confirmation. It is a challenging measurement, but with some care you might be successful.  (Back to top)

Is it possible to quantitatively describe a color just by its reflectance or radiative spectrum? (852)
Perceived color depends on the reflectance of the object (or the radiance of a source), but it also depends on the illumination, the viewing geometry, and the response and adaptation of the human observer. The spectral properties alone are not enough to fully describe a color.  (Back to top)

How a mixture of R, G & B single wavelength sources (such as RGB laser) can produce white light even though they do not cover the whole spectrum? (851)
Three appropriate lasers are sufficient to stimulate the three types of cone receptors in our visual system to the same degree that they would be by a continuous spectrum. Thus, we are not able to perceive the difference. This property is known as metamerism.  (Back to top)

Is there a formula for calculating Delta E in Munsell? (850)
This person answered their own question before I got to it, but It seems worth pointing out the Nickerson Index of Fading on our FAQ. Here's what she wrote...

"Hello Dr. Fairchild,
: I've submitted another questions to your wonderful "Ask a Color Scientist" page, but I thought I would also send a note to you on the same topic, but with a bit more detail.
: I am looking for an easy color difference formula for Munsell. I know that it won't be as spot-on accurate as the advanced formulas used with more modern color spaces, but that's OK, I want something that is generally reliable, and that will be good enough.
: I've done a bit of Googling on this and the only thing that I have found is the 1936 Nickerson "Index of Fading" formula:
: ΔE = 2/5CΔH + 6ΔV + 3ΔC

Where H/V/C are Munsell coordinates.  (Back to top)

I would like to convert from CIELAB coordinates into the approximate spectral red green and blue component curves. Is this possible? (849)
It's not really possible since a given set of CIELAB coordinates can be produced by an essentially infinite variety of spectral power distributions. The only way this problem can be solved is to assume a limited form of the spectral curves and then compute mixtures of those properties.  (Back to top)

Why do we see colors when we spin a disc that contains figures in black & white? (848)
These colors are referred to as Benham's colors or Fechner-Benham colors. Here is a nice explanation of the Benham top. Also, if you do a web search on those terms you can find a lot of information and theories.  (Back to top)

When job requirements demand color discrimination, how often should employees be tested on their color perception? (847)
A person's overall color vision (normal vs. defficient) rarely changes in their lifetime. However color discrimination ability might. So everyone should be checked at least once and it is probably reasonable to repeat the tests every few years, but not really necessary to do it more often than that.  (Back to top)

Is there a color model which would allow me to do the following? Given L*a*b*/XYZ/whatever coordinates of a color stimulus, given specified viewing conditions and color, what is the shape/size of the JND "ellipsoid" around the color, and what are the directions of the hue/chroma/lightness axes? (846)
The CIELAB color space would be your best bet as a starting point. The L*a*b* rectangular coordinates can be converted into L*C*h cylindrical coordinates to represent lightness, chroma, and hue angle respectively. Predicting the size and shape of JND ellipsoids is another matter that is a bit more complicated. You might want to look into the CIE2000 color difference equations to understand the relationships between color difference perception and appearance specification in CIELAB.  (Back to top)

Is there still a copyright on the Munsell color system? If so, who owns it? (845)
X-Rite currently produces and sells Munsell products. The colorimetric coordinates of the colors have been published, so the colors themselves are not copyrighted. However, it is likely that X-Rite does have the various products protected by copyright. It would be best to check with X-Rite.  (Back to top)

Which light colors are absorbed by a yellow tulip? (844)
Blue.  (Back to top)

A one dimensional halftone cell has 50% coverage with reflection density 2.0 and 50 % coverage with reflection density 1.0. How is L* calculated for this cell? (843)
You first need to convert from visual density, D, to luminance factor, Y for each cell. (Y = 10^-D). Then you would average the two Y values (since both are 50% coverage, otherwise you would do a weighted average according to the areas) to get the overall Y value, which is then plugged into the L* equation.  (Back to top)

Are you aware of any software packages or even just charts, that list Munsell colors by both code and name? (842)
I can't vouch for the the completeness or accuracy, but I have found an online table of the ISCC-NBS color naming system with Munsell designations. It might be helpful for you.  (Back to top)

What colors (in order) would best absorb a green 529nm laser light? (841)
You haven't provided enough information to answer that question. You would need the spectral reflectance of the material (or transmittance if it was not opaque) for each color and then you would just order them from least reflective to most. For example, a black object would absorb almost all the energy at any wavelength, but a red material might well absorb even more at that particular green wavelength. You can't tell from just the color names.  (Back to top)

Could you please tell me if there is any function that computes the chromatic adaptation and the color appearance for the various types of color deficiencies? (840)
I am not aware of any models specifically designed to compute chromatic adaptation and color appearance for color deficiencies. The closest I can think of is the model implemented and described at vischeck.com. While it does not produce predictors of appearance, one might take the simulated output of their model as calibrated RGB input into a color appearance model such as CIECAM02 in order to obtain approximate appearance.  (Back to top)

How could I provide (or display) the approximate color of a star given it's Kelvin temperature? (839)
Here's a website that describes how to approximate various color temperatures in sRGB.  (Back to top)

I am interested to convert a hex html or decimal rgb value to cie 1931. How can I do this? (838)
You might try easyrgb.com.  (Back to top)

Is 0.05ppm a suitable tolerance for measuring absorption on a spectrometer for beverages? When is it no longer a visible difference to the naked eye 1ppm, 0.1ppm, 0.01ppm, etc? (837)
This would depend on what color the beverages are and what you are measuring. The only way to evaluate visual tolerances is with colorimetry, PPM on spectral absorptance functions tells you very little (perhaps nothing) about visual perception.  (Back to top)

What is the best color to paint behind a High Defintion tv mounted on the wall? (836)
The best thing to have back there is a little bit of light that doesn't fall on the front screen (therefore washing out the image). If the lights are off in the room, then a small light behind the TV is helpful. This lightness helps to increase the perceived contrast of the display over what you see in a completely darkened room. If the room is illuminated, a light grey is probably the best choice. Light, but not too bright like a white, and neutral to not bias the color perception of the display.  (Back to top)

What dyes are mixed to get Dark Shell Gray? (835)
I'm sorry but that's really impossible to answer for a couple of reasons. First, color names are generally not defined precisely so "dark shell gray" could be a wide range of colors. Second, the dyes used would depend on the type of materials being used to make the color and even within a single material type it is possible to make the same color with many different combinations of colorants.  (Back to top)

I am a retinal surgeon. We sometimes stain the retina with trypan blue in order to highlight superficial membranes that need to be peeled (over the retina). Is there a colored contact lens that I could wear that would highlight the blue spectrum more so that it would be easy to see faint staining on trypan blue on the retina? (834)
If you are trying to see a light blue on a pale background, then the best thing to do would be to use light that is absorbed strongly by the blue, but not by the background. That would be yellow or red light (which would also be reflected strongly by any red tissue). You can accomplish the same effect with either lighting or lenses, although it might be a little easier to discriminate by briefly changing the color of the lighting since you won't adapt to the color change as much as you would wearing colored lenses.  (Back to top)

I would like to come up with a color scheme that assigns an absolute color value to common color words. In this scheme I would want scarlet to have a distinct and absolute value different than crimson or puce. Is there any site or program that has assigned absolute color values to the English names of colors? (833)
For this purpose, I would highly recommend the "ISCC-NBS Dictionary of Color Names". It is well out of print, but used copies can still be found online and in libraries. It provides Munsell designations for a wide collection of common color names. However, you should be aware that color names are very imprecise and there is a range of designations for each name. It is quite likely that scarlet and crimson will have some overlap.  (Back to top)

We measure several prints for our printer characterizations. Two questions: (1) what is best space to average - spectral reflectance, LAB or XYZ and (2) if we wanted to do a weighted average to reduce the weight of data which tended to be "inconsistent", how would we best do that? (832)
There is no single "correct" answer. If you are interested in color appearance, then it makes most sense to average in a perceptual space like CIELAB. If you are interested in the physical characteristics of the printer, then it makes more sense to average in spectral reflectance. (However it would make even more sense to average in toner amounts after deriving an accurate model of the printer!). For color applications, CIELAB is probably the best choice. To do a weighted average, you could compute color differences from the mean measurement for each color and then recompute a weighted mean using a weight that is inversely proportional to those differences.  (Back to top)

In the CIELAB color space, what is the approximate difference for each of L*, a*, b*, and ∇E that most people would not percieve as a different color. (831)
It depends significantly on the color in question and the viewing conditions and could range anywhere from less than 0.5 to as much as 3 or 4 units. The general rule of thumb is about 1 CIELAB unit.  (Back to top)

Are there guidelines or rules of thumb for how colors are perceived as "dominant" in an image? Are there differences in the perception of color dominance among different colors? If so, how is this described in the CIELAB color space? (830)
I am not sure what you mean by "dominant", but it would likely be the colors that occupy most of the image area and would probably be defined by hue (as in ... there is a lot of orange in that image). I don't know of any algorithms, but I would suggest looking at a histogram of hue angle in CIELAB and finding any dominant peaks. You might also search for the "Color Reproduction Index" that was published by Pointer and Hunt which does a similar division of colors.  (Back to top)

Where can I find a chart showing color names, the colors themselves and CIELAB color coordinates? Ideally, this would also show the RGB color coordinates. I have found many RGB charts like this but none with the CIELAB coordinates. (829)
I don't know of any since it is a very difficult procedure to calibrate such a chart and RGB coordinates are generally undefined. I would suggest taking one of the RGB charts you have found and performing a conversion from sRGB values to CIELAB values (along with viewing them on a calibrated display).  (Back to top)

Are there established color values for the color of skin for various ethnic groups (like Lab or Munsell for example)? This is for a makeup manufacturer who wants to design packing for specific groups of people. (828)
I am not aware of any specific standards although some might exist. You might look at some of the color imaging test charts (e.g. the Munsell Digital ColorChecker SG from X-Rite that has several skin-tone samples, see this web page to get an array of reasonable colors.  (Back to top)

I dyed 100% cotton red, yellow and blue and placed them in direct sunlight for a week. The red appeared to fade the most, but I can't explain why. Can you help? (827)
It's hard to say since it depends on the specific colors and how you measure "fading". The yellow one wouldn't look like it faded much because it is hard to see yellow fading toward white (they are already similar colors) and the red one might have faded more than blue because it is absorbing more blue light, which is made up of the shorter wavelengths, which have more energy and tend to cause more chemical changes such as fading.  (Back to top)

Why does red disappear when viewed behind red glass? (826)
It actually doesn't disappear at all ... it still looks red doesn't it? What happens is that red text (or other patterns) on white backgrounds disappear since the red glass makes the white background appear red as well and no contrast remains. If the red text or pattern was on a black background, it would remain just as visible through the red glass.  (Back to top)

Are grey shades actually constant, because they are nearly achromatic, or is there something about eye biomechanics that create a visual perception not in agreement with spectral readings? (825)
Grays, like any colors can be very color inconstant. It really depends on the combination of reflectance characteristics, light sources, and observers. The most likely samples to be nearly constant would be grays with flat reflectance curves (known as nonselective materials). However, even those are likely to not be perfectly color constant.  (Back to top)

I need to know the Munsell color for 'school bus yellow.' The unsourced quote I have says it is 9.4YR, 7.5/9.2. (824)
I'm sorry, I don't have the standard in terms of Munsell coordinates, but the Yxy values are 40.2, 0.4882, and 0.4205 respectively (From Color Standard SBMTC 008, 1997). Your Munsell coordinates seem about right, but I can tell from the Y value of 40.2 that the Munsell Value should be 7 instead of 7.5. The rest can be converted using the Munsell Conversion Freeware available from X-Rite. Unfortunately that won't run on my computer!

Update!
From The Physics and Chemistry of Color:The Fifteen Causes of Color. Second Edition by Kurt Nassau. John Wiley & Sons, Inc. 2001 p 395: "A full designation for an orange-yellow school bus in the sequence H V/C and using interpolation might then be 9.4YR 7.5/9.2."  (Back to top)

What does LCC stand for in the color science world? (823)
To my knowledge, it stands for nothing. I did a little online searching and found a few references in patents to an LCC color space as a "luminance-chrominance" space. It appears the term might be used in a generic sense to refer to "any luminance chrominance space".  (Back to top)

Where can I purchase a poster size print of the CIE 1931 Chromaticty Diagram? I realize that the diagram is only a 2D projection of 3D matching data however I still find it useful to illustrate some of the other attributes of the data depicted (color mixing, color temperature, color gamuts)? (822)
Not only am I not aware, but I probably wouldn't tell you if I was!

Seriously, we really don't like them because they badly misrepresent the meaning of a chromaticity diagram. The diagram tells us nothing about color appearance since it is only based on matching data. It only tells us whether two colors match, but not what color they appear.  (Back to top)

At what distance, target size and viewing conditions (illumination level/type) does the HVS's ability to resolve color breakdown? Put another way, what is the "chromatic" resolving power of the HVS and how is that defined in terms of distance, size, viewing conditions? (821)
In your observed scene, there are two things happening. The stimuli are getting smaller, but they are also becoming physically more similar due to the intervening atmosphere. If they were far enough away, all you would see is atmosphere (or imagine the atmosphere was more optically dense, like on a foggy day) and you couldn't discriminate them at all. So, first of all, that physical change needs to be accounted for. After that, there are changes in your visual system as well. Viewing conditions make all the difference in the world and cannot be explained in a short answer (I wrote a whole book on that entitled "Color Appearance Models"). However, as the stimulus becomes small in viewing angle, you do lose your ability to distinguish color. Once the stimulus is smaller than about 0.25 deg, you are close to becoming a dichromat (it's called small area tritanopia) due to the small numbers of blue-sensitive cones. Then when it gets even smaller, maybe 1-2 minutes of arc, you really can't discriminate color at all (only brightness). This is why stars look far more colorful in images than they do to the naked eye (even those that are bright enough to see with cones).  (Back to top)

Is black a natural color or is it just a very dark brown that people just see as black? (820)
Technically, color is defined as a perception, so if something appears black to you, it is black. Beyond that, there are many naturally occurring materials that we would consider to be black (not just dark brown or other color). Obsidian is one thing that comes to mind (although it does occur in colors other than black as well). So, yes, black is a natural color.  (Back to top)

We would like to know the complete Reflectance Measurement principle and formula for the dual beam color spectrophotometer including Light trap, PTFE standard and sample. (819)
I would suggest texts like Berns' "Principles of Color Technology, 3rd. Ed." for a fundamental introduction and Wyszecki and Stiles' "Color Science, 2nd Ed." for more details and references.  (Back to top)

Is there a relationship between visual lightness (when we judge lightness of color) and Light Reflectance Value (a measurement that is expressed mathematically? (818)
Yes, they can be related mathematically. One way is through the CIELAB L* equation. L* is intended to represent perceived lightness (your grayscale judgement) with a perfect white at 100, a perfect black at 0, and a perceptually middle gray at 50. It is a function of the Y tristimulus value, or luminance (which is also your LRV). That perceived middle gray with an L* = 50 has Y = 20. The particular equation is a cube root function with an offset. L* = 116(Y/Yn)^(1/3) - 16.

Yn is the Y value for a perfect white under the given viewing conditions. I hope this helps.  (Back to top)

I am a massage therapist. After a massage a customer said that when I had massaged her on a certain part of her body she could see a certain color, for ex. red, & at another spot she saw the color blue. Is there a reason to this? (817)
There are two possible reasons that I can think of. One is something called a pressure phosphene. They usually happen when you gently touch the side of your eye and you can see some colors. It is possible that the massage in different areas cause differing pressure on the eyes (depending on the position your client was in). A second possibility is a phenomenon called synesthesia. This is when stimulation of one sense produces a response in another sense. For example massage in certain areas might produce the red impression along with a feeling of slight pain (as a random example) while the massage in another area might have produced a blue sensation along with a soothing feeling (another random example).  (Back to top)

How do you find a RAL equivalent of an NCS colour reference? (816)
The RAL system is proprietary, so it is not easily possible to convert to/from RAL from/to any other system. That is even true with one that has been published, like NCS. The best I can recommend is to purchase the RAL swatches and visually match to the desired NCS color (you could also do the matching instrumentally).  (Back to top)

Why can't humans see ultraviolet or infrared energy? (815)
There are a few reasons for the range of wavelengths that we can see. One is that much of the interesting interactions between energy and objects happen in those wavelengths, so there is a lot of information for us to see there. There are also more "practical" reasons. UV energy is often damaging to biological tissue, so it would be potentially dangerous for our visual systems to absorb that energy. Some insect respond to UV energy, but they don't live nearly as long as us, so maybe the potential damage is not so much an issue for them. At the other end of the spectrum it becomes difficult for biological photoreceptors to respond reliably. Basically, because of the temperature of our bodies, receptors that could respond to IR would also be very noisy (IR cameras are often cooled for this reason) and it would be difficult for us to differentiate noise produced by our visual system from objects out there in the world.  (Back to top)

What color is chroma key paint for green screen back drops? (814)
Green! Seriously, it can be just about any color that stands out from the foreground objects or people. A high-chroma green is most commonly used, but sometimes other hues are selected. The camera/processing system is set up in the studio to use the selected color as the region to substitute in other video content. Here is a link to one source of green and blue chroma-key paint.  (Back to top)

Is it possible to determine the ink densities (or transmissivities) in a small area (~20 mm^2) of a four color image (CMYK) from its reflectance spectrum? The relative areas of coverage and overprint of the inks are not known. Illuminant and substrate spectra are available. (813)
Yes it is possible. However, in addition to the information you mentioned, you would also need to know the characteristics of the inks and their interaction with the substrate. Essentially, you need to have a model of the printing system and you could use that to determine the CMYK coverages needed to produce any given color. It is not a simple matter, but it is possible.  (Back to top)

How would I measure the yellowness of the sclera (whites) of the eye? Also, what instrument would be an appropriate measure haziness of the cornea? (812)
The best instrument would probably be a spot spectroradiometer (also sometimes called a telespectroradiometer). You point it at the object of interest like a you would a camera and measure the indicated spot. This would allow you to choose the appropriate spot on the sclera (or cornea) to measure. With controlled lighting, or a reference white measured next to the eye, you could get quite reliable measurements.  (Back to top)

What is the color temperature that makes the road most clear during driving, even under unconditional weather? (811)
I am not familiar with any research on this topic, but some might exist. My understanding is that moderate color temperatures (say around 4000K) are being used as a trade-off between the traditional halogen lamps that are too yellow to render colors well and the bluer discharge lamps that tend to produce a lot of visual glare for oncoming traffic. That might well be the best compromise.  (Back to top)

How do I match the actual fabric color in PhotoshopTM? Images are digitally shot, and the final output is four color offset printing. (810)
This is not a simple matter, but it can be accomplished with carefully calibrated and characterized imaging systems (camera, display, printer, and software). This is the domain of color management and I would suggest you look further into that area to learn more about it. "Real World Color Management" by Fraser et al. is one book that provides a useful introduction.  (Back to top)

What is the theoretical principals of the color change test in chemistry? (809)
How Stuff Works provides a great explanation and experiment. Here is an example: Printing Industries of America RHEM Light Indicator  (Back to top)

I am interested to convert a hex html or decimal RGB value to CIE 1931. How can I do this? (808)
You might try this site easyrgb.com  (Back to top)

Is 0.05ppm a suitable tolerance for measuring absorption on a spectrometer for beverages? At what concentration is the difference not perceptible? (807)
This would depend on what color the beverages are and what you are measuring. The only way to evaluate visual tolerances is with colorimetry, PPM on spectral absorptance functions tells you very little (perhaps nothing) about visual perception.  (Back to top)

What would an acceptable max display luminance setting be for a display that does not see direct sunight or much ambient light so that colors and text are easily readable? (806)
There is no set, agreed upon, answer. However it certainly is possible to make displays that are uncomfortably bright in dim surroundings. The useful maximum is probably somewhere between 500 and 1000 candelas per square meter.  (Back to top)

What is the unit of the viewing angle? Is it degree, radian or steradian? (805)
"Degree" is the unit typically used. However degrees and radians are interchangeable (like yards and meters they measure the same thing). Steradians, on the other hand, are used to measure solid angles like cones and are not appropriate for viewing angles.  (Back to top)

Using the color bar on Powerpoint 2003 I drag the brightness bar on the Device RBG mode, I get a darker color of apparently the exact hue. Is the hue is truly staying the same? Also, if I drag from R 200, to R150 to R50, is it decreasing in the amount of red equally in each interval? (804)
No, hue is not staying the same. Display devices are not linear, so that causes one problem, and even if they were, constant ratios of linear RGB wouldn't necessarily appear the same hue since the human visual system is nonlinear (although they would be closer). The only colors for which this will be approximately true are when you have only one non-zero RGB (i.e., just red, just blue, or just green being adjusted). On the second point, equal RGB intervals are not equal intervals in either amount of light (luminance) or perceived amount (brightness).  (Back to top)

"The Desktop Color Handbook" mentions using a "light source color rendering card" which has patches that look the same under light of the appropriate CCT, but different under other lights. Do you know who makes such cards, or how one could create one? (803)
I have seen such cards, but I don't know of any current manufacturer. "color rendering" really isn't the correct name. These cards are based on a pair of metameric samples that will match each other under the appropriate lighting. They are produced by having the samples prepared with differing spectral reflectance functions, but designed to match under a certain light. For example, gray metameric pairs designed to match under a chosen light could be made on a CMYK printer by printing one sample with black ink only and the other with a combination of cyan, magenta, and yellow (but no black).  (Back to top)

I am a Color Technician for a woman's retailer which specializes in black, white, and neutral only clothing. We test our associates with the Munsell 100 Hue test. Is there a similar test to evaluate for neutrals only? (802)
There appear to have been some developed. If you do a search on "lightness discrimination test" you can find some references. However, I am not aware of a commercially available test similar to the Farnsworth-Munsell test. It wouldn't be difficult to create one of your own. You would just need to make a set of gray samples closely spaced in lightness (perhaps 100 or so from white to black) and then ask people to put them in order. You could score it in the same way as the normal Farnsworth-Munsell test.  (Back to top)

How can I convert RGB or CMYK shade to Pantone? (801)
Pantone is a proprietary system, so unless you license their software (or buy software from someone who has licensed their conversions), you cannot make this conversion. In addition without accurate display or printer characterization, the RGB or CMYK values would be inaccurate.  (Back to top)

Is there an accepted universal standard for the calibration of color Spectrophotometer? Are there white standards available with certified reflectance values? (800)
The universal standard is the theoretical perfect reflecting diffuser (PRD). A PRD is a perfect reflector (reflects 100% of the incident light) and a perfect Lambertian diffuser. No such material actually exists, but pressed PTFE (sometimes called Halon) is a good approximation. Materials are calibrated by national standardizing laboratories to provide reflectance factor data relative to the PRD for a given illumination and viewing geometry. In the USA the national laboratory is NIST. Other examples are NPL in the UK, and NRCC in Canada.  (Back to top)

Are there maximum/minimum values for a* and b* like the min and max for L*? (799)
No, there are not specific limits. The limits for given types of materials (or lights) would depend on both their material properties and the viewing conditions. You might look at the MacAdam Limits to see what some theoretical limits for reflecting objects look like. The a*b* limits depend on the hue and lightness of the object. In imaging applications, limits for 8-bit encoding are often set as the range from -127 to +128 in both a* and b*. This is adequate to cover the gamut of most systems.  (Back to top)

Is color a quality in physics? (798)
The main definition of color is that it is an attribute of the human perception of light. It is not a property of physics (although physics can be used to describe the light stimulus), but a property of human perception. So, according to that definition, the answer is a very definite "no". However the word color is unfortunately sometimes used with other definitions. For example the term "color" is used in physics to describe a property of the theoretical subatomic particles known as quarks. In that sense of the word, "color" would be a quality of physics.  (Back to top)

I have noticed a couple of different species of early-flowering trees have pink blossoms which turn white literally overnight. It is the speed of the process which is intriguing. Do you know how it works? (797)
I don't know the mechanisms for sure, but it could be that the pink colorants are very light sensitive and fade shortly after the flowers bloom. Also, it could be that they are transported quickly to different parts of the flower or spread across the petals as the petals grow (thus diluting the color from pink to more of a white). Plants grow incredibly quickly this time of year.  (Back to top)

Do you know where I can purchase Hess Ives Color standards, both physical standards and papers publications? (796)
I'm afraid I don't know of a source, but I would suggest contacting Tintometer since they make instruments capable of measuring Hess-Ives units.  (Back to top)

How can tulips be so many different colors when they are the same plant? (795)
It is for the same reason that people look different from one another. Part of our appearance (and that of a tulip) is genetically programmed and another part is due to environmental factors. The genetic code of the tulip has the information necessary to produce the chemical colorants in the petals of the flower (or lack thereof) allowing different tulips to express different colors. Humans have spent many years selecting tulips for color and other features to assure that certain tulips have the desired genes. In humans, things like eye color, skin color, etc. are encoded in the genes we inherit from our parents.  (Back to top)

Is there such an instrument as a hand-held spectrometer for measuring the color temperature of light sources? If so, how much do they cost? (794)
Yes, there are handheld colorimeters from companies like KonicaMinolta that are capable of such measurements. They range in price from several hundred to several thousand dollars. At the higher end, there are portable spectroradiometers from companies like PhotoResearch that are in the tens of thousands of dollars range, but also provide spectral radiance data in addition to color temperature.  (Back to top)

Densitometers can be purchased with or without polarizing filters. Why are both types used? Which type provides the best correlation to perception? (793)
The polarizing filters help eliminate gloss in the measurement. This means less light is being measured and therefore density is higher. This measurement is used sometimes to allow the measurement of wet inks that are still glossier than the finished product, or to compare surfaces with different gloss characteristics. Observers tend to move the samples to minimize gloss when judging color.  (Back to top)

How do you convert ΔL ΔC ΔH values for colour differences in CIELCh to the equivalent ΔL Δa Δb colour difference values in CIELab, when you do not have access to the absolute values for the original measurements? (792)
Unfortunately, you can not.  (Back to top)

Are there some basic averages for the LRV of human skin? (791)
I don't know of any specific data and there would be quite a range, but an old photographers trick is to use the back of their hand to measure the light level if they don't have a standard gray card with them. This is equivalent to assuming that the skin has a reflectance of about 20% and that might well be a reasonable assumption on average.  (Back to top)

At what age do children develop good color perception? (790)
The visual system of infants is still developing after birth. It is difficult to say exactly how long it takes color vision to become "normal", but by about 6 months of age the visual system is essentially the same as an adult. More significant, probably, is that visual acuity improves rapidly during those first six months and stimulation of all types from the visual world does aid in this early development. Here is a link to a good page on infant vision.  (Back to top)

Can you define what is Multiflux math model used in color mathing? (789)
Here are a couple of suggested references from our faculty:

• H. G. Völz, Industrial Color Testing: Fundamentals and Techniques, 2nd ed, Wiley-VCH, New York, 2001.
• Articles by Willard Richards
(Back to top)

I have used three online color space conversion calculators and I get three different sets of numbers when I convert from XYZ to RGB. What's happening? Who's right? (788)
It is quite likely that they are all accurate, however they all might be using slightly different definitions of RGB or assumptions on how the XYZ values were obtained. This is one problem with RGB; it is not specifically defined and is device dependent. It is probably best to just choose one converter and stick with it.  (Back to top)

Can light be a "Brown" color? If not, why not? (787)
This is one of my favorite questions. The short answer is "no". Brown cannot be perceived as a light source because it is only perceived as a related color that is "dark" and "low in chroma". This can only occur for colors that are less luminous than the white we are adapting to. Here is some more detail from one of my other websites.

Q: What is brown?
A: Brown is dark, low-chroma (desaturated), orange. In other words, an orange light that is perceived in an environment that makes it look dark and less saturated will appear brown. A brighter surround is such an environment. Brown is a special sort of color perception that requires this relationship to its environment in order to be perceived. That's why you can't go to a hardware store and buy a brown light bulb!"  (Back to top)

What is the RAL number for PMS 268 plum? I've attempted to find a cross-reference tool and have not been successful! (786)
Both RAL and Pantone PMS systems are proprietary. They make their money on selling the samples that embody their systems (or software to do conversions like you are looking for). They are both protective of their systems in order to retain their market. That is why you cannot find a conversion tool and also why I can't help you.  (Back to top)

When purchasing an encapsulated Haz-mat suit, Level A, what colors have the best visibility for the different environments in which they might be used, i.e, sandstorms, snow, heavy rain, thick smoke, etc.? (785)
The best color for each situation will be different. The thing to keep in mind is that the visual system functions on contrast, the differences between colors of objects. For example in a snowstorm with snow on the ground, black would be by far the best color (most contrast with white), even at night. In thick smoke or heavy rain at night, black would probably be the absolute worst color. White would be the best then with respect to contrast. However a secondary consideration is that you want them to stand out from other common objects. In that case, you would want a very light color that is rare (like blaze orange or fluorescent yellow-green).  (Back to top)

Roughly how large is the overall remaining perceptual non-uniformity of color tolerances in CIELab space, when using the ΔE94 and ΔE2000 color distance formulas? (as compared to the 6:1 non-uniformity of simple ΔE*ab)? (784)
It is a serious estimate, but if you are assuming 6:1 in CIELAB ΔE*ab, I would say that it would be about 3:1 or 2:1 in either ΔE94 or ΔE2000. Whether or not ΔE2000 is a significant improvement over ΔE94 is questionable. Both are significant improvements over ΔE*ab.  (Back to top)

How can I have someone create a paint for me using a theoretical spectral reflectance curve? (783)
There are computer color matching systems that are capable of determining the best spectral match possible for a given paint medium and colorants. In any given paint system, there are limitations, but many spectra can be matched. The best advice is to find a manufacturer of the type of paint in which you are interested and send them the spectrum. They should be able to compute the best match they could make for you.

Hardware and paint stores have color matching systems that can be used to measure a sample and formulate a match. However, it is likely that such systems are aiming for color matches, which are not always spectral matches. That would be an easy way to get something close if you have a sample of what you are interested in producing.  (Back to top)

How does chameleon paint (i.e. paint which color depends on viewing angle) work? What is its composition? (782)
Such "effects pigments" work on the basis of interference in the layers of material that are dispersed in the paint. This is the same effect that causes soap or oil films to exhibit a range of colors. The wavelength of constructive interference varies with the angle of illumination and view and that results in the range of colors we see. This website provides a nice overview. You might also search for terms like "interference pigment" or "pearlescent pigment" to learn more.  (Back to top)

How do I calculate the dominant wavelength of a color sample with X,Y,Z or x,y? I do not want to draw it in the colorspace. (781)
I am not aware of any code to do this directly. You would have to do the computation in terms of xy. The task would be to define the line that connects the sample xy with the white point xy and then solve for the intercept with the spectrum locus (which is defined by the color matching functions and not analytically). It would have to be some form of numerical solution. There is also some information and suggestions from visionscience.com.  (Back to top)

Why does your hair change color during the season? (780)
I found this nice answer on the MadSci Network.  (Back to top)

How to convert CIELAB data to Yellowness and Whiteness index? (779)
The equations are available in most textbooks on colorimetry, such as Berns'"Principles of Color Technology, 3rd Ed.". You will have to first convert the CIELAB coordinates back to CIE XYZ values and then compute the yellowness and whiteness indices you are interested in (there are several) from there.  (Back to top)

Which minimizes solar heat absorption better, polished aluminium or white paint on it? (778)
It would depend on the specific materials, but in general a good polished aluminum will reflect more light than a typical white paint. The aluminum should be well above 90% reflectance while the paint will be between 80% and 90% reflectance. You also have some control over where the reflected energy goes with the mirrored surface whereas it is reflected diffusely in all directions from a white surface.  (Back to top)

How does the eye determine the various colors of visible light? (777)
Our perception of colors comes from a combination of the sensing of light in the eye and the processing of that information in the brain. The first step is that we have three types of cone receptors in the eye that respond to different wavelengths of light. Approximately, they respond to red, green, and blue light respectively. It is the responses of these three receptor types, in various proportions, to the light present in a scene that initiates the process of color vision.  (Back to top)

Could you please tell me, what color would 1.2Y 6/11 be? (776)
Since it is a Munsell designation, which is a published color system, I can. Munsell also has the advantage that you can estimate the color appearance directly from the notation. This color would be a slightly greenish yellow that is fairly light and very saturated. This link includes a nice explanation of the Munsell system.  (Back to top)

Why do black clothes appear blue in the artificial light of a clothing shop? Then when you take the item into natural light it is black? Why is that? (775)
There are many types of artificial light and the interactions of light with objects can have very significant effects on the perception of the object color. It all depends what the material is made of and how it is colored. However, you might be seeing a purely perceptual effect. If the lighting is yellowish (like incandescent lamps), then there is a visual contrast effect (technically called the Helson-Judd effect) that makes darker colored objects appear bluish. What is happening is that the in contrast to the overall yellowness of the scene, our visual system adapts and where there is little light (the dark objects) our we notice the perceptual opposite of yellow, which is blue. It is always best to judge colors of objects under the variety of lighting in which they will be used (or worn).  (Back to top)

What type of Pantone Guide shall we refer to & which instrument is suitable to measure the values of colour on aluminum foil with a flexo printing process? Since Aluminum has glossy as well as the matt finish then is there any need to change the settings of the instrument? (774)
I can't help you with the Pantone guide; I'd recommend contacting Pantone on that one.

On the measurement geometry, that is difficult with a metallic substrate. I suspect you are interested in the color of both the diffuse reflection and the specular reflection (especially if your ink is transparent). You really do need to make two measurements to evaluate both. I would suggest that you might use a traditional 45/0 geometry to evaluate the diffuse reflectance and a diffuse/0 (specular included) measurement to include the specular reflectance. It might also be insightful to look at the differences between the two measurements. As an alternative, you could use the specular included and specular excluded measurements on a single integrating-sphere instrument and compare those.  (Back to top)

We print on Aluminum foil with a flexo process. What type of Pantone Guide shall we refer to & which instrument is suitable to measure the values of colour? Since Aluminum has glossy as well as the matt finish then is there any need to change the settings of the instrument? (773)
I can't help you with the Pantone guide; I'd recommend contacting Pantone on that one.

On the measurement geometry, that is difficult with a metallic substrate. I suspect you are interested in the color of both the diffuse reflection and the specular reflection (especially if your ink is transparent). You really do need to make two measurements to evaluate both. I would suggest that you might use a traditional 45/0 geometry to evaluate the diffuse reflectance and a diffuse/0 (specular included) measurement to include the specular reflectance. It might also be insightful to look at the differences between the two measurements. As an alternative, you could use the specular included and specular excluded measurements on a single integrating-sphere instrument and compare those.  (Back to top)

How do I convert from spectral reflectance (from a hyperspectral imager) to CIELAB colorspace? (772)
You integrate the product of the reflectance, a selected illuminant, and the CIE color matching functions with proper normalization to obtain CIE XYZ tristimulus values. These values are then used in the computation of CIELAB coordinates. A colorimetry text such as Berns' "Principles of Color Technology, 3rd Ed." explains this procedure in detail.  (Back to top)

Can a consumer grade flat bed scanner be used as a colorimeter? (771)
The general answer is no, but you can make a reasonable approximation if you know the characteristics of the material you are scanning. For example, if you are always scanning photographic prints that are made of the same three dyes, then it is possible to derive an accurate conversion from the scanner readings to XYZ values for that material. However, when you scan other materials, the conversion would not be accurate.  (Back to top)

The lighting goal for our new indoor shooting range is to have 100 footcandles of light on each target. Our question is this: should the walls and ceiling around the target be light in color or dark in color? (770)
I would make them light in color since this will increase the perceived contrast of the targets themselves and make them essentially more visible. Although with the intense focus on the target that usually accompanies shooting, the effect might be very small.  (Back to top)

Can you suggest an ideal color scale for a digital elevation map to be printed in a scientific paper? Is it possible to have a color scale that looks good in color and in black and white? (769)
I'd suggest you check out the information at colorbrewer.org. It is based on good perceptual studies and should help with your needs. What you need is a scale that is monotonic in luminance/lightness so that the information is preserved in black and white images.  (Back to top)

Suppose I introduce the same amount of noise in individual channels of a YUV image of 420 format (chroma has been downsampled). Which image is the most noticeable, the one with noisy Y, U, or V? (768)
Noise in the Y dimension will be most noticeable. However, YUV is often poorly defined and the differences in noise perception are often not as large as expected since there is luminance information in the U and V channels.  (Back to top)

Can you point me to the actual formula the Photoshop Hue/Sat tool is using to change given colors in an image?Can you point me to the actual formula the Photoshop (tm) Hue/Sat tool is using to change given colors in an image? (767)
Sorry, I can't. The inner workings of programs like Photoshop are usually kept proprietary and it is not always obvious at all what these controls do.  (Back to top)

What are the colors of a complete spectrum? (Starting with the color with the longest wavelength.) (766)
The classic answer is ROYGBIV. That's shorthand for red, orange, yellow, green, blue, indigo, and violet. However, it really isn't that simple. When humans view the spectrum, they don't see just 7 discrete hues, but rather a continuous range of hues from red through violet. The number of distinct hues we can see would depend strongly on the specific viewing conditions, but it is safe to say that it could be well over 100! When naming colors, we tend to group them into categories of similar colors and that is why the ROYGBIV shorthand is so popular.  (Back to top)

What color measurement system would be best to define colors for a liquid, to be described in a US patent? (765)
First, you would have to measure the spectral transmittance of a sample and define the measurement conditions such as the path length through the liquid that you are measuring. You could then do normal colorimetric computations with the measured spectral transmittance. You would have to select and appropriate illuminant and observer and then you could compute CIE XYZ and/or CIELAB coordinates to define the color.  (Back to top)

Why is blue ink most commonly used for writing? (764)
As with many historical questions, there are probably many factors involved. This website gives some history on ink and suggests that blue ink was the first color developed after black. That probably goes a long way toward explaining its popularity. Also, the utility of ink is in its legibility (in most applications) and legibility comes from contrast. After black, blue would provide the most visual contrast on white (and light) papers and thus be the easiest of the non-black colors to read. Beyond that, blue has also traditionally been one of the most popular "favorite" colors. It is likely that all these factors, and more, combined to make blue ink popular for writing.  (Back to top)

What is the difference between the L*a*b* values and the L a b values when using a Minolta colorimeter? (763)
L*a*b* refers to CIELAB coordinates that are commonly used as the basis for color difference measurements. Lab refers to an older color space known as HunterLab. It was a predecessor of the CIELAB space and, unless someone specifically asks for HunterLab values, it shouldn't be used. The values are not equivalent.  (Back to top)

Do you do research on ink jet printer color consistency? We are interested in printing solid colors for visual evaluations. We have a high-end ink jet and use photo paper, but we see color differences of up to 0.5 Units ΔE* between prints using the same printer, inks, profile and paper. (762)
We do some research on related topics. Given the variability in papers, inks, and the printing process, I would be very surprised to see performance better than what you are quoting. In fact, I would suspect it is difficult to print the same color on two edges of a piece of paper within a tolerance of 0.5 ΔE*. You should be very happy with the performance you are seeing.  (Back to top)

My spectrodensitometer states its measuring geometry is 0/45, and its standard observers are 2 and 10. Are these angles related? Which should I use? (761)
0/45 refers to the geometry of illuminating the sample (0 degrees from the normal, or perpendicular) and detecting the reflected light (45 degrees from the normal). That is unrelated to the two and ten degree observers. Those refer to the field size when the original experiments were done to determine the standard observers for colorimetry. Think of them like have two different people look at the matches. Either observer would be appropriate for your application. It is best to be consistent, record which one you use, and use whatever is requested by your customers/suppliers or any standards you are following. More on all these topics can be found in books on colorimetry such as Berns' "Principles of Color Technology, 3rd Ed."  (Back to top)

Is there a way to determine what a symmetrical circle 1% dot, 120 line screen should measure? I am looking for a published table or a equation as I need to determine this on several different screen rulings. (760)
Well. that would depend on what you assume a 100% dot is. If you assume 100% means that the dots just touch when they are diagonal from one another (adjacent dots overlap), then you can take a right triangle with two sides equal to the dot spacing, compute the hypotenuse and divide that by two to get the radius of a 100% dot (skip the divide by two if you want diameter). Then dividing that by 100 will give you the radius of a 1% dot. Of course, this ignores any sort of irregularity in the dots and dot gain.  (Back to top)

I am working on a plastic matched automotive black (GM 848) where the color is visually darker, but it measures LIGHTER than the standard. Can you please explain why? (759)
It's difficult to say without seeing the samples, but two possibilities come to mind. One is simply that there is some sort of problem with the measurements or the instrument. Assuming that is not the case (e.g., both samples measured at the same time, on the same instrument, in the same way), then I would suspect that there is a difference in the surface characteristics of the two samples. For example, the one that looks darker might have more of a glossy (or less matte) surface than the one that looks lighter. Depending on the instrument geometry you use, this difference might not show up in the measurements. If you take a glossy and matte surface that measure the same for total reflectance (0/d), the matte surface will usually look lighter. If you measure the two on a 0/45 instrument, the reading for the matte surface will be higher, more in line with the visual assessment.  (Back to top)

What is the mathematical equation to convert Lab or XYZ to Density? (758)
I'm sorry, there isn't a direct conversion from CIELAB or XYZ to density. CIELAB and XYZ are based on visual color responses, while densitometers are designed to measure material properties (ink or dye amounts) of various systems.  (Back to top)

I sometimes catnap at my computer or while reading and when I wake everything looks green tinged for about 20 seconds. Why? (757)
This can be explained by chromatic adaptation. If you close your eye and look toward a light source you will notice that you can perceive some red light passing through your eyelids. When your eyes are exposed to this red light for a while (while you nap), they become relatively less sensitive to red light and therefore relatively more sensitive to green light. When you open your eyes, they are exposed to the world of fairly neutral light, but since your eyes are more sensitive to green light, everything appears to have a tinge of green for 20 seconds or so. This is the same effect as chromatic afterimages that cause you to see one color after staring at another for a while.  (Back to top)

Does the color of a laser affect the velocity of the light wave projected by the laser? (756)
In a vacuum, the answer is "no". The speed of light is the same for all wavelengths (laser colors). In other materials, the speed of light does become a function of wavelength which is why we have dispersion and prisms can separate white light into a spectrum. The change would depend on the material and is defined by the material's index of refraction and how that index varies with wavelength. The index of refraction of air is essentially identical to that of a vacuum for visible wavelengths, so for all practical purposes the answer in air is also "no".  (Back to top)

What is the physics behind a blue-yellow polarizer such as the Singh-Ray Gold-N-Blue filter which photographers use to add yellow or blue highlights to polarized parts of a scene? (755)
I am not very familiar with those particular filters and I couldn't find any details on them. However, it sounds like you are halfway there and what you need is a second polarizer on the other side of the cellophane. When you rotate one of the polarizers, you will get a range of colors coming through the whole sandwich and it will work in either direction. Here is a little experiment from the Exploratorium that does something similar.  (Back to top)

Is is possible to convert R, G, B values of an image captured using a flatbed scanner to densities? I can convert R, G, B to CIELab using the ICC profile of the scanner. Is there a way to convert CIELab to Densities? (754)
Since the spectral sensitivities of your scanner most likely differ from those for a given density standard, it is very unlikely you would be able to directly compute densities from the RGB measurements. The best you could do would be to construct a lookup table that would be appropriate for a given type of input medium (much like the ICC profile you mentioned). The same goes for a CIELAB to density conversion. It would be medium dependent since the two are derived with different spectral responses.  (Back to top)

Where could an art reproduction facility obtain an optical device that used spectral-based imaging? (753)
There isn't a commercial implementation as yet. Several companies are considering such a device including Sinar and Geospatial Systems. At this point, you would need to implement this on your own. See the Art-SI page for details.  (Back to top)

We know density is derived from the transmitted spectral power distribution. Can you suggest a reference that provides the responsivity functions for Status A and Status M densities, as well as the specific function for the calculation of density? Also, what is the proper illuminant to use? (752)
The standard procedures for computing density are published by ISO (ISO 5 Photography - Density Measurements). It appears that part 3 (ISO 5-3) includes the spectral details. The procedures and data are also described in Hunt's book, "The Reproduction of Colour, 6th Ed." in chapter 14.  (Back to top)

What is "camera gamma" and its relationship to NTSC/HDTV video? (751)
Gamma is the nonlinear relationship between the light intensity in the scene and the encoded camera responses that is normally described with a power function. The exponent of that power function is called "gamma". In video, the camera signal is encoded in this nonlinear way to compensate for a similar nonlinear relationship between video signal and display intensity in traditional CRT displays. However the compensation is rarely complete since it is often desired to increase the physical contrast of the video images. Hunt's text, "The Reproduction of Colour" provides much more background on this topic and Poynton's FAQ provides many answers on the confusing use of the term "gamma'.  (Back to top)

How does coloured text affect accommodation? (750)
There are two ways that the color of text impacts accommodation. The first is simply through luminance contrast. Text that has more luminance (or lightness) contrast is easier for us to focus on. For example black on white has far more contrast than yellow on white (or blue on black). Secondly, there is chromatic aberration in the eye, so if we are focused on text of one color, it is quite possible that text of another color will be out of focus and require us to accommodate again to read it.  (Back to top)

How can I meaure the color of clear solutions? We want an objective measurement (wavelength) of colors of various juices and wines. (749)
You would want to use a spectrophotometer or colorimeter capable of transmittance measurements using cells that can hold the liquids. There are a number of manufacturers of such instruments. Two that are well-known for these types of measurements are Hunterlab and Tintometer.  (Back to top)

When choosing an umbrella, what color is best to protect yourself from UV light? (748)
Technically it's UV radiation, not UV light, since the term "light" is only used for visible wavelengths. There is not a direct correlation between color and UV absorption/reflection so there is no simple answer to your question. I would suggest you stick with an umbrella that allows no, or very little, light to pass through (e.g., you can't see the sun looking through it) and it will also be very likely to stop the UV energy as well.  (Back to top)

I am teaching color theory to our press operators and I am wondering if there is a full color spherical L*a*b model illustration that I can download, and if necessary purchase. (747)
If you do a google image search on "CIELAB", you will find a collection of images. I will leave it up to you to determine which have copyright restrictions.  (Back to top)

I would like to know if the color of the water (by adding food color dyes) will affect the absorption rate of water and thus the color of the white flower over time. (746)
There is no reason to think that the color of the water will change the rate of uptake by the flower. However, the color imparted will appear different for different colors of dye. Some simply make more visible changes than others. It will also depend on the amount of dye that you put in the water and this is a difficult thing to equalize across different colors. What you are seeing is the different strength of the dyes to produce a visibly noticeable effect. There are many websites with experiments on coloring flowers this way (carnations are a common choice), for example KidZone.com.  (Back to top)

What is the Kubelka Munk Theory? What its use in textile dyeing process? Would you please provide me any literature on the Kubelka Munk theory? (745)
Kubelka-Munk theory represents a series of mathematical models that can be used to predict the spectral reflectance properties of materials from measured absorption and scattering characteristics of the medium and colorants. Predicting color matches in the textile industry is one application of the theory. Much has been published on Kubelka-Munk theory and computer colorant formulation (or color formulation). A good starting point is an introductory textbook on colorimetry such as Berns' "Principles of Color Technology, 3rd Ed.".  (Back to top)

Is there an existing paint industry standard system for expressing (and thus potentially matching) all the visual characteristics of a metallic or metallic flake paint, whether one-coat or two-coat (colored transparent over metallic or flake)? (744)
I am not sure if there are any industry standard procedures for these measurement and matching procedures. I would recommend checking with ASTM for the existence of such standards. However, there are commercial multi-angle instruments available for characterizing these materials. One example is the the X-Rite MA-68. You might contact X-rite to learn more about that instrument and any related standards and applications.  (Back to top)

I am trying to calculate the colour of a coating. I have a graph of wavelenth versus reflectivity. Is there a simple way to convert this graph into RGB values or some similar standard? (743)
Unfortunately this is not quite so simple. RGB values are not standardized, so their meaning as a color specification is dubious at best. There are fairly straightforward colorimetric calculations that are performed to specify color matches (CIE XYZ) and color appearance (CIELAB) from spectral reflectance measurements. These might be helpful in your application. I would recommend a good introductory text on Colorimetry (such as Berns' "Principles of Color Technology, 3rd Ed.") to learn more about the details of these computations.  (Back to top)

Since a mirror reflects most of the light that falls on it, as does any white body, what is the difference between the two? (742)
This is a matter of geometry. A perfect mirror reflects all of the light that strikes it, but reflects it as regular, or specular, reflection. That means all the light is reflected at an angle equal to the angle of incidence and opposite the normal to the surface. A perfect white also reflects all the light incident on it, but reflects that light diffusely. In other words, the white scatters the incident light in all directions. That's why a white object looks white from all angles, but you can only see yourself in a mirror when you look at it directly (in line with the normal to it's surface).  (Back to top)

I know that the color of a body depends upon the color that it reflects. What causes some bodies to be transparent, others translucent still others to be opaque? (741)
There are many factors involved, but the main one is scattering of light. A material with no scattering can be transparent, one that scatters a little is translucent, and one that scatters a lot is likely to be opaque. And the amount of scattering depends on various material properties such as surface roughness, homogeneity of the material, changes of index of refraction within the material, etc. Also, another important factor is the amount of light absorbed. Something that doesn't scatter at all, but absorbs 100% of the incident light, will also be opaque.  (Back to top)

I found a source that says an RGB of 194, 32, 11 for Calder Red. Can you tell me what proportions of red, yellow, and blue paint should be mixed to get that RGB? (740)
Unfortunately not. Neither RGB, nor red-yellow-blue paint, are standardized specifications of color appearance. Both would have to be well calibrated and characterized to define a match. Your best bet is to find a sample of the color you are looking for and then experimentally mix the paint to make a visual match (or take the sample to a paint store for matching if you are interested in large amounts of paint).  (Back to top)

Where can I buy Baker-Miller pink paint? (739)
I don't know of any particular source for Baker-Miller pink paint or even a standard definition of what it is. Your best bet would be to find a sample of the color you want and then go to a paint store to have them custom match it for you. However, you might want to do some more research on the topic as well. The reason there is no standard color is probably because the effects of calming people are not consistent from person to person and the pink exposure has also been shown to enrage people if they are left in pink rooms too long. I think the effect is more folklore than well-understood science. There are certainly emotional effects of color, but they are not consistent from person to person. Faber Birren's book "Color and Human Response" is a good introduction to the topic. There is also this Color Matters page with a little more information on Baker-Miller pink.  (Back to top)

I want to make a slate grey color from white exterior latex paint. What additional colors and quantities of color will I need to mix? (738)
You would need to mix in some black paint (or more simply some black colorant). The amount would depend on just how dark you think "slate gray" is. You would probably need to do some experimenting. I would also recommend mixing all the paint together to get consistent color. Since that would be one big vat of paint, it might be easier for you to go to a paint store and ask them to make a match for you. They could add colorant to each gallon in a consistent manner and convert that paint from white to gray for you.  (Back to top)

This pertains to colors being used for home theater projection screens. Neutal grays have been suggested as the best color since they should not shift the color of the reflected image. Munsell grays are reported as neutral for illuminant C. What would the RGB value be for Munsell N8 and N9 under D65? One source lists N8 as 202 202 202 (L*ab- 81.3256 0.0044 -0.0087, Yxy as 59.0619 0.31272 0.32900). Is this correct? (737)
The Munsell neutral samples are quite nonselective, meaning they will remain neutral under a wide variety of illumination levels. Your numbers seem reasonable, but entirely too precise. As a good approximation, N8 will have L*a*b* values of 80, 0, 0 and N9 will have L*a*b* values 90, 0, 0. Conversion to RGB values will depend on the particular display characteristics and setup, but your values of 202,202,202 are not unreasonable. You might want to adjust them all up or down slightly if you would like a bit lighter or darker background.  (Back to top)

I am an artist interested in painting with photochromic inks or paints. Do you know of a supplier? (736)
I don't know of any specific vendors, but a search seems to suggest that some are available. See this Google search.  (Back to top)

We have two items painted with Munsell N7 which are significantly different in shade. Is this normal? How can I measure if the differences are within specification? (735)
It is not unusual for materials that are supposed to have the same color to have perceptibly noticeable differences. Whether or not they are within specification depends on the tolerances set for a particular application. Such tolerances are normally established by setting acceptable lightness, chroma, and hue ranges in a color space such as CIELAB and using spectrophotometry to measure the samples. Alternatively, tolerances are sometimes setup visually. You could select the darkest and lightest samples that are acceptable to define the range that you would accept in your application.  (Back to top)

I am building a pit solar greenhouse and need to decide what color to paint the non-glazed walls. White would be the best for reflection onto the plants but I am also looking for more pleasing colors. What is the best color for growing plants? (734)
Most plants are green, which means they are reflecting more green light than other wavelengths. Those other wavelengths are being absorbed by the plant to help them produce energy. Taking away the green light (which the plants don't use as much) leaves you with purple. So while you are correct that white would result in the most light getting to the plants, as a second choice I would go with a light purple.  (Back to top)

Why isn't E point (x=0.3333 - y=0.3333) used as a standard illuminant for color matching and ΔE? (733)
It is used on occasion, but since it represents a purely theoretical illuminant (equal energy at each wavelength) it never occurs in a natural viewing environment. Therefore many prefer to use something like an average daylight distribution (such as Illuminant D65).  (Back to top)

I am working on honeybee color vision. Using a honeybee specific color diagram, we know how to calculate dominant wavelength. How do I calculate the dominant wavelength of a color sample which resides in my color diagram in the "magenta" (or purple) zone. (732)
The way it is done in normal colorimetry is to draw a line from the color through the white point to the spectrum locus (rather than the other way) and then the wavelength of intersection is called a "complementary wavelength" or "complementary dominant wavelength". Thus you will end up with "green" complementary wavelengths for those "magenta" colors.  (Back to top)

Why are the colors of Christmas typically red and green? (731)
There are various stories and theories, but this explanation seems to capture the essence of several of them.  (Back to top)

I work for an automotive finishing company and use an X-rite MA68II for measuring color. Which should I use, L*a*b*, or L*C*h�? Everyone seems to use L*a*b*, but I'm not sure why. (730)
The two represent different ways to represent the same CIELAB coordinates. L*a*b* are the rectangular coordinates and L*C*h are the cylindrical coordinates of the same space. So, in a sense, it doesn't matter which you use. L* is the same in both cases and correlates with our perception of lightness. Some people feel that L*C*h is more intuitive for describing colors and tolerances (C* representing chroma, and h representing hue) and choose to work in those coordinates. However, at times it is simpler to work in the rectangular coordinates where a* represents redness-greenness and b* represents yellowness-blueness. It really is a personal preference based on the application and user's experience, but normally L*C*h are a little bit more intuitive and effective (especially for tolerances).  (Back to top)

How long does it take for your brain to recognize color? (729)
This is not an easy question to answer since it will depend on many variables including the way the color is presented and the response required from the observer. For example, you might be able to recognize a color (or color change) in a very short pulse of light, but it could take you significantly longer to produce the word required to describe the color. Unfortunately there is no simple answer for this question. However, if you type the words "color", "reaction," and "time" into google, you will find lots of resources that describe and demonstrate various experiments that might be helpful.  (Back to top)

Can you provide a mathematical formula to convert a RGB color value in CMYK color value? (728)
Unfortunately there is no single simple conversion formula to go from RGB to CMYK. It depends on the particulars of the devices and their primaries. If you do a search on "RGB to CMYK Conversion" on the internet, you will find a number of resources explaining the procedures. Here is just one example that might get you started.  (Back to top)

How do mood rings work? (727)
They are liquid crystals that change color with temperature. A nice explanation can be found at owstuffworks.com .  (Back to top)

At what illuminant temperature are Munsell neutrals (N6, N7, etc.) designed to be neutral? (726)
The Munsell samples are specifically designed to be viewed under CIE illuminant C (simulated daylight with a correlated color temperature of about 6774K). However the actually samples are quite spectrally nonselective (i.e., flat spectral reflectance curves) and should appear neutral under a variety of white light sources.  (Back to top)

Obviously having a single colour on two different stimuli (each encompassing a different meaning) on a workstation will cause confusion. What are these problems from a psychological, cognitive, or ergonomic perspective? (725)
I received the following suggestions from a colleague far more knowledgeable than me in this area....

Here's a relevant article: Smallman, H. S. & Boynton, R. M., Segregation of basic colors in an information display, J. Opt. Soc. Am. A, Vol. 7, No. 10, 1990.

This site is also very helpful: colorusage.arc.nasa.gov

Also of interest: colorbrewer.org

There is also a book by Colin Ware called Information Visualization: Perception for Design that is supposed to be quite good.  (Back to top)

Are the colors that we see during the day, present at night - only we cannot see them? Or are the colors that we see during the day not present, or altered, because of the low light? (724)
The physical stimulus for color is still there, it's just that the amount of light is reduced to the point that we can no longer perceive color. For example, if a photograph is taken with long enough exposure time the colors will look just like a daytime photograph. So in terms of your question the "colors" are still there, we just cannot see them.  (Back to top)

What are the names of pigment suppliers who mfg or distribute metameric pigments? (723)
They all do! Metamerism is not a property of individual pigments, but a property of pairs of color samples (they match under certain viewing conditions despite spectral differences). You can make them out of all sorts of colorants.  (Back to top)

What are the evolutionary advantages and disadvantages of impaired color vision? (722)
It is thought that normal trichromatic color vision provides advantages for finding healthy food and healthy mates, both of which are selective advantages for evolution. However, it is clear that people with color vision deficiencies can function quite well in the world and therefore there hasn't been enough of a disadvantage of these deficiencies to make them fade away through evolution. There are even reports of some situations in which color deficient observers can see patterns that are masked by all the color variation that normal observers might see.  (Back to top)

I am developing paint color regulations to help buildings constructed on mountainsides blend into their surroundings to minimize their visual impact. What maximum LRV do you recommend for an environment typically consisting of darker browns, grays, and greens as the "base" colors in order to keep buildings from "standing out"? What would be the most appropriate instrument for measuring the LRVs of paint/stain for enforcement purposes? (721)
You would want the paint LRV to be similar to that of the background environment. In general this is taken to be about 20% for natural scenes. I would use that as a guideline and perhaps allow slightly higher values as well. Perceptually, a 20% reflectance appears to be midway between white and black, so this is a reasonable number. There are a variety of instruments that could perform this measurement, but a handheld spectrophotometer would be the most accurate, flexible, and easy to use choice.  (Back to top)

How does the color of a room affect people's moods? (720)
Emotional responses to color can be very real, but they do vary in type and magnitude from person to person. Many times these responses are from learned associations. Faber Birren's book, "Color and Human Response" is a good source of well-grounded information on this topic.  (Back to top)

What are the equations for McAdams ellipses? Are they location dependent in the 1931 chromiticity diagram? (719)
There are no equations for MacAdam's ellipses. They represent data on the variability of color matches for an observer when matching lights of various chromaticities and yes, they are location dependent in the 1931 xy diagram. I would suggest reading about this type of work in Wyszecki and Stiles' reference book "Color Science" and then exploring the original MacAdam papers on the topic if you need further details.  (Back to top)

It makes sense that I should want window glass with the highest Color Rendering Index available in the performance category that I need; but how do I judge the relative CRI when all options are between 87 and 95? (718)
Color Rendering Index is computed for light sources, so I assume you are referring to daylight transmitted through the various glass samples. In your case, there isn't much to compare since all your CRI values appear quite high. If CRI is the most important variable in your application, then simply choose the highest. If there are other issues, such as cost, that would incline you to lean toward the lower CRI, then you won't be losing too much in this case.  (Back to top)

What mixture of acrylic painiting do I need to achieve a copper tone? (717)
The appearance of metallic objects, like copper, depends not only on the diffuse color, but the color of the highlights. Metals look like metals because their highlights are colored. That means that you can't directly mix a non-metallic paint to have the same appearance as copper. You would need to have a reddish-brown color and then paint a representation of a 3D object that illustrated the metallic highlights. In that sense you could mimic the appearance of a metal.  (Back to top)

Can CAM02 to predict Appearance Correlates of a color on different colored backgrounds? (716)
CIECAM02 is only capable of accounting for changes in the relative luminance of the background, not its color. To account for background color changes you would have to adjust the adaptation white point to be some weighted combination of the color of the light source and that of the background (essentially that is the same as incomplete adaptation to the background).  (Back to top)

Is it proper to state a Munsell notation for a metallic or pearlescent color, or is Munsell a solid color only notation system? (715)
It can be done, but it should be done with great care. With metallic or pearlescent materials, the color changes with changes in illumination and view angles. Thus, if you wanted to use Munsell notations for such materials, they too would have to change with illumination and view angle and the material would have quite a large number of Munsell notations.  (Back to top)

Is it possible to draw plankian line in CIE diagram? How? (714)
Yes, it is possible. The full procedure would be to use Planck's equation to compute spectral power distributions for a range of color temperatures and then compute the desired CIE coordinates from those spectral distributions. This is described in several places in Wyszecki and Stiles reference book, "Color Science". Conveniently, they also have a table of the xy chromaticities for a range of Planckian radiators in chapter 3 of that book.  (Back to top)

Is there any source of spectral reflectance data for samples in the Munsell Book of Color? (713)
I don't know of any and the reflectances are not uniquely defined, so different books might have difference reflectance properties (while still having the colorimetry of the renotation).  (Back to top)

Does the Munsell system account for the Helmholtz-Kohlrausch effect? (712)
No. The Helmholtz-Kohlrausch effect is an anomaly in the Munsell system. A gray at value 5 will look darker than a high-chroma color at value 5. At some point in the process of defining the Munsell system and renotation, it was decided that value would be directly related to luminance and that created this issue. It is best to think of the Munsell hue, value, and chroma scales as three independent scales, rather than as a three-dimensional space to avoid this issue. In other words ... only compare value when hue and chroma are constant, etc.  (Back to top)

How closely do the Munsell Renotation data reflect the actual tristimulus values of samples in the Munsell Book of Color as viewed under Standard Illuminant C? (711)
The Munsell samples are made to very tight tolerances with respect to the Munsell renotation data. There is some indication of this accuracy level mentioned in the Munsell books themselves.  (Back to top)

Why does the sheen of a finish affect the appearance of a color? Do the wavelengths of color get magnified, refracted, or intensified as the pass through the layers of a paint film? (710)
The overall perception depends on both the diffuse reflectance of the material (what we often call color) and the surface properties (sheen or gloss, which is not colored). For a glossy material, all that surface reflection goes off in one direction and we can then see the nice saturated, or dark, color of the underlying material. As the material becomes more matte, more of that surface reflection is scattered in all directions and that results in the material looking lighter and less saturated no matter how it is viewed. Consider a black car and a white car both with equal levels of sheen on the final finish. The black car appears to be more shiny. Here, the sheen of the white is masked by all the diffuse reflection that makes the material look white. In the black material since there is really only surface reflection (the sheen), it is very apparent.

Generally for normal materials, there is no effect on the wavelength composition of the light.  (Back to top)

Can you explain the difference between value and luminance? Is Light Reflectance Value different than luminance? (709)
LRV is related to the percent of light reflected and is proportional to luminance. Luminance would refer to the absolute amount of light, while LRV refers to the percentage that is reflected by the paint regardless of how much light is present.  (Back to top)

I understand that the visible spectrum is comprised of the wavelengths red, orange, yellow, green, blue, indigo and violet. Combined they make up white light. So does that mean that white is then a wavelength too? Do all the other colors/wavelengths add up to something that we recognize as white? (708)
There is actually a continuous range of wavelengths between violet and red. The seven names are just a convenient way to divide up the spectrum. White light cannot be described as a single wavelength, but is instead a combination of many wavelengths. Interestingly, you don't need to have energy at all of the wavelengths to make white (for example some displays can make white out of a combination of red, green and blue wavelengths). To answer your last question, yes, all the wevelengths can combine to make something we recognize as white.  (Back to top)

If you are in the dark, it won't matter at all. If you are out in sunlight, then darker hair will absorb more of the sunlight and convert it to heat ... making you feel warmer. However, I would guess that the amount of hair would have a larger influence than the color in most circumstances.  (Back to top)

What's a good reference for information on the Munsell system? (706)
I'd suggest this article from American Scientist to get an introduction to Munsell and his system along with references to more details.  (Back to top)

Where can I buy Color Standard for measurement calibration? (705)
Measurement standards are general sold by national standards laboratories such as NIST in the USA, NRC in Canada, and NPL in the UK. There are also companies that sell more affordable secondary standards that are traceable to the national labs. If you do a Google search on terms like "reflectance standards", you can find a lot of information.  (Back to top)

To measure process color is it better to use a spectrometery or density, hue, and gray? (704)
It is better to use spectrophotometry if you are interested in measuring the color appearance, but it might be better/simpler to measure density if you are only interested in the amount of ink present (and you don't measure different types of inks). Densitometers are designed to measure amounts of colorant (ink, dye, pigment, etc.) and not what the color looks like to humans. Spectrophotometers can be used to provide colorimetric parameters that correlate more closely with human perception. Thus, if you are measuring different sets of inks or different types of materials it is possible for density readings to match when the color doesn't.  (Back to top)

Where can I find absorption or reflectance data for inorganic chemicals or pigments? (703)
Sorry, I don't know of any specific source for such information. The best I can suggest is contacting some of the pigment manufacturers to see if they have data they can share or suggestions about where typical data can be found. You might also inquire with these folks. I'm not sure if they include spectral data or not.  (Back to top)

Is there an index for the degree of yellowing? I want to characterize the degree of yellowing in trees as an indicator of poor health from aerial RGB photographs. (702)
There are yellowness scales that could be used. For example, there is a description on page 70 of Berns' "Principles of Color Technology, 3rd Ed." You could also look at color changes in a color space such as CIELAB (also described in the same book). The difficulty you will run into is calibration of your system to consistently convert from your RGB images to device-independent colorimetric coordinates. If the imaging system is not stable, or calibrated, you might have more variability in the RGB images than is present in the foliage of interest. Looking through Berns' text (or a similar text on colorimetry) will give you a sense of the issues involved.  (Back to top)

Please give an example of how to begin with a tertiary color,subtract one color and then subtract another color but don't end up with a primary color. (701)
Well, it depends on how you define "tertiary color". There are a few definitions out there. If you define it as a mixture of 3 primaries, then removing two of them will always leave you with a primary. I've also seen definitions that consist of mixing two secondary colors or mixing a secondary with an adjacent primary. So it really all depends on how you define "tertiary" and how you define the "colors" you are removing. It also would depend on how the colors are being mixed.

Looking at the most common definition, a tertiary only has two primary colors in it, so if you remove all of one, and then all of the other, you are left with either black (additive mixing) or white (subtractive mixing) and that would answer your question.  (Back to top)

Where can I buy the Munsell color chart that has the 14 samples used for CRI tests? (700)
Munsell products are sold by GretagMacbeth. I don't think there is a specific chart with all the color rendering samples, but it is possible to by individual Munsell samples.  (Back to top)

Where do the beautiful colors of fireworks came from? (699)
Various chemicals in the fireworks produce colors due their temperature of burning or atomic/molecular emission. There is a nice article on fireworks here and if you go to the second page there is a link with many more details on light and color in fireworks.  (Back to top)

Why does the greenery of parks and gardens look beautiful to our eye? Please elaborate with regard to perception and function of the eye. (698)
Those same green colors might look quite ugly in a different context. It is the context of the objects, their setting, their meaning to us, and our associations of colors with all of those features that helps us perceive beauty. Ultimately, all of those stimuli are being associated with with pleasant experiences. It could also be considered that the perceived beauty of the plants is an evolutionary advantage for them because it encourages humans to take care of them.  (Back to top)

Do you a reference for this equation used to get color decimal value from RGB values: "color_code = (r*65536) + (g*256) + b" (697)
I don't have a reference for that equation, but looking at it what it is doing is converting 3 8-bit RGB values (0-255) into a single 24 bit number where B is encoded as the least significant 8 bits, G the middle 8-bits, and R the most significant 8 bits. If that number was then expressed in hexadecimal notation, you would get the normal hex color codes that are used on the internet.  (Back to top)

How precise areMunsell color charts meant to be read? (696)
One step in Munsell Chroma is approximately 5 CIELAB units (very approximately). In general , people accept that approximately 1.0 CIELAB unit is a visual threshold (again very approximately and dependent on the color in question). Thus, observers should be able to fairly reliably discriminate about 0.2 Munsell Chroma steps under the best of viewing conditions. So what are the best of viewing conditions? A nice bright daylight booth with fairly large samples set up right next to each other.

Now, I've seen a few of the Munsell soil books, and generally they are ... "soiled". I would be very surprised if the samples in the real world could be considered precise to much better than one Chroma step. On top of that you need to factor in the varying lighting and observers. Certainly with all those variables there is no way the judgements can be considered precise to 0.1 Chroma step. Your "opposing viewpoint" is the one that makes sense to me as a color scientist. It is certainly in line with what I would expect if I was doing the judgements.  (Back to top)

What makes colors fade in plastics? What part or parts of the light spectum? (695)
A precise answer would depend on detailed properties of the material, but it is probably fairly reasonable to say that any of the wavelengths absorbed by the material will contribute to its fading. Thus, the wavelengths most strongly absorbed are probably doing the most relative damage.  (Back to top)

Are there any instruments or methods which measure the intensity of gray color? (694)
Yes. These measurements are usually performed in the field of colorimetry using instruments called spectrophotometers. Spectrophotometers measure the percentage of light reflected by an object and then the results are used to compute colorimetric coordinates. In the case of grays one could use the Y tristimulus value to look at the relative luminance of the grays or the CIELAB L* parameter (which is a nonlinear transformation of Y) to have a number that correlates better with perceived lightness. All of this can be found in more detail in texts on colorimetry such as Berns' "Principles of Color Technology, 3rd Ed."  (Back to top)

What the physical meaning of sharper cone reponse in CIECAM02? (693)
There is no direct physical meaning to these sharpened responses in terms of cone responses. They are not cone responses at all (as is clear from the negative parts of the curves). Instead, they represent combinations of cone responses that could potentially exist at higher levels of the visual system. The result is that these responses, together with multiplicative normalization, seem to fairly well model the overall processes of the visual system. Sharpened responses produce a chromatic adaptation transform with predictions closer to perfect "color constancy" and it makes sense that von Kries scaling in the actual cone responses combined with other mechanisms of adaptation at higher levels would also result in such responses.  (Back to top)

How do paint colors affect the room temperature? (692)
The paint color probably doesn't have a huge physical effect on room temperature overall. However, when there is plenty of light in the room, darker colors will absorb more light and convert that to heat so there could be some effect. There might actually be larger perceptual effects. Some colors are perceived to be cooler than others (but this will vary from person to person) and therefore people might feel that the temperature is higher in a red room (a warm color) than a blue room (a cool color). I have heard that this perceptual effect can be significant (and applies to lighting color as well).  (Back to top)

How can I mathematically find the best match for a CMYK color from an array of CMYK colors? (691)
As you have discovered, this is not an easy task. What you really want to do is convert your CMYK colors into a perceptual color space like CIELAB and then use color difference equations to find the nearest selections. This would require either some for of characterization or profile of your printing system.  (Back to top)

I have some images taken with a digital SLR at the wrong white-balance setting (tungsten instead of daylight). Is there a fixed set of RGB correction factors that I can apply to the entire batch? (690)
Unfortunately not. There are nonlinear steps between the in-camera white balance adjustment and the final output image. Thus it is not possible to obtain the same results after the fact with a simple RGB scaling (unless you happen to have camera raw data, which is saved prior to white balance and other processing). This is illustrated by the fact that you can't get satisfactory results by trial and error. You probably can come up with a reasonable set of RGB scalars, but the results won't be the same as images collected with the proper setting. I'd suggest finding one image of the set that has something that you want to make neutral and using that to get approximate factors to apply to the rest as a first step.  (Back to top)

Why do boys choose blues, grays, black, and browns when coloring and girls choose more brighter colors like greens, reds, oranges when coloring? (689)
I don't know of any research on the topic. There is some information on color popularity on the Crayola website, so there might be some information there. I would suspect the preferences are not well separated by gender at a young age.  (Back to top)

Do some people, through genetics, have dramatically different "rods and cones" in their eyes and see colors totally differently than "most people". This does not include colorblindness, but that some people might see red as yellow and blue as orange, etc. Is this true? (688)
I am not aware of any research that would suggest such unique visual systems, so to answer your question I'd have to say "no" it's not true. Beyond typical color vision deficiencies, there are small genetic differences in the color sensitvities of the three cone types, but these are very small differences and nothing that would produce qualitiatively different visual percepts (like seeing blue as orange, etc.). Of course, we can never be sure that everyone's internal perceptions are identical, but there is plenty of evidence to suggest that our overall responses to color stimuli are very similar.  (Back to top)

I am trying to establish a color system criteria for silicon parts which are white. What do you suggest in setting up a criteria for the color white? (687)
I'd suggest treating it like any other color. Choose a standard to aim for and characterize it colorimetrically (perhaps under multiple light sources) and then use your knowledge of the process and/or customers to set reasonable tolerances in CIELAB coordinates or using a color difference equation. There's nothing unique about white to make this procedure different than it would be for any other color.  (Back to top)

What is meant by plankian locus? What is its importance? (686)
The Planckian locus is the curved line on a chromaticity diagram that represents the colors of Planckian, or blackbody, radiators. As a blackbody radiator is increased in color temperature, its color changes from red, through yellow, to blue and the Planckain locus maps out this change. These color temperatures are also used as descriptors of the colors of other types of light sources. I'd suggest you read about blackbody (Planckian) radiators, color temperature, and correlated color temperature in a text on colorimetry such as Berns' "Principles of Color Technology, 3rd Ed."  (Back to top)

Are there any estimates of the probability that a photon impinging upon a cone photoreceptor will be absorbed by the visual pigment within the receptor? How does this probability change as the radiance is increased so that more of the pigment is in the bleached state? (685)
This question is not as easy to answer as it might seem. I am not aware of any specific estimates. For the optimal conditions with a cone dark adapted, optimal wavelength, etc. the probability that a photon incident directly on the cone will be absorbed is probably quite close to 1. That probability would gradually reduce to zero as the photopigment is bleached, but it takes an extreme amount of exposure to bleach a significant amount of photopigment. The real limits on visual perception are actually getting the photons to be incident on the cone in an optimal way. The probability would depend on the direction and location of the photon in addition to its wavelength. The classic work of Hecht, Shlaer and Pirenne, "Energy, Quanta, and Vision", The Journal of General Physiology 25, 819-840 (1942) describes how photons are lost in the visual process and how single photons incident on each a collection of rods are adequate to produce a visual response. It is generally accepted that cones are only slightly less sensitive to light, but respond individually, rather than in groups and therefore need more photons in a single receptor. You might also look at this website for a brief overview and chapter 7 in the classic text "Vision and Visual Perception" edited by Graham also provides some more details.  (Back to top)

Can a person with a color deficiency learn colors and shades and be able to distinguish differences even if they are not able to see the exact color that a normal vision person might see? (684)
Yes. Many color deficiencies still allow the observers to see quite a range of colors and learn to associate appropriate names with those colors. There will still be some cases where there is confusion (depending on the particular deficiency), but it is really quite amazing how well people can get by. You can learn some more and see some images at vischeck.com.  (Back to top)

Would it be possible to correct color blindness (for computer users) by measuring a user's color sensitivity at various wavelengths, then adjusting the output of the monitor to compensate for it? (683)
Unfortunately not. For some observers, a simple boost in one of the primaries might help, but it is more likely that their own mechanisms of chromatic adaptation have already taken care of that. For dichromats, they are missing one of the cone responses and there is nothing you can do with the stimulus to correct for that. The best you can do is make sure that the stimulus doesn't contain colors they would confuse with one another. You can find lots more information on this topic at vischeck.com.  (Back to top)

How do I calculate whiteness index (WI by ASTM standards) if I don't have a spectrophotometer but am in possession of reflectance data for materials that have <95% reflectance @ 600nm? (682)
I would recommend two texts to get more information on whiteness indices and colorimetric computations. These are Berns' "Principles of Color Technology, 3rd Ed." and Hunter and Harold's "Measurement of Appearance, 2nd Ed." The general form for computing a whiteness index is:
W = Y + 800(xn-x) + 1700(yn-y)
where Y is the sample relative luminance (Y tristimulus value), x and y are the sample chromaticitiy coordinates, and xn and yn are the white point chromatcities. The Yxy values can be computed from your spectral reflectance measurements using standard colorimetric procedures described in the texts. You might also want to refer to the original ASTM standard for more details.  (Back to top)

I am a massage therapist trying to do research on what color is best to have in a massage room and how it affects the client, such as improves relaxation, promotes healing. Any guidance? (681)
There is no single answer. It would really depend on the particular client. You are probably best off with a muted color (near neutral) that is not likely to seem offensive to anyone. Blue is the most popular color as a "favorite color", so maybe a grayish-blue would be good. You then will probably have the best results by allowing the client to decide the lighting level (again keep the color fairly neutral to avoid being offensice). Personally, I would close my eyes and the color of the walls wouldn't matter at all.  (Back to top)

Does a sheet of colored glass reflect, refract or absorb light? Or all 3? (680)
Yes, it does all three. It will reflect light at its surface, refract the light that enters the material, and absorb some of that.  (Back to top)

Could you possibly tell me the formulas for the harmonic equivalents (tones) of red, orange, yellow, green, blue, violet? (679)
To my knowledge, there is no scientifically valid link between color and pitch. However a lot of mythology and non-scientific theories exist. You might choose one of the relationships described on this web page. harmonics.com.  (Back to top)

I need a fairly fool-proof way for a moderately skilled technician to measure a large number of samples and present the data using the CIECAM02 coordinates. I am willing to buy a new instrument/software. Any thoughts? (678)
There are some examples of research code available on the internet. One option would be to adapt one of those to your needs and write the code in house. Since you are willing to buy a new instrument, it might be easier to convince the manufacturer to add the code to their system, if someone hasn't already. To my knowledge, vendors often add features to their software to make a sale. I'd suggest checking with several manufacturers to see if one hasn't already added CIECAM02, or would be willing to to get your business.  (Back to top)

Is pink a shade of red? (677)
Generally "shades" are colors produced by adding black to the color and "tints" are those obtained by adding white (in a subtractive system like paint). Thus, pink would be considered a "tint" of red and not a "shade". See practicalpainting.com  (Back to top)

I am studying the colour of abalone shells. How can I statistically differentiate cbetween two shells? Eg: this shell is bluer than that one. I plan to use digital photos, D65K light with the camera at the normal angle. (676)
As long as you keep your illumination, geometry, camera settings, and processing constant, then the LAB values in Photoshop would be adequate for your purposes. As far as statistics, you can use any normal statistical test on the LAB data. I would, however, suggest using multivariate tests on all three dimensions at once. You might also consider reducing your data down to color differences between shells (or some other standard). The equations for color difference computations can be found in any good text on colorimetry such as Berns' "Principles of Color Technology, 3rd Ed.".  (Back to top)

Does the color of one's eyes (light colored vs dark color) have any effect on vision? (675)
There is no evidence that eye color has an effect on visual performance.  (Back to top)

Can the color of the bedroom walls prevent proper sleep? My husband sleeps fine in a blue room, but has trouble in a red room (same bed!). (674)
Can the color of the bedroom walls prevent proper sleep? My husband sleeps fine in a blue room, but has trouble in a red room (same bed!).It is possible, but not likely just becuase of the color stimulus. For example, if he entered a room that was already dark he would have no way to know the color and it could not possibly affect his sleep. However, that is not the case here. I am sure he knows the color before he tries to sleep and it might have some impact. Perhaps he is just less comfortable in a pink room. Since you have asked the question, this might indicate that it is an issue for him (for whatever reason). Also, you shouldn't necessarily jump to the conclusion that it is the color since I am sure there are other things that are different about the two rooms ... it might be something else. Some even think the direction the bed is pointing matters.  (Back to top)

Are car color choices determined by age, gender or ethnic group? (673)
Almost certainly "no". There would definitely be societal preferences and since people often tend to group themselves according to the dimensions you listed and "like what their friends like", you could probably find some correlation. However, it is quite rare that someone purchases a car that is their preferred color. First of all, not that many people special order a car. Instead they purchase what is on the lot and available for a perceived better price. Secondly, most cars only come in a limited range of colors. These two facts of the automobile market mean that measures of the colors sold and peoples' preferences bear little relationship to one another. It is an interesting topic and there are those who spend a lot of time "predicting" what colors people will like in forthcoming products. However these predictions are self-fulfilling prohpecies since the manufacturers use the predicitons to put the products out into the market and people are forced to purchase what is available. They will capture longer-term trends, but not short-term predictions. One organization involved in this sort of market research is the Color Association of the U.S. See colorassociation.com to learn more about them. Also, here ppg.com is an interesting article on recent automobile color preferences, or at least sales numbers. One other thing to note is that dealers like to stock "conservative" colors like white, silver, gray, etc. since they are more likely to be "not objectionable" to a larger group of potential buyers. I might prefer red, and you green, but we both might be OK with silver.  (Back to top)

What is linear tiff data. What does it record? (672)
Generally when someone refers to linear TIFF data, they are saying that the image data (normally RGB) is encoded linearly with respect to the amount of light in the scene (be it real or synthesized). Since most displays are nonlinear, it is common for image data to be nonlinear to compensate. Thus normal images are nonlinear, but in some applications it is more useful to have linear image data.  (Back to top)

What are the differences in perception of color by males and females? (671)
The most significant difference is in terms of color blindness. Color blindness is a sex-linked genetic trait that is carried by women and normally expressed in men. So while about 8% of males have some form of color vision deficiency, there are almost no females with color vision deficiencies. There are a few other differences, but they seem to have little meaningful impact on perception. For example, it is possible for a woman to have 4 types of cone receptors instead of the "normal" three. However, there is no evidence that they make any special use of them. Their color vision is still trichromatic (3 dimensional) and the fourth type is very similar to one of the others anyway.  (Back to top)

Does the color of glass effect the index of refraction? (670)
Generally, it is assumed that colorants in glass do not affect its index of refraction. However this cannot possibly be true. Depending on the type of colorant and how it is dispersed within, or on top of, the glass, it is almost certain to have some effect. I suspect the effect is often quite small however, but it could be very significant if colorant concentrations were high.  (Back to top)

Why do colors abosorb and/or reflect heat? (669)
That's similar to asking why colors are colors? The chemical structure of atoms and molecules allows them to interact with different wavelengths of electromagnetic energy differently. Some absorb, some transmit, some reflect. There is nothing else that can happen to the energy. I can't say "why", that's just the way nature works.  (Back to top)

What trends are typical when Munsell testing a group of people? Are there effects due to age, gender, education, or ethnicity? (668)
I haven't noticed anything systematic like that in our testing over the years, but we haven't looked for such effects either. You might inquire with the folks at GretagMacbeth about whether there are data on such effects (much of this sort of thing is described in the manual that accompanies the tests). I've never seen data on age vs. results, but as people age their lens yellows, so that would suggest that the blue colors would become harder to discriminate with age. However, a little more light in the blue end of the spectrum might counteract that effect. That brings me to one thought on your results. You might be seeing a systematic trend to more errors in the blue due to some aspect of your viewing conditions being different from normal (e.g., maybe your daylight sources are at a lower correlated color temperature or the luminance is low). There are definitely more errors at lower luminance levels. There should be no correlation with ethnicity or education, but there can certainly be a correlation with motivation. Could it be that your observed correlations are really correlations with some other variable, like motivation to perform the test. If one is not interested in it, they can have wonderful color vision and still score quite poorly.  (Back to top)

Why doesn't Pointer's gamut (CR&A, 1980) fully cover all real points in Munsell system as defined by real.dat? (667)
You should look at the Munsell colors in the file "1929.dat" on style="#0000aa"> available here, to compare reasonably with the Pointer gamut. Those in "real.dat" are not necessarily "real" at all. Those are any colors within the MacAdam limits. Those limits were defined by theoretical reflectance spectra that are 100% or 0% at every wavelength. This range is far more saturated than any object colors that can actually be made. The Pointer gamut is accepted as a reasonable approximation of object colors that can actually be produced.  (Back to top)

I heard that HDMI for HDTV's can have billions of colors but I also heard that our eyes can only see around 10 million colors. Does this mean all that extra color doesnt matter or will it seem clearer in some way? (666)
The number of colors you can see at any given instant is fairly limited, perhaps in the tens of thousands, but your visual system is capable of adapting to different viewing conditions to make those tens of thousands depend on what is around them. For example, in a very dark scene in a movie, you might be able to distinguish many dark colors that would all look alike in a bright scene. There are also issues about how the color information is encoded, processed, and displayed that make those seemingly extra colors helpful. The bottom line is, yes, the added capabilities do indeed improve image quality.  (Back to top)

What color do most people answer when asked to name a color quickly? (665)
I am not aware of any research on that specific question, although some might exist. This website suggests that the answer might be "red" since it is usually the first color name to evolve in a language after black and white. That makes some sense and is based on classic work on categorical color naming by Berlin and Kay. Another reasonable guess might be "blue" since that tends to be the most popular "favorite" color. You might also look into the research in this paper (Boynton, R. M. and Olson, C. X. (1987) Locating Basic Colors in the OSA Space, Color Research and Application, 12, 2, pp. 94-105.) in which people were asked to name colors as quickly as they could and people were quicker and more consistent when the using Berlin and Kay's eleven basic color terms.  (Back to top)

How can I get spectral reflectance information about Color-Aid samples? (664)
Unfortunately, I don't know of any source for these data. The best solution would be to buy a set of Color-Aid samples (very inexpensive) and then measure them with a spectrophotometer (unfortunately relatively expensive). Perhaps you can find someone that will allow you to use a spectrophotometer if you don't have one readily available.  (Back to top)

Is there any standard by which Munsell colors are called "light" or "dark" or are those terms that are too subjective so that they don't get used? (663)
I'd suggest that you take a look at the "ISCC-NBS Dictionary of Color Names" for the most systematic relationship between Munsell designations and common color names. It is out of print, but can often be found from used book sellers and in libraries. Also, to clarify, in Munsell Value 0 is perfect black, Value 10 perfect white, and Value 5 a middle gray perceived to be halfway between white and black. Thus anything above Value 5 would be a light gray until it is called white (maybe above Value 8 or so) and anything below Value 5 is a dark gray until it is called black (maybe below Value 2 or so).  (Back to top)

Do you know of any freely available color matching software? (662)
I am not aware of any free formulation software. The website matchmycolor.com does provide an inexpensive subcription solution. Other than that, you can obtain "free" software from many of the instrument manufacturers when you purchase an instrument.  (Back to top)

sRGB has a gamma of 2.2. Why do the equations have an exponent of 2.4? (661)
The simple answer is that the equations include both linear and power function parts and in combination they approximate a simple power function with an exponent (gamma) of 2.2. In other words, the 2.4 in the sRGB equations is not a "gamma" since it is part of a more complex function. Here is a more detailed explanation.  (Back to top)

Can you comment on how does the following colors affect human psychology when considered in relation to fashion designing: red, yellow, blue, green, orange, violet, brown, pink, white, black, grey? (660)
The psychological/emotional effects of color in any context (fashion design or other) are not really that systematic and precise. There is no question that there are psychological responses to color, but they are very individual and come from different learning situations (e.g., society, personal experiences, etc.). You will probably be able to find books that give you whatever answer you want for the colors you listed. I'd recommend taking a look at Faber Birren's "Color and Human Response" to get a well-written and technically-balanced summary of these types of effects.  (Back to top)

Are there any filters that can be placed over the eyes to make a substance, which is invisible to the naked eye, visible through the filter? Can you give an example? (659)
Sure, there are plenty. For example you could print a fine pattern in yellow on a white background that would be normally invisible and then view the object through a blue filter which would make the yellow areas very dark. My daughter has a game that has hidden answers unless you look through a red filter. The text is cyan on a white background with a high-frequency red masking pattern on top of it. The red filter makes the white and red match (the masking pattern goes away) and the text becomes plainly visble. You could also use metameric pairs that match for a certain viewing condition. When the viewing condition changes (i.e. you look through the filter), the colors would become discriminable. Lastly, you could use a fluorescent material and then changes in either the illumination, or the illumination with filtration could make the material visible.  (Back to top)

Can you provide me with a formula for RGB values given Munsell coordinates? (658)
Actually, I can't. First of all, RGB is not defined and depends on the monitor, operating system, etc. Even if you had calibrated RGB, there are still no formulas to convert to/from Munsell. A lookup table is required. GretagMacbeth provides some conversion freeware and you might also look at the tools at easyrgb.com to get some approximations.  (Back to top)

Is there a range of Munsell or L*C*h coordinates that would describe browns? (657)
Yes, there is a range, but I don't know of a strict definition. I would recommend a very interesting paper on the topic (CJ Bartleson, Brown, Color Res. Appl. 1, 181-191 (1976)) that includes scaling of brown and defines the boundaries of brown in Munsell space. You could also just have a few people look at samples and estimate boundaries yourself. You might also take a look at the "ISCC-NBS Dictionary of Color Names." It is out of print, but an internet search may find copies.  (Back to top)

With all things being equal, is there any reason to see an increase in black dot gain simply by changing the print sequence from CMYK to KCMY? (656)
Yes! There is every reason to think that the absorption and spreading of the black ink would be very dependent on what ink (or lack thereof) has come before. It makes perfect sense that you would see more dot gain if black is printed first since the black ink might spread in the paper more than it would if there were already three ink layers down.  (Back to top)

What is the ideal working space for digital painting in Photoshop and painter is. I tried using Adobe RGB but my reference images look so saturated. (655)
The "ideal" space depends on the user and application. If you are finding Adobe RGB results in images that are too saturated, you might try sRGB, which has less chromatic primaries.  (Back to top)

What color is best to paint a room with hardwood floors to enhance productiivity? (654)
There are plenty of theories on these sorts of things, but very little hard science. I would say that the best color is one that makes you comfortable and keeps the room fairly bright (but not uncomfortably so). When there is more light, you see better, there is less eye strain, etc. and all of that should make you more productive. It also saves energy since lighter paint will reflect more light and require less output from your lamps.  (Back to top)

Is there a guideline, perhaps application-based, for the number of patches needed to make adequate ICC profiles for printer output? Is there an upper limit on the number of patches beyond which more patches do not necessarily improve the ICC profile? (653)
There are no set rules. I have heard of people being successful with 5x5x5x5 sampling of a CMYK process (or 5x5x5 for CMY) as long as they are careful in selecting the samples and in how they interpolate between them. It is not unusual to measure as many as 11x11x11x11 to build a good profile. The number required really depends on the printer's behavior (e.g., linearity, stability, etc.)  (Back to top)

I am looking for products to protect my eyes from UV-C light (254 nm). What substances are opaque or reflect to UV-C light? How harmful is UV-C light to human skin and eyes? Can you suggest any protective measures I should take? (652)
I did a google search on "UVC Safety" and found many products and a lot of information. UVC is extremely harmful radiation, so you should be careful with it and take every possible precaution. In nature, it is absorbed by the atmosphere and does not reach the Earth's surface. In human hands it is used to do things like kill bacteria because it is so good at destroying biological tissue. Apparently it can even cause changes in DNA structure. Here is a link to a document that outlines some safety procedures. Good luck with your work and be careful; protect both your eyes and your skin.  (Back to top)

hat is the correlation between Hunter whiteness (L, a , b) and CIE whiteness (L*, a*, b*)? Can I relate them mathematically? (651)
The HunterLab and CIELAB color spaces to which you are referring (they are more than whiteness metrics) are both computed from CIE XYZ tristimulus values for the sample and the illuminant. To go from one to the other, you would need to invert the first space to compute the sample XYZ values (assuming you know the illuminant values) and then use the normal forward equations for the second model. These could probably be concatenated into one set of equations, but I have never seen that and there would be little benefit. I would recommend Hunter and Harold's "Measurement of Appearance, 2nd Ed." as a good reference for details on these spaces.  (Back to top)

I have a Minolta 2600d portable spectrophotometer. Can you tell me the best way to measure the fluorescence of teeth in vivo? Also, can I use the 2600d to measure the opalescence of both natural teeth and dental materials? (650)
The KonicaMinolta website doesn't give many details on that instruments and I am not personally familiar with it. However, the website does mention UV adjustment, so I assume it is possible to make measurements with and without UV energy in the illumination. Such measurements are one way to evaluate fluorescence. Any difference between the two indicates that the UV energy is being absorbed and re-emitted as visible light through fluorescence. As far as measuring the color of fluorescent materials, keep in mind that the measurement is only valid for the light source in the spectrophotometer. (The website doesn't say, but often these are D65 simulators.) I assume by opalescence, you mean changes in color with changes in illumination and view angle. There is little you can do with this particular instrument, but you could compare the specular component included and excluded measurements to see if there are any significant color changes across that change in geometry.  (Back to top)

Is there a certain color that has been proven to be least tiring to the eye on web page backgrounds? (649)
I don't know if any research has been done on web pages specifically, but it is well established that dark text on a white background is least fatiguing for extended amounts of reading. That's why most computer systems arrange the text that way. For presentations in a darkened room (and short amounts of text), bright text on a dark background is easiest to read.  (Back to top)

What are the calculations used to convert spectrophotometric spectral data to density, CIE XYZ, and RGB variables/values? (648)
All of these calculations share the same basic structure. You take the spectral data, weight it (multiply) by a set of responsivity functions, and then integrate to get the result. In the case of density, you do a logarithmic transformation after the integration and in the case of RGB, you have to do a nonlinear transformation (often) to account for display properties. The weighting functions are different in each case. The best way to learn about these calculations is through a good text such as Berns' "Principles of Color Technology, 3rd Ed." and Hunt's "The Reproduction of Colour, 6th Ed.".  (Back to top)

Why can't humans see colors that fall in the UV or IR part of the electromagnetic spectrum? (647)
It is difficult to answer briefly, but it has to do with the visible range of wavelengths being were a lot of the important information about our world is and with the reactions of biological tissue to electromagnetic energy. UV is damaging to tissue and is best absorbed to protect us rather than being used in our visual system. IR is also produced emitted by warm objects, like our bodies, and it is difficult to overcome this background emission (noise) to detect IR signals in the world.  (Back to top)

Could you please tell me the equivalent Federal No. (Std 595) for Munsell No. 2.1 YR 5.6/16 (Orange)? (646)
I don't know of a conversion table, so I really can't ... but looking at this page, the only orange that is close is FS 32473. You can buy the FED-STD-695 fan deck at chassis-plans.com and make a visual match to a Munsell book. Again, looking at that page (and my Munsell book) I would say that the only one that is close is FS 32473.  (Back to top)

How can I convert NCS color mode to RGB mode in Photoshop? (645)
The NCS system is not defined by any equations that could be used to directly relate it to RGB (or other color specifications). The conversion would have to be made via look-up table. NCS provides some products and software that might help. See ncscolor.com.  (Back to top)

Can you explain how detergents would cause color fading? (644)
Fading from detergents can happen in several ways, but the most likely causes are simply that the detergents are removing the colorants (chemicals producing the color stimulus) from the material or that the detergents are facilitating a chemical reaction that changes the colorants into a form that is colorless.  (Back to top)

Do HSV and HSI refer to the same model? If not, what is the difference? (643)
They are slightly different and both examples of "color spaces" derived from device-dependent RGB color specifications that bear no real relationship to accurate colorimetry or color perception. They are very gross approximations, but sometimes more useful than the simple RGB specifications. You can learn more about them in texts on computer graphics or get a quick overview here.  (Back to top)

How often should color vision testing be assessed if the ability to see colors is important in one's occupation? (642)
Most color vision deficiencies are acquired and do not change through our lifetime. For those cases, once is enough to diagnose the presence or absence of a color vision deficiency. If you are interested in color discrimination ability (e.g. some of the information given by the Farnsworth-Munsell 100-Hue Test), then that can change with both age and motivation. I would suggest that doing that exam once every two years or so would be more than adequate. (As a note, I have done the test for more than 20 years and my results have not significantly changed when I am motivated to do the exam carefully.)  (Back to top)

I would like to make a children's computer program emulating colour mixing of paints. Since coputers generally use the RGB colour model, I don�t know how to achieve this. Can you help? (641)
The technical details of paint mixing are quite complex, but you can make an approximation with simple subtractive mixing. The trick will be to convert from the subtractive model for paints to the additive model for your computer program. What you need to do is abstract the color combinations in the subtractive model and then convert the result to RGB. For example your "blue" paint is generally really a cyan color (G + B to display it) and yellow is G + R. If you did additive mixing of lights, then the result would be R + 2G + B (just add up the values). However, a subtractive mixture behaves differently. You have to look at the light that is missing. The cyan has removed the R and the yellow has removed the B, so starting with white (R + G + B) and removing those amounts you end up with (R+G+B) - B - R = G (the green you are looking for). You then have to factor in various amounts of these mixtures.

I hope that is helpful. There are also some demos at the SDC Colour Museum website. You have to click on the little floating icon to get to the demo.  (Back to top)

Can chromatic discrimination change over time or can it improve through practice? (640)
It can certainly change over time, but I am not aware of any data that suggest an improvement with practice. What might improve with practice is the awareness of certain attributes or the motivation to be concerned with the differences. The raw color discrimination capability doesn't improve with practice, just the ability to do related tasks.  (Back to top)

Any correlation between age and chromatic discrimination ability? (639)
In general, as we age our color discrimination ability decreases (except for young infants where it is still improving). This is largely due to the gradual yellowing of the lens in our eyes, which also results in more scattering and reduced luminance sensitivity.  (Back to top)

Do any illnesses/physical characteristics/disabilities impact one's chromatic discrimination ability? (638)
I have heard of illnesses or various other injuries impacting chromatic discrimination. For example, there are cases of tumors in certain locations in the brain that cause the patient to have achromatic perception (a black and white world). I don't know much about details, but it is possible.  (Back to top)

How can a color display with three monochromatic lasers produce a finely rendered colorspace, with what seem like the appropriate pastels and everything? (637)
This is because our eyes cannot tell how a stimulus is made. We have three types of cones in our retinas and as long as the energy absorbed in the three types is equal for two stimuli (e.g. a pastel made with paint and illuminated with daylight vs. a pastel made with three lasers, then the colors will match. This is the definition of metamerism, which is the basis of modern colorimetry.  (Back to top)

What are the different textures of colour? (636)
Texture is usually considered separate from color as the spatial distribution of colors, the spatial distribution of gloss, or the spatial distribution of the surface height of the object. Color is normally treated as a distinct measurable value (a perception).  (Back to top)

How can I convert CIELCH to CIE 1931 x,y coordinates? (635)
The first step is to convert CIELAB L*C*h (there is no such thing as CIELCH) back to CIELAB L*a*b*. That is a simple coordinate transformation from the cylindrical coordinates (L*C*h) to the rectangular coordinates (L*a*b*). You will then need the CIE XYZ tristimulus values of the illuminant (which were used to compute CIELAB coordinates). With those, it is fairly straightforward to invert the CIELAB equations to compute the CIE XYZ values of your sample. From there you need to convert XYZ to xy in the normal way. All the necessary equations can be found in any good text on colorimetry such as Berns' "Principles of Color Technology, 3rd Ed."  (Back to top)

What is the utility of converting tristimulus values to L*a*b* values? (634)
Converting to CIELAB (L*a*b*) allows you to better describe what the samples look like, their lightness, chroma, and hue, and to set tolerances for the production of those colors (i.e., measure color differences). It is another way to represent the same information, but it is a potentially more useful way.  (Back to top)

Can my company's color references be accurately reproduced 10 years from now based on the tristimulus values we maintain ? (633)
Your references will be quite stable over 10 years. The CIE just celebrated the 75th anniversary of the 1931 system and it is still very widely used and will certainly be around for a good long time. No worries there.  (Back to top)

How do the eye and the brain work together to create the chromatic adaptation 'illusion' effect with colours? Also, why does looking at certain stimulus for certain lengths of time effect the time span of the adaptation? (632)
Here's a nice little demo of chromatic adaptation .

To try to answer your question. The eye accomplishes chromatic adaptation through adjustment of the sensitivity of the three types of cone receptors based on their previous exposure to light (more exposed to blue, the receptors become less sensitive to blue). This is like an automatic exposure control (or more precisely and automatic white balance) in a digital camera or camcorder. The brain helps by interpreting the world and discounting illumination effects (such as shadows or clearly colored light sources) to help us better recognize objects (which are usually more interesting to us than illumination). This is called "discounting-the-illuminant" and is essentially a sub-conscious cognitive effect (it can be learned and unlearned in certain situations ... normally in the form of illusions).

The time-course of chromatic adaptation is not really dependent on the stimulus. It is essentially complete in about two minutes. However, often when looking at adaptation effects, we don't take the time to fully adapt and the illusion, or after-effect, goes away in roughly the same amount of time we took to adapt. There are some higher-level after-effects that can take longer to go away. You might search on the "McCullough Effect" to learn about one of those.  (Back to top)

What makes wet grass greener than dry grass? A scientific explanation of more than just refractive index would be much appreciated. (631)
Actually, refractive index doesn't explain it very much at all. There is surface reflection at the interface between grass and air that is due to the change in index of refraction (grass different from air). The index of refraction of water is closer to that of grass than air, so it helps the situation. Surface reflection is not colored (not green in the case of grass), but the same color as the illumination. Dry grass has a fairly diffuse surface that scatters that surface reflection in all directions so what you see is a combination of the green (subsurface reflection) plus the white (surface reflection). When the grass is wet, it becomes glossy since the water smoothes the surface. That means all the surface reflection goes off at one angle and becomes a glossy highlight. At other angles all you are left to see is the subsurface (green) reflection and thus the wet grass looks greener (more chromatic) than dry grass.  (Back to top)

What does the ratio b/a give or mean? Here a is negative and b positive. (630)
I assume you are asking about CIELAB a* and b*? The ratio itself has little meaning, but the inverse tangent of the ratio is called hue angle and is a good indication of the color's hue. When a* is negative and b* is positive, you are in a range of hues between yellow and bluish-green.  (Back to top)

Can you provide list of non toxic pigments that are insoluble in water measuring about 1 micron particle diameter? (629)
The definitive list of dyes and pigments (colorants) and their properties is the "Colour Index International".  (Back to top)

I am preparing a 3 band fluorecent lamp powder recipe for color point x=0.312, y=0.336, Ra= 82,SDCM<3. Can you provide any useful recipe development procedures, methods or any useful website for me about the color coordinates adjustment? (628)
This is a very specific application and I would suspect that very little information is in the public domain regarding this topic as it is probably a competitive secret among lamp manufacturers. I am not sure how the fluorescent powders interact, but if you know the color of the individual phosphors (Yxy) then you could predict the color of additive mixutres by simply summing the tristimulus values (XYZ) in absolute units. You could use the same procedure to predict the amounts (or luminance levels) of the three phosphors required to produce your aim chromaticity. This all assumes the phosphors don't interact when they are coated simultaneously.  (Back to top)

Can you help me transfer color temperature to 1931 CIE xy-coordinates? (627)
You would use Planck's equation to compute the spectral power distribution of a blackbody at 6500K and then use the CIE color matching functions to compute the chromaticity of that spectral power distribution.  (Back to top)

I am shopping for a new motorcycle helmet and want to know the color's that would keep my head cooler in the sun. Where do colors such as grey or silver fit in as far as reflecting light/heat and staying cooler? (626)
The coolest (temperature wise) will be white because it reflects all the light. A good shiny silver would probably be close. Black will be the hottest and gray will be in between (probably hotter than silver). For colors, it is hard to say because the color might not necessarily correlate perfectly with heat absorption. However, they will all be hotter than white. Red, for example is nearly as hot as black because it absorbs a lot of energy (all the blue and green light and only reflects the red).

If you really want to keep your head cool ... go with white.  (Back to top)

Is there any research on changing the colors of a light to improve a surgeons abilility to see in different surgical enviroments? Can you improve their contrast sensitivity like skiers do with colored lenses. (625)
I am not aware of any research on this topic and couldn't find any in a brief internet search. It seems the main concerns in surgical lighting are the elimination of shadows, high luminance (improving acuity and color discrimination), and white (best for rendering all colors). A specific color other than white might be helpful for certain types of surgery (depending on the colors of the features being operated on), but it would also change the appearance of everything else and that might make it less comfortable/familiar for the surgeon.  (Back to top)

How do I assign a RAL number to a standard color used by my company? (624)
RAL is a proprietary color naming system. You would have to find out from them how to set a designation. See ral.de.  (Back to top)

I'm measuring several 3000K white lamps. I'd like to know the color variation of these lamps. Is there any other way to measure color variation of a population of lamps, aside from standard deviation and MacAdam ellipse? (623)
You can use the CIELAB color space and color difference equations for lamps, but you will have to make a selection of one to use as a reference white. Perhaps the ideal 3000K lamp could be your reference. You would then use its tristimulus values as the CIELAB white point and the other lamps as stimuli. You could then compute CIELAB color differences between each lamp and the reference, or between various lamps. If there are differences in absolute luminance this computation might get tricky, so I would encourage you to normalize all your lamps to a relative luminance of 100 (any number would do) before the CIELAB calculation. This will result in every lamp having an L* of 100 as well, but you can examine the color differences between the lamps by looking at the a* and b* differences that will remain.  (Back to top)

I require formulation for a Paint that absorbs magnetic rays or RF energy used by RADAR and do not reflect back. (622)
This is not really a color question since absorbing radar and producing color happen at different wavelengths and they need not be related. There is a description of radar-absorbing materials here.  (Back to top)

Is it possible for a liquid subtance to appear clear or colorless when placed on a white piece of paper with the naked eye, but will appear a unique color when seen through a filter? (621)
The only way to do this would be if the substance was opaque (like the paper) and had a spectral reflectance curve metameric to the paper for the first viewing condition, but not metameric when viewed through the filter.  (Back to top)

I am interested in obtaining a very accurate spectral match of my color sample. Some of the color matching systems I have used in various paint stores do a pretty good job of matching the color but not necessarily the spectrum of my sample. (620)
Many, if not all, of the commercial color matching software packages (such as what you find in paint stores) use some form of spectral matching algorithm and such algorithms can be quite effective. However, the only way to get accurate spectral matches is generally to use the same material and colorants as the original. For example, if you were to match house paint with one of the systems in a paint store, you would have a good chance of getting a spectral match. In many cases you don't have the luxury of using the same materials and colorants so you have to settle for a colorimetric match (probably metameric).  (Back to top)

Can you explain which coloured dyes change colour or fade and why when used as a gamma radiation dose indicator? (619)
Well, I don't know anything about this topic (and I don't think it is familiar to anyone else in the lab), but I found this webpage that seems to explain it pretty well.  (Back to top)

What is multispectral image? (618)
"Multispectral" is an ill-defined term in imaging. In general it refers to an image that has more than 3 channels (which are typically RGB) sampling the visible spectrum (and it could extend to other wavelengths as well). When the image is sampled a relatively fine increments through the spectrum (say 20nm or less) with a bandpass equal to the sampling increment, that can be referred to as simply a "spectral image" (just like we talk about spectral reflectance or spectral power distributions).  (Back to top)

What is the problem if a person sees both gold and silver as silver? This person also has red-green defects. (617)
Since you mention that the person has a red-green deficiency, that really answers the question. A red-green deficiency can be consistent with a confusion of silver and gold. Although it would depend on the illumination, the discrimination between silver and gold (which is a color discrimination) could well fall along a line of confusion for a person with red-green color blindness. You might take an image with silver and gold in it and process it through the tools at vischeck.com.  (Back to top)

What is the top that is black and white and makes pale colors when spun? (616)
It's called Benham's top, or Benham's disk. It's easy to find information on it on the internet. Here's an example.  (Back to top)

I would like to perform the Farnsworth-Munsell 100 hue test on my monitor. Will this work? Can I have the values of each colour caps to form the colours? (615)
You could, but you would have to be very careful about monitor calibration/characterization, adapting conditions, and luminance level in order for the results to mean anything. It is probably cheaper and easier to simply buy the real test from GretagMacbeth. I don't believe they publish the color definitions, but there might be some academic papers on the topic with data. I would suggest a search in Google Scholar to see if you can find references.  (Back to top)

I am creating a colorant database for my matching software. How do I reduce the bronzing of Pth Blue at high concentrations? (614)
The usual cause of this is that you have reached too high a concentration of colorant for your substrate and it is coming to the surface. Since this is happening, you are perhaps trying to construct your database at levels of concentration that will not be usable in your final products. You might well be able to build a usable database without going to such high concentrations.  (Back to top)

Which colored paper reflects more light? (613)
White.  (Back to top)

The color of an object depends on the reflection and the absorbtion of light. How? (612)
Actually, the color of an object depends on three things, the light source, the object properties (as mentioned in the question), and the human observer. All three are necessary for an object to have "color." As far as the object itself, it is the property of absorbing different amounts of the various wavelengths of light that impart color to the object. Whatever light is not absorbed is either reflected or transmitted and that is what we see.  (Back to top)

What are good books to get detailed information on both the Munsell and CIE systems? I'm an artist and have a general education of both systems but would like to know indepth knowledge of each, including the math and reasoning behind the constructions of each system. (611)
A good starting point for the CIE system is Berns' "Principals of Color Technology, 3rd Ed." and it also includes some background on the Munsell system. Specifically on Munsell, there is "The New Munsell Student Set" by Long and Luke (which also includes an abridged set of samples) and you can still purchase "A Color Notation: An Illustrated System Defining All Colors and Their Relations" by Munsell and Farnum.

If you want to get deeper into the subject you might look at "Color Space and Its Division: Color Order from Antiquity to the Present" by Kuehni.  (Back to top)

I am building a fiber glass airplane. The kit mfg says paint it any color you want as long as its white due to heat build up in the sun. Any idea of how far off-white I can get? How can I calculate the increase in heat absorbtion if I go a little blue or yellow off of white? (610)
This is not a simple question to answer since it depends on more than just the color. Heat build up will come from light absorption (color), but also from absorption of ultraviolet and infrared radiation (which might have little correlation with color). So the answer can't be given simply in terms of color. However, if you select a paint that is a fairly light shade (near white) as you suggest, then it's properties in the UV and IR are likely to be similar to those of the same type of white paint. That is because there simply is not that much colorant in the paint to make a change at any wavelength.  (Back to top)

Is there a specific color that could be called Munsell green? What is its Munsell designation? I understand this color is commonly used at places like university campuses for exterior electrical equipment enclosures. (609)
I understand why you are perplexed. There is no single color known as "Munsell Green". There is a principal Munsell hue called green that would have the hue designation of 5G. However, even when narrowed down to that single hue, you have many colors because you can still vary value (lightness) and chroma. You would need a designation of all three to have a specific color.  (Back to top)

How is the location of an illuminant on the Planckian Locus and R-G Axes related to colour vision tests? (608)
The illuminant location defines the appearance of white in a given viewing condition. The appearance of samples in a color vision test is then determined relative to the adaptation point. Since these tests are designed to probe certain color discrimination properties of deficient observers, it can be important that they are used with the proper illumination.  (Back to top)

What is the "straight line rule" when referring to the CIE 1931 Chromaticity Diagram? (607)
It refers to the fact that the chromaticity of additive mixtures of two colors fall on the straight line between the chromaticities of those colors. The exact location along that line is proportional to the relative luminance to each of the two colors in the mixture.  (Back to top)

Which RGB values or others make color that is visible only under ultraviolet ray on the white paper? (606)
Making a color that is visible only under UV radiation is not a matter of RGB values, it is a matter of selecting a material that is fluorescent when illuminated with UV, but matches the background under normal visible light. In that case, the RGB values would be the same as the background, but you would only use the fluorescent material for the parts you wanted to be visible under UV.  (Back to top)

If one measures same color on various spectrophotometers the spectral profile is the same,but there are differences of 2-3 % in reflectance. Can you explain why? Which is the most correct reading? (605)
The most correct reading is that obtained by a national standardization laboratory on a reference instrument. Results from different instruments vary for many reasons. These include subtle differences in illumination and viewing geometry, sample area, sample presentation, calibration differences, and systematic errors in the instrument (e.g., wavelength, bandpass, photometric scale linearity, etc.). For fluorescent objects, the light source in the instrument will make a difference as well.  (Back to top)

Can I use BCRA standard tile for calibration of spectrometer? Where can I buy it? (604)
Yes, they are available from the National Physical Laboratory in the UK.  (Back to top)

The chromaticity diagram provides an abridged method to specify the hue and saturation of an unrelated stimulus or of metameric matches. That is really its only meaningful application. It has far more disadvantages such as having no representation of luminance, having no relationship to appearance, and having perceptually nonuniform spacing. I would recommend a review of the topic in Berns' "Principles of Color Technology, 3rd Ed" for more details.  (Back to top)

Can you please define "dominant wavelength" and "wavelength of maximum absorbance"? Which should be used in the calculation of strength? (602)
Dominant wavelength is a colorimetric measurement from a chromaticity diagram. If you draw a straight line on a chromaticity diagram from the white point through the chromaticity of the sample to the spectrum locus, the wavelength at which this line intersects the spectrum locus is the dominant wavelength. The wavelength of maximum absorbance would be derived from simply finding the maximum on a spectral absorbance function. Dominant wavelength would have no correlation with strength and neither would the wavelength of maximum absorbance. However, the absorbance value at that wavelength might be useful in a calculation of strength.  (Back to top)

I am writing a story about colour for children. Can you help explain (in simple terms) how a rainbow works and how the mixing of just a few colours in printing pictures in papers works? (601)
I like howstuffworks.com for reasonable explanations of many things. That link is for rainbows and I'm sure they have one on printers, too.

As far as a simple explanation, I just like to say the different colors of light are bent to different angles by the raindrops in the air (for a rainbow). For color printing, you can say that we have only three types of color receptors in our eyes and the primary colors in a printer work to control the responses of those three receptor types. Since there are only three receptors types, only three primaries are required.
<>br> Another good source is the "Magic Schoolbus" books on color and the senses.  (Back to top)

Would you recommend a source for standards tile for calibration of spectometer? (600)
There are a series of National Measurement Laboratories around the world, some of which provide standards for spectrophotometry. For example NIST in the USA, NPL in the UK, and NRC in Canada provide these services. I am sure there are others as well. Also, some instrument manufacturers provide calibration services.  (Back to top)

How long does it take to perceive color? Are some colors perceived more quickly than others? (599)
It takes about 1/10 of a second to perceive any visual stimulus. But that is the latency for the signal to reach awareness. But the visual system can follow changes, such as flicker, at very fast rates depending on their color and intensity. So even though there is a lag for the signal to reach consciousness, the visual system itself can follow rapid change.

Your question is hard to answer because the answer depends on the state of adaptation of the eye and the intensity of the flash of light. It really doesn't matter how brief a flash of light is if it is bright enough because the response of the eye is accumulated over time.

But it is safe to say that under certain certain laboratory conditions, one can show that red and green are perceived more quickly than blue and yellow and that white/black differences are the fastest. But under normal viewing conditions, these differences will not be observed.  (Back to top)

I have an RGB digital SLR camera. Is it possible to find relationship between the camera output and the spectral reflectance of objects? (598)
It is possible if you have some more information about the objects. For example the spectral reflectance for many objects can be fairly accurately represented with three basis functions. If you know these three basis functions, then you can use the RGB responses of the camera to determine the necessary weightings to recover a spectral reflectance curve. It would be an approximation in most cases and you can do better with more responses.  (Back to top)

How can I determine color names corresponding to hex color values? (597)
This is easy to figure out yourself (your names are the best for your purposes) since there are several tools on the web for conversion between decimal and hex RGB notations. You just need to parse the hex numbers such that the first two are R, the second two G, and the last two B and use a tool like easyrgb.com to view the colors.  (Back to top)

What is the meaning of NET profiler of a color spectrophotometer? (596)
Netprofiler is a commercial software application sold by GretagMacbeth.

Check the GretagMacbeth site for many more details. Briefly, this application communicates with GretagMacbeth via the internet. Information is exchanged which tries to improves the measurements and maintenance of your spectrophotometer.  (Back to top)

When I measure delta L on low-gloss (matte finish) parts, I notice more variation between measurments than with higher gloss parts. Is there a scientific reason for this? Does the gloss play a roll? (595)
The measurements of the lower-gloss parts might be more sensitive to variations in measurement geometry. That would mean that differences in instrument geometry, sample presentation, sample flatness, etc. would be more likely to add variability to your measurements.  (Back to top)

How can I extract a specified color from an RGB image using Matlab? (594)
There are two parts to this question. First, I assume you have an RGB image file to extract the data from. This file could be output from a camera, scanner, or simply an image you found on the web somewhere. Second, you need to transform the RGB data into a color space, such as CIELAB.

To extract the RGB data from an image file, use the Matlab function imread(). Details on this function can be found on this Mathworks page. This is a smart function which looks at a file and determines which procedure to use to read in the data. It will automatically handle jpeg, tiff, gif, and many other image formats. Imread() will usually return a 3D array, M rows, N columns, 3 image planes. For example: myImg = imread('myImageFile.jpg'); The RGB coordinates of a pixel are myImg[m,n,1], myImg[m,n,2], myImg[m,n,3], respectively.

To extract the actual color of that pixel, you will need more information. If you know the image is from certain scanner or camera, you can derive a characterization for that scanner which will predict CIELAB or CIEXYZ from the RGB data. If you know the data are sRGB, there are transformations to convert the data to CIELAB. If you have no other information about the RGB image file, then you cannot recover the color accurately. RGB values are device-dependent, and without information about the device, RGB data cannot be processed any further.  (Back to top)

How does one go about using use color measurements to determine the concentrations of colorants/dyes in aqueous solutions? (593)
This is essentially the topic of the area known as color, or colorant, formulation. The relationship between colorant concentration and spectral transmittance (in this case) is characterized for all colorants of interest and then an appropriate model (the Beer-Lambert Law in this case) is used to predict the characteristics of mixtures. Given a mixture of known colorants (unknown concentrations), it is possible to figure out the concentrations by inverting the model. Berns' text, "Principals of Color Technology, 3rd. Ed." provides a good overview of these procedures.  (Back to top)

I evaluate llighting systems by measuring u'v' with a spectroradiometer. How can I calculate deltaE color difference from this data? (592)
It is a tricky calculation, but what I would recommend is normalizing the sources to the same luminance (Y=100) and then use Illuminant D50 (with Y=100) as the CIELAB reference point (XnYnZn). Then convert your u'v' measurements for other sources back to XYZ (scaled to Y=100) and use the test source as the stimulus (XYZ) in the CIELAB computation. You can then compute the delta-E between the reference D50 (L*=100, a*=0, b*=0) and your test source (which would be slightly different). This delta-E will be equivalent to the idea of illuminating side-by side white samples with the reference and test sources and assuming you adapt to the reference source. That is a completely valid and meaningful computation to do.  (Back to top)

How is the Light Reflectivity Value (LRV) measured? Why do building codes require a certain percentage of LRV in the materials used? (591)
LRV is ill-defined, but the best metric appears to be the CIE Y tristimulus value (or the luminous reflectance). I am not familiar with the building codes, but I would suspect that it is an issue of energy conservation since lighter building materials would result in less energy required for achieving desired illumination levels and to cool the building interior.  (Back to top)

Do people with light colored eyes have better night vision than people with dark colored eyes? (590)
I don't believe there are any data to suggest that eye color has a significant systematic effect on visual performance.  (Back to top)

Do you know of a color/temperature chart showing various colors and their related heat reflection/absorption properties? (589)
Unfortunately, I am not aware of any such chart and it is unlikely that it is even feasible to make one. Heat absorption properties do depend on color, but they also depend on the material's properties in the ultraviolet and infrared parts of the spectrum and the nature of the illuminating source. So there is no way to consistently match up color and heat absorption properties without knowing other details of the material and illumination.  (Back to top)

What sorts of jobs would a color scientist do? (588)
Thanks for your inquiry. For one thing you made me realize we have to make a page on our website that gives examples of the types of jobs our graduates accept. We do have a page listing our sponsors and companies that students have gone to work for (there is a big overlap between the two).

Most of our graduates at both the M.S. and Ph.D. level end up going into industrial research and development positions. There are a variety of fields represented by the list I mentioned above. Recents grads have ended up working on things like printing/color reproduction of greeting cards at Hallmark, formulation of paints at Benjamin Moore, imaging systems at HP, cell phone cameras at Qualcomm, display systems at TI, electrophotographic printers at Xerox, imaging software/hardware at Apple, television quality at Philips and Samsung, etc. Many have gone on to help found technology companies or to very high level research positions at places like Kodak, HP, Microsoft, etc.  (Back to top)

What is the relationship between the CIELAB a*-b* opponent axes and the perceptual unique hues? (587)
The unique hues do not directly line up with the a*-b* axes. One reason for this that we perceive different numbers of discriminable hues between each of the four unique hues (e.g. we see more purple hues between unique red and unique blue than greenish-yellow hues between unique yellow and unique green). In terms of CIELAB hue angle, the unique hues fall at approximately R:24 deg., Y:90 deg., G:162 deg., and B: 246 deg. Only unique yellow happens to line up with a CIELAB axis (positive b*).  (Back to top)

When the 3 hands on a spinning toy (which are 3 different sizes, long, medium and short) spin around really fast, they leave 3 "trails" of light in concentric circles. Why do we see 3 complete circles of light all the time? (586)
What a great question! The effect you are seeing is called "Persistence of Vision". A web search on that topic turns up a bunch of good explanations. Here is one experiment about it from the Exploratorium.

Essentially, our eyes take a certain amount of time to respond to stimuli in the world. It is roughly 1/30 of a second. So if your lights make a complete rotation in less than 1/30 sec. then your eyes basically add up all the light for that time period and produce a single image (the circle formed in that time period). It's the same reason that things moving quickly look blurred. You should be able to see similar trails of light by just moving lights around with their hands ... the faster they move the lights, the longer the trails. If they make a full circle quickly enough, then it will appear like a full circle of light.  (Back to top)

Why do colors fade in the sun? is it the heat or the light? (585)
There are a lot of causes of fading, but it is probably more common that it is the light exposure more than heat exposure. Beyond that, exposure to ultraviolet energy is even more damaging in most cases (this is the same stuff that gives us sunburns).  (Back to top)

Do humans have a natural affinity for certain sorts of colors more than others as compared to other creatures? (584)
Our affinity for colors is very much influenced by society, so you could say that we do since our society is different. If there are inherent affinities, then they might well be similar for other animals that have similar visual systems (which turn out to be relatively few in number).  (Back to top)

Do insects that have UV sensitivity in their visual system see the UV portion of the spectrum as "color"? (583)
As far as insects, it is really impossible to say for sure, but it seems that there visual systems are designed such that the UV part of the spectrum gets processed as another color response. Of course it is possible that they don't see "color" at all and just use the different responses for different functions (sort of different sets of black and white images for different purposes).  (Back to top)

Is color limited to the interaction of light with an object and our eyes? (582)
Yes, we need to have our eyes involved in color but we do a lot of manipulation of lighting and objects to control those color perceptions. So there is plenty of color science that just involves the light and objects without worrying about the eye too much. You need light and the eye, but you don't necessarily need an object ... lights can be colored all on their own.  (Back to top)

If no light is falling on an object, does it still technically have a "color"? (581)
Color is technically defined as a perception and if there is no light to initiate that perception, then there is no color. So I would say that if there is no light falling on an object it doesn't have a color. If there is light on other objects in the scene, but none on that one particular object, then its color would be black.  (Back to top)

How does a color TVthere is a great website describing how TV works at . work? (580)
There is a great website describing how TV works at How Stuff Works.  (Back to top)

How can I know the spectral sensitivity of a common CCD camera? (579)
You could ask the manufacturer. They might have published specs for the camera and/or sensor. Other than that you can measure them yourself, but it takes a rather elaborate laboratory setup to do so.  (Back to top)

Does the color green inspire thought? (578)
No more so than any other color. Affective responses to color are very real, but they are not consistent from person to person. If green inspires thought in you, that's great, but it might just put someone else to sleep.  (Back to top)

Do objects that are the same color have the same chemicals (elements) present? For example if I looked at the chemicals in red paint would I find the same ones in a red flower? (577)
Generally no. Simply put, there is more than one way to make the same color. For example, grass is green due to chlorophyll, but my color TV can make that same green color with a combination of red, green, and blue phosphors and a color printer can make grass green with cyan, magenta, and yellow inks or dyes that include no chlorophyll at all. There are lots of different materials capable of producing any given color.  (Back to top)

How I can mathematically convert from CIELAB (D65,10�) to XYZ (D65,2�) or RGB? (576)
You can't. CIELAB can be converted back to XYZ using the simple inversion of the CIELAB equations, but there is no way to change from the 10� observer to the 2� observer in the process. As far as conversion to RGB, that requires a characterization for the monitor you are using. A common choice is to convert to standard RGB (sRGB) values and there is a set of equations defined for that conversion.  (Back to top)

How do the colors you wear affect your appearance? What colors make you look certain ways? (575)
Really, the colors you wear affect your appearance however you believe they do. There are no specific scientific rules that apply in all cases. Some people look and feel good in certain colors, while others will have the same experience in completely different colors.  (Back to top)

What is an easy way to create a color scheme using an elliptical path and a Munsell conversion program? (574)
I assume you mean an elliptical path through the color space to select the colors in your scheme. An easy way would be to draw (either by hand or electronically) the polar coordinates of the Munsell system and then select the range of hues, etc you want to traverse and simply sketch out the path. There is no need to have a precise ellipse. You can then select the Munsell notation that your path crosses and use the conversion program to convert those over to RGB, CMYK, or whatever other coordinates you would prefer to work in.  (Back to top)

Is the temperature the same in shadow under a dark colored fabric awning as under a a light colored awning? (573)
As long as you are some distance from the awning and/or their is plenty of ventilation then it wouldn't matter at all. However, a dark awning is going to heat up more (it absorbs light and converts it to heat where the light one reflects most of the light) and then re-radiate some of that heat. Thus, if you are close to the darker awning, or their isn't good ventilation under it, then it will definitely feel warmer under the darker awning.  (Back to top)

Sailors know from experience that darker colored fabrics last longer in full-sun exposure than white or very light colored fabrics. I am specifically referring to Sunbrella bimini and sail cover fabrics. Why? (572)
I am not familiar with the longer lifetime of the fabrics you mention, but it makes perfect sense that the colorants might well protect the fabric itself from harmful radiation (light and UV). The colorants probably end up fading first and then any damage that the light/UV causes the fabric would begin.  (Back to top)

How do grey scale images differ from color (RGB) images? (571)
Grayscale images have one dimension of information for each pixel (light-dark) while RGB images have three dimensions (channels) of information (amount of red, green, and blue). There is a wide variety of methods for deriving grayscale images, but generally they are equivalent to some weighted combination of the RGB channels of a color image.  (Back to top)

How can I match a paint color to Munsell N8? (570)
You could purchase a Munsell N8 sample from GretagMacbeth and then take that to a paint store to be matched. This is usually very effective and accurate. Alternatively, if the store can match a numerical specification, Munsell N8 translates into CIELAB values of L=80, a*=0, and b*=0 under daylight. You might be able to provide these numbers to the paint matcher and they could enter them into their system to formulate the match.  (Back to top)

I'm looking for a Circular Spherical Vision Spectro Colorimeter for analyzing luminance and other parameters (axis etc...) of reflective materials in different colors for road construction. Can you help? (569)
It sounds like you are looking for a 3D goniospectrophotometer. To my knowledge, there are none available commercially but rather research labs custom build instruments for their own use. There are 2D multi-angle instruments on the market that are used for applications like metallic automotive finishes. (2D means that the illumination and measurement angles are always in the same plane relative to the surface.) One example is from Datacolor, but there are others from manufacturers like GretagMacbeth and X-Rite. Perhaps one of those instruments would be helpful for your application.  (Back to top)

Is there a source for a neutral gray paint designed to be used on floors and/or walls in a TV broadcast studio? (568)
I am not aware of a specific standard or source for gray paint for TV studios. If you take a sample of the desired gray to a reputable paint store, they can measure it and produce a very close match. In your situation, and 18% photographic gray card might be a good choice to match. That is a bit darker than typical viewing booths, but I would think a slightly darker surround would be good in a TV studio.  (Back to top)

I would like to find pigments with relatively narrow spectral aborption bands. The goal is to create paint which offsets the peak emission lines of different artifical light sources. Can you help? (567)
I've never come across such pigments. In general it is difficult to produce such narrow absorption bands with the exception of some materials like rare earth metals. This is really a chemistry question and I am not sure who can best answer it. There is a department of colour chemistry at the University of Leeds. Perhaps someone there can point you in the right direction.  (Back to top)

As an Interior Design student I know that paint manufacturers, geologists, anthropologists, and color designers use the Munsell system of color notation. What other industries currently his system to code their color? Automobile industry? Textile Industry? (566)
I think you covered most of them. The system is also used in imaging (color reproduction) to help test and improve imaging systems. You might look at this paper I recently co-authored on Munsell and the system for American Scientist.  (Back to top)

Why do we recognize uniqueness only for four colors, blue, yellow, green, and red? Is there any color map of unique hues in a visual system? (565)
There seems to be a lot of physiological and psychophysical evidence supporting the special nature of the unique hues in the human visual system. I'd suggest a google scholar search to find lots of interesting references. "Why" is a difficult question, but it is probably related to the nature of the cone photopigment absorptivities and the transformation of them necessarily to maximally decorrelate the spectral signals in the natural world. It is hard to say there is an exact physiological locus for the unique hues, but there are certainly various types of opponent encodings that have been identified and there is certainly a map somewhere in the visual system because the response is such an easily accessible percept.  (Back to top)

How do we see metallic colors like gold and silver? (564)
We see metallic colors like gold and silver just like we see any other colors. It turns out the difference between metallics and other colors is a physical, not a perceptual, difference. The highlights (where the light sources are reflected) on metals take on the color of the material, while the highlights on other objects take on the color of the light source. That's due to the physical nature of how metals, as opposed to other materials, interact with light. There's a little bit more on this in another FAQ answer.  (Back to top)

Why is a red Popsicle red? (563)
Because, in our culture and language, "red" is the name chosen for the color perception generated by that Popsicle.  (Back to top)

What is the difference between value and chroma? If I add black or white to a color I am changing its value. How do I change its chroma? (562)
Value (or lightness) refers to the perceptual difference between white (high lightness or value) and black (low lightness). Chroma refers to the difference between a neutral gray (low chroma) and a color of the same lightness that is more colorful (like a vivid red ... a high chroma). Stimuli can, of course, vary in both dimensions at once when physical changes are made. If you add black or white to a color (regardless of what type of color mixing you are doing) you will certainly be changing the value, but you will also be changing chroma. If you want to just change chroma, you should mix the color with a gray of equal lightness.  (Back to top)

I measure pigments from bird egg shells with a spectrophotometer CM-2600d, should I work with 100%UV or 0%UV? What about pigments from bills and legs of birds? (561)
The reason for the UV selection in spectrophotometers is for the evaluation of fluorescence (some materials absorb the UV and re-emit that energy at visible wavelengths). I am not sure if the materials you are interested in are fluorescent, but measuring with the UV included and excluded will give you that answer. If the results are the same, then there is no fluorescence present and it won't matter which you select for future measurements. If the results differ, you will have to decide which is more relevant to your work. I would think that including the UV would make most sense since these materials are normally viewed in daylight, which has plenty of UV. But, again, it only matters if the materials are fluorescent ... otherwise the results will be the same either way.  (Back to top)

Out of blue, green or peach, which color is more likely to attract flies or insects and why? (560)
I don't know of detailed research myself. But I did a google search on the terms color, attraction, insects, and found quite a few pages of interest. This one (scroll down to the color part), suggests that the attractive colors vary by type of insect.  (Back to top)

What effect does light have on white paper: does it reflect or transmit? (559)
If the paper is a perfect white, then it would reflect 100% of the light incident on it and would reflect that light mostly in a diffuse (scattered in different directions) way. Real paper is not perfect and typically reflects something like 80% to 90% of the light that falls on it. The rest of the light is either absorbed or transmitted. If you can see light through the paper (i.e., it is not completely opaque), then you know it is transmitting some light.  (Back to top)

What is chromatic textile? What properties do this textile have? (558)
An achromatic color is a color devoid of hue (e.g., white, gray, black) and a chromatic color is a color with some appreciable amount of hue (e.g., different from gray). A chromatic textile would simply be a textile that has a hue.  (Back to top)

How do I compute the correlated color temperature of an Lab/Luv color coordinate? (557)
You can't compute correlated color temperature from CIELAB or CIELUV coordinates. Correlated color temperature is a metric associated with light sources and is computed from tristimulus values (XYZ) or chromaticity coordinates (xy, or u'v'). CIELAB and CIELUV are computed by normalizing out the light source so those coordinates no longer have the information you need to compute correlated color temperature.  (Back to top)

I'm trying to understand whether there is an optical/physical basis for this "chameleon effect" (the ability of the materials to blend with surrounding tooth structures to create naturally appearing restorations) and whether there may be way to measure it. (556)
This sounds like a fascinating color measurement question. I am not very familiar with the materials used or color measurement of teeth, but I can imagine a few things that might cause the "chameleon effect" you refer to. The first one would be scattering. It is possible that two materials have the same measured color and translucency, but one achieves it more through absorption and the other more through scattering. Whichever is more similar to the natural tooth material would blend in better under a variety of viewing conditions. Related to this could be index of refraction. The material with an index of refraction more closely matching the natural tooth would blend in better at the edges of the filling/repair. The third thing that comes to mind is metamerism. It is possible to produce the same color and translucency from different materials and end up with different spectral reflectance/transmittance properties. This could mean that a given material is a good match to the tooth under one viewing condition and a poor match under another. Difference materials could exhibit greater or lesser degrees of metamerism.  (Back to top)

I'm looking for the paint formulation for the official colors of West Virginia University. The Pantone colors are either: A) PMS 873 metallic gold and PMS 295 Navy Blue or B) PMS 124 Gold and PMS 286 Blue. Can you help? (555)
Since it sounds like the color matching system that the paint formulator has is not capable of matching Pantone colors directly (that would require the system developer to have licensed the Pantone system), the next best solution would be to use a Pantone book and have the formulator measure the two color patches on their spectrophotometer in order to formulate a match. If your formulator can't do that, go to another one. Most hardware stores have a spectrophotometer available to allow matching to customer samples. Go Mountaineers!!!  (Back to top)

In your opinion what pantone color best defines TEAL? (554)
The only opinion that matters is yours, or your customer's. The best thing to do is look through a set of Pantone samples and select the one that best represents teal to you. Color names are not generally used that consistently and there is usually no scientific way to pinpoint the best example.  (Back to top)

Can you help me calculate the color difference between my Macbeth Colorchecker and its image after capture and display on an LCD monitor? (553)
You need either an instrument to measure the XYZ values of the displayed chart (a colorimeter or spectroradiometer) or a characterization of the display that will allow you to convert the displayed RGB values to XYZ. You can then compute CIELAB values for both the original chart and the LCD display and from their calculate color differences using any of the recent CIE color difference equations. You might look in Berns' "Principles of Color Technology, 3rd Ed." for details on each of these steps.  (Back to top)

I would like to purchase a grey scale color variation kit for comparing the change in color wash for Denims and Textiles. Can you recommend a source for this kit? (552)
I believe the kit you are referring to is produced and sold by the AATCC and can be directly ordered through: this site. They also have other related products, so you might want to look around their website beyond the single item mentioned.  (Back to top)

Why does eye (iris) color appear to fade with old age? Is it because of the yellowing of the lens? (551)
I found a brief explanation online at wonderquest.com. Eye color is determined by the amount and distribution of melanin in the iris (melanin also gives our skin and hair its color). Apparently with age, the density (amount) of melanin can change in the iris resulting in the observed change in eye color. Similarly this can cause our hair to change color as we age. However, the perceived color of our iris does depend to a degree on having a dark area behind it and as we age the lens does yellow. That would make the area behind the iris less dark and therefore change the appearance of the iris. I would suspect that the changes in pigment density are far more important than changes in the lens, however. There are also other causes for changes in eye color that are mentioned in the linked column.  (Back to top)

Where can I find a tabulation of all pairs of distict spectral monochromatic lights that are perceived as achromatic? (550)
First of all, I assume you mean pairs of spectral lights that appear monochromatic when mixed together, in other words additive complementaries. I am not aware of any tabulated sets of complementary wavelengths. There are two reasons for this. One is that there is a large range of chromaticity coordinates that are perceived as achromatic due to chromatic adaptation in our visual system. Thus, the first step in this process is to decide which achromatic chromaticity is of interest. Once that decision is made, the complementary wavelengths are fairly easily computed from either the spectral chromaticities (a straight line through the achromatic point connects complementary wavelengths) or the color matching functions.  (Back to top)

At our paint-supply company we need to match a color to existing color on a clients' wall. Is there a portable instrument we can use to determine the required mix to match the exisitng paint? (549)
Yes. Most, if not all, of the instrument manufacturers make portable hand-held spectrophotometers that could make the necessary measurements. You would then need to match those measurements with your existing paints or use your current colorant formulation system to predict the paint recipe. If you currently have a non-portable system that does this job, then you should be able to use the colorimetric values measured on the portable system with your current software to predict a match. The instrument manufacturers should be able to help you put together an appropriate system.  (Back to top)

How can I know the tricolor chromaticity coordinate of a common CCD camera? (548)
A CCD camera does not have chromaticity coordinates. This is a common confusion between the analysis (measurement) of color and the synthesis (display) of color. Display primaries (and all displayed colors) have chromaticity coordinates. Camera signals do not have chromaticity coordinates until they are rendered to a display. With accurate analysis through characterization of the camera and accurate synthesis through characterization of the display it is sometimes possible to accurately recreate the chromaticity coordinates of the imaged scene on the display. The intermediate representation can be almost anything that preserves the data.  (Back to top)

How does color affect our vision? (547)
Color is our perception that results from vision. So you can't really say that color affects our vision since it is the product of our visual system. Color, in terms of the color of objects or light sources, can affect our visual performance. For example it is easier to read black text on white paper than it is to read yellow text on white paper. There are many ways that color affects visual performance. So many, in fact, that there is no way to give a short answer to this question.  (Back to top)

How do I calculate the chromaticity coordinates of a single wavelength of light? (546)
To compute the chromaticity coordinates for a monochromatic stimulus (a single wavelength, wl), simply read the values of the color matching functions (x-bar, y-bar, and z-bar) and do the following computation:

x = x-bar(wl)/(x-bar(wl)+y-bar(wl)+z-bar(wl))
y= y-bar(wl)/(x-bar(wl)+y-bar(wl)+z-bar(wl))  (Back to top)

What factors affect the intensity of color? (545)
Many factors influence the intensity of a color. These include the physical properties of the stimulus itself as well as the viewing conditions (color and amount of illumination, surrounding colors, etc.) All of these physical and perceptual factors together determine our perception of color appearance.  (Back to top)

What do I tell the paint store to mix in order to get a color that best matches a wavelength (5040 Angstroms)? (544)
You really can't make paint that is a perfect match to light of a single wavelength. In this case, the wavelength you selected (5040 Angstroms or 504 nm) would appear cyan, or bluish-green, if viewed in isolation. Since paint doesn't reflect just a single wavelength, you can't reproduce this physical stimulus with paint. Even if you could, it would be very dark since it would be absorbing most of the visible spectrum. The best you can do is make a visual approximation. For this wavelength, you could go to the paint store and select the paint sample that is bluish-green (about half way between an ideal green and ideal blue) and as bright and chromatic (saturated) as possible.  (Back to top)

Do different people see the same color in different shades? Why? (543)
While normal color vision (that is for observers who are not color blind) is very similar from person to person, there are differences large enough to be noticed. Most often this happens in color matching when one person sees two stimuli as a match and another does not. Simply put, these differences are caused by a number of physiological differences in the visual system (just like people are different in many other ways). Some of the causes are differences in the transmittance of the eye lens (which gets yellow with age and also varies from person to person), a protective yellow filter layer on top of our retinas, and the photoreceptor responses. In addition, there are also differences in the ways individuals assign names to various colors.  (Back to top)

How does one resolve the concepts of the 'scientific' (linear) color frequency model and the 'artistic' (circular) color wheel model in regards to color perception? (542)
The resolution is quite simple and close to what you suggested:

[the rest of the question, in the questioner's words] "If we look at a color wheel and assign a corresponding light frequency to each point on the wheel, high frequency blue around to low frequency red, it seems that somewhere in 'purple/violet' range that there would be a point which represents both the very highest perceptible color frequency and the very lowest!?"

The linear representation of color is in fact simply a representation of the physical stimulus in terms of wavelenght (or frequency). This does help define the stimulus, but not the color perceptions. To fully describe color in a scientific way, one must factor in the the human visual system. Once this is done, systems of colorimetry easily represent the circular nature of hue by connecting mixtures of the two ends of the spectrum to produce purples. So both scientists and artists describe the perception of hue as circular.  (Back to top)

I have a thin PET film that is colored with dyes. I am suprised by the amount of dye required to match the opacity of the current film. Would you say that dyes are more linear than conventional pigments are when referring to thickness and saturation? (541)
I don't know that I would say more linear, but in general dyes do behave differently from pigments. In general, dyes are more transparent and pigments more scattering. Thus, if you are trying to make something opaque, you will have more difficulty with dyes unless the substrate itself is opaque.  (Back to top)

Do you have a table of NCS color code corresponding to RAL color code? (540)
No, to my knowledge there are no published conversions to/from RAL. On their website, RAL recommends buying all the other systems and doing visual comparisons.  (Back to top)

What does RAL stand for? (539)
According to their website:

"In 1925, the private sector and the German government of that time founded the "Reichs-Ausschuss f�r Lieferbedingungen" - RAL - (Committee of the German Reich for Terms and Conditions of Sale)."  (Back to top)

I am a member of an internet forum dedicated to the Rubik's cube. What is the set of 6 colors (including black/white) that human eye is best able to discriminate? (538)
Generally research on color contrast involves only two colors (such as text on a background), so this is definitely an interesting question. You have already identified black and white and they should certainly be included in your set of six. The next step would be to include 4 chromatic colors that are of intermediate lightness (i.e., not very dark or very light so that they are not easily confused with the white or black). While it is difficult to say what the optimum choices would be, there is probably very little to gain in performance once the colors are separated into distinct hue categories that would not be confused. For example, choosing yellow, green, blue, and red would be about as good as you can do. You just want to make sure that the blue and green are clearly distinguishable which might be accomplished best by making the blue slightly purplish.  (Back to top)

What is the equation for plot a MacAdam ellipse? Also, what steps are the Macadam ellipses on the standard x,y chart and what do these "steps" mean? (537)
There are not equations to plot out the MacAdam ellipse data. Instead the data for each ellipse are tabulated in the original paper (D.L. MacAdam, Visual sensitivities to color differences in daylight, J. Opt. Soc. Am. 32, 247-274 (1942).). The steps refer to standard deviation of the color matches with 1 step meaning 1 standard deviation, etc. Please keep in mind that these data do not generalize for color discrimination of the population as a whole, but rather represent the experimental results for one observer in a particular viewing situation.  (Back to top)

I am a competitive rifle shooter. Are there any colored filters to place in my rifle sight that would help to reduce eye fatigue or even simply aid in aligning things? (536)
The best answer will probably vary a bit from person to person and with the particular details of the lighting. However, a good general rule for both the outdoor and indoor situations would be to use a yellow filter (or something similar). The yellow filters will remove short-wavelength (blue) light that is most detrimental to your visual acuity for many reasons. The blue light is scattered the most in the environment and in your visual system and thus produces the most flare that reduces the contrast of objects. Also, your visual system has very poor acuity for blue light (try reading blue text on a black background or yellow text on a white background, which is detected by the difference in the blue light, to see how poor your blue vision is) because of chromatic aberration in your eye and because of the distribution of different photoreceptor types. So looking through a yellow filter will give you the best contrast and acuity in general. You might be able to fine tune this a little bit for individual observers, objects (targets), and lighting. I'd suggest trying a range of yellowish filters and choosing the one that is most comfortable for you and produces the best results. Good luck with your training for the Olympics and I hope this was helpful.  (Back to top)

What is the best method to measure the opaque black color of a plasic material? (535)
The best method is to measure spectral reflectance with a high-quality benchtop spectrophotometer. The benchtop instruments will usually have larger measurement apertures, more powerful light sources, and more sensitive detection systems in comparison with handheld instruments. These features will help to make the measurement of very dark colors more accurate and precise. The measured spectral reflectance can then be used to compute CIELAB coordinates to describe the color of the material.  (Back to top)

What distance can humans still distinguish between different colors? (534)
There are a couple of areas for you to explore further. One is called "small-area tritanopia." That refers to the fact that the central area of our fovea has no blue-sensitive cones and our vision for small objects (less than 0.25 degree or so) becomes tritanopic (yellow-blue) color blind. The second are the "chromatic contrast sensitivity functions." These provide data, like those you cited for acuity. Unfortunately the results depend very much on the definition of chromatic contrast and the viewing conditions. As a very rough estimate, you could assume that the ability to discriminate colors has about half the acuity of our ability to see the objects. So, roughly speaking, if you can just see the 2.5m object at 1700m, it would probably have to be 5m in size to reliably distinguish colors.  (Back to top)

What wavelength out of red, green, blue and full spectrum would purple cabbage reflect, and what would it absorb? (533)
I'll assume you are asking about the purple parts of the cabbage leaves. A dark purple color like that will absorb a large percentage of most wavelengths. It will reflect some small fraction of the blue, very little to no green, and some red (probably a bit more than the blue). The same could be said for a full spectrum. It will absorb most of the energy in the green region of the spectrum, reflect some in the blue end, and a little bit more in the red end. Of course the best way to quantify this would be to measure a spectral reflectance curve using a spectrophotometer.  (Back to top)

Is black paint recommend for a boy's bedroom? (532)
First of all, whether the child is a boy or girl makes no difference. Black is an unusual choice for room paint, but if it is the child's preference and it is not related to any other behavior you want to discourage, then there should be no problem. There are two reasons to avoid black paint though. First the room will be very dark and it will take more lighting (and therefore more energy) to illuminate it to the level needed to do homework, etc. (White walls are very energy efficient since they reflect almost all the light and higher levels of illumination can be obtained with fewer lighting fixtures.) Second, when it comes time to paint the room again it will be more difficult to effectively cover the black paint (more coats of new paint required). White, on the other hand, is very easy to cover.  (Back to top)

What's the RGB composition of RAL 7005 & RAL 1021? (531)
Unfortunately I cannot directly provide the RGB values you are looking for. For one thing RGB is is a device-dependent metric and the correct answer will vary from display to display. Secondly, the classic RAL designations are simply arbitrary color names with no published colorimetric coordinates (the system is proprietary and samples for each designation must be purchased). There is a newer RAL design series with designations that can be related to CIELAB coordinates thus enabling more flexible use of the numbers. There are a number of websites with example RAL colors, but I cannot confirm the accuracy of their representation or the methods they used to obtain RGB values for display. One such site, that includes the colors you mentioned, is martinindoor.com. That site might give you approximate RGB values that would be good enough for some applications.  (Back to top)

How can I measure the LRV value of something. Or can I convert CIELAB data into an LRV value? (530)
LRV doesn't seem to be strictly defined (see this FAQ answer), but assuming that it is the CIE Y tristimulus value it can be easily calculated from the CIELAB L* value:
Y = Yn*((L*+16)/116)^3
Yn is the Y value for a perfect white under the same illumination and is normally 100.  (Back to top)

What material has the most uniform reflectance spectra, including visible, IR, and UV? (529)
A material with the chemical name polytetrafluoroethylene (PTFE) that also goes by trade names of Halon and Spectralon is recognized as a very good and uniform (nonselective) reflector through the visible spectrum and into the ultraviolet and infrared wavelengths as well. It is sometimes mixed with carbon black to make a good, nonselective gray. As an example, Labsphere produces a selection of their Spectralon standards at various gray levels. They could provide more information on the reflectance properties outside the visible wavelengths.  (Back to top)

Why are common soccer balls black and white? (528)
I don't know the answer for sure, but I would guess that the black and white pattern makes them easiest to see against a variety of backgrounds and when moving quickly. This because black and white have the most visual contrast. I also found this web site with a more extensive explanation.  (Back to top)

Can you help me convert between Pantone and RAL colors? (527)
Both Pantone and RAL are proprietary systems. I have no way to convert between them and I am not aware of any.  (Back to top)

Do clear objects reflect or absorb heat? (526)
Yes. Clear objects reflect and absorb heat. It is impossible to tell from color alone (clear being a lack of color in this case) whether an object is going to reflect or absorb heat or in what proportions since generally no object perfectly reflects or absorbs. For example some types of windows are coated to reflect heat better than others while both will look clear.  (Back to top)

Is there a simple formula to take numbers obtained by matching paint samples to the 1929 Munsell book and convert them to the numbers that would be obtained using the current Munsell book? (525)
There is not a simple relationship. The Munsell renotation experiments published in the Journal of the Optical Society of America in the 1940s included the revised (current) designations, but not a relationship back to the 1929 book.

(While not a formula or simple relationship, there is a list of corresponding notations that could be used to build a look-up table in the JOSA 1943 final renotation report.)  (Back to top)

Is it possible to make a color that is not visible to the human eye? (524)
No. The definition of color specifies that it is an attribute of visual sensation. If there is no visual sensation, then there is no color.  (Back to top)

Do age and gender matter when seeing color? (523)
There are some gender-dependent differences in color vision, but not of general significance. The most significant being that color vision deficiencies are far more common in males than females (usually a sex-linked genetic trait). With respect to age, there are differences due to the natural yellowing of our lens as we age. However, these differences are also relatively small when compared to the variation from individual to individual at any given age.  (Back to top)

I need a color swatch for 10YR 9/1, do you have this? (522)
We don't sell/provide Munsell products, but GretagMacbeth does. Please see this web page.  (Back to top)

We are trying to measure color reproduction for digital cameras using the Macbeth ColorChecker. We image the Color Checker under various illuminants. Is there a better metric to calculate the color reproduction than ΔE? (521)
The ΔE metric you are using is a typical practice and probably adequate for your needs. As far as what reference to use, that would depend on your particular application. Normally, if accurate color reproduction is the goal, then color differences from the target under each of the illuminants (rather than using D65 as a reference each time) would be more appropriate. If however, you are interested in testing how well the system color corrects and white balances to produce a daylight appearance, then the color difference to the D65 reference might be more appropriate.  (Back to top)

I am trying to image a white fabric bag using a light source inside of the bag. Are there any wavelengths of light that would be most appropriate? That is, which wavelengths might be absorbed by a white acrylic fabric? (520)
The fact that the material is white is an indication that it doesn't absorb any visible wavelengths well.  (Back to top)

What colour paint dries the fastest? (519)
If color is the only variable, then the paints should dry at equal speed if they are in the dark. The lack of light removes any effect of color difference. If they are drying in a lighted environment, then the colors that absorb the most light will dry faster (again all else being held equal). This is because the absorbed light is converted into heat within the paint, which means there is more energy in the water in the wet paint, which causes it to evaporate more quickly. Darker colors will absorb more light and therefore should dry faster.  (Back to top)

What hue can the human eye dicern the most shades of? (518)
This question is impossible to answer definitively, which is why you find conflicting answers. The answer will depend on how one defines hue, the type of stimuli, and the viewing conditions. If forced to give an answer, I would select purple. I base that on the structure of a color space in which perceptual hue differences are equally spaced around the hue circle. In such a space, the greatest area is devoted to hues that would be called purple. That indicates that we can perceive the most distinct hue differences in what we would call purple.

Please note however, that with other definitions of the visual task and viewing conditions it is quite possible to derive and defend other answers. I'm sorry I can't give a single, definitive answer, but I do hope this helps you understand the difficulties in exploring this topic.  (Back to top)

What does the color blue mean? (517)
The color blue means whatever you want it to mean. A color by itself does not have meaning. However, within a culture and a context a color can develop a meaning. However, that meaning usually is only valid within the specific context (e.g. red usually means stop when it is a signal light, but not when it is the paint on a sports car).  (Back to top)

When reading a violet, what does the Δa and Δb values represent? Example is the Δa dirty/clean or red/blue? (516)
With a violet color you are going to be in the positive a*, negative b* quadrant of the CIELAB space. While the precise answer will change with the precise color, the best general answer would be: +Δa* = more red, -Δa* = less red, +Δb* = less blue, and -Δb* = more blue. This is assuming the difference is computed as sample minus standard.  (Back to top)

In literature about discoloration of wood (furniture or parquet floors), UV-light is mentioned as the cause for the discoloration. But doesn't the glass window act as a UV-filter? So is the discoloration caused by visible light instead of UV-light? (515)
You are correct that glass absorbs most UV energy. For example, we don't get a sun burn through a window. However some UV does get through and this might be enough to cause some discoloration. Also short wavelength visible energy (the violet end of the visible spectrum) definitely gets through the glass and might well be capable of producing similar changes in your wood samples.  (Back to top)

How I can find the formulation of paint after coating? (514)
This is the process that a colorant formulation system performs. Based on the measurement of spectral reflectance of a sample, and a database of possible colorants, the system computes the best selection of colorants and the required concentrations to produce a match. If the colorants in the sample are already known, then such systems can do an even better job of predicting the required concentrations. Berns' text "Principles of Color Technology, 3rd Ed." provides an introduction to how this works and the various instrument manufacturers provide software solutions.  (Back to top)

I am wanting to know how broad the primaries were for the 1931 color matching experiment. Were they monochromatic, as in a laser? If not, what was the spectral half width at half max for each of the colors? (513)
There wasn't a single experiment that resulted in the 1931 CIE color matching functions. There was one done by Wright using narrow-band primaries (essentially monochromatic light, but not lasers in the 1920s) and a second done by Guild using broad-band primaries (rough guess would be about 50nm full-width at half max). Wright performed the transformation of primaries necessary to mathematically represent his results in Guild's primaries and showed that the results were essentially identical. The CIE then took the averaged results and transformed them to two new sets of primaries, one a set of monochromatic RGB primaries to establish the seldom-used CIE RGB color matching functions and one a set of imaginary primaries that allowed for all-positive color matching functions resulting in the commonly-used (and equivalent) CIE XYZ color matching functions.  (Back to top)

Does the color of an ice cube affect the rate at which the ice cube will melt ? (512)
Assuming the coloring does not change the chemical nature of the ice in any significant way, then ice cubes of various colors will melt at the same rate if they are in the dark. When illuminated, the darker colored cubes (those absorbing more light and converting that energy into heat) will melt faster.  (Back to top)

Can you tell me please what the CIELAB reading is of matte Munsell gray to be used inside color viewing cabinets for the print industry? (511)
Viewing booths typically have a Munsell N7/ background. This translates into CIELAB values of approximately L*=70, a*=0, and b*=0.  (Back to top)

I would like to measure the color intensity of various parts of birds. What instruments will I use? (510)
This is a very challenging color measurement problem. Assuming the various areas are to be measured on live birds in their natural habitat, it is extremely difficult to make accurate measurements. The challenges include measurement duration (getting it done before the bird moves), variation in lighting, variation in illumination and view geometry, and issues about the size of the measurement spot. If all of these can be overcome, then a spot spectroradiometer (also called a telespectradiometer) would be the instrument of choice. However, it is probably unlikely that all these issues can be resolved to the degree necessary to meaningful measurements. A viable alternative would be to use a camera system. A high-quality digital SLR could be characterized to provide reasonably accurate colorimetric data for the hornbill features. The problems of illumination and geometry variations could then be overcome by capturing a large number of images and computing average colors for the features of interest.  (Back to top)

Why does some plastic yellow over time? (509)
Materials change color over time for a number of reasons. Some chemical reaction is happening to change the structure of some molecule or molecules in the object. These reactions could be facilitated by exposure to light, oxygen, heat, or other things. Without knowing the chemical composition of the material in question and the storage conditions, it really isn't possible to write the specific chemical reaction(s).  (Back to top)

Can you help me convert between candella and power wattage as measured by photodiode? (508)
You can't do this conversion unless your photodiode is filtered to have a photometric response or you are always measuring light sources with the same relative spectral power distributions. Just about any text on "Radiometry and Photometry" will provide the details necessary to understand these relationships.  (Back to top)

I am trying to characterize my computer display. I have a failure of additivity. That is, the sum of the individual R, G, and B XYZ values does not equal the XYZ of white. Can you help? (507)
What you have run into is a lack of additivity in your display and you are correct that it might be difficult to accurately characterize. However, there might be hope. For LCDs there is typically a significant black level that is always present and an additive display model can work if this black level is corrected. What you need to do is measure the black and subtract it from each primary (and the white) measurement before checking for additivity (and then add it back in to assess the final color). What your calculation is doing is incorporating that black level 3 times when you sum R + G + B and only once when you measure white. Looking at your numbers, it would appear that the black level is quite high, but that might be possible for this display. It is worth a try. If you look at the technical report from Dec. 1996 on this page you can see a fairly simple example of this procedure.  (Back to top)

Can the brain take a 2D image (eg: a picture) and convert it to a 3D image? (506)
Yes! Our visual system (of which the brain is a key part) is capable of fairly accurately computing 3D geometry from 2D images. There are times it will be tricked, or there won't be enough information available in the 2D image, but generally (especially if there is motion, or multiple views) we can do quite well. Often what are called visual illusions are stimuli designed to trick our visual systems into making an "incorrect" interpretation. There is much research on topics like "shape from shading", "shape from motion", etc. that describe how we use the clues available in an image to compute 3D geometry.  (Back to top)

Assuming a monitor calibrated to the sRGB standard, is there a combination of available adjustments (gamma/contrast and/or saturation) that can be calculated and used to provide a better 'ballpark' display of an NTSC signal on an sRGB monitor? (505)
You should be able to fairly easily derive a reasonable transformation from NTSC to sRGB as you suggest. Essentially, you would use the following steps.
1. Convert NTSC to linear RGB using the NTSC gamma definition
2. Convert NTSC linear RGB values to sRGB linear RGB values using a 3x3 matrix
3. Convert those linear sRGB values to nonlinear sRGB for display using the sRGB gamma definition.
Each step could be done with various degrees of precision. Step 2 will cause some out of gamut colors and might not be terribly accurate since the NTSC signal might not have been specifically coded for the NTSC primaries anyway. If you want to eliminate that conversion of RGB values, then steps 1 and 3 can approximately concatenate into a gamma adjustment on the incoming data. For a real shortcut, you could determine the required gamma adjustment visually for typical displays and apply that generally.

Poynton's book, "Digital Video and HDTV: Algorithms and Interfaces", has all the detailed specifications if you want to get that precise.  (Back to top)

Can you tell me what color eye shadow would look the best for a given complexion? (503)
This really isn't a question of science, but one of preference. A color you like might look quite distasteful to others, etc. There is one perceptual phenomenon that might guide you little. An effect known as "spreading" causes perceived colors to be enhanced by similar colors that are nearby. That would suggest that to enhance your eye color, you might want to consider eye shadow of similar color. Strangely enough, the effect reverses itself when the colors being viewed are larger, but since your eyes are normally viewed from some distance, the spreading effect probably does come into play most of the time. Of course other criteria might also be important in deciding the colors for cosmetics, so ultimately you need to do what you like.  (Back to top)

Can you tell me what the cmyk formula for silver is? (502)
The short answer is "no". The appearance of silver is dependent on the three-dimensional distribution of reflected light. For a flat patch of color viewed via diffuse reflectance, silver and gray are the same. Only on 3D objects, or by tilting 2D objects, can the difference between gray and silver be observed (or measured). So there isn't a simple set of CMYK values to produce silver. Instead you would need an image of a 3D object to produce the appearance of silver on a CMYK print. If you want flat printed patches to appear silver, you need to use a metallic ink. This answer also addresses the same question from the RGB perspective.  (Back to top)

Do different colors affect your eye sight different over time? (501)
No. As long as you avoid extremes, like staring directly at the sun, and view a variety of colors, then your visual system will not be affected. Lack of variety in exposure to color can produce long-term adaptation. For example, if you always wore red lenses, your visual system would adapt over the long term to be less sensitive to red. However, this effect is reversible.  (Back to top)

Can one measure color through heat frequencies? (500)
No. There is no correlation between color and reflection or emission in the thermal infrared.  (Back to top)

What are the colour combinations or chemical substances that can be used to obtain jet black colour in a cotton flannel material? (499)
I'm afraid specific colorant formulations are somewhat outside the expertise of our laboratory. However, given that cotton flannel tends to have a very diffuse surface, it will be difficult to achieve an extremely dark black. This is because the material surface is always scattering some light in all directions.  (Back to top)

What is the real meaning of color blind? (498)
In the vast majority of cases, "color blind" refers to people who have a deficiency in their color vision. In fact, "color vision deficiency" is a more appropriate term. People with deficiencies generally confuse certain hues. For example most have some form of red-green confusion while others exhibit a yellow-blue confusion. There are very rare cases of people with monochromatic vision who can only see in shades of gray (they confuse all hues). There is more information in this answer.  (Back to top)

Did humans use chromatic adaptation long ago, for example in hunting? (497)
The apparent advantage of chromatic adaptation is that it helps humans (and other animals) to reliably identify objects across significant changes in the color and intensity of light sources. These changes happen with natural illumination (daylight is very yellowish in the morning and evening and bluish at mid-day and also changes color significantly with weather conditions) as well as more modern artificial illumination. Chromatic adaptation to natural variations in daylight is very helpful for activities like hunting, gathering food, and protection from predators. There's really no doubt that the visual systems of humans (and other animals) have been taking advantage of chromatic adaptation for a very long time.  (Back to top)

Does the color of your eyes affect your vision? Do people with blue eyes have better vision than those with green? (496)
I have never heard of any research to suggest that the quality of your vision (for either acuity or color) is affected by eye color. While theoretically, it might be possible for their to be some effect, other variables in our visual system are almost certain to overwhelm any variability due to eye color. So the short answer is "no".  (Back to top)

What is the minimum ΔE that a human eye can detect? Which &Delta E" (495)
This is not at all a simple question with a single answer. As you note, the answer depends on the ΔE method or equation used, but it also depends on many other factors such as the viewing conditions for the samples (size, color, separation, luminance level, gloss, texture, background, etc.). As a very general rule of thumb, people take a simple CIELAB ΔE*ab of 1.0 to approximate a just noticeable difference. However, the CIELAB space is known to be non-uniform for color differences and it might take a significantly larger ΔE*ab for the difference to be noticed for a high chroma color. This non-uniformity is what prompted the development of weighted color difference formulas such as CIE94, CMC, and CIE2000. Since those weightings tend to reduce the average color differences, a rule of thumb for just noticeable color differences tends to be a bit smaller, closer to 0.5, when using those equations. In all cases, these are very general rules and shouldn't be taken too seriously. It is always best to derive a tolerance for the specific samples, colors, viewing conditions, application, etc. to get a reliable answer.  (Back to top)

Do you know of any table of rgb value pairs that make up visual complementaries (colors that are afterimage of each other)? (494)
I am not aware of any such tables and it would be difficult to construct one since RGB values are device dependent and a pair of RGBs that would be afterimage complementaries on one display might well not be on a second display. There is also significant inter-observer variability (and difficulty in judging the colors of afterimages) that would complicate matters. Sets that you construct yourself are likely to be just about as good as those constructed by anyone else. (And would certainly be better for your visual system and your display.)  (Back to top)

What color of construction paper fades the fastest? (493)
It is impossible to say without knowing a lot more information. What type and color of paper? What is being exposed to? How are other variables (e.g., temperature, humidity) being controlled? How is "fading" being defined and measured? Etc. The best thing to do is define these variables for the case that is of interest and do an experiment.  (Back to top)

Why is blue light plus yellow light equal white light, but blue and yellow paints mix to green? Why do the color wheels from various systems use different primaries? (492)
Yellow and blue light mix together via additive color mixing to make white light since the yellow light is missing blue wavelengths that are filled in by the added blue light. Subtractive mixing of yellow (which removes the blue light) and cyan (which removes the red light) results in green. A subtractive mixture of a true yellow (removes blue) and blue (removes red and green) would result in black.

The term "primary" is used differently in different systems. For example, Munsell and NCS don't actually use the term at all. Munsell has principle hues and NCS uses the unique hues. Since primary can mean different things in different systems, the term needs to be interpreted carefully. The best generic definition is that of a "set of primaries" such that no one of the primaries can be made by mixing the other two.  (Back to top)

When refering to "pure" colors (red, green, blue, orange) what is the corresponding notation in the Munsell system? (491)
Here is a description of the Munsell hue scale directly quoted from Fairchild's book, "Color Appearance Models, 2nd Ed.".

"The hue circle in the Munsell system is divided into five principle hues (purple, blue, green, yellow, and red, denoted 5P, 5B, 5G, 5Y, and 5R, respectively) and is designed to divide the complete hue circle into equal perceptual intervals. Five intermediate hues are also designated in the Munsell system as 5PB, 5BG, 5GY, 5YR, and 5RP for a total of 10 hue names. For each of the ten hues there are ten integral hues with notations as illustrated by the range between 5PB and 5P, which is 6PB, 7PB, 8PB, 9PB, 10PB, 1P, 2P, 3P, and 4P. This type of sequence continues around the entire hue circle resulting in 100 integer hue designations that are intended to be equal perceived hue intervals. Hues intermediate to the integer designations are denoted with decimal values (e.g., 7.5PB)."  (Back to top)

What's method that appropriate for measure color of gemstone? (490)
Measuring gemstones is not a trivial matter and some have constructed special instruments and sample holders for the purpose. It is reasonable to measure both transmittance and reflectance to get an idea of the gemstone color. The most perceptually meaningful measurement would be one carried out with more typical illumination and viewing conditions. This could be accomplished by arranging the desired illumination environment and then measuring the light from the gemstone at various angles using a telespectroradiometer. The Gemological Institute of America has developed measurement techniques and published on the topic in the past. Perhaps an inquiry to them or a search for their publications would be helpful.  (Back to top)

How does color affect our mood? (489)
There is no question that color can affect our mood, but there is no specific relationship between particular colors and moods. A color that affects one person one way might affect another a different way, or not at all. I'd recommend Faber Birren's book, "Color & Human Response" as a good starting point if you are interested in learning more about this and related topics.  (Back to top)

Which of these Munsell soil designations is the reddest: 5R 6/4, 5R 5/2, 5R 7/3, 5R 6/3? (488)
They are all the same hue, 5R, which is pretty close to a unique red. The second number is value, or lightness, and again they are about the same ranging from a middle lightness of 5 (half way perceptually between white and black) to slightly lighter at 7. The key number is the third one, chroma. A chroma of zero is a neutral gray and as the number increases the sample becomes more colorful. A chroma of 4 is significantly more colorful (more red in this case) than a chroma of 2. So, I would say that your 5R 6/4 sample is the reddest.  (Back to top)

Can you help me explain (in simple terms for a 5 year old child) why some colors of popsicles melt faster than others? (487)
Light colors look light because light from the sun or other light sources mostly reflects (bounces) off them and comes to our eyes. Dark colors look dark because that light doesn't bounce off, but instead gets absorbed by the object. Since the light can't get to our eyes, the object looks dark. That light that doesn't bounce off gets turned into heat and ends up helping raise the temperature of the popsicle faster (therefore making it melt faster). So, darker colored popsicles should melt faster than lighter colored popsicles.  (Back to top)

Can you provide the pantone equivalent to GE plastic FXM171R-GY2A360M? (486)
I'm sorry, we can't help with that. The PANTONE system is a proprietary color naming system and we have no way to convert to it even if we knew the color of the GE plastic (another proprietary designation).  (Back to top)

Can you help me find a cost-effective way to measure black-gray-white and all shades in between. My son is doing an science experiment to determine how well erasers work. (485)
I can think of two techniques that might work. One would be to use a computer monitor and some graphics software to adjust a patch until it matches the sample and then record the RGB values (or other control ... some programs let you control a single value for light-dark) required for a match. The second would be to use a spinning disk. By making white and black disks that can be varied in proportion and spun to mix into gray, the gray level can be measured by recording the proportion of black and white. This web page gives an example for mixing red, green, and blue. The same can be done with just black and white. There are many more examples of spinning disks to mix colors on the web.  (Back to top)

Can you help me create the color green that is exactly in the middle of the green color spectrum using the RGB values of my display? (484)
Unfortunately there is no simple answer to your question. First of all, all RGBs are not created equal. RGB is a device-dependent measure of color and a given set of RGB values (such as 0,255,0) will be rendered as different colors on different displays. Secondly, RGB displays cannot in general produce spectral colors. They fall outside the gamut of RGB displays. The best you could do is to choose a particular display, characterize it carefully, and then compute the most chromatic gray that has the same hue as the wavelength you are interested in (hue could be defined in a color space such as CIELAB). As an alternative, you could simply approximate this by visually selecting the green you desire.  (Back to top)

I need an example or a graph help to explain why humans cannot see colors that fall in the ultraviolet or infrared part of the electromagnetic spectrum. (483)
This web page seems to illustrate it fairly well. I would also recommend the first chapter of Berns' book, "Principles of Color Technology, 3rd Ed."  (Back to top)

Specifications on the web given by SAE, ECE eg, mention that the colour should fall within the given boundary:
y = 0.335 (yellow boundary)
y = 0.980 - x (purple boundary)
What does this mean? (482)

Under typical viewing conditions red colors tend to plot in the lower-right side of the chromaticity diagram. So it is possible to figure this out given the color names you used (yellow, purple, red) and the numerical boundaries. The chromaticity coordinates for your red sample would need to be below the yellow boundary and to the right of the purple boundary. So a sample would pass if the following inequalities held true:

y < 0.335 and x > 0.980 - y.

In your case y = 0.151 which is less than 0.335 so the sample is below the yellow boundary and x = 0.789 which is less than 0.829 (0.980 - 0.151) so your sample is to the left of the purple boundary (too purple). Therefore your sample fails to be within the defined red region. It would either need to have an increased y, and increased x, or some combination of the two to fall within the boundaries.

I'm not sure if the example you gave was just hypothetical, but those values fall outside the spectrum locus of the chromaticity diagram so cannot be a real object or light source. If they are intended to be real values, you might want to check for errors in your measurement or computation procedures.  (Back to top)

Is there anyone who I can pay to convert 12 colors with LCh values into Lab or Munsell values? (481)
I could certainly put you in touch with someone, but the L*C*h to L*a*b* conversion is a very simple calculation given in this answer and the L*a*b* to Munsell conversion can be performed using GretagMacbeth's Munsell conversion freeware available from www.gretagmacbeth.com.  (Back to top)

In my prepress company we have to convert fabric samples by eye in to CMYK values. This is time consuming. Is there a software hardware combo that can read a piece of fabric and give the best cmyk values in a specific profile? (480)
Thanks for your inquiry. It sounds like you are on the right track. What you need to do is profile your CMYK printing process to allow you to set up a transformation for CIELAB values to CMYK. You then need to measure the fabric swatches and process those measured CIELAB values through the profile to convert to CMYK. Since the fabric and the printed images will have different surface characteristics, the profile might need some fine tuning and that will likely take some trial and error. I suspect you are doing the right thing with your current product, but running into some of the limitations of reproducing color across different types of objects. Unfortunately I can't recommend any particular product that would better solve your problem, but you might want to explore options from other manufacturers to see if their performance is better in your particular situation.  (Back to top)

How does the eye see colors that are moving very rapidly? (eg: a spinning color wheel) (479)
I am assuming you mean a color wheel made up of several colored segments that spins rapidly and appears as a single color. That is called a Maxwell disk. Maxwell used such a device to study how the human visual system perceives color and determined that all colors can be matched by additive combinations of three primaries. What happens is that the disk is spinning so quickly that we are no longer able to resolve the individual segments and they blur together. The color effect is the same as superimposing three projected lights and is known as additive color mixing. There is a good explanation/description at www.handprint.com. Another way colors are produced with spinning disks is by using black-and-white patterns that result in different stimulation of the three cone types in our eyes and thus produce colors where there physically are none. That demonstration is known as Benham's disk (as well as other names). A demonstration is described at exploratorium.edu. Web searches on either of these topics will turn up a lot of good demonstrations and explanations.  (Back to top)

Why does light create color? (478)
This question can be answered on many levels. At the most fundamental level color is a human perception and that perception arises from the responses of the three types of cone photoreceptors in our eyes. Those photoreceptors respond to light of different wavelengths and therefore it is this light that creates our perceptions of color. Why there are three types of cones and why they respond to the wavelengths of energy they do (light) are questions of the physiology of human cells, the information content in scenes and objects and how they interact with light, and evolution.  (Back to top)

What is neutral grey colour shade and the pantone number for neutral grey in pantone formula guide solid uncoated and uncoated ? (477)
The PANTONE system is a proprietary color naming system with no mathematical way to compute notations for desired colorimetric coordinates. You have to refer to PANTONE samples or PANTONE software to obtain the notations you require.  (Back to top)

How can I make a colored chromaticity diagram? (476)
Since a chromaticity diagram does not represent the information necessary to define color appearance (but rather defines color matches ... and only a 2D projection of the 3D match data), we do not recommend making colored chromaticity diagrams. They are always incorrect and misleading.  (Back to top)

Could you recommend a good website for the glossary of color imaging terms? (475)
Here is one that seems reasonable.  (Back to top)

Do you know of any research into the quantity of a particular colour eg., yellow that is in a painting and whether that painting is one that gives pleasure? (474)
I am not aware of any research on this topic. It would require some psychophysical experimentation to quantify the amount of "pleasure" (or other variable) produced by a range of paintings (and across a large population of observers) and then some image analysis to determine if there was a correlation between the amounts of any given color and the psychophysical results. Typically some sort of histogram analysis would be performed to measure the proportions of various colors in an image. This would require the definition of your color regions for categorizing the histogram (e.g. do you divide only by hue or do you use a 3D histogram). Even if there was a correlation, you would then need to explore the question of causation. Sometimes there is a correlation between two variables, but one actually does not cause the other.  (Back to top)

Which colour has the most easily visually discriminable levels of saturation? The literature seems to suggest blue violet as this has the most steps when white light is added in 'just noticeable' amounts - would you agree with this? (473)
This is a difficult question to answer precisely since there are multiple ways that saturation could be defined. The definition you mention, number of perceivable steps between an additive mixture of white and a spectral color does suggest that the violet region is the "best". This, as you mentioned, has been reported in the literature and also seems to be consistent with object-color perception (although that adds many other complications in defining the "maximum" saturation.) The historical literature on this topic is reviewed in section 7.10.3 of Wyszecki and Stiles "Color Science" (in case you haven't already looked there).  (Back to top)

What do the various CIE illuminant designation mean (D55, D65, etc)? How are they related to each other and to CCT? (472)
There is no simple correlation. On a clear day, the CCT of daylight will increase as the sun gets higher in the sky. However, other variables such as season, location, and weather conditions will also play a role.  (Back to top)

What colors should I use for my experiment on which colors absorb more heat? I already know I am doing black and white. (471)
There is something to be learned from any colors you select. Choose the colors you are interested in learning about.  (Back to top)

How do colored objects reflect heat? (470)
The answer all depends on how you define "heat". If you define heat as infrared radiation (which is only one of many ways that objects can be heated up), then objects reflect or absorb this radiation based on their chemical structure, which defines their ability to absorb, reflect, or transmit various wavelengths of energy. It's difficult to get more specific than that without a particular object and situation in mind.  (Back to top)

Can you help me transform Lch values to Lab? (469)
CIELAB L*C*h and L*a*b* are two representations of the same color space. They are related by the transformation from cylindrical (L*C*h) to rectangular (L*a*b*) coordinates. The equations are:
L* = L*,
a* = C*cos(h),
b* = C*sin(h).  (Back to top)

Does the first color you see when you wake up effect your mood? (468)
Color can influence mood, but not in the same way for everyone. So I would say that it is possible that the first color you see could influence mood (although probably not consistently in most cases), but that it doesn't necessarily do so. Probably, for most people, the first color has little or no influence on its own.  (Back to top)

Does the color of your car determine the temperature in the car? (467)
The color is one factor, of many, that influences the temperature of the car. In general darker colors will absorb more sunlight and become warmer on sunny days.  (Back to top)

Do you think people can remember words better from colored paper or white paper? (466)
Text and/or paper of various colors will certainly influence the legibility of words. However, once the word is read I would suspect that the color of the paper would have no influence on how well it is remembered.  (Back to top)

I would like to do a science experiment on "Will male and females chose the color that has been associated with their gender?" For example, pink for girls and blue for boys. Can you help be to create a 5th grade science experiment to test this idea? (465)
An important question to consider will be the context of the question. For example, you would likely get one answer if you just ask people to rank their favorite colors from a set and another if those colors are associated with some particular object (like a doll or other toy) or application (like paint for your bedroom wall or clothing). I'd suggest selecting a range of colors and then asking people to select their favorite both when the colors are simple patches (like pieces of paper) and when they are associated with specific objects or uses. Then you can examine the frequency that each color is selected for males and females (and perhaps see if there is an influence of age) for each question. I would also include a range of colors, not just pink and blue.  (Back to top)

In literature it is mentioned that the difference between SCE and SCI color measurements gives an indication for the gloss of the surface. Which parameters are used to evaluate this: the difference between the SCI and SCE values for CIE-L, a, b or a combination? (464)
While a change in gloss would have an affect on all three CIELAB dimensions, the most significant (and consistent across colors) affect will be on the L* dimension. The glossier a sample, the larger the difference in L* between an SCI and SCE measurement will be.  (Back to top)

What would be the reason to use CIELAB, CIELCH, CMC, CIE94, and CIE2000 (all of the above together) to calculate color difference from sample to standard? (463)
There really is no reason to use all of the color difference metrics together. The reason that many instruments/software systems provide all of those metrics is that different people are accustomed to using different metrics and the instrument manufacturers like to provide all of them. Generally in most applications you could settle on consistently using just one of the metrics.  (Back to top)

What is the best way to test color fading in construction paper? My child is making an experiment on what color will fade the fastest when exposed to sunlight. (462)
There are instruments to measure color, spectrophotometers, that would provide a variety of data to measure the changes in color due to fading. You could also use a simpler instrument, like a photographic light meter, to measure the change in "lightness" of the paper. Even with the measurements there will be issues of interpretation. For example a 2% change in black paper might look far more objectionable than a 2% change in white paper. The PANTONE system is really a color naming system, so it doesn't provide a systematic way to measure changes and compare them across different colors. It might be best to do the evaluation visually and simply have some observers rank order the changes from smallest to largest after various periods of exposure to sunlight.

As far as preserving the colors with sunlight exposure, it is possible that some type of varnish or clear coating would help but it is hard to say and would require some experimentation. Even a piece of glass might help (e.g., the paper in direct sunlight might fade faster than that covered with glass since the glass will absorb some ultraviolet ... the same reason we don't get sunburned through windows).  (Back to top)

What chemicals are used to create fluorescent paint? (461)
The specific chemicals used by various manufacturers are probably kept somewhat as trade secrets. A wide variety of substances are fluorescent and it is mainly the molecular energy levels that determine the fluorescence. The book "The Physics and Chemistry of Color: The Fifteen Causes of Color" by Nassau is an excellent resource to learn more about the chemical nature of fluorescence. You might also inquire with manufacturers of fluorescent paint to see if you can find out their material properties. For example, you can request technical documentation on products from DayGlo at dayglo.com.  (Back to top)

Can you provide the CMYK formula for ANSI Safety Green? (460)
Unfortunately, I cannot help. Even if I had the CIE specification for ANSI Safety Green, the transformation to CMYK is not unique. The CMYK values required to print a given color depend on the particular inks, the paper, the printing technology, and the algorithms used to drive the printer. The Munsell designation for ANSI Safety Green is apparently 7.5G 4/9 (found in a web search, so you might want to confirm that) and the GretagMacbeth conversion freeware, gretagmacbeth.com, can give you a conversion from Munsell to a generic CMYK that would be a reasonable approximation. That's probably the best I can recommend.  (Back to top)

Is there direct co-relation between CIE 76 and ANLAB 40 and ANLAB 50 color spaces? (459)
The CIELAB equations can be thought of as an evolution from the ANLAB color space. However, there isn't a direct mapping between them. One could always invert one space back into XYZ tristimulus values and then compute the other and it might be possible to derive a set of equations for that whole process if that is really necessary.  (Back to top)

What is the primary calibration of a spectrophotometer? (458)
The primary reference standard for spectrophotometry is the perfect reflecting diffuse (PRD). A PRD is a theoretical material that is perfectly diffuse (Lambertion) and reflects perfectly (100% of all incident light). Properly calibrated spectrophotometers report values relative to the PRD.  (Back to top)

Please explain the Bradford Chrmatic adaptation transform as a physical phenomenon and provide the mathematical basis. (457)
The Bradford chromatic adaptation transform is not a physical phenomenon. It is a mathematical model of the process of chromatic adaptation in the human visual system. Essentially, it consists of a 3x3 linear transform from XYZ tristimulus values to an optimized set of RGB responses. These RGB values are then normalized to the RGB values for the illuminant (or chosen white point) using a simple von Kries normalization. One additional twist in the Bradford transformation is that there is an adaptation-dependent exponential nonlinearity on the B channel (while the R and G channels remain strictly linear). A modified form of the Bradford chromatic adaptation transform was used in the CIECAM97s color appearance model. Details on both can be found in "Color Appearance Models, 2nd Ed." by Fairchild.  (Back to top)

Are HSL values that the Paint accessory of Windows system use recognised by CIE ? (456)
HSL is transform from device-dependent RGB values to an approximation of hue, saturation, and lightness. It is also device dependent and not an accurate perceptual space. Conversions can be found in most texts on computer graphics and at easyrgb.com or many other places on the internet. HSL is not recognized by the CIE.  (Back to top)

Is there a mathematical conversion from RGB to HTML color code? (455)
HTML color codes are RGB values expressed in hexadecimal notation (00-FF, which is equivalent to 0-255). For example, black (0,0,0) converts to 000000, white (255,255,255) converts to FFFFFF, and a full blue (0,0,255) converts to 0000FF. A web search will turn up plenty of conversion utilities. One example is easycalculation.com.  (Back to top)

Can you derive CIELAB values directly by visually inspecting the reflectance curve? (454)
With experience, one can learn to estimate CIELAB values for various illumination conditions by viewing spectral reflectance curves. However, this could only be very approximate. The actual values are computed from the spectral reflectance curves multiplied by a standard illuminant and weighted by the color matching functions. Finally a nonlinear transform is applied to obtain CIELAB values. While there is a direct computational relationship, it is not easily apparent from visual inspection of the reflectance curves and there is certainly no way to visually derive accurate CIELAB values from the curves.  (Back to top)

Can you explain the mechanism of UV filter in color spectrophotometer, to get universal Whiteness values on all spectros. Is it a wedge filter or an attenuator filter. (453)
Generally the UV filter in a spectrophotometer is a cut-off filter that eliminates ultraviolet radiation incident on the sample below a certain wavelength (approximately 380nm). When the filter is in place, no UV radiation is incident on the sample and when the filter is removed, whatever UV is in the instrument's light source strikes the sample. For non-fluorescent materials, the resulting spectral reflectance measurements will be identical with or without the filter in place. For fluorescent materials, the measured reflectance factor (and therefore computed whiteness for white samples) will be higher when the UV energy is allowed through the system (no filter in place). Essentially, the UV filter allows a measure of the sample with and without fluorescent emission (approximately, since the filter might not remove all fluorescence, but just attenuate it).  (Back to top)

Does the color of light affect how much snow is melted? (452)
Not much since the snow is white and will reflect all wavelengths of light more or less equally. However if there are colored objects in contact with the snow, then the color of the light will affect how much they heat up and therefore how much they melt the nearby snow.  (Back to top)

Can human eyes see noise? Or, can our brain recognize what's signal and what's noise? (451)
Yes, the human visual system can see and perceive noise, both noise internal to the visual system itself and noise present in stimuli. We have no problem recognizing signal from noise in most situations. Keep in mind that "signal" is often defined simply as the part of the stimulus we are interested in.  (Back to top)

I work in printing. Can you tell me what the effect of dot gain is on delta E? Also, can we get an accurate delta E measurement from a coated sheet to an uncoated sheet? (450)
Dot gain has an effect on the color of the print and this will effect the measured Delta-E, but there is no direct relationship between the two. The measurement of Delta-E is independent of whether or not there is dot gain. If the dot gain happens to bring the printed color closer to the proof, then the measured Delta-E will be smaller (and vice versa). You can accurately measure Delta-E across changes in paper surface. However the perception of the color differences will depend on the viewing geometry (due to the gloss differences) and that might effect the correlation between visual assessment and computed Delta-E. It is important that the measurement geometries (location of light source and detector) and the viewing geometries (location of light source and observer) are similar.  (Back to top)

What colors absorb more heat, when a piece of ice is sitting on the piece of paper on a hot and sunny day? (449)
Dark ones.  (Back to top)

How specifically does a magenta-dyed silver halide crystal absorb green energy? I know it has to do with wavelength ... is magenta somehow 'dovetailing' with green light to chemically interact? (448)
First of all, in the fully-developed film (or print) the silver halide crystal has been removed and all that remains is the dye. The magenta dye is magenta because of its spectral absorption properties. It is the fact that it absorbs green light that makes it look magenta. The reason it absorbs green light is due to its chemical structure. Essentially an allowed and stable energy level transition in the magenta dye corresponds with the energy of wavelengths that are perceived as green and those wavelengths can be absorbed.  (Back to top)

How do I figure out what color a reflection of another color might be? For example, if I have a yellow car and something red is reflecting off of it, what color will show up as the reflection on the car? (447)
The process of reflection is properly modeled with a mathematical multiplication in this case. If you know the spectral power distribution of the red light striking your yellow car, then the resulting light would be the product of that distribution with the spectral reflectance of the car. In color terms this would work like subtractive color mixing. Your yellow car is absorbing blue light and the red light is already missing green and blue. Thus when the red light strikes the yellow car, no more light will be removed and the reflected light will be red (approximately since it will depend on details of both the red light and yellow car).  (Back to top)

What effect renders UV lamp when selecting the colour with use light booth? (446)
Generally the UV lamp in a light booth is used to check for the presence of fluorescence in a sample. If the sample is viewed under the UV lamp (only) and appears to glow in some color other than a deep violet, then it is fluorescent (absorbing the UV and emitting at different wavelengths). The UV lamp is also used with the daylight illumination in the booth to enhance the UV content, making it more like real daylight, to better render the appearance of fluorescent materials in daylight. It won't make any difference for non-fluorescent samples.  (Back to top)

Which will melt faster, blue Popsicles or red Popsicles? (445)
In the dark, they should melt at the same rate. If illuminated it would depend on the color of the light.  (Back to top)

What would be the ideal color for drafting rooms or any rooms used by art students? (444)
Really, the ideal color is one that makes the students most comfortable and therefore able to focus on their work. However, that answer is likely to vary significantly from person to person. From a color science and perception perspective the best color would be a neutral gray and generally a light gray is preferable. The reason for this is that the gray will not bias color perception for any other colors being viewed or used in the artwork. This is why standard color viewing booths have a light gray interior. However, making a room completely gray might be a little too drab and cause students to not enjoy being in the room. So a little bit of color in the room is probably helpful. It's probably best to just avoid an overall bias toward a single highly chromatic hue.  (Back to top)

What is a full-spectrum paint and what is it good for? (443)
The term "full-spectrum paint" makes no technical sense, but an internet search turned up some paints being marketed with this term. Apparently full-spectrum paint is paint in which no black pigments are used to create the final color mixture. A given paint color could be formulated from a mixture of white, black, and one or two chromatic pigments or from a mixture of several chromatic pigments with white. This later mixture is what is being marketed as "full-spectrum" paint. There is probably little practical difference between the two methods for producing a given color. However, the paints would most likely have different spectral reflectance properties (neither is more "full" than the other) that would make them behave differently when the color of the illumination is changed or when the paint reflects upon itself (as in the corner of a room). These differences are likely to be very subtle and go unnoticed by most people. It's just a different way to formulate certain colors and full-spectrum paint is good for the same things as regular paint, making our interiors more interesting and enjoyable (or not).  (Back to top)

Is there a concordance listing the ISCC-NBS color name for each chip in the Munsell Book of Color? (442)
"The ISCC-NBS Method of Designating Colors and a Dictionary of Color Names" provides charts that allow one to look up the ISCC-NBS for any Munsell designation and then to look up synonymous color names. The book is out of print, but can often still be found in used bookstores or on the internet at places like bookfinder.com.  (Back to top)

When visually comparing colors, why is D65 light source better to use than Illuminant C? (441)
A D65 simulator isn't necessarily better than an illuminant C simulator. For evaluating fluorescent materials, D65 might be better since it has UV content that more closely represents real daylight. However, you would need to be sure that the D65 simulator you are using truly has that UV content. The best choice for visual evaluation is always the light source that most closely approximates the standard illuminant data used in your colorimetric computations. So if your colorimetry (XYZ, L*a*b*, etc.) is computed using illuminant D65, then view samples under a D65 simulator. If you only have an illuminant C simulator to view samples, then it would probably be best to do your colorimetry with illuminant C.  (Back to top)

We are looking at colour and redness of scars on human skin. Which is better for the job, a hand held spectrophotometer or a hand held chromameter? Some literature validates the use of a chromameter CR300 in skin research, but spectrophotometers are apparently more accurate. (440)
You are correct that spectrophotometers are generally more accurate and flexible in the colorimetric values they report. However, since you will be measuring a fairly limited range of colors, a colorimeter will likely provide you with sufficient data and accuracy. So, a spectrophotometer would be better for the job, but the colorimeter might be adequate if you are looking to spend a bit less on an instrument.  (Back to top)

Can you tell me the name of any software for changing a gray image into color. Is any software to convert a real time gray video to color? (439)
There are a number of software packages and utilities that can "colorize" images and video. A google.com search with the keywords "colorizing" and "software" will provide you with quite a list of possibilities. From a color science perspective, this process is not uniquely or correctly defined. It is simply an artistic process to associate arbitrary colors with objects in the black and white images. You would need information beyond that available in the black and white images alone to make accurate color images.  (Back to top)

I can get x,y,z color coordinates. So how would one relate to the materials response for a particular wavelength? (438)
First of all, given the values in your question, I think you are referring to CIE tristimulus values, XYZ (upper case), and not CIE chromaticity coordinates, xyz (lower case). The values mean very little without more information. For example you need to know the illuminant, standard observer, and measurement geometry used in their computation to define the numbers. With that information (or similar information if it is a self-luminous stimulus) you could compute CIELAB coordinates that would give you some indication of the color appearance of the stimulus. However, you have asked a more complicated question. You cannot uniquely define the spectral properties (usually reflectance) of a material from its tristimulus values. This is really the fundamental definition of metamerism. There are many spectral curves that would map to the same sets of tristimulus values. You can't go backwards in the computation. There is an exception to this when you have a limited number of colorants with which to produce the stimulus. In such cases you might be able to find a one-to-one mapping between tristimulus values and spectral reflectance.  (Back to top)

How can I calculate hue angle from CIELAB? What is pi? (437)
Hue angle is computed as the angle between the line that connects the a*b* origin with the stimulus a*b* values and the positive a* axis. It is computed as the inverse tangent of b*/a*. Since most programming languages will return the computed angle in radians, it then becomes necessary to convert to degrees. The conversion from radians to degrees is to multiply the angle in radians by 180/pi. pi is the numerical value of the ratio of the circumference of a circle to its diameter (approximately 3.14159).

Another way to look at this as a conversion from rectangular coordinates L*a*b* to cylindrical coordinates L*C*h (lightness, chroma, hue angle) in the CIELAB color space.  (Back to top)

Is there something like color adaptation? I was wearing a blue light filter over one eye and felt that the differnce in color perception between the right and left eye disappeared after some time. When removing the filter, color perception in the previously uncovered eye felt like wearing a blue light filter for a short period of time. (436)

What is the RAL number that should be used in the background of a light cabinet? (435)
I don't have any way to convert from colorimetric quantities into RAL numbers. However, the most common color for light booths is a neutral gray with Munsell Value 7 (a light gray). In CIELAB that would translate into an L* of 70 and C* of 0 (hue is undefined for neutrals). You might be able to index into some RAL notations using the CIELAB values.  (Back to top)

If someone walked down the street, what color clothing or facial features would that person have for me to easiest remember him/her? (434)
I think that would depend on the observer somewhat. In general, the most memorable person would be the one that stands out the most. In other words, colors that contrast most with the background and with other people on the street are most likely to be noticed, and therefore remembered. (e.g. If everyone is wearing gray, then the one person in red might be best remembered. However, if everyone is wearing red, then the one person in gray will be the one to stand out.)  (Back to top)

Does age and sex determine what color car a person buys? (433)
Not to my knowledge in any set physiological way. However, societal/cultural norms and trends will always influence preferences, which in turn influence purchasing decisions. Also the manufacturers dictate these choices to a degree by the colors that they make available for the various models.  (Back to top)

We are a printing company. We print offset (litho), gravure and flexo, both on coated and uncoated paper. What spectrophotometer should we buy spectrometer for Quality Control and Color matching? (432)
0/45 spectrophotometers are essentially the "standard" in the printing industry. There are probably two main reasons for this. First, the 0/45 geometry is best for comparing the colors of materials with various levels of gloss (common with different paper types) and secondly, the standards for densitometers in the printing industry also use the 0/45 geometry. In your case, it seems like the clear choice is to go with 0/45 instrument.  (Back to top)

I am doing a science project on what colors are easiest to see in the dark. Can you help me figure out what experiment I should do? (431)
There are a variety of experiments you could do. I would suggest two things to consider. One is the visibility of the colors in comparison to how visible they are in daylight illumination and the second is contrast of the colors with their background (really that is the definition of visibility). For example, you could select several different hues (say red, green, yellow, and blue ... more if you like) that look like they are equally bright under daylight illumination to use in the experiment. Construction paper or paint samples from a hardware store make good inexpensive color samples. You would then want to compare their visibility in the dark to that in daylight. This is where contrast comes in. Some colors will be more visible against a black background and others against a white background. It would be interesting to try all the colors on each background. If you mount the different colors on white and black backgrounds and then view them in a very dark room, I think you will find some interesting results regarding which colors are easiest to see. Good luck with the experiment.  (Back to top)

I want to purchase an LCD HD TV. But I can't decide between 2 TVs. One boasts 3.2 billion colors and the other 16.7 million colors. Do I care? (430)
The human visual system is not capable of distinguishing 3.2 billion, or even 16.7 million colors in a single display. (In fact, there aren't enough pixels on the display to show all the colors at once!). However, that doesn't mean that the capability to display more colors isn't helpful in producing better image quality. For example, if you were watching a DVD with a dark scene that had a lot of subtle gradation you might well see some unnatural contouring on a TV capable of only 16.7 million colors. The more expensive TV with finer color resolution might well produce a significantly better image. It is not so much the total number of colors that matters, it is how they are used. You also might be less likely to see the difference in a store since viewing conditions are not optimal. However that difference might show up when you get the TV home. If it were my decision to make, I would spend the money and get the better color resolution.  (Back to top)

Does the color of water effect its heating rate? (429)
The heating rate of water should only be influenced by color if the water is being heated by the absorption of light (darker colors will absorb more light and convert that into heat). However, if the water is being heated on a stove for example, then the color of the water in the pot should have no influence.  (Back to top)

I capture video camera RGB values through a microscope for image analysis of translucent microfossils. If I repeat this using a different camera I get different RGB readings. Are there any translucent standards available that I can use to calibrate between the two setups? (428)
You have come upon the fundamental different between device-dependent color (e.g. RGB) and device-independent color (e.g. CIE XYZ or CIELAB). Unfortunately there is not an easy solution to your problem. Video RGB is simply not standardized to the point of providing consistent color measurement results across various cameras. Your best bet is to stick with using a single camera (or single manufacturer and model if you need more than one) to keep your measurements as consistent as possible. Barring that, you might try to make a simple linear transformation between images from the two cameras in order to approximately equate the data. This could be derived from images you collect on samples that you think are relatively uniform and stable. However, there is no guarantee that the signals from the two cameras will be linearly related to one another.  (Back to top)

I would like to ask if colour filters use to photograph - the filters we use to put in front of a lens in a camera- is a subtractive method? (427)
Additive and subtractive methods refer to how colors are mixed, not the properties of a single stimulus (like a filter). Photographic filters work by absorbing light (some wavelengths more than others if they are colored). That is the simple physical process that gives a filter it's color. If you were to stack two or more filters together, then you would use the principles of subtractive color mixing to predict the color of the combination. If instead you were to project spotlights through each of the filters individually and then superimpose the spotlights, you would use the principles of additive color mixing to predict the resulting color.  (Back to top)

Where can I purchase a "student model" Munsell Color Tree? (426)
I am not aware of a student model of the Munsell Tree, but you might contact GretagMacbeth, the company that sells Munsell products to inquire. There is another product called "The New Munsell Student Color Set" that includes a number of hue pages, but not in the form of the 3D Munsell tree. It is a very useful and helpful resource and might be what you are looking for. It is published by Fairchild Books (no relation) and can be found here.  (Back to top)

I need to measure human skin color directly. What instrument should I use? (425)
There are a number of small, hand-held spectrophotometers available on the market that would be very capable of making the measurements you need and reasonably priced. To find the best instrument for you and your budget, you would need to contact the manufacturers. A number of them (such as GretagMacbeth, KonicaMinolta, X-Rite, Hunterlab, BYK-Gardner, Datacolor, etc.) are linked on our sponsors web page.  (Back to top)

I am building a color & B/W headshot photo lab. My primary light source is 5000K fluoresent tubes with a CRI rating of approx 93. Since skin tones are critical, what color should I paint the walls? (424)
Since you have taken great care to specify and install high-quality illumination, then you don't want the walls to change the effective color of the illumination. That would indicate choosing a neutral gray for the wall colors. Then you need to decide how light or dark to make them. If you want the illumination to be strictly from your light sources, then make the walls darker, or even black. If you want a little more diffuse ambient illumination on your subjects, then make the walls a lighter gray. Either way, it is probably best to stick with neutral wall paint.  (Back to top)

How do yo measure ΔE*cmc, ΔE*94, ΔE*fmc2 from the L*, a*, b*, C*, h* ΔH*, ΔE*? (423)
CMC and CIE ΔE94 color difference values are computed from the CIELAB values you mentioned. The FMC2 color difference equation pre-dates the CIELAB color space and is based on earlier CIE colorimetric coordinates. I would recommend the texts "Principles of Color Technology, 3rd Ed." by Berns and "The Measurement of Appearance, 2nd Ed." by Hunter and Harold as good places to learn more about the history of these equations and the equations used for computation.  (Back to top)

How can people distinguish edges of objects? Is this a function of rods or cones, or some organs else? (422)
We detect the presence of edges when the light detected by our rods and cones changes abruptly across space (location on our retina) or time (due to our eyes moving). These changes in light intensity are what produce the signals ultimately transmitted to our brains and interpreted as objects (defined by their edges). So, yes, it is a function of our rods and cones with our cones being a little bit better at it than rods.  (Back to top)

I have a mapping website. Is there any function or algorithm that produces a collection of discrete colors that all have some minimum level of perceptual contrast between each other? (421)
I am not aware of any systematic way to produce such a list of colors. You might be getting up to the point where you are simply trying to produce too many distinguishable colors. You might start with the Munsell system and use equal steps in Value (lightness from 1-10) and then select a number of hues and also trace out those Value steps at the maximum chroma. I would suspect you could reasonably generate 100 colors that way (10 grays and then 10 each of 9 different hues). GretagMacbeth (who sells the Munsell system) provides free software for the conversion from Munsell into other coordinates that might be helpful (the page is linked in several places on our FAQ).

You might also look at Cindy Brewer's work and on her linked personal research page, that describes strategies for selecting colors for maps. I don't think her examples get up to 100 colors, but they still might be helpful.  (Back to top)

When measuring the color of colored plastic plaques with a spectrophotometer, how thick should the plaques be and why? (420)
Ideally you would like the plaques to be thick enough that they are completely opaque (no light passes through them). This way it is only the plaque itself that influences the color measurement and not whatever happens to be behind it. If it is not possible to use opaque samples, then be sure to always be consistent in how the samples are backed (e.g. with a black object) when measurements are made.  (Back to top)

On what things color depend so that only some colors disappear while others remain? (419)
To quote the CIE, "perceived color depends on the spectral distribution of the colour stimulus, on the size, shape, structure and surround of the stimulus area, on the state of adaptation of the observer's visual system, and on the observer's experience of the prevailing and similar situations of observations."  (Back to top)

Are there metrics that quantify whether items in an image are discriminable or identifiable? (418)
I am not aware of formal definitions, but there does seem to be some common practice. Discriminability refers to the observers ability to detect objects in the image and determine that they are indeed different from one another. Identifiability would imply that the observers can also identify just what the objects are. Another term sometimes used in perception is detectability, which implies that the observer can tell that something is present, but not discriminate it from other objects, or identify it. Overall performance of an imaging system would have to be defined based on the application of interest and some metric the capability for the images to assist in that task.  (Back to top)

Where can I buy Baker-Miller Pink? (417)
I am not aware of any specific supplier of Baker-Miller Pink. Previously it was believed that only a certain shade of pink had the desired effect. Further research showed that the effect was short-lived and, in some cases, the pink could cause more distress upon prolonged exposure. I suspect that's why there aren't specific suppliers of paint in that color. This web page provides some information. Some say it is the pink of Pepto-Bismol. If you really want some paint that color, I would suggest looking at paint chips to pick the one closest to the desired color, or finding another material in the desired color and taking that to a paint store to be matched.  (Back to top)

I am doing a problem on image permanence for large format inkjet. Should I use Delta E or Delta E 2000? Why and what is the difference between the two? (416)
Using either the simple CIELAB Delta-E or CIEDE2000, would be appropriate for your work. The CIEDE2000 equations incorporate some corrections for nonuniformity in the perception of color differences across the CIELAB space. Thus, if it is important for you to compare the magnitude of a color differences for widely varying colors, CIEDE2000 might be a better choice. However, if you are simply looking at quantifying change in the samples and you will always be comparing like colors (e.g. red changed by so much for one technology and a different amount for another technology) then the simpler Delta-E equations will be more than adequate. You really can't go wrong with either. If I were to choose, I would use the simple Delta-E equations for this application since the added complexity of the CIEDE2000 equations are not likely to provide any additional insight.  (Back to top)

Could you help me convert the Munsell codes to RAL codes? (415)
I'm afraid not. We do not have the capability to convert from any color system into RAL codes and it is not clear that any utilities for the conversion exist. GretagMacbeth does provide free software for conversion between Munsell and other common color systems.  (Back to top)

Will we ever be able to see colours we have never have seen? When I tell other people about my wonder to have different perceptions they think it is bogus. Will genetics and medicine make it possible in the future? When? (414)
This is a fascinating question. Essentially, in order to have different color perceptions we would need to have a different visual system. That could mean photoreceptors sensitive to different wavelengths or different types of photoreceptors and processing sensitive to the same visible regions of wavelength. There are certainly examples of animals that have very different color responses (e.g. birds and insects), so it is physiologically possible. There would have to be some evolutionary advantage for such capabilities to develop in humans and it is probably unlikely such changes would happen anytime soon. However, you never know.

There are certainly examples of medication that can give us illusions and perceptions beyond normal experience, but probably not with the control required to give us meaningful new color perception capabilities. Of course there is always technology and imaging systems such as infrared, x-ray, etc. give us ways to extend our visual system's capability right now.

If you are curious about possible differences in human color vision, do a web search on "tetrachromatic females" and explore the results a bit.  (Back to top)

Why is it more difficult to say the color of the word when it is printed in a different color? (413)
This is known as the Stroop Effect. An internet search on "Stroop Effect" will turn up plenty of examples and explanations. This page aculty.washington.edu has an interactive experiment and explanation. Briefly, the effect is due to interference between your perception of the words and the colors of the letters. Apparently the words are either processed more quickly or more easily in our brains than the colors are.  (Back to top)

We always measure the color difference by using d/8 spectrophotometer with included specular reflectance. But why do we always measure the white with excluded one ? (412)
I don't know. Normally an instrument will have calibration values for the standard for both specular included and excluded. A more normal procedure is to set up the instrument in the desired configuration and then calibrate with the tile. It might be possible that your instrument can be calibrated in one geometry and used for measurements in the other, but I would encourage you to use the same setup for both the calibration and measurements if that is possible.  (Back to top)

I have an emission spectrum from a sheet of luminescent material. Can you help convert the emission spectrum to an RGB color to display? (411)
The easiest way is to first convert your spectral curve into CIE XYZ tristimulus values (see texts such as Berns' "Principles of Color Technology, 3rd Ed." for details) and then convert those tristimulus values to default RGB values such as sRGB (see w3.org). You could do better by having a specific calibration for your display to convert XYZ to accurate RGB values for your display, but the sRGB values would be a simple, and commonly used, approximation.  (Back to top)

Can white light be captured inside an opaque container. If not, why? When the source is disrupted it becomes dark. why? (410)
Theoretically, yes! Unfortunately the container would not only have to be opaque, but a perfect reflector (reflecting 100% of the incident light) on the inside. If the inside was anything less than a perfect reflector, then there would be some probability that light would get absorbed each time it strikes the surface. Since light is traveling so quickly, it wouldn't take long for all the light to get absorbed. Unfortunately there are no such materials to build the container from and, even if there were, you'd have to figure out a way to remove the light source from the container and seal it (so that it was perfectly reflecting on all surfaces) before the light escaped from the opening. That would be quite a challenge indeed. Also, once you opened that theoretical container the light would all escape instantaneously so that the most you might see is a very brief flash.

Real materials quickly absorb all the light when a light source is extinguished or blocked.  (Back to top)

What is the best color model for use with colors that have a low saturation? CIELAB is not working for me because when a* and/or b* are close to zero I get poor values for hue. (409)
I assume that by poor hue angles you are suggesting that the CIELAB hue angles do not match the your perceived hue angle. I am not familiar with any deficiencies win the CIELAB equations that cause systematic problems with hue angles for near neutrals. There are two difficulties that you might be running into. One is that the actual neutral for your viewing conditions is slightly different from that computed by CIELAB. A small shift will produce very large discrepancies in hue angle (as much as 180 degrees). Another might be observer metamerism. This is the difference between your visual response and that of an average observer represented by the CIE standard colorimetric observer. That would also result in significant sensitivity for near neutrals. I would suggest sticking with CIELAB, but to use the a*b* dimensions to evaluate the relative redness-greenness and yellowness-blueness of your colors rather than using the C*h dimensions. If you see a systematic trend that your perceived neutral is always at a given set of a*-b* values, you could subtract those values from the other a*-b* pairs prior to computing chroma and hue angle to make an adjustment for your purposes.  (Back to top)

Can you hep me match the PPG paint color "burnt copper" with a CMYK color so I can print material that match the paint on my motorcycle? (408)
I'm guessing from the paint name and application that this is a metallic paint (on with some metallic flakes in it to give some nice angle-dependent effects on the bike). Such paints can't be reproduced with a simple CMYK value since their appearance changes with angle. If you picked one CMYK value then you would end up with something that would look like a fairly boring brown. Companies like PPG make their marketing material by coating color cards with actual paint samples to illustrate the angle effects. In other printed material, or on their web page, you will notice that they show the paint on three-dimensional objects (either simple shapes or finished products) to communicate the color effectively. So, the best I can offer you is to reproduce some images of the bike for your marketing material and perhaps do some visual matching for other artwork.

Even for simple colors, there is probably no conversion between specific paints and CMYK or Pantone colors. If there is, you would probably have to get that from the paint manufacturer. Sorry I can't be more helpful.  (Back to top)

I have to purchase a system to compare shades of yellow in plastics with rough surfaces. Would you please recommend a company and/or instrument that would have good repeatability and be low maintenance (407)
There are a number of manufacturers of color measurement instrumentation that can provide instruments capable of making the measurements you describe. We don't have the data, or the access to all current instruments, that would be required to make a recommendation of a particular manufacturer or model. Links to a number of instrument manufacturers can be found on our Sponsors page.  (Back to top)

Are RAL 7032 and ANSI 61 are same colours? Where I can I locate a colour cross reference table between RAL & ANSI standards? (406)
The ANSI colors are defined by Munsell notations and standards that are available from GretagMacbeth (see gretagmacbethstore.com) and I am not aware of any direct conversion between those designations and the RAL numbers. Unfortunately all I can suggest is an inquiry to GretagMacbeth. Perhaps they can help you sort this out.  (Back to top)

Why are there chromaticity values of zero or less than zero in the Munsell data on the MCSL web site? (405)
There are such values in the file all.dat. As explained on the Munsell renotation data page, this will contains unreal colors. That is, colors whose chromaticity coordinates lie outside the Macadam limits. One such color: 2.5PB 0.2/20 is shown at the lower end of the red line in the diagram below. The unreal colors are only useful if you want to interpolate color that lie very close to the Macadam Limits. Note that for this very dark color, value=0.2, the spectrum locus is essentially the same locus as the Macadam Limits at this value.
(Back to top)

How do I measure contrast? (402)
Contrast is normally measured using the luminance dimension of color (the Y tristimulus value) or a transform of that to better predict perceived lightness (L* in the CIELAB space). Generally, regardless of color, the greatest visibility is when there is the most luminance (or lightness) contrast. The point at which contrast can be noticed is a determination of the contrast threshold. Much has been published on contrast thresholds and there is no simple answer. The amount of contrast required for detection depends on the configuration (size, shape, flicker, etc.) of the stimulus. In the best cases, it can be as low as a .1% change in luminance. In other cases it might require 10% or more. Unfortunately I can't give you a simple answer. In terms of CIELAB L*, people generally say that a difference of 1.0-2.0 units is just perceptible for relatively large stimuli.  (Back to top)

Do the tiles in the Farnsworth Munsell 100 Hue test kit require calibration? If so, what can change that would require their recalibration? If they require calibration, what is the frequency? (401)
To my knowledge, and in my experience with our F-M 100 Hue tests, they are very stable with respect to color change. Our very old tests appear to provide results consistent with out new tests. (That said, note that the manufacturer of these tests, GretagMacbeth, certifies the caps for a period of two years.) The color chips could fade over several years if left exposed to light, but that would be rare for F-M 100-Hue chips since they come in boxes, and should be stored there when not in use. More likely of concern than fading of the colors would be simple wear and tear damage from the handling of the chips. Sometimes they get dirty or damaged by finger prints. When such damage becomes noticeable, it is likely time to get a new test. The tests, and more information on their stability, are available commercially from GretagMacbeth. They produce and sell all Munsell products and are not associated with our laboratory (we're just named after the same person).  (Back to top)

Does there exist a (known) correlation between a Minolta CM3500D and a BYK Gardner or does this depend on the product type? (400)
Theoretically, instruments of the same measurement geometry from different manufacturers should provide similar results. However there remain differences in the details of the optical configuration and calibration procedures that will likely result in differences in measured values. In general, the best inter-instrument agreement is obtained by using instruments of the same manufacturer and model. Obviously, this is not always practical. There are some techniques (described in Berns' "Principles of Color Technology, 3rd Ed.", Chapter 3) to evaluate the accuracy of instruments and then correct measurements from various devices to improve agreement.  (Back to top)

What are "suprathresholds" and "thresholds," with reference to color difference? (399)
"Suprathreshold" refers to stimuli that are clearly visible. In the case of color differences, it would refer to a pair of stimuli in which the difference was clearly perceptible. "Threshold" on the other hand refers to color differences that are just barely perceptible. The magnitude of a color-difference threshold depends on details of the viewing conditions, stimulus configuration, and color in question.  (Back to top)

Can the Munsell 7GY3.29/1.5 Green be converted into a ANSI number? (398)
There are a set of ANSI colors that are defined in terms of Munsell notation (with standards sold by GretagMacbeth. However it appears that each ANSI number is simply a name with a Munsell designation and there would be no way to go in the opposite direction for a conversion (unless one happened to match up perfectly).  (Back to top)

What color will a solution that absorbs all the green, yellow and red light appear? (397)
The simple answer is blue. The more complex answer is that appearance will also depend on the lighting and other viewing conditions. However, simply looking at subtractive color mixing, if you have absorbed all the green light and all the red light, then you are left with blue (generally people divide the yellow into the green and red regions, but since you said that was all absorbed as well, the answer wouldn't change).  (Back to top)

CIE chromaticity diagrams are usually exhibited in a shape of a hoof. Why are the single waves spread on the ellipse side instead of the straight? (396)
The shape of the chromaticity diagram is an artifact of the shapes of the XYZ color matching functions and the projective transformation to xy. The curved part represents the gradually changing responsivities of the three cone types in the visual system as we sweep through the spectrum. It is curved due to the overlapping sensitivities. Near the long-wavelength end of the spectrum it becomes a straight line since only two visual responses are active for those wavelengths and not three. The purple-line (the straight line connecting the red and blue end of the spectrum) is simply derived from the additive mixture of the two ends of the spectrum, which is visible and has to fall on a straight line due to the laws of additive color mixture (Grassmann's Laws).  (Back to top)

Do you know the RAL Number for a cream colored paint known as Queen Ann or Magnolia? (395)
No, I don't. Color names are a very imprecise description. The best I could recommend is to find a sample that closely matches what you are looking for and then take that to your paint dealer and ask it to be matched.  (Back to top)

Of papers colored blue, white, red, green, purple, yellow, and black, which would be easiest to remember things if they all had black letters (except the black paper, which would have white letters)? (394)
I am not aware of any research on remember things written on various colors of paper. However, you are probably most likely to remember something that is most legible and the most legible text is that which contrasts most with the background. Thus, white on black, black on white, and black on yellow are likely choices. Black text on red, green, blue, or purple paper would be harder to read because the paper itself is likely to be significantly darker than white or yellow paper.  (Back to top)

Why do flat black objects get hotter than lighter colors? (393)
This is addressed in a number of the answers on our FAQ. A black object is black because it absorbs most of the light that falls on it (light colors absorb very little light). All this absorbed light is converted into heat and therefore the black object heats up more than lighter colors.  (Back to top)

Which color model is best suited for tonal changes (i.e. curves) without changing the perceived color (hue, saturation; however defined)? (392)
It would be best to use the CIELAB color space (LAB in Photoshop) to do these manipulations on the L*, or lightness, channel. This space is the most likely to give you perceptually meaningful results and changes in images without changing hue and chroma.  (Back to top)

Suppose that we want to measure the difference between the color of a test object and a reference object. How would I choose between using RGB color space and CIE Lab for color difference? (391)
RGB is a device color space and will vary from device to device. The values have no perceptual meaning and the meaning of differences between them will vary from device to device. CIELAB is a color space that describes color in terms of human perception independent of the device used to create the color. Color differences in CIELAB have perceptual meaning. So, if you want to know about perceived color, rather than an arbitrary color designation, you need to use CIELAB.  (Back to top)

Can I determine the color rendering index of this source purely from the light-emission spectrum? (390)
Yes. You can compute the CIE color rendering index from the emission spectrum of your source. The technique is described in CIE Publications 13.3 and 15:2004. (See these CIE pages for more information.  (Back to top)

We capture colour images printed on paper through a CCD camera. How can we eliminate the scattering effect of the paper and to account for optical dot gain? (389)
Essentially you can't. The scattering and optical dot gain happen before the image is captured and there is no going back. You could try some image enhancement in a program like Adobe Photoshop, but that would be an aesthetic judgement on success since getting back to original CMYK data used to make the print would depend on far too many variables (ink, paper, separation algorithm, camera sensors, etc.) to make it feasible to automate.  (Back to top)

Is there a chemical compound for invisible uv red ink compound. How we can get some or make our self? (388)
I have no idea what an invisible UV red ink would be. If you mean an ink that is invisible, but fluoresces red when illuminated with UV energy, then you need to find a manufacturer of such a material. A web search on "red fluorescent ink" turned up this site riskreactor.com  (Back to top)

I want to create 3D look up table for CMYK data using Matlab. Can you help? (387)
I would recommend you open the "Help" window in MATLAB and type "ICC" in the search field. It will tell you about several functions in MATLAB that provide built-in support for ICC color management. You could then implement your look-up table as an ICC profile.  (Back to top)

Are there any objects that can physically reflect UV light to be able to be seen with the unaided eye? (386)
Since we cannot perceive UV light directly, the only mechanism by which an object can convert UV radiation into visible light is fluorescence or phosphorescence. Thus there are no objects such as you describe.  (Back to top)

Can you provide an example showing how to calculate ΔE using Δa*, Δb*, ΔL*, ΔC*, Δh for textile pigment? (385)
A color difference, ΔE*ab is computed as the Euclidean distance between the CIELAB coordinates. Mathematically, that is ΔE*ab = (ΔL*^2 + Δa*^2 + Δb*^2)^(1/2). I would recommend a basic colorimetry book such as Berns' "Principles of Color Technology, 3rd Ed." for more details.  (Back to top)

I want to see what make-up shades and colours suit which skin tone, for a school project. Please get back to me with a colour wheel and and some useful information. (384)
I'm afraid this is outside the normal realm of color science. Proper cosmetics colors for various skin tones are a matter of personal preference and a variety of opinions. I would suggest that you look at some of the tips and ideas that can be found on the websites of various cosmetics manufacturers. Sorry I can't be more specific.  (Back to top)

I'm a wood scientist. Can you help me convert values from NCS to CIELAB? (383)
I am not aware of any publicly available conversion utilities. NCS does sell software for the conversion ncscolour.com, but it is rather expensive. There have been some papers written on the topic (e.g., N.S. Smith, "A Colour Conversion Program Revisted", Color Research and Application 17, 405-409 (1992).) that might get you started to learn about what has been done. I'm sorry I can't give you a better answer.  (Back to top)

Should ideally conditioned standards (kept humidity controlled, dark envelope, minimum exposure to light, minimum handeling) develop metamerism? (382)
I've never heard of cases of metamerism developing over time, but there is no fundamental reason that it wouldn't be possible. If two samples were colored with different colorants, it is feasible that the colorants in one could change over time differently from the colorants in the other. That could result in more significant metamerism. However, if both were made of the same colorants, then one changing should only result in a simple color mismatch (one changed, one didn't) and not metamerism.

How long samples last in a controlled environment is really impossible to say since it depends both on the material and the colorants. Again, I wouldn't expect metamerism to develop where none existed before, but the magnitude might change over time.  (Back to top)

I have a Munsell paint specification of 7GY3.29/1.5. My paints use IF numbers. Can you help me transform this request to an IF number? (381)
I'm sorry to say that I am not familiar with the "IF" paint designations that you mentioned and I couldn't find any information on them. I can tell you that the Munsell designation is for a fairly dark color. A middle gray has Value 5 and black has Value 0. The desired color at Value 3.29 would look fairly dark, but not black. It is also low in Chroma. A Chroma of 1.5 is very nearly gray. So you are looking for a green that is fairly dark and close to gray. That might be enough to help you decide which of your two candidates is closer.  (Back to top)

Does the melting rate of a substance vary by color? (380)
Yes. If the objects are otherwise identical and exposed to light, then the color will impact the rate at which they heat up (and therefore melt if they get hot enough to melt). In general, darker colors will absorb more light and that light is converted into heat. In the dark, it wouldn't matter what color they are.  (Back to top)

Why does the spinning Benham top look colored? Is there any simple explanation of that phenomenon? (379)
There is not a simple explanation for the colors observed. Clearly they are based on some mechanisms in the visual system responding at different temporal rates, but the correlation with the patterns and simple assumptions about the rate of cone responses is not easily shown. The color probably depend on differences in higher level visual mechanisms as well. There is a short discussion with links here.  (Back to top)

How is a star's color different based on the temperature? (378)
Stars, like other objects emit light based on their temperature. For certain types of objects, called blackbody, or Planckian radiators, you can compute the spectral energy distribution (and therefore the color) of their emission based only on temperature. As these objects heat up, they are first dull red, then orange, then yellow, then white, and eventually blue. There is an equation known as Planck's equation that allows computation of the spectral power distribution as a function of absolute temperature (K). That equation is what let's us know that a blackbody at 10,000K will be bluish and one at 3000K will be yellowish. Since stars are very nearly blackbody radiators, this scale, known as color temperature, allows estimation of the star's surface temperature based on its color.

I hope this helps. This page This page has a little more explanation that might help get you started in exploring this topic more.  (Back to top)

What size print can I make with a 8 megapixel camera? If I print in high resolution (300dpi) and low resolution (72dpi). There is any formula to do that transformation from megapixel to inch? (377)
First, all 8 megapixel cameras are not created equal. You need to start by finding the actual number of pixels in each dimension of the image for your camera. I looked up the Nikon Coolpix 8700, which is an 8 megapixel camera. and found that the images are 3264 x 2448 pixels. If you multiply those two numbers together you will get the total number of pixels in the image, which is just under 8 million pixels (8 megapixels) in this case. Other 8 megapixel cameras might have slightly different numbers. Then to find the print size for a given print addressibility, you just need to divide the number of pixels in each dimension of the image by the pixels per inch on the print. So at 300 dpi, this camera would make images that are about 10.9 x 8.2 inches. At 72 dpi, it would be 45 x 34 inches, and so on. I like to use 200 dpi as a rule of thumb for print quality equivalent to typical analog photographic processes without visible artifacts on the print. This camera would allow you to print 16.3 x 12.2 inch images at 200 dpi. This is roughly equivalent to a very good quality 35mm negative. It's pretty safe to say you can make prints of 8 megapixel images that will match or exceed the image quality of those you could make with 35mm film. In most cases the image quality will be significantly better than consumer 35mm film/camera combinations (assuming you are using a high-quality printer ... preferably one that prints on photographic paper).

(Note that DPI, dots per inch, is often used to describe the number of pixels per inch, PPI, in printed output. Since, depending on the printing technology, each pixel on a printed image might be made up of many dots, it is reasonable to say that the DPI of the printer exceeds the PPI of the image. Thus, PPI might be considered more precise terminology for describing images independent of technology used to render them despite the common usage of DPI and PPI interchangeably in most situations.) That's not as simple a question as it first seems. There are several ways to determine complementary colors. Perhaps the most technically accurate definition of additive complementaries is in the CIE chromaticity diagram. You would need to have a monitor characterization to convert from RGB to CIE xy and a definition of a preferred white point in xy (the complementary is defined by the white you choose) and then you could find the complementary chromaticity. Even then, you would still have undefined variables of luminance (how bright to select for the complementary) and saturation (how far from the white) unless you decided that the complementary hues had to add together to make an exact white.

If you want to work in RGB, you can make a simple approximation by subtracting each of your RGB values from the maximum for white (typically 255). This would assume that your display is linear, which they usually aren't. If you can linearize (gamma correct) the display first, your estimates will be more meaningful. So if I had a red color with RGB of 200, 50, 30, then an approximate complementary could be 55, 205, 225 (a nice cyan color). That will at least get you in the ballpark and always keep you with a simple rule that adding the color and it's complementary sums to white (255, 255, 255).  (Back to top)

Can you help me compare RAL specification paint with ANSI paint for color match? (376)
The most direct, and accurate, way to compare the two specifications is to obtain a sample of each paint and have it measured colorimetrically. You could then compute a color difference, such as a CIELAB ΔE* to quantify the magnitude of the difference.  (Back to top)

Please can you explain what Macadam limits are (as mentioned in description of the 'real' set of Munsell values on your Munsell Renotation Data page)? (375)
The MacAdam limits are a definition of a theoretical maximal color gamut for object colors. In fact the MacAdam limits are far beyond any realizable object colors since they were computed, by David MacAdam, from theoretical reflectance curves that were equal to either zero or 100% at every wavelength (with a maximum of 2 transitions). Any color designations outside the MacAdam limits cannot possible be created with non-fluorescent object colors. Thus they provide a way to limit color spaces, or samplings of color spaces, to colors that might be realizable as objects. (Note that light sources can have colors that exceed these limits.)  (Back to top)

I am searching for paint color mixing recipes for liquid (bottle/jar) opaque water based pigment paints. I have found a book of colored paint mixing recipes for paste (tube) bodied paints with a volumetric proportioning system for only paste consistency (tube) paints: Color Mixing Recipes ; by Walter Foster Publishing Staff, (48 pages, Trade Paperback); 2005. Do you have any resources and/or pertinent leads that you could provide me? (374)
I have asked a number of people and found no good answer to your question. We are not aware of any books of paint recipes that would solve your two problems. I'm sorry I can't be of more help.  (Back to top)

I am a dental student. As part of my project I need to convert the RBG values obtained from Adobe Photoshop into CIELAB values in order to compare with values obtained with a spectrophotometer? (373)
Performing this conversion accurately requires knowledge of the specific RGB specification used. You can get a reasonable conversion in Adobe Photoshop itself (which will depend on the color settings of Photoshop to define the exact transform). Simply take an image in with the RGB values and change mode to LAB. (Under Image->Mode->Lab color.) That will give you CIELAB coordinates. You can also find both sets of coordinates in the Photoshop color picker. Just type in the RGB values and you will see equivalent LAB values for the current color settings.  (Back to top)

Can you explain how to select the color center (or sample color) which can be used to evaluate color difference formula? (372)
Various researchers use different strategies to determine the color centers. The most important objective is to be sure you have a variety of colors that reasonably sample the range of color space you are interested in. There are also 5 color centers that were defined by the CIE for coordinated research. You can read more about them in A.R. Robertson, "CIE guidelines for coordinated research on colour-difference evaluation," Color Research and Application 3, 149-151 (1978).  (Back to top)

I am trying to find a precise, systematic way to describe the colors of inks and metals used in medieval manuscripts. For example, I want to be able to define "red" as a range of similar colors and "scarlet" as a different range. (371)
I would recommend that you refer to the ISCC-NBS Dictionary of Color Names for a systematic technique to relate color specifications (in that case Munsell) to more commonly used color names. You can find a little information on it hare www.colorsystem.com. Unfortunately it is out of print, but you might be able to find a copy in the library and their appear to be some more online resources that might help if you do a Google search for it.  (Back to top)

Can you help me to discriminate between color space, color model, and color system? I'm confused! (370)
Your confusion probably arises from the fact that these terms are often used interchangeably.

According to the International Lighting Vocabulary, where such terms are defined, a color space is a manifold of three dimensions for the geometrical representation of color. That might not seem very helpful, but in general a color space is some mathematical representation of colors aligned in a geometrical (usually 3D) spatial structure. I would then add that color models or color systems (terms which are not in the International Lighting Vocabulary) would be names of specific color spaces. For example, the Munsell System defines a particular color space, the CIELAB system another color space, and so on. It is probably safe to say that "model" could be used interchangeably with "system" in this context although some might limit the use "model" to an actual 3D construction of the space with colored samples (as in the actual Munsell Book of Color). I would also note that it might be possible to have a color model, or color system, that describes color in some way other than using a space. For example a color system might just give names to color or values to indendent dimensions of color appearance with no intention of representing them as a 3D space.

For a lot more information and history on this topic, you might want to look at Kuehni's book, "Color Space and Its Divisions: Color Order from Antiquity to the Present".  (Back to top)

People of my area prefer reddish black on textiles. What kind of black do people in your area prefer on textiles? (369)
I have heard of people preferring either warm (reddish) or cool (bluish) blacks in various situations. However, I am not aware of any specific trend in that preference based on geography or application. At least in my experience, it seems that the preference depends on the person and situation.  (Back to top)

As we know black colours, for example on textiles, usually have a pale tint. Is there any formula for measuring blackness? (368)
I do not know of any specific blackness indices. I would recommend simply using the CIELAB L* metric to quantify how dark a black sample is. You can also use the C* metric to quantify how close to neutral the sample is.  (Back to top)

How can light affect an interior color? For example: what is the effect of natural light on a red wall, blue wall, etc. (367)
Unfortunately your question cannot be answered simply. The lighting has a tremendous effect on material colors, but they are not always predictable without detailed knowledge of the material properties and the lighting. For example one blue paint might still look blue upon a change from daylight to incandescent light and another might become purple. There is also a large variation within a given type of lighting (i.e. different phases of daylight, different types of fluorescent lamps, etc.). Simply put, color science can predict the changes that will be seen, but only when the specific material and light sources have been well defined and/or measured.  (Back to top)

Where can one buy an anomaloscope? Does an instrument exist that has a bipartite field like an anomaloscope but allows any combination of one or two monochromatic lights in each half of the field? (366)
It seems that most researchers end up building one themselves on an optical table. This would probably end up being more cost effective and flexible (given your desire for multiple wavelengths) than purchasing a commercial instrument. There are a couple available that we found with a web search. www.neitz.co.jp and www.oculus.de
Both appear intended for clinical settings and are likely quite expensive. Some searching on the internet and in the literature will turn up examples of simple instruments that others have built.  (Back to top)

What is colour? How is it detected? (365)
According to the official CIE definition in the International Lighting Vocabulary, color is an "attribute of visual perception consisting of any combination of chromatic and achromatic content. This attribute can be described by chromatic color names such as yellow, orange, brown, red, pink, green, blue, purple, etc., or by achromatic color names such as white, gray, black, etc., and qualified by bright, dim, light, dark, etc., or by combinations of such names. Perceived color depends on the spectral distribution of the color stimulus, on the size, shape, structure, and surround of the stimulus area, on the state of adaptation of the observer s visual system, and on the observer s experience of the prevailing and similar situations of observations."

Color is detected by the human visual system. The physical attributes of light and objects that produce the stimulus for color are measured with spectroradiometers and spectrophotometers. Correlates of our color perceptions are then computed from those physical measurements using the CIE system of colorimetry.  (Back to top)

Does color affect the reflection of heat? (364)
Does color affect the reflection of heat? Color mainly affects the absorption and reflection of light. Dark colors absorb a lot of light and convert that light into heat. Therefore dark colors tend to heat up more when exposed to light. Lighter colors reflect much of the light and absorb very little, thus there is little absorbed light energy that is converted into heat. A black object outside on a sunny day will warm up more than a white object.

There is more to the story however, infrared energy is sometimes referred to as heat and it could be possible for a black object to reflect a lot of the infrared energy and thus not absorb much of that heat. It would also be possible for a white object to absorb a lot of infrared energy and warm up more than the black object in the presence of another hot object (emitting infrared energy).

So, the general answer is that dark objects reflect less heat than light objects, but it is possible for their to be exceptions.  (Back to top)

Would CIECAM02 or CIEDE2000 be better at predicting spot color differences? (363)
Most of the research on predicting color differences has been aimed at optimizing equations in the CIELAB color space (such as the DE94 and DE2000 equations). Similar work is being done within the CIECAM02 color space, but there is not yet a recommended technique for performing the computations. There is certainly reason to believe that one will be developed in the future. For now, and for the application you mention, the CIEDE2000 equations would be the recommended technique. For a simpler equation, that is likely to perform just about as well, you might want to consider the CIE DE94 equations.  (Back to top)

How many colors does Munsell solid establish? (362)
The Munsell Book of Color is an embodiment of the Munsell system with approximately 1500 color samples. However, that is a limit of the pigments available and the sampling chosen. The Munsell system is not limited to any specific number of colors. It's definition extends well beyond that typically represented with painted samples and the intervals can be subdivided as finely as one likes.  (Back to top)

Can mixed paint be measured while it is still wet? In our factory we mix paint, we want to measure it's color before painting parts. (361)
Certainly paint can be measured wet (preferably with a non-contact instrument). However, for most paint the color will change significantly as the paint dries. Thus you will either have to learn how the colors change and compensate for that when you make wet measurements, or simply wait for samples to dry before measuring them. The best way to evaluate the color of finished paint is to create a sample as close as possible to the intended application and then measure it after the paint has dried.  (Back to top)

I would like to know if there is a way to determine the amount of ultraviolet light absorption by causing a change in the color of a particular substance and what the substance would be. (360)
You might be able to do this by using fluorescent or photochromic colorants. A fluorescent colorant absorbs UV and emits visible energy. The amount of energy emitted would depend on the amount of UV exposure. Photochromic colorants simply change color in the presence of UV and I would expect that the amount of color change could be quantitatively related to the amount of UV exposure. I am not aware of any quantitative systems based on these materials, but this should give you a starting point to search for them or think about making something. Some information about these sorts of colorants can be found in this article.  (Back to top)

I am trying to describe the colour of my sediment cores and have L*, a*, b*, and C* data from a spectrophotometer. I am looking for some directions on how to 'describe' the colour of my mud! Is there some tool or guide to convert numbers into a colour category? (359)
The CIELAB coordinates correspond approximately with color appearance descriptions so it is possible to make some assessment of categories directly from the values. L* is lightness with 0 being black, 100 being white and 50 being a perceptually middle gray. The a* dimension is redness-greenness with positive values indicating redness and negative greenness. Likewise for b*, which is yellowness (positive) and blueness (negative). If the a* and b* values are both zero, you have a neutral gray. The C* metric is chroma, which is simply a measure of the distance from the origin in the a*-b* plane (C* = 0 for gray). The magnitude of the C*, a*, and b* values is similar to the L* dimension. With some experience, it is not too difficult to learn to correlate CIELAB values with appearance.

Another option is to convert the CIELAB values into Munsell coordinates, which are direct descriptors of lightness, chroma, and hue. Munsell descriptors are also commonly used in soil science., so your mud would fit right in. GretagMacbeth provides free software for these conversions.  (Back to top)

I painted a very pale yellow on one of my bedroom walls. I do not like the contrast with the country blue on the three other walls. Can I paint a brown that complements the blue over the yellow without affecting the brown color? (358)
Most home paint is nearly opaque so it should be no problem. Some light colors do show through, but that can usually be fixed by painting a second or third coat. Since the paint is designed to be as opaque as possible, the intent is that the color underneath would have no effect. Brown, being darker than the yellow should have no problems.  (Back to top)

I wish to have a white bedroom, where in the walls ceiling and floor are white. The windows doors and all wooden furniture will be painted white too. Kindly advise me on the suitability of the scheme. Should I be using stark white or would the use of ivory white be better. What should be the finish of the paint? (357)
I won't ask you why you want to do this. It is pretty well established that we are more comfortable and relaxed in an environment with some visual contrast. You are proposing to remove as much of that contrast as possible. It probably won't be a pleasant room to be in. Of course, if you are asleep it won't matter. I don't think it matters at all what white you use and would advise simply choosing what you prefer.

An interesting note, if the room was completely uniform, it wouldn''t matter what color you chose since it would disappear from your perception anyway. Our visual system cannot perceive color where there is no spatial contrast (no edges) and everything quickly fades to a middle gray. You might do some reading on the Ganzfeld or Troxler fading to learn more about that.  (Back to top)

Is it true that only some dreams are in color, while others are in shades of gray? (356)
Yes. I regularly experience both black-and-white and color dreams. There has been folklore that people only dreamed in black and white, but that appears to be the result of poor dream recall and cultural influences. There are some interesting references on this page on the topic. Some say that all dreams are in color, but we sometimes don't remember the color. Since we experience black and white images both in natural settings (as in low luminance levels where we lose color vision) and through technology (black and white photographs and video), there is no reason to believe we couldn't replicate that perceptual experience in a dream.  (Back to top)

Sign language interpreters have always been taught that they should wear colors that are in contrast to their skin tone to create maximum visibility and minimize eye fatique. Is there a more scientific way to teach these principles? What does research tell us about clothing color and eye fatique? (355)
It makes perfect sense to have maximum contrast between the hand color and the background clothing. This will make the hands themselves and movements of the hands easiest to perceive. That is true of any object, printed text, etc. I have never heard of research on particular clothing colors and eye fatigue, but there is no question that lower lightness contrast (i.e. clothing that is not significantly lighter or darker than the hands) will be harder on the eyes. The scientific way to teach these principles would be to explain the idea of lightness and contrast between the background an object of interest. Here is a web page that talks about it for text. The principles would be the same for the perception of anything else.  (Back to top)

I am trying to get the appearance of gold and silver on my monitor but they always look like yellow and silver. How do you get metallic gold on a monitor? (354)
I think you meant to say they always look "yellow and gray." Otherwise you already know the answer!

It turns out that if you look only at diffusely reflected light, gold is yellow and silver is gray. That is exactly what you are seeing on your monitor. The appearance of metals depends on their unique characteristic that their first-surface (or specular) reflection (the highlights on an object) are also colored. This is the appearance difference between a yellow piece of plastic (white highlights ... the color of the light source) and a piece of gold (yellow highlights ... the color of the object). The only way to render these appearances on a display is to add some dimensionality. In other words you can render a 3D object on your monitor that looks like a piece of gold, but you can't make a flat patch of color that looks like gold (unless you render different views of it).  (Back to top)

How does one measure color of ink in its raw liquid form? I need to measure the color in that form. What tools are required? Are there standardised methods to sample ink to measure it? (353)
I guess I would ask what meaning the color of ink in raw liquid form has? Does anyone really care what it looks like then? It makes much more sense to measure the ink in a way similar to how it will be used and viewed. There are probably standard methods for preparing and measuring ink samples. Unfortunately I am not aware of details. I would suggest you look for ASTM standards on the topic or make an inquiry with an ink manufacturer to find out about standard techniques.  (Back to top)

Somedays my eyes look green and other days they look more blue. What color of clothing or eye shadow would I wear to make them appear more green? What colors should I NOT wear if I don't want them to look blue? (352)

Most of all, I would encourage you to pay attention to the lighting. It might be that your eyes look more green under inside (incandescent) lighting and more blue under daylight. That might be the more important issue.  (Back to top)

Each of our spectrophotometers has its own calibration standard. If all of the spectros are not calibrated to the same standard, aren't we calibrating in variation between the spectros? We can see anywhere from a .2 to over a .9 difference in our ΔE readings when we read the same areas of a sample on the different spectros. What would happen if we lost one of the standards since they are suppose to be unique to the manufacture of one specific specto? (351)
You are correct that you might be calibrating in variation between your instruments. However, they will each have different data for the calibration standard to compensate for some of that. If you were to decide to use one standard for all your instruments, then you would also need to make sure they were all using the same internal calibration data for that standard. Such a practice might well help you to obtain better inter-instrument agreement.

If you were to lose a standard you could get a replacement from the manufacturer and they could update the calibration data for the new standard.

Unfortunately, the color differences you have observed between various instruments are not unusual.  (Back to top)

My customers set up the upper limit of ΔE=1. This a very difficult to match, and I've got the information that the human eye can't detect a deviation below ΔE=2. Which would you recommend to set-up? Also, what is the grayscale-measurement for plastic parts? (350)
You are correct that a ΔE of 1.0 is a very tight tolerance. Unfortunately there is no easy answer to your question. The size of a perceptible color difference depends on the particular configuration of viewing and the color in question (not to mention the particular color difference equation used). You really need to establish a tolerance that is agreeable to both you and your customer. I don't blame them for saying 1.0 because they are sure to get what they want, but I would bet they would be happy (in many cases) with a less stringent tolerance. Berns' text, "Principles of Color Technology, 3rd Ed." provides more details on establishing visual tolerances.

I have never heard of "greyscale-measurement", but I would suspect it is just referring to the lightness dimension in CIELAB. In other words the L* value and its tolerance.  (Back to top)

I have read that a digital darkroom should be painted Munsell 8 gray. What should I tell my paint dealer so he can mix a color close to this? (349)
You could just tell him "light gray" or look at their color cards and select the one you are trying to match. That would probably be good enough given the variation in lighting and geometry in any given room. However, if you want to be more precise and accurate, you could tell him to match the CIELAB values of L* = 80, a*=0 and b*=0. I would think they can do that with their color matching system (or they could measure some of their gray color cards to find the closest one). Another alternative would be to purchase a sample of Munsell N8 from GretagMacbeth and then take that in to be matched.  (Back to top)

I have a paint is specified as Munsell colors and I need the RGB equivalents to display on my computer. Can you tell me the RGB for 1BG 6.3/1.4 and 4.6G 9.0/0.5? (348)
There is freeware available from GretagMacbeth can perform this conversion.  (Back to top)

What is digital colour? The history of digital colour? Digital colour challenges? Problems encountered with digital colour? (347)
"Digital color" is just a term used in the color reproduction industry to refer to processes that are based on digital systems rather than historical analog systems. For example photography, printing, and television were all invented and developed as analog systems. The current state-of-the-art in each of those technologies is now digital. During the transition time people would use the term "digital color" to refer to the newer systems and the challenges posed by them that weren't present in the analog systems.

Color is color, it doesn't matter if it is produced by a digital or analog system. As such there really are no specific differences in the challenges and problems of digital color. Especially since all new systems are now digital.  (Back to top)

I've seen so-called UV energy beads change from white to various colors when exposed to UV light. What type of chemicals are embedded in those plastic beads, and are they toxic? (346)
The colorants imbedded in the beads are known as "photochromic colorants". A web search on that term will turn up lots of information on them. I couldn't find details on toxicity, but the colorants themselves are embedded in plastic prior to use, so they are probably not readily accessible. However, I would encourage you to be careful with these as you would with any man-made chemical. In other words, I wouldn't eat them! This web page gives a good overview of how photochromic colorants work (scroll down to the section on photochromic colorants near the bottom) and lists what some of the specific chemicals are. You could do some toxicity searches on those chemicals. Another page, gives some more specifics and explains how different colors are made.  (Back to top)

I am testing whether colored (red and blue) backgrounds enhance or detract from a person's ability to assemble puzzles. Have these experiments like this have been done before? Any idea which age groups to test? (345)
I have recently heard of some research that showed that people exposed briefly to red (as opposed to green) before an IQ test did not perform as well. Apparently the red exposure produces a kind of avoidance response that might also happen for other tasks such as sporting events, or in your case, assembling puzzles. Given those results, I wouldn't be surprised if you find something significant in your experiment. That work has not been published yet and I don't think there is a lot similar to what you are planning. I don't have any insight into the best age groups, but would expect that any result would generalize across ages. Keep in mind that there will be individual variation. Red might be faster for one person and slower for another. However, when you average across enough people you might see a significant result.  (Back to top)

I need a means to estimate the difference in general light reflection between walls in a windowless room painted matt magnolia (Munsell 10YR9/2), compared to matt brilliant white. (344)
Your magnolia is very light with a Munsell Value of 9. I would guess that a good white would also have about the same Munsell Value, so the two would be about equivalent. (With a Chroma of 2, your magnolia color is pretty close to white anyway). Munsell Value 9 is approximately a luminous reflectance of about 76% percent if that might help in your calculations.  (Back to top)

What is the Reflectivity Index and what is the index for various colours, particularly silver? (343)
"Reflectivity Index" is not a term commonly used in color science. It probably refers to the luminous reflectance factor, or CIE Y tristimulus value. The value itself depends on the measurement geometry and can range from 0-1 (or 0 -100%). A good silver surface, measured in a total reflectance geometry, might have a reflectance factor as high as 0.98 (98%) or so.  (Back to top)

Blue colors wouldn't have to have higher UV reflectance than other colors and I am sure exceptions can be found. However, in general blue objects are reflecting highly in the short-wavelength end of the spectrum and that reflectance continues into the UV. Another way to look at it is that the wavelengths they tend to absorb are mainly longer than the the blue and UV wavelengths while other colors are largely absorbing the shorter wavelengths. (342)
There really are not any well established rules with respect to color and it's effect on people. Color certainly has psychological effects, but they vary substantially from person to person. The best bet is to make sure that the brochures are pleasant and legible. Following basic rules of good design, including harmonious use of color will probably serve you best.

You might also inquire with the Color Marketing Group to see if they have any guidelines or suggestions.  (Back to top)

Why does the color blue have higher UV reflectance properties than other colors? (341)
Blue colors wouldn't have to have higher UV reflectance than other colors and I am sure exceptions can be found. However, in general blue objects are reflecting highly in the short-wavelength end of the spectrum and that reflectance continues into the UV. Another way to look at it is that the wavelengths they tend to absorb are mainly longer than the the blue and UV wavelengths while other colors are largely absorbing the shorter wavelengths.  (Back to top)

I am an artist, and I would like to display the color of the universe. Can you send me the RGB values? (340)
The RGB or CMYK color would depend on the display or printer. The color is very close to a daylight at 5000K (standard illumination for printing). On an sRGB display (which has a white point at 6500K daylight ... bluer than the universe color) it appears beige and the RGB values are 255, 227, 210 (in R, G, B, order).  (Back to top)

When I transform D65 illuminant values (in CIELAB) to RGB, I do not get R=G=B. Why? (339)
The CIELAB L*a*b* values for an illuminant are always 100, 0, 0. Whether or not a given white point (e.g., D65) transforms to R=G=B linear values depends on the definition of the RGB system. Some RGB systems are defined with D65 white points (meaning that, by definition, D65 maps to R=G=B) and others are not. White points do lie at the origin in CIELAB because CIELAB is normalized to the white point. This is essentially an assumption that chromatic adaptation is complete and the white point appears .... "white."  (Back to top)

Various color systems (NTSC, SMTPE, CIE) use different white points. How are these defined? (338)
The CIE system does not have a fixed white point. White is defined by the user for a given situation and application. It is not fixed to any one chromaticity coordinate. The SMPTE and NTSC systems you mentioned are examples of specific applications in which a given white point has been selected. Both CIE illuminants D65 and C represent average daylight (D65 being the newer standard) and they were selected for TV systems since this is a good whitepoint to represent many typical scenes and for displays that appear neutral under a wide range of viewing conditions.  (Back to top)

How precise are Munsell color sample meant to be read? (337)
One step in Munsell Chroma is approximately 5 CIELAB units (very approximately). In general , people accept that approximately 1.0 CIELAB unit is a visual threshold (again very approximately and dependent on the color in question). Thus, observers should be able to fairly reliably discriminate about 0.2 Munsell Chroma steps under the best of viewing conditions. So what are the best of viewing conditions? A nice bright daylight booth with fairly large samples set up right next to each other.

Now, I've seen a few of the Munsell soil books, and generally they are ... "soiled." I would be very surprised if the samples in the real world could be considered precise to much better than one Chroma step. On top of that you need to factor in the varying lighting and observers. Certainly with all those variables there is no way the judgements can be considered precise to 0.1 Chroma step.  (Back to top)

Can you provide information on Ultralume U5000? Also, what are ΔE GE, ΔE Audi,and ΔE PQ?) (336)
The only reference to Ultralume U5000 I can find on the internet is in brochures about the KonicaMinolta color control software. Perhaps someone there could let you know exactly what the light source was. Ultralume sources are usually narrow-band fluorescent sources and I would guess that U5000 might refer to a 5000K correlated color temperature.

Likewise, the color difference metrics you mention appear to be customer specific. Sometimes instrument manufacturers will add specific equations to their software for certain large customers and then just leave it in for everyone in case they want to use them. I would assume that GE refers to some General Electric specific equation, Audi to the automobile manufacturer, and PQ ... I have no idea. They are not CIE color difference equations.  (Back to top)

Has research been done to determine how closely the color receptors in various species match what one might expect given their evolutionary environment? (335)
This is a fascinating question. Essentially the answer is "yes." There are a couple of classic vision papers that address the issue. One is H.B. Barlow, "What causes trichromacy? A theoretical analysis using comb-filtered spectra," Vision Research 22, 635-643 (1982). Barlow shows that the the spectral frequency content in typical color stimuli is adequately sampled with 3 band-limited mechanisms similar to cone responsivities. The second is G. Buchsbaum & A. Gottschalk, "Trichromacy, opponent colours coding and optimum colour information transmission in the retina," Proceedings of the Royal Society (London), B220:89-113 (1983). They show that opponent channels optimally encode visual information. A more recent reference on the topic is T.-W. Lee, T. Wachtler, and T.J. Sejnowski, "Color opponency is an efficient representation of spectral properties of natural scenes," Vision Research 42, 2095-2103 (2002).  (Back to top)

What are these devices: densitometer, spectrophotometer, and colorimeter? (334)
A spectrophotometer is the most fundamental of the three instruments you mentioned. Spectrophotometers measure reflectance or transmittance as a function of wavelength. From those data, one can compute other quantities such as colorimetry (XYZ, CIELAB, etc.) or densitometry (Status A, Status M, etc.). A colorimeter directly measures colorimetric quantities through filtration of the sensors. Thus a colorimeter directly reports XYZ or CIELAB values, but spectral data are not available for additional computations. Likewise, a densitometer reports integrated values, called densities. There are various standards for densitometers, but they are not designed to measure color. By that I mean reported densities are normally not intended to correlate with human color perception, but rather to act as measurements of the quantities of dye or ink present. (Of course, with added knowledge about the system it is sometimes possible to convert densities into colorimetric quantities.) Again, densitometers do not provide spectral data that could be used for further computations.  (Back to top)

When driving at night, what colors on cars are easier to see and what colors are harder to see? (333)
At night, your rod system often becomes more important than your cone system for seeing objects. The rods are sensitive to shorter wavelengths than the cones. (This is why red objects tend to look black at night, the rods simply don't respond to red light.) The easiest things to see are those that contrast most with the background. Assuming the background is dark, you want cars that would appear light. White will always be a good choice for this since it is lighter than the background for both rods (when no extra light is on the car) and cones (when headlights are shining on it). As far as chromatic colors, a light blue would probably be then next best choice. As I mentioned above, red would not be good since it would look black to the rod system. A yellow color would probably end up somewhere in between; nice and bright for the cones, but a bit darker for the rods.  (Back to top)

Why was Munsell's colour system the most widely and globally accepted one? (332)
It is difficult to say exactly why, but it is probably due to its systematic nature, the perceptual uniformity of the three perceptual dimensions, the degree to which it has been studied and refined over the past decade, and the commercial success of the Munsell Color Company. Part of this is that the system was not only well specified theoretically, but in practice. The samples have always been accurately produced and are very stable over time.  (Back to top)

I want to scan the Ishihara plates and create the same colors on my monitor. Can you help? (331)
First of all, I'm not sure the publisher of the Ishihara plates really wants you to be doing this. You should check into any copyright issues if you are planning more than personal use. Your question is a synopsis of the problem that color management systems are designed to solve. You first need to have your scanner characterized so that the scanned RGB values can be converted to meaningful colorimetric values (like XYZ or CIELAB). This characterization can be embedded in an ICC profile. The characterization must also be created for the lighting under which the plates are properly viewed. Then the same process is completed to characterize the display. Another profile allows conversion from the colorimetric values to the RGB signals required to drive the display to produce the proper colors. You will need to learn more about color management and then purchase a system to profile your scanner and display to complete this task. Berns' "Principles of Color Technology, 3rd Ed." provides some introductory material on color management and there are many other books out there on the topic.  (Back to top)

What is the equivalent RAL paint code for Munsell 3Y7.8/1.1? (330)
I cannot give you an answer. First of all there are different types of RAL designations, some of which are not related to colorimetry. Some RAL designations are directly related to CIELAB coordinates. If you are looking for one of those designations, then you could convert from Munsell to CIELAB (GretagMacbeth provides software that does that) and then from CIELAB to RAL. Both these conversions are addressed in this FAQ.  (Back to top)

I want to write a paper on color measurement. Can you tell me the latest developments on this, and what we can do? (329)

I have a severe color vision deficiency. Can you help me modify images so I can see the world the way others do? (328)
Unfortunately there is no way for you to see the world the way others do. There have been filters, or filter combinations, suggested that would help people with color deficiencies to discriminate the colors they normally cannot (e.g. a different filter on each eye). I am not aware of any of these being particularly successful. There is a little information on that topic in this FAQ . Going the other way, showing others what the world looks like to you (or at least which colors you can't discriminate) is a bit easier. Here is a website that has tools to take images and convert them into versions that illustrate appearance to color deficient observers. There is also a program to help people make images/slides more friendly to those with color deficiencies. It is a very interesting site and the software is technically sound. It might be worthwhile to take some images of your pathology slides through their system to show your colleagues what you are up against.  (Back to top)

Does color affect reflection of heat? (327)
Objects appear different colors because they absorb or reflect different amounts and wavelengths of light. A black object absorbs nearly all the light that hits it while a white object absorbs almost none. A red object absorbs the blue and green wavelengths while not absorbing the red wavelengths. So what happens to that light that gets absorbed? Usually, it is turned into heat and the object warms up. So if you were to put identical objects out in the sun with one painted black and the other white, the black one would absorb a lot of light and heat up much more. So you could say the black object is absorbing more heat from the sun. It's really absorbing more light and converting it into heat.

That's only part of the story, there is a lot of infrared energy out there and that is absorbed or reflected too. The properties of the object in absorbing infrared will influence how much it heats up. However, the properties in the infrared are not directly related to color.  (Back to top)

To what degree is violet light magnetified to change into UV heat radiation? (326)
First of all, magnetified is not a word, so I can only assume you mean "transformed." Electromagnetic radiation is sometimes transformed from shorter to longer wavelengths through processes such as fluorescence or phosphorescence. This type of transformation would convert violet light to something like green or orange light, but not to the shorter-wavelength ultraviolet. It would be very rare indeed, but not completely impossible, for radiation to be converted to shorter wavelengths (which are higher-energy photons). Secondly, heat radiation is normally considered to be infrared radiation (longer wavelengths than visible) and not ultraviolet. It is true that absorption of ultraviolet radiation (or any energy) can raise the temperature of an object, but this temperature change itself is not considered heat radiation (the object might then emit some infrared energy, depending on its new temperature).  (Back to top)

Can you help me separate Yu'v' values into Y, r-g, and b-y opponent channels for fourier analysis? (325)
Given Yu'v' values you already have the most important separation done. That is the separation of luminance, Y, from the chromatic components, u'v'. Since this is a linear space, you could perform your Fourier analysis direclty on the Yu'v' dimensions. Unfortunately u' and v' do not correspond directly to red-green and yellow-blue opponent dimensions. It turns out that the perceptual unique hues are not orthogonal to one another in any linear color space, so their is no easy answer. There are a variety of other color spaces used for this type of analysis and they all have their advantages and disadvantages. YCbCr is one such space used in digital video. I would suggest you look at chapters 21-26 of Charles Poynton's "Digital Video and HDTV: Algorithms and Interfaces" book for more details on some of these color spaces and transformations.  (Back to top)

Is there such thing as a perfect hue angle for Cyan and Magenta, in Lch? (324)
There is no such thing as a perfect hue angle for cyan and magenta (or yellow for that matter) in CIELAB LCh or any other color space. The optimal hues for cyan, magenta, and yellow would depend on the application and the technology. Take yellow as an example. Perceptually unique yellow (a yellow with no perception of red or green in it) is at a hue angle of just about 90 degs. in CIELAB. So that is one definition of an optimal yellow. However, in a color reproduction system the best yellow to optimize the volume and nature of the color gamut might not be perceived as unique yellow. For example, in photographic systems the yellow primary tends to be a bit to the red side (lower hue angle).  (Back to top)

We design commercial kitchens with lots of stainless steel, white tile, and fluorescent lights. Why do our cooks prefer dark, mat finish tiles? (323)
I would imagine that the preference is mainly aesthetics. People do like to have some contrast in their environment and white tiles and stainless steel are both very light colors, thus not contrasting much with one another. The glare from the stainless would be unpleasant and tiring. The darker tiles would reduce this glare by absorbing some of the light that is bouncing around the room. This is particularly true with the mat finish. It is simply because the tiles are absorbing some of the light that might have bounced off the stainless (becoming glare) had you used the white tiles.  (Back to top)

What is LRV? (322)
LRV is Light Reflectance Value. It seems to be a term commonly used for paints and signage. It is defined as the amount of visible light a surface reflects, but not more strictly than that. I could not find a strict definition, but it appears to be the CIE Y tristimulus value for appropriate viewing conditions.  (Back to top)

What is the Pantone equivalent to color RAL 3020? (321)
I cannot give you an answer. First of all the Pantone system is proprietary and not related to colorimetry. Second the RAL designation you gave is also a simple index and not related to colorimetry. Some RAL designations are directly related to CIELAB coordinates, but even with such an RAL designation, it would be impossible to convert to Pantone numbers without their proprietary look-up tables (or licensed software that includes them). Your best bet is to inquire with Pantone on how to do this conversion.  (Back to top)

I use a colorimeter that returns x,y,Y values. What units do they express? (320)
The x,y,Y values you get from your instrument are chromaticity coordinates (x,y) and luminance (Y). The chromaticity coordinates describe the hue and saturation of the stimulus with no information about the luminance. They are unitless, normalized versions of the XYZ tristimulus values. They are computed as follows:
x = X/(X+Y+Z)
y = Y/(X+Y+Z).

There is no way to compute Y from x an y. The x,y values range from zero to one and are generally reported to 3 or 4 digits beyond the decimal point. You can read more about chromaticity coordinates in almost any introductory text on colorimetry. One good example is Berns' "Principles of Color Technology, 3rd Ed.".  (Back to top)

I have a densitometer that measures visual density. How do I convert the density values to CIE L*? (319)
The full CIELAB L* equations are as follows:

L* = 116(Y/Yn)^1/3 - 16 for Y/Yn > 0.008856
L* = 903.29(Y/Yn) for Y/Yn <= 0.008856
Y/Yn is the relative luminance of the stimulus (relative to white) in a range from 0 to 1.0. You can convert density to relative luminance using:

Y/Yn = 10^-D

This gives you the equation from Poynton for Y/Yn > 0.008856, which happens to correspond fairly closely with a density of 2. So the Poynton equation is limited since it does not change to the alternative form for Y/Yn <= 0.008856. All you need to do is first convert from density to Y/Yn and then use the two-part L* equations given above and you can compute L* for any density values.  (Back to top)

Why are plants green. It seems they should be shades of black. (318)
Here is a website that explains why plants are green. It is interesting, but it really doesn't address your question of why they aren't black. I assume you are thinking that black leaves would absorb even more sunlight and therefore be able to complete even more photosynthesis. I don't know the answer, but I can certainly speculate as a color scientist. There are two reasons that come to mind for me that would make green leaves preferable (and thus evolutionarily advantaged) to black leaves. The first is heat. If leaves were black, they would absorb a lot more light, but they would also have to figure out a way to dissipate the extra heat. Many plants wilt and die if they get too hot, so this would limit the areas plants could live. Green plants reflect/transmit the wavelengths were there is the most visible energy, thus keeping them cooler. That also makes them more visible. The second is translucency. If the leaves absorbed all the light, then there would be none transmitted to the leaves below. Every plant would end up being a single layer of leaves. I would suspect that being an imperfect absorber makes it possible for layers upon layers of leaves to effectively survive. I would also think that these layer's of leaves that are green are making much more oxygen for us all than a single layer of black leaves could. Lastly, you could simply say that green leaves are more attractive to the animals and humans that interact with them and promote their survival. The question to then ponder is why the flowers and fruits are not green!  (Back to top)

I have a white printing on LDPE which is yellowing. How can I determinate the yellowing in a datacolor instrument? (317)
I would suggest simply using your instrument to measure CIELAB values and examine the L* and b* values. As it yellows, your sample will get lower in L* (lightness) and higher in b* (yellowness-blueness). With some experience, you will be able to establish tolerances for changes in these dimensions that are unacceptable for your application. There are also whiteness indices that you might look into. The software with your instrument can probably calculate them. The only advantage of a whiteness index is that it might give you a reasonable single number to look at.  (Back to top)

What is the differance in reflective properties between white and orange hard hats working in the sun? I know white reflects more but how much more does it reflect? (316)
You are correct that white will reflect more light than orange since the orange helmet is absorbing green and blue light in order to appear orange. However, that doesn't give the full story on perceived brightness (which I assume is your interest for safety). Vivid colors, like a bright orange, look bright not just because they reflect a lot of light, but also because they are colorful. Our perceptions of brightness depends on both the amount of reflected light and colorfulness. The orange helmet is far more colorful than a white one and this colorfulness might be enough to make it look brighter than the white. Also if the orange happens to be fluorescent (like a hunting jacket), then it might look significantly brighter than the white. Ultimately, the best choice is the one that contrasts most with the surroundings. I would expect, in most situations, that bright orange, will stand out more from the surroundings than white. If it was my head, I'd pick an orange helmet. The best thing you can do is to have a few people look at the two choices in a typical environment and choose which they think is brighter (or which is more noticeable). If there is a clear consensus, then you know which to pick. That's a better answer than anything you'll get by simply looking at amounts of light reflected.  (Back to top)

In performing a mechanical task with several different colored backrounds, which colors would bring the most contrasting results in the efficency of which the task is performed? (315)
Generally objects can be most easily perceived as separate from a background when there is maximal contrast in lightness. In other words, put light objects on a dark background and dark objects on a light background in order to best see the objects. The lightness dimension tends to be more important than the hue and chroma dimensions for the discrimination of objects (especially as they get smaller). There is another effect, known as crispening, that can become important when trying to compare two objects on a background (rather than seeing a single object). I suspect your tasks might involve discriminating difference in objects on a background. In that case, crispening is an effect whereby the apparent differences between objects appears largest when the background is of a color intermediate between the two objects. Thus, if trying to discriminate two middle gray objects, put them on a middle gray background, two dark objects, put them on a dark background, and two light objects, put them on a light background. Also, in general neutral (gray) backgrounds tend to be best overall for viewing a variety of objects and colors. So I'd pick a gray background with lightness about in the middle of the range of the objects that are being manipulated.  (Back to top)

What is the maximum opacity for 1 micron thick white ink? is there a theoretical calculation? (314)
The best method to explore questions like this is to study and apply Kubelka-Munk Theory. A good reference to explore is "Industrial Color Testing: Fundamentals and Techniques" by Hans G. Völz. (reference 4 at the bottom of this page)  (Back to top)

Is there a way to express Lch differences, in terms of units? (not referring to ΔL*, ΔC* or ΔH*) Hue for example is expressed in degree?s (eg 5°). What unit do I use to express the difference in lightness and chroma? (313)
The units of color difference in the CIELAB space are an interesting bit of color science trivia. Looking at the CIELAB equations, you can see that there are no explicit physical units for the color correlates. That is because all of the tristimulus values (which can have units) are divided by the tristimulus values for the white (which would have the same units). People tend to just refer to the magnitudes of differences in CIELAB as "CIELAB units". This is necessary to indicate the specific color space. There are no physical units to attach to those dimensions (which are correlates of perceptions).  (Back to top)

The LAB values output by the GretagMacbeth Munsell conversion software are different than those I can input in Photoshop. Any solution for this? (312)
The CIELAB space in Adobe Photoshop is the same space as that used everywhere else including the GretagMacbeth conversion software. All I can imagine is that you are having issues with the encoding. In CIELAB, L* ranges from 0-100, but is encoded as 0-255 in Photoshop. Thus, the Photoshop L values need to be divided by 2.55 to get ordinary L* scaling. For the a* and b* dimensions, there is a simple offset. Photoshop values of 0 to 255 are equivalent to CIELAB a*b* values of -127 to +128. This allows the positive and negative values of CIELAB a* and b* to be represented as simple 8-bit (0-255) numbers in Photoshop. Thus Photoshop a*b* values need to have 127 subtracted from them to get ordinary a*b* scaling. Perhaps it is this conversion that you are missing. (Note that this conversion might only be approximately correct depending on the color management settings in your system, Photoshop, and your software versions, OS, etc.)  (Back to top)

What is the best test for screening color vision in employees using penetrant fluorescent dyes to inspect manufactured metal items for defects? (311)
I am not familiar with the specific task involved, but I assume you are interested in measuring your employees' ability to detect and recognize small color changes. Probably the best test for that would be the Farnsworth-Munsell 100-Hue Test. It is a fairly simple to administer test that evaluates color discrimination ability and also screens for color deficiencies. It is currently sold by GretagMacbeth.  (Back to top)

What is the best color to paint walls in a machine shop to enchance productivity? (310)
This is an interesting question and I am sure you could find research to support almost any color choice. To be strictly scientific, the only changes that should enhance productivity are those that make the work more visible. That would be light colors. There is an effect known as the "Hawthorne effect" (see wikipedia.org) that suggests what is truly important is the attention given workers. In studies about lighting and wall color, it was found that workers in areas had increased productivity regardless of the changes in their environment. It was the fact that they were put into the special environment that made them happier and more productive. One suggestion is to allow the workers themselves to choose the wall color that they feel would make them more productive. It also seems that changes are important, so maybe asking them to select again every year or two would also help.  (Back to top)

For color rendering/reproduction purposes, is it better to use a high-end CRT or LCD monitor for accurate color creation? What are the pros and cons of each system, and why have high-end CRT manufacturers discontinued their CRT products in favor of LCD? (309)
In either case, it is important to use a high-end display. You do get what you pay for. Since about 1998 or 1999 we have exclusively used high-quality LCD monitors for our critical visual experiments on color reproduction, color appearance, and image quality. That is about the best endorsement for the technology I can give. In general, LCDs are higher luminance (brighter), higher contrast (for real images), more uniform, and sharper than CRTs. The only advantages of CRTs are that they can easily handle multiple pixel resolutions, are more uniform across changes in viewing angle, and sometimes have better temporal responses. The temporal issues are largely moot now, so as long as you can work with the native LCD resolution, they are a clear better choice for critical color work.

LCDs still suffer from angular dependencies, but those are only an issue in situations with multiple observers viewing the same display. In critical situations, the viewing angle is usually easily controlled. Also, the angular dependencies have been reduced greatly in modern LCD monitors. Desktop LCDs have significantly larger color gamuts than CRTs as well (not necessarily true for laptops, since power saving trade-offs can reduce the gamut volume).

In our experience, the colorimetric characterization of LCD monitors is both more accurate and stable. You should have no concerns about switching to high-quality LCD systems for your critical color rendering and reproduction work.  (Back to top)

How can I transfer from Munsell notation to a single number indicator of overall color? (308)
Color, by its nature, is not uni-dimensional and cannot be expressed in a single number. The Munsell designations show this by expressing color in terms of three dimensions, hue, value(lightness), and chroma. For example, one of your designations was 2.5GY 5/4. The hue is 2.5GY (a greenish-yellow), the value is 5 (a lightness midway perceptually between black and white), and its chroma is 4 (indicating that it is significantly different from gray, chroma=0, but still not too saturated). Perhaps one of those single dimensions would suit your purpose (like chroma, if you are interested in how much color is present), but you can't describe the overall color with just one number, it takes all three. The website coloracademy.co.uk provides some overview on the Munsell system. It can also be found in introductory texts on colorimetry like Berns' Principles of Color Technology 3rd Ed.  (Back to top)

I am working on matching a number of colours for plastic production. How do I convert my CIELAB values to colour pigments for production? (307)
There is an entire field known as colorant formulation that addresses the question you have asked. Essentially colorant formulation software takes the desired CIELAB values (or spectral reflectance curve) for your standard and uses information about your available pigments and mixing system to predict the required pigment amounts to formulate a match. Companies that sell spectrophotometers typically also sell color formulation software. You might check with companies such as Hunterlab, BYK-Gardner, Datacolor International, GretagMacbeth, etc. to learn about their products. You might also look at Berns' book, Principles of Color Technology to learn more about the process.  (Back to top)

What are the general norms for pale & pastel colours?Which are the colours (generic names) are considered as pale & pastel colours? (306)
There are not specific scientific definitions for pale and pastel colors. In common usage the two words are considered synonyms and this is consistent with their use in color naming. Of the basic color terms, none are exclusively pale, or pastel, colors. Instead, pastels would be colors of any hue that are light and near neutral. You might also look at the Colorcurve and Munsell systems, which both define sets of pastel colors.  (Back to top)

If the fovea contains no S cones why don't humans have functional dichromancy in foveal vision? (305)
The visual system is indeed dichromatic for very small stimuli. It is called small-field tritanopia. The key is that most of the fovea does contain S cones. It is only the very central area, less than 0.25 degrees that has no S cones. To observe small-field tritanopia the stimulus must be entirely contained within that small central region of the fovea. Even brief eye movements to areas that will expose the stimulus to S-cones will destroy the effect due to perceptual filling-in (just like the blind spot).  (Back to top)

The freeware from gretagmacbeth.com cannot convert RGB values less than (34,34,34), the software tells me that the values are "Out of Munsell Range". Does Munsell support no color below (34,34,34)? (304)
I am not familiar with the inner workings of the GretagMacbeth freeware, but it is reasonable to make a couple of assumptions. The first would be that they are using the sRGB model for RGB values and the second would be that they are limiting the conversion to Munsell samples that are physically realizable. The RGB code values of (34,34,34) in sRGB correspond approximately to a neutral with a reflectance of 1%. That is a very dark black and impossible to produce as a hard-copy (painted paper in the case of Munsell) sample in the standard Munsell viewing conditions (not a flare-free environment). Thus, it does make sense to me that code values below (34,34,34) are outside the range of the Munsell system. I would simply suggest calling anything that dark essentially Munsell Value 0.  (Back to top)

How can I measure the color temperature of a image? (303)
Images don't really have a color temperature. Color temperature, or more properly "Correlated Color Temperature", is a measure of the color of light sources. It is based on the idea that the spectral power distribution of blackbody radiators (and therefore their color) can be defined as a function of one variable, their temperature (using Planck's equation). That temperature is measured on the absolute scale in Kelvins (K) and generally ranges from about 2000K to 10,000K. The variation in color tends to be from reddish-yellow (2000K) through white (a large range around 6000K) to bluish (10000K). The color of other light sources are measured using correlated color temperature by assigning them the color temperature of the blackbody that is the nearest color match.

So (correlated) color temperature is a reasonable short-hand way to describe the color of typical light sources and is often used to describe the white-point color of a display. Computer displays are often setup with 6500K white points to closely simulate average daylight. Older CRT displays were often more blue (around 9300K) due to technological limitations in CRTs. It is rare for displays to be set up with white points yellower than 6500K, but sometimes 5000K is used in the publishing/printing industries to match the color of the standard light sources used for evaluating prints.

Thus, an image doesn't have a color temperature, but the light source illuminating the scene does, the light source illuminating a print does, and the white point of a display does.  (Back to top)

How does the color pink affect the minds of males and females? (302)
Exposure to various colors can have significant and very real physiological and psychological effects on people. However, the rules by which these effects can be predicted are not firm and the effects can be different for different people based on previous experiences, etc. There are many books on the topic, one of the most well-known being "Color and Human Response: Aspects of Light and Color Bearing on the Reactions of Living Things and the Welfare of Human Beings" by Faber Birren. In particular regard to your question about pink, you are probably thinking of Baker Miller Pink, which has been shown to have a calming effect under certain circumstances. There is a good discussion of Baker Miller Pink with some references at colormatters.com. I have not heard of any differences between the reactions of males and females, but it is possible there could be some psychological differences due to cultural conditioning. It has been suggested that Benjamin Moore's Deco Rose #1328 closely matches the historically used shade of pink.  (Back to top)

Are you aware of an algorithm in the public domain for calculation of NCS notation from reflectance spectra? (301)
I am not aware of a specific algorithm. There are products available from NCS Colour to do the conversion ncscolour.com, but they are not inexpensive, nor in the public domain. There are conversions and explanations at the EasyRGB site easyrgb.com that might help you. One possibility is to purchase the relatively inexpensive Munsell-NCS conversion data from NCS and then use the free XYZ-Munsell conversion software from GretagMacbeth gretagmacbeth.com to get to Munsell and implement the second conversion yourself. Sorry I can't be more helpful. Let us know if you find something.  (Back to top)

Can you give me a ballpark magnitude of color difference in ΔEcmc from monitor to monitor (crt) that one might experience? (300)
Unfortunately I cannot give you a simple answer. Two well calibrated and characterized CRT displays might agree with each other to as little as 0.5 Δcmc units for the colors that both can display. Two "off-the-shelf" CRT displays might not agree nearly so well. It is nearly impossible to guess how large the Δcmc values might get, but the displays could be different in their RGB primary colors, their white point, their tone transfer characteristics (gamma), and the characteristics of the graphics cards. These differences could result in very large Δcmc values between the displays when the same RGB values are used to drive them. I would not be at all surprised to see average Δcmc values of 20 or larger.  (Back to top)

Which is the most appropriate measurement geometry for quantitative evaluation of metallic colors? What are the variables that impact the viewing conditions (size of sample, illuminant, surround, intensity) and is there any research that addresses this questions? (299)
It is generally accepted that multi-angle spectrophotometry is both necessary and sufficient for most metallic colors. However, some very strange materials might require measurements at additional angles and therefore goniospectrophotometry. With a single-geometry measurement (either 0/45 or sphere) you do not have the information necessary to distinguish a metallic sample from a non-metallic sample of a different color.

All of the variables you mentioned are important in visual assessment. Perhaps most important for metallic samples are the illumination (color and luminance) and the viewing geometry. Special viewing booths are made for evaluating metallic materials that allow careful control of the viewing geometry.

There is research on these topics, but perhaps not too much has been published. Much of it remains proprietary since the companies believe there is still competitive advantage in keeping their techniques to themselves. I would suggest looking in the literature for research on the measurement of automotive finishes to find the most information.  (Back to top)

Does the color of ice affect its melting rate? (298)
The short answer is "yes". If there is something causing the ice to be darker, then that means it is absorbing more light. That absorbed light is converted into heat, which raises the temperature of the ice more than similar ice that is not absorbing the light.

There are several related questions this FAQ.  (Back to top)

How do I get the value of Munsell renotation coordinates? (297)
The Munsell renotation data are available here. One can obtain the values by spectrophotometric measurement of CIE XYZ or CIELAB values followed by lookup in the tables. GretagMacbeth also provides freeware for this conversion. Alternatively, Munsell designations can be obtained by visual matching between a stimulus and reference samples in the Munsell Book of Color.  (Back to top)

Is there any standard Colorimetric values of this GSC (Gray-Scale Comparison Method) method used for visual assessment was used by Luo and Rigg? (296)
The technique is general and there are no specific requirements or standards for the gray samples used as references.  (Back to top)

I am a geologist working with Munsell colour chips and need to know how accurate and precise manufactured chips are and how long they can be expected to last (i.e. before fading). (295)
The specific tolerances on chips in the Munsell system are generally very tight with the books being both accurate and precise. However, I do not know the exact tolerances. Perhaps and inquiry with the current manufacturer, GretagMacbeth , would produce an answer. This web article discusses accuracy and precision of Munsell soil charts. It suggests there could be significant variation from book to book, but this could be due to mistreatment in the field. It also quotes the Munsell company as saying the books are only good for 4 years. Again, this is assuming field use. The Munsell chips are made from very stable materials and should last several decades if stored carefully in the dark when not in use. Our lab has 1929 Munsell books that still appear quite accurate and colorful.  (Back to top)

Please explain to me what the Munsell Color Disc and Munsell Charts are. The discs used are: red (5 R 2.6/13), yellow (2.5 YR 5/12), black (N1), grey (N4). I need to measure the color of tomato paste by this method. (294)
Judging from your description you are referring to a form of a disk colorimeter using Munsell papers for the primaries. A disk colorimeter consists of a circle made up of segments of different papers (in this case 4, red, yellow, black, and gray) that can be adjusted to cover different fractions of the circle. The disk is then spun at a rate fast enough to produce temporal fusion of the colored segments (i.e., they all blur together into one color). By adjusting the fractions of the disk made up of each color, the resulting color mixture can be changed to match the tomato past in question. The color designation of the tomato paste would then simply be the fractional areas of each of the four primary colors.

This technique was often used by Albert H. Munsell in the creation of the Munsell system. The disk colorimeters are also sometimes called Maxwell's spinning disks. Some images and more explanation can be found on this web page www.handprint.com.  (Back to top)

Could you recommend a book that would serve as a good introduction to colour science for the lay reader? I am interested in learning the basics of colour science but am not a scientist or mathematician. (293)
One of my favorite introductory books, and a fun one to read, is "Colour: Why the World isn't Grey" by H. Rossotti. It is also very affordable. There are several other books that might also fit the bill. In order of increasing complexity/mathematical content I would suggest: "Color: Essence and Logic" by R. Keuhni (out of print, but used copies are available) "Color: An Introduction to Practice and Principles by R. Keuhni (an extended and revised version of "Essence and Logic") "Principles of Color Technology, 3rd Ed." By R. Berns (contains some more math, but is also very readable)

For a more general range of related topics, you might also want to look at "Seeing the Light: Optics in Nature, Photography, Color, Vision, and Holography" by D.R. Falk et al.  (Back to top)

Where I can find the exact value of Xn, Yn (the luminance factor for the illuminant or perfect white) and Zn? (292)
Xn, Yn, and Zn are the tristimulus values of the illuminant used in the computation of the XYZ tristimulus values of the sample. They are computed the same way as the sample's tristimulus values with the exception that reflectance is removed from the equations (equivalent to setting it equal to 1.0 or 100%). Due to the normalization of the tristimulus value equations, Yn is always 100.0. Xn and Zn vary above and below 100 depending on the particular illuminant. For CIE standard illuminants, the XnYnZn values can often be found in tabulations of the illuminant spectra in standard texts such as Berns' "Principles of Color Technology, 3rd Ed."  (Back to top)

What's the minimum Δfreq or Δwavelength an average person can differentiate so as to recognize two different colors? (291)
Our ability to perceive changes in wavelength depends on several variables, most important of which are the wavelength itself and the luminance level. As a general rule of thumb, it is reasonable to say that we are able to detect changes in wavelength of about 2nm in the middle of the visible spectrum. At the ends of the spectrum this number increases and it becomes undefined at the very ends of the visible spectrum where only a single type of cone photoreceptor is responding to the stimulus. Some classic measurements of wavelength discrimination are given in Wyszecki and Stiles, Color Science, 2nd Ed. on pages 570 and 571. The show, among others, results from a 1934 study by Wright and Pitt indicating that wavelength discrimination was as good as about 1nm near 500 and 600nm, around 2nm for much of the visible spectrum and then in excess of 5nm for wavelengths below 430nm and above 650nm.  (Back to top)

Where I should look for for the energy distribution of commercially available sources? (290)
This is a great question. There are really only two sources. Many of the manufacturers will publish nominal spectral power distributions for their light sources. Sometimes you can even find these specs on the internet, but you should be able to get them by making a request directly to the manufacturer. The second option is to measure sources yourself. That's expensive and time consuming, but will actually provide more useful results for real-world settings. I believe the CIE did a measurements of a lot of real fluorescent light sources to create the newer F illuminants in publication 15:2004.  (Back to top)

What is the best method (color space, sci vs. sce, degree observer, Illuminant,etc.) for calculating Light Reflectance Value (LRV) on a spectrophotometer? (289)
I can't find a specific definition of light reflectance value (LRV). Based on what I can gather, it is an overall measure of the average reflectance of a sample in the visible wavelengths. Most likely it is computed from the Y tristimulus value (0 for perfect black and 100 for perfect white). Unless there are specific specifications to answer your question, I would select the following:
• Color Space = XYZ tristimulus space, Y is the light reflectance value
• Specular Component Included, measure total reflectance
• CIE 1931 2-degree Observer, this is the strict definition of luminance so is probably slightly more appropriate
• CIE Illuminant D65 (you might want to pick others for specific applications, but this is the generic average daylight choice)
(Back to top)

Why do we have color vision, and what is the evolutionary drive for color vision? It is striking how colorful the man made environment is compared to natural settings. What was the selective pressure that made individuals with color vision survivors-breeders in our natural habitat? (288)
This is a fascinating question that cannot be answered easily or succinctly. It is a fun topic to ponder over an ale or two at pub. Much has been written on the topic and I would suggest that it is worthwhile and fascinating to review some of the literature. A brief introduction is given by Gouras on this website. Also Chapter 6, "The ecology and evolution of primate color vision," in "Color Vision: From Genes to Perception" by Gegenfurtner and Sharpe is a nice summary.

It is generally accepted that the short-wavelength sensitive cones evolved first as a distinction from a single cone type sensitive to longer wavelengths. The evolutionary advantage of this might have been a better capability to sense changes in the illumination (and also to discount the effects of such changes when searching for objects). Knowing about changes in illumination helps one to better respond to changes in weather and time of day. Perhaps it helped those early primates with simple color vision to be on time for dates with primates of the opposite sex and therefore fair better in the selective breeding process? The ability to perceive red-green differences evolved later and that is often attributed to improving the ability to find good (as in ripe) food more quickly. Ripe food tends to vary from it's surrounding vegetation in a red-green direction in color space. It might also be useful in judging which prospective mates are available and interested in mating.

I don't think the idea of improved spatial resolution is consistent with the evolution of color vision. The best spatial resolution would be obtained by having a single cone type with a relatively narrow spectral response. This would minimize the effects of chromatic aberration and maximize the spatial sampling of the cones. Of course there are tradeoffs between color vision, spatial resolution, and overall sensitivity that must be made during the evolutionary process. Plenty to ponder when considering how we ended up with the very efficient and functional visual system we have.  (Back to top)

Can you tell me the RAL Number which is most closely match with Munsell N9.5? (287)
Munsell N9.5 would be a very highly reflecting white paint that will probably be hard to obtain. It would have an L* value of approximately 95, a chroma, C*, of 0, and an undefined hue (since it is neutral). I am not sure how hue is handled in RAL, but I would suspect the RAL designation for this white would be 0,95,0.  (Back to top)

How is color temperature of a monitor set to a particular temperature, say 6300k or 9300k? Is there any specific formula for this? How can I change RGB values of a particular temperature to anothertemperature? Is there is any concept of change in offset of R,G,B? (286)
Monitors are set to specific white points through their characterization that relates RGB values sent to the display with displayed CIE XYZ tristimulus values. Specific white-point correlated color temperatures can be related to tristimulus values and then the display (or video card look-up tables) is adjusted such that maximum RGB values produce the desired white (e.g. desired XYZ values). It is sometimes done by changing the gain on RGB, but not the offset. More information on all these concepts can be found in Berns' text Principles of Color Technology, 3rd Ed.  (Back to top)

How does color affect heat absorbtion? (285)
Darker colors absorb more light than lighter colors. Normally this absorbed energy is converted into heat. Therefore, darker colors will normally heat up more than lighter colors. However, there are exceptions. Absorption of energy in the ultraviolet and infrared (some of which actually is heat), also will influence how much an object heats up and it is possible for light colors to absorb a lot of energy in those wavelength regions and vice versa. Therefore the correlation between heat absorption and color is imperfect.  (Back to top)

I am looking for color matching software for matching translucent materials. Are there any commercial packages that employ multiflux theory to aid in prediction and correction of CIELAB values as well as contrast ratio in formulations? (284)
I am not very familiar with all the technical specifications of the available commercial color formulation software. Many of the details are kept proprietary for competitive reasons. You would need to inquire with the manufacturers to find out if their software is applicable to your application. I believe that a previous package from X-Rite used multiflux theory and the webpage on their current formulation software mentions matching for translucent materials www.xrite.com. There are not many details on their page, but it might be worth an inquiry to see if their software, or that from other manufacturers, can be of use.

UPDATE: From a colleague at Benjamin Moore ...
"We do use our own, but I believe Xrite ColorMaster and GretagMacbeth Propalette are both multiflux color matching systems." By "our own" he means matching software they wrote in house.  (Back to top)

When is white not white? (283)
"Mu!"  (Back to top)

Why is yellow the lightest color of the spectrum? (282)
The short answer is, it's not.

Your question could be interpreted many ways. That answer is technically correct, but there are other ways to look at it. If you just took spectral lights and looked at them one wavelength (essentially hue) at a time and had equal amounts of each, then the brightest would be a green wavelength around 555nm. If you look at objects like artists paints, then a saturated yellow color often looks lighter than a saturated green (or other hue). This has several reasons. Mainly is that the yellow will be reflecting more light than the green (often green, yellow and red light to produce a yellow). However, whether or not it looks lighter will depend on the particular pigments being used and the lighting conditions. It might also depend on the observer.

For further study, I would recommend a book on color order systems. You might look at some of the books on the Munsell System, or a book like "Color Space and Its Divisions" by Keuhni for a more detailed treatment.  (Back to top)

How does the yellowing of the lens with age affect: colour vision, scotopic vision and the contrast of tests charts that are spectrally neutral? (281)
In general as the lens ages it becomes more yellow and also scatters more light. It also hardens such that we can no longer accommodate to near viewing distances and have to wear reading glasses. The yellowing is caused by absorption and scattering. To answer your questions. (a) Color vision is not affected as much as we might think because our visual systems adapt to the yellowing and the world still appears fairly normal (it doesn't take on a yellow cast). However, certain colors will be influenced more than others. Blues will look darker and purple colors will look more like reds. The main effect is that colors that matched when we were young might no longer match as we age. This is just like what happens if we put on yellow sun glasses and looked at various color matches. (b) For scotopic vision, there is less light that the rods are sensitive to in the blue region of the spectrum. This results in a decrease in scotopic sensitivity. In other words, it gets harder to see at night. The shift in spectral sensitivity for scotopic vision is probably not important perceptually. The increased scattering also makes it harder to see at night when light sources (like oncoming headlights) are present. (c) The perceived contrast of all stimuli goes down due to the increased scattering. Since much of this scattering is in the blue region of the spectrum, some of the perceived luminance contrast can be recovered by wearing yellow lenses that remove the blue light before it can be scattered in the eye.  (Back to top)

Where I can find the ultraviolet reflectivity index of various colors? How about the heat absorption index for various colors? (280)
Unfortunately I can't help you. While there might be some correlation between UV and IR absorption and an object's color (absorption in the visible), it is not perfect and colors can have one property in the visible wavelengths and different in the UV and IR. In general, colors that reflect more light in the short-wavelength regions of the visible spectrum are more likely to reflect more UV as well. These would be blues and purples. Likewise, colors that reflect in the long-wavelengths (yellows, oranges, reds) are also likely to reflect a lot of IR (and therefore absorb more heat). The IR end is much more difficult to predict however. An object heating up is not just because it absorbs IR (regardless of wavelength) but also because it absorbs other wavelengths. For example, black objects that absorb all the visible wavelengths will heat up more than white objects that reflect all visible wavelengths (assuming their behavior at non-visible wavelengths. is similar.  (Back to top)

In a chromatic adaptation transform, how can we measure the white reference of the incoming video signals? (279)
It is not possible to measure the white reference value for video signals. Video signals themselves do not specify a color until they are used to drive a display (whether a real or theoretical display). It is the white point of the display that provides the white reference for a chromatic adaptation transform. That is determined by driving the display with maximum RGB values and measuring the color with a spectroradiometer or colorimeter. It might also be available from reading the display specifications (if you trust the accuracy).  (Back to top)

Is there a reference offering a method for the bi-directional calculation of densitometric quantities (particularly status M) and spectral transmittance to be used as weighting coefficients in the integration to solve for XYZ tristimulus values? (278)
In general it is not possible to perform the reverse computation of spectral transmittance from densities. The density values are a weighted integration of spectral transmittance and it is impossible to recreate arbitrary spectral transmittance functions (perhaps 31-dimensional) from the 3-dimensional density data. If however, you are working with density measurements for a single system, particularly a 3-color system like photographic film, it is possible to derive an empirical relationship (based on sound theory) between the density measurements and the dye or pigment concentrations. You could then compute spectral transmittance (or reflectance) from the dye concentrations and knowledge of the dye properties. This has been previously done in the context of media-dependent scanner calibrations. A reference on that topic is R.S. Berns and M.J. Shyu, "Colorimetric characterization of a desktop drum scanner using a spectral model," Journal of Electronic Imaging 4, 360-372 (1995).  (Back to top)

What is the difference between perceptual colour gamut mapping and categorical colour gamut mapping? (277)
These terms can mean different things to different people. However, in general you could think of categorical color gamut mapping as one form of perceptual color mapping. Categorical color mapping aims to make sure that colors in the reproduction do not cross color-naming category boundaries defined typically according to the 11 basic color terms of Berlin and Kay (white, gray, black, red, green, blue, yellow, purple, pink, orange, brown). Presumably, if the colors all remain in the same categories, the reproduction will approximately match the original perceptually. Perceptual color gamut mapping tries to best reproduce the appearances of the original image by minimizing changes in some color space such as CIELAB. Generally some rules are adopted to set priorities for the distributing the necessary errors (e.g., preserve hue, use a sigmoidal lightness mapping, and clip chroma). A more general perceptual gamut mapping algorithm wouldn't specifically concern itself with color-naming categories although the result of a good algorithm should also be reproduced colors that do not change categories from the original.  (Back to top)

What is the basis for 18% reflectance being the center of the lightness scale in many different systems? For example: Munsell, Ansel Adams zone system, and CIELAB. Coincidence? I doubt it. (276)
You are correct that it is not a coincidence. The number you are referring to ranges between about 18% and 20% reflectance in various systems. There are two bases for this number. The first is that the average reflectance of natural scenes tends to be about 18-20%. This was measured by various researchers, perhaps the most famous of which was by Jones and Condit at Kodak in 1941. This value has been used to set proper exposures in photography. The second basis is human perception. The appearance of middle gray (perceptually half way between white and black) has a reflectance of about 20% (and hence the definition of Value 5 in Munsell or L*=50 in CIELAB). Is it a coincidence that the average reflectance of natural scenes and the human perception of middle gray have the same reflectance factor? I don't think so. The human visual systems functions by sensing contrast from the background average and evolved in a world with this average reflectance. No coincidence there.  (Back to top)

How you can relate the color temperature to RGB values of video signals? Will the color temperature vary frame by frame or will it vary pixel by pixel of the signal? Can you give the mathematical equation by which color temperature and RGB values are linked? (275)
Color temperature (or more correctly in this case Correlated Color Temperature) is often used to describe the color of the white point of a video display. For example 6500K is similar to an average daylight, while a higher color temperature would represent a more bluish white and a lower color temperature would represent a yellower white point. Since this is really a property of the display and not the video signals, it doesn't make sense to compute it on a frame-by-frame basis and there is no directly link between RGB values and color temperature since the same values can produce different color on different displays.  (Back to top)

I measure animal colouration, in particular contrast, which is dependent on spot size, not just area coverage. I cannot measure the spots on a lizard belly, but I can get a numerical matrix from a digital photo. Can you help me calculate perceived contrast? (274)
Given your point about equal areas of black and white producing different contrast depending on the spot size, you will not be able to do simple statistics on just the colors and area of colors. Instead you need to also factor in the size of the patterns. In image processing and visual perception, we would say that you need to analyze the spatial frequency of the pattern and compare that to the human contrast sensitivity function (CSF) to see which patters we are more, or less, sensitive to. This can get quite complex and requires assumptions about the viewing distance. That would be one approach, but I suspect it would make your analysis more complicated than it needs to be. Perhaps you can simply look at the average colors of the background and features (dots) in the photos (either in RGB, or LAB if you want something a little more perceptually meaningful), the relative areas of each, and the number of features (dots). That would give you five pieces of data you could analyze statistically (although area of background and area of feature might really be the same data if there are no other colors on that area of the animal). I would suspect that would give you something to correlate with your perception of contrast.  (Back to top)

Would you explain the meaning of ΔH*? (273)
Color differences in CIELAB, ΔE*ab, represent Euclidean distances between the coordinates of the two colors in question in the L*, a*, and b* dimensions. These differences can be expressed in terms of lightness, ΔL*, chroma, ΔC*, and hue, ΔH*, difference components in the same Euclidean units. ΔH* is computed as the residual difference in the overall color difference, ΔE*ab, after the lightness, ΔL*, and chroma, ΔC*, components can be removed. That's the long way to say that ΔH* is a measure of the perceived hue difference in Euclidean CIELAB units (same size as L*, a*, and b* units) as opposed to a difference in hue angle in degrees (Δh ... note it's a lower-case h for hue angle difference).  (Back to top)

Can visual color discrimination be taught? Does color discrimination become better through practice? What affect does the aging of a person's eyes have on color discrimination? (272)
You can't really teach someone to improve their color discrimination. However, in some applications you can teach observers how to better look for the differences. For example in image reproduction, some observers typically overlook huge color differences that are clearly perceptible once they are pointed out. When observers are trained to look in the "right places" it seems like they have improved their color discrimination. They really haven't, they've just improved their observing skills. Also an observer's motivation can have an effect on their apparent ability to discriminate colors.

As people age, their ability to discriminate small changes in color often deteriorates to a degree. More importantly their color vision changes due to yellowing of the lens of the eye. This can result in metameric color matches for one observer being significantly different from those for another. Of course this is just as true for multiple observers of the same age as it is for a single observer as they age.  (Back to top)

What is the scientific termonology for when you fold a piece of fabric and two areas appear to be entirely different colors? I've seen this demonstrated: after folding, both sections of fabric appeared to be different colors. (271)
I've asked a couple of people and we can't come up with a specific term for that situation. One possibility could be "goniochromatic", which means something changes color with viewing angle. Another possibility might be "geometric metamerism", but that would really be a misuse of the term "metamerism" (which would imply a pair of samples that match for one viewing geometry, but mismatch for another).  (Back to top)

I need to find a way to measure the saturation and brightness of printed colors and pen ink. I need to select hues of equal brightness and saturation in my studies. Can you provide information about machines that can make such readings? (270)
For starters, you can check this answer to get a list of instrument manufacturers. Your best bet is to look at their web pages to see what's available that could meet your needs. Konica-Minolta is one company that makes a line of rather inexpensive colorimeters and spectrophotometers that might meet your needs. The other companies mentioned might have appropriate instruments as well (the GretagMacbeth Eye-One is an instrument that comes to mind).

What you will probably want to measure are CIELAB coordinates. This color space provides three parameters, L* which correlates with perceived lightness, h which correlates with perceived hue, and C* which correlates with perceived chroma. Chroma and saturation are similar perceptual dimensions. Color scientists worry about the differences, but for most applications the chroma dimension is the one that people are most interested in.  (Back to top)

I have used a spectrophotometer to analyse colour samples. How do you interprete the curves, for example prussian red, since there are many peaks shown? (269)
Unfortunately I cannot answer your question without more details on what you are hoping to accomplish. Some applications, like identification of pigments or their concentrations, are performed directly with the spectral reflectance measurements. Others, like color evaluation and control are completed using colorimetric parameters such as CIE XYZ or CIELAB L*a*b* values computed from the spectral measurements (often by software that comes with the instrument). A great introductory reference to learn about these different ways to use spectrophotometric data is Berns' Principles of Color Technology, 3rd Ed.  (Back to top)

My technical director asked if I could find an "easy to understand" explanation of "CIE 2000." He may mean the CIE DE2000 mentioned in this publication. Is there a publication or expert that could assist us? (268)
I think you are correct in assuming that he was referring to the CIE DE2000 color difference equation. I don't know of any other reference to "CIE 2000". There is another CIE publication that describes the equations in more detail. Unfortunately I don't know of any textbooks that yet have an explanation of the equations. (Berns' "Principles of Color Technology, 3rd Ed. describes much of the background work, but was published just before DE2000.) You may also want to look at a paper that one of my colleagues and I wrote on color difference measurement for images. It contains a lot of spatial imaging information that is not necessary, but starting on page 14 is a fairly brief discussion of DE2000. This paper can be found in Color Research and Application in 2003 (G.M. Johnson and M.D. Fairchild, "A top down description of S-CIELAB and CIEDE2000," Color Research and Application, 28 425-435 (2003)).  (Back to top)

Can you provide me a graph of grayscale Values (0-255) verses density? I need it to evaluate a grayscale calibration wafer. (267)
Unfortunately the answer is "no." There are simply too many undefined variables. Luminance (or at least relative luminance) would be the common characteristic, but you need to have a monitor calibration and characterization in order to relate the digital counts (0-255) to the relative luminance values displayed. On the density side, if you have visual density, then that is logarithmically related to relative luminance, but the relationships for other density metrics might not be so direct.

So, to derive the answer, you need to first convert density to relative luminance using the definition of density you are interested in, and then relative luminance to digital counts using the display characterization. That can give you the relationship for your particular devices and measurements.  (Back to top)

How can I specify a color so that manufacturers the world over will use the same color paint? (266)
Yes, there is a standard way to match your color. What you want is a colorimetric specification and tolerance. Typical specifications are in terms of CIELAB coordinates of L*, a*, and b* and tolerances are normally set around the desired color in terms of some color difference equation. When the paint manufacturers matched your samples in the past, it is likely they measured existing samples and then matched those CIELAB coordinates. Perhaps you can get those values from one of the paint manufacturers to set your standards. Alternatively you can choose a standard sample and send it to a paint supplier and they can measure it. Regardless, it is always good practice to have a physical sample as a standard to allow visual checks of the instrumental color matches.  (Back to top)

Could you recommend a source for calibration tile standards for a camera-based color measurement system? (265)
The answer really depends on how much money you want to spend. If you want the best (and very expensive) standards, then calibrated white and black standards are available from the National Institute of Standards and Technology (NIST) in the USA, the National Research Council of Canada, the National Physical Laboratory (NPL) in the UK (they provide a set of ceramic tiles known as BCRA tiles), and other national standards laboratories.

For many practical camera characterization questions, people use the GretagMacbeth ColorChecker Chart, which includes white and black samples among several others and has nominal calibration data. Another inexpensive option for black and white might be a Kodak gray scale target.

Alternatively, for black you could just use a box painted black on the inside and shoot it in a darkened room. It would be safe to assume that represents an ideal black with a reflectance of 0%, but maybe that's too dark for your application.  (Back to top)

How do I create a value Intensity scale illustrating three dimensions of colour? Could you define a cool and warm colour and whats a colour scheme? Could you give me examples of complementary, split-complementary colors? (264)
The terms you are asking about are commonly used in color theory for artists. I am not terribly familiar with how they are all used and they are often used in inconsistent and imprecise ways. I would suggest you find a good book on color theory for artists and review that. Probably you should find two books and compare the definitions. Also, these websites seem to have some insight painting.about.com, and www.sanford-artedventures.com.

I'll try to provide some insight on your three questions from a scientific viewpoint. (1) Value and intensity both refer to the light-dark dimension of color, so it doesn't make sense to use those terms in conjunction with three dimensions. You could create value or intensity scales for any colors and fill a three dimensional space showing how any given color varies from light to dark. The Munsell Book of Color provides a nice example of this. (2) Cool generally refers to colors that are more bluish or greenish, while warm refers to colors that are more reddish or yellowish. As far as I know color schemes are just sets of colors that "work well together" for a given application. (3) These refer to relationships of various hues that are part of the color theory mentioned above. Monochromatic would refer to a range of colors of a single hue and achromatic refers to neutrals (from black, through grays, to white).  (Back to top)

Where do I find a definition of the term CIEJab? I found the term CIEJab in the Microsoft Longhorn specification in their document "Gamut Boundary Description and Gamut Shell.doc" (263)
The simple answer is that there is no such thing as CIEJab as an official CIE designation. Microsoft has apparently taken it upon themselves to define a new term. Given the context you provided, I would assume they are referring to use of the CIECAM02 color appearance model without tying themselves down so specifically (you could get similar parameters using CIECAM97s). J is the lightness correlate in CIECAM02 and would be similar to the L* correlate in CIELAB. There are no a and b correlates in CIECAM02, but there are several sets of similar parameters depending on whether the dimension being described is colorfulness, chroma, or saturation. Chroma is the one most commonly used in imaging, so I would assume that Microsoft is referring to the rectangular chroma dimensions in CIECAM02, which are designated aC and bC (with the C as subscripts). These are described in Chapter 16 of the second Ed. of "Color Appearance Models".  (Back to top)

Did Munsell ever design and make public an eight-hue color circle? If not, such false facts are circulating on the internet. For example, see mauigateway. (262)
I looked at the website you referenced and it mentions the system developed by Munsell in 1905. This is when Munsell first published the "Atlas of the Munsell Colors", which has evolved into the current "Munsell Book of Color". Munsell designated 5 principle hues (red, yellow, green, blue, and purple) and 5 intermediate hues to complete the hue circle. Each of these 10 hues then had 10 subdivisions resulting in the 100 hues in the Munsell system. Thus, you might call the Munsell system a 5-hue circle, a 10-hue circle, or a 100-hue circle (probably the best answer), but it certainly is not an 8-hue circle and never was. I am not aware of Munsell having ever developed an 8-hue system. That reference, as well as most of the other information on the website you referenced, is erroneous.

The website attempts to describe a "true" hue circle. The "trueness" of a hue circle depends on the objective for its use and the type of materials that are being mixed to produce the colors. There is little value in the website other than for mixing the specific transparent colorants mentioned by the artist. The color names on the circles are completely arbitrary.  (Back to top)

Why do painted walls of saturated complementary colors cause eyestrain when placed close to one another? (261)
It would depend upon the specific complementary-color combinations, but the most likely cause is accommodation changes due to chromatic aberration in your eye. Essentially, different colors are focused at different positions within your eye and your visual system must adjust its focus to view those different colors in optimal sharpness. If you are quickly looking back and forth between two very saturated colors, you are forcing your eyes to change accommodation (focus) rapidly just as if you looked back and forth quickly between near and distant objects. This is tiring and strains your eyes. Another contributor might be changes in chromatic adaptation since your eyes are trying to adapt to the two strong colors in succession. This effect is similar to sitting in a room and flipping the lights on and off repeatedly. That would also strain your eyes.  (Back to top)

In relation to Kodak absorption filter graphs; What does "Transmittance" & "Diffuse Density" mean? and the difference between them? (260)
Our collective wisdom is that the Wratten filters were measured using a GE Hardy spectrophotometer with the filters up against the sphere port. Thus the geometry was 8° incidence and total hemispherical collection. Assuming Helmholtz reversibility of optics, this is equivalent to diffuse illumination and nearly straight on viewing, a common photographic geometry, e.g., transparency viewer. Transmittance is the amount of light transmitted compared to the amount incident. Density is the "common logarithm of the reciprocal of transmittance" according to the Kodak Filter guide. So, 10% transmittance is equal to a density of 1.  (Back to top)

Regarding the use of Kubelka-Munk theory and reflectance values, if I used the spectral reflectances for the Macbeth Color Checker, how do I determine the K and S values needed for the calculation (if I want to used the two-constant theory)? (259)
It is not possible to compute separate K and S constants for a single sample. Instead you need to have samples with several different concentration levels of the pigments involved in order to figure out the K and S constants. You can convert a single reflectance factor measurement into a (K/S) value using the normal equation for single-constant theory.

(K/S) = ((1-R)^2)/2R

There is more detail on this and further references in Berns' Principles of Color Technology, 3rd Ed. chapter 8.  (Back to top)

Outside my building, someone has spray painted the letters USAF with blue paint. The snow under the paint has melted putting the letters in relief. How come? (258)
I am assuming they painted USAF in blue on the snow. When the sun came out, the white snow, since it reflects essentially all wavelengths of light, did not absorb very much energy from the sun and therefore did not heat up and melt. The snow painted blue, however, would absorb most wavelengths of light (absorbing green and red energy is what makes it look blue) and this absorbed light is transformed into heat, raising the temperature of the snow and causing it to melt. Therefore, where the blue letters were, the snow would melt away leaving impressions of the letters.  (Back to top)

A customer has asked for these colors: Light Green: 0.5G6.25/6.3 and 0.5G5.6/7.0 and 0.5G5.1/7.5, but I don't know how to translate it. He told me it's from Munsell and color is light green. How can I transfer to Pantone or NCS etc. to make a sample for him? (257)
Those are Munsell designations for various greenish colors. You can download free software from GretagMacbeth to convert from Munsell designations into a variety of color spaces. It does not include Pantone or NCS conversions. Pantone does not publish conversions to colorimetric spaces. I am not sure if an NCS conversion is available since their website is not working as I write this. However, you might be able to work with the CIE XYZ or CIELAB values from your the Munsell software to formulate your plastic pellets.  (Back to top)

Can I increase the opacity of a white Ink (at a given thickness) without affecting the color? I use TiO2. Are there other pigments that will increase opacity without affecting the whiteness index or CIELAB values? (256)
First of all, you can't really increase opacity without changing the color. Opacity, by definition, affects color since more or less of the substrate is showing through. Presumably you were referring to not changing the color of the ink (i.e., keeping it white). To my knowledge, TiO2 has, by far, the largest refractive index of these pigments. As such, it would be the best scatterer (and therefore produce the most opacity). All the others are much poorer, especially when mixed with clear ink. Another possibility might be zinc oxide. There are also synthetic scatterers that are polymer beads. Perhaps they can be mixed to provide a good result. Yes, there are optimal pigment/volume ratios and it would probably require some experimentation with your particular inks to determine the best for your application. Pages 8-10 in Berns' book, Principles of Color Technology, 3rd Ed. have some introductory material about scattering. A more detailed reference that might be of value is "Industrial Color Testing 2nd Ed." by Völz. It includes a lot of material on scattering and particle size.  (Back to top)

Digital image sensors (such as those used in digital cameras)use red, green, blue ink-based color filters to generate color. Do they therefore have a color gamut that limits the range of colors that they can detect? (255)
Let's start with the short answer to your question; there is no such thing as a camera, or scanner, gamut. A gamut is defined as the range of colors that a given imaging device can display. To say that a camera had a gamut would be to imply that you could put a color in front of it that it could not possibly respond to. While it is certainly possible that two colors that are visually distinct might be mapped into the same color signals by a camera, that does not mean that the camera could not detect those colors. It just couldn't discriminate them. For example, a monochrome sensor will map all colors into a grayscale image and encode it as such. Certainly the encoding has a gamut (in this case a lightness range with no chroma information), but did the camera responded to all the colors put before it. It is the encoding that imposed the gamut. In the color world, encoding is based on some explicit or implied display. For example, sRGB is a description of a display and therefore defines a gamut (but only if the sRGB values are limited in range). If a camera encodes an image in sRGB, that doesn't mean that the range of colors the camera detected are only from within the sRGB display gamut, but it means the camera data have been transformed to best use that sRGB encoding. As long as a camera has three or more sensors that span the visual spectrum, then it will respond all the same stimuli as our visual system. Whether the camera can discriminate colors as well as the human visual system will depend on the encoding of the camera signals, quantitization, and the details of the camera responsivities. (To return to the black and white system, that camera encodes all the colors into a gray scale. They could then be displayed as any color within a given display, but many colors from the original scene would be mapped to the same values.)

Since there is no such thing as a gamut for an input device, then there is no way to compute it or calculate a figure of merit. Generally, the accuracy of color capture devices is assessed through the accuracy of the output values for known inputs in terms of color differences. Also, sensors are sometimes evaluate in terms of their ability to mimic human visual responses (and therefore be accurate) using quantities with names like colorimetric quality factor, that measure how close the camera responsivities are to linear transformations of the human color matching functions. Doing an internet search on "colorimetric quality factor" will lead you in the right direction.  (Back to top)

Do you have any training materials to illustrate color that I could use as an educational tool? (254)
We don't have any specific educational materials that we provide. Also you might be confusing us with the Munsell Color Company, part of GretagMacbeth. That's probably where you have color chips from. They don't provide educational materials, but there is a lot of information on their website. If you are looking for materials on the Munsell system, there are student sets available from Fairchild Books called "The New Munsell Student Color Set, 2nd Ed.". (Just as our lab is not related to the Munsell Company, Fairchild Books has no relationship to me!). A good general reference on color science and measurement is Roy Berns' Principles of Color Technology, 3rd Ed.. Lastly, if you are looking for educational materials that allow you to provide interesting demonstrations of various, I have had good luck with the material available from Arbor Scientific.  (Back to top)

I have made four boxes out of colored clear plastic and will be putting glass containers inside full of snow inside each one, then leaving them in the sun to see which melts faster. The colors are red, blue, yellow and clear. Can you help me find information on how color will make the snow melt faster? (253)
The answer to your question depends on the fact that as the plastic absorbs light, that light energy will be converted into heat. That's why a black object (which absorbs almost all light) feels warmer on a sunny day than a white object (which absorbs almost no light). To find the answer to your question, you need to find out which of your containers will absorb the most light. There are two aspects to this. One is the colors (or wavelengths) of light that the container absorbs and the second is the amount of each of those colors present in the sunlight itself. It is reasonable to assume that the sunlight has about equal power for all the colors, so you shouldn't have to worry too much about that part.

So, to figure out which of your containers cause the snow to melt fastest, you will need to figure out which one absorbs the most light. One easy way to do this is to simply look at your containers and put them in order from darkest to lightest in color. That should be the order in which the snow melts as well.

If you want to be a bit more scientific about figuring this out, you need to look at how each container absorbs light of different colors. In a laboratory like ours, we have specialized instruments to measure this, but there is a website where you can get a rough idea. It is this one www.rosco.com, a company that makes filters for spotlights and other uses. If you look at all the filter colors they have (lots of them) and find some that look similar in color to your containers, you can then look at the graphs of how they absorb different colors of light. The filters that absorb the most light will have the smallest values on the graphs and the number at the top of the graphs gives an overall percentage of the light that is not absorbed. The color with the lowest numbers on these graphs should melt your snow the fastest.

There is another complication in your project. How much the glass heats up will also depend on how much of the invisible ultraviolet and infrared energy from the sunlight is absorbed. It is probably safe to assume that all your containers have similar absorption of ultraviolet and infrared, but if you discover any results that can't be explained by the colors alone, it could be due to differences in the absorption of invisible infrared or ultraviolet energy. Unfortunately, you can't figure that out by looking at the containers since your eyes do not detect ultraviolet or infrared energy.

Update! The experiment was completed. The red glass melted first in 1 hour 45 minutes. The blue glass was second and melted in 2 hours. The clear glass melted third in 2 hours and 5 minutes and the yellow one melted 2 minutes later.  (Back to top)

Can you provide spectral reflectance values for the Farnsworth-Munsell 100 Hue test? (252)
We don't have them, but I recall someone asking this question recently and the best I could find on the internet was this reference www.ncbi.nlm.nih.gov to a paper that did eigenvector analysis of the reflectances. I haven't looked at the paper itself, but the abstract says they measured the reflectances and even the eigenvector analysis (if it is fully presented in the paper) might let you make reasonable reconstructions.  (Back to top)

I need a teaching aid that uses three areas of colored cellophane (R,G,Y; or R,G,B) which is set on top of a background with text of different colors. How can I find the right cellophane and text color combinations that will allow the user to see only one group of text, while that same text is not visible through the other two cellophane colors segments? (251)
I'm sure it would be possible, but it will be difficult. To make text disappear when viewed through a given filter, you essentially want to make the text the same color as the filter. For example, red text is visible on white paper because it absorbs green and blue light. If you view the red text through a red filter (also absorbing blue and green light), then ideally you will not be able to see the red text because the stimulus reaching your eye is now the same for the text as it is for the paper. I say "ideally" because it is difficult to perfectly match the text and filter colors. Since you want the text to disappear for two of your filters and be visible for only one, it will be very difficult to find appropriate combinations. It would be much easier to make two parts of the text visible for each filter and one disappear. I won't say it is impossible, but it might be impossible in a practical sense. To get some more information on available filters and their spectral transmittance properties (useful for choosing filters and text colors), I would suggest Rosco as a source of affordable filters. Details on their gels can be found here www.rosco.com.  (Back to top)

I am working on a digital conversion for raw files from RGB digital cameras. Can you assist me with a formula to convert floating point data from a digital camera into LCH space and help with transforming that space through a LUT to 4-channel printer separations? (250)

Is there an analytical method for calculating relative powers needed to achieve a specific white balance for monochromatic sources (eg: lasers) of known wavelengths? (249)
First, this answer assumes you are using three monochromatic sources, such as lasers. I will use the wavelengths of the three lasers are red = 671 nm, G = 532 and blue = 473. Further, we need to select a target whitepoint, for which I will use D65.
Basically, the solution is to use the CIE color matching functions to find out how much relative power of each of the three wavelengths you need to achieve a desired ratio of X:Y:Z. In your case the desired result is the tristimulus values of CIE Illuminant D65, X = 95.05, Y=100.0, and Z=108.88. The color matching functions define a 3x3 matrix transformation from RGB power levels (on a relative scale) to the XYZ values. What you need to do is invert that matrix in order to compute the required RGB values to produce your given set of CIE tristimulus values. Using your three laser wavelengths the forward equations are:

X = x-bar(671)*R + x-bar(532)*G + x-bar(473)*B
Y = y-bar(671)*R + y-bar(532)*G + y-bar(473)*B
Z = z-bar(671)*R + z-bar(532)*G + z-bar(473)*B

So you have three equations and three unknowns, RGB, since XYZ are defined by your desire to match D65 and x-bar, y-bar, and z-bar are defined as the CIE 1931 color matching functions. FOr convenience, I inverted the matrix and solved this equation in the this Excel file. My results for RGB are 802, 75, and 93 respectively. Normalizing these to be power levels relative to your green laser's power I get ratios of 10.7 to 1.0 to 1.2 for R to G to B. That's a lot of red, but the red wavelength we are using is getting close to the end of the visible spectrum.

Now, here's where a little more understanding of colorimetry is needed. If you set up the lasers to those ratios, you might still need some fine tuning by eye. There are two reasons for this. One is that the CIE color matching functions are an average, their is significant variation from person to person, and you are stressing their accuracy to the extreme in having a stimulus with only 3 wavelengths. The second is that your red laser is at such an extreme wavelength that it is reasonable to expect the color matching functions to have large uncertainty there. I wouldn't be surprised at all if you had to adjust your red laser by a factor of two relative to the above computed ratio. However, the above ratio should get you close to the right answer.  (Back to top)

My spectrophotometer is getting old. I need a new instrument and software for evaluation and formulation of color paints and ceramics. What recommendations can you make? (248)
There are several companies currently manufacturing spectrophotometric hardware and color matching software capable of replacing your system. Several of these are linked on our lab's sponsors page. The companies most likely to have systems with the flexibility you require are BYK-Gardner, HunterLab, DataColor, and GretagMacbeth. Minolta and X-Rite also manufacture instruments, but I am not sure they have software for your applications. Unfortunately, I am not aware of any "consumer's report" on this instrumentation, however I can say that the quality of all of the instruments is very high and likely substantially improved from the time you purchased your system. I would recommend looking at options from all of the manufacturers and making your decision based on the availability of instruments that can measure the variety of samples you have, the capability of their software for your applications, and the ability to transfer any necessary data from your old system.  (Back to top)

I have a reflectance spectrum of a soil sample. Can you help me convert this reflectance to CIE X,Y,Z and then to Munsell HVC? (247)
First, note that this answer is not specific to soil samples, and will generally work for any reflectance values.

The basic computation is to multiply your sample reflectance by the spectral power distribution of an illuminant (I would recommend CIE illuminants C or D65, average daylight, since you are interested in Munsell designations) and each of the three CIE 1931 color matching functions (x-bar, y-bar, and z-bar). You then sum these products across the spectrum and multiply each by 100 divided by the sum of y-bar times the illuminant. This gives you relative XYZ tristimulus values. The only way to convert to Munsell designations is to use a look-up table since the Munsell notations are not related to CIE tristimulus values by simple mathematical formulae. Fortunately, GretagMacbeth provides free software for this part of the conversion. For a reference on the calculation of CIE tristiumulus values, XYZ, I would recommend Roy Berns' text, Principles of Color Technology, 3rd Ed. If you are interested in software to perform the computation, there is a book by Westland entitled "Computational Colour Science" that provides some example MATLAB code. Westland's website includes a description of the book, some FAQs that address similar questions, and downloadable code.  (Back to top)

How can I determine the whiteness/blackness of CMYK inks? (246)
As it turns out the answer may be easier than you realized. While the exact answer will depend on the specific CMY inks being used, you can use Photoshop to estimate the relative lightness (or darkness depending on how you think about it) of the three colors for typical printing processes. The best metric to use is CIELAB L*, which is a measure of perceived lightness based on the relative luminance factor of the color (which is the CIE Y tristimulus value). Photoshop has an Lab mode that allows conversion between typical CMY values and CIELAB values (the Photoshop L corresponds with CIELAB L*). For typical CMYK inks, you might get approximate L* values of 62, 52, 95, and 12 respectively. This is on a scale where a perfect black is at 0 and the paper white would be at 100. The increments in between are designed to be perceptual such that an L* of 50 looks perceptually half-way between white and black in lightness. These numbers show that the yellow is very light, which is observed in the difficulty of reading yellow text on white paper. Magenta tends to be the darkest of the colored inks, but clearly not as dark as black with an L* of 12.  (Back to top)

Can you help me convert from a Munsell color specification to a RAL color specification? (245)
Thanks for your inquiry. I am personally not that familiar with the RAL specification, but I found these links: paintcenter.org/rj/oct04hh.cfm, and paintcenter.org/rj/apr05a.cfm that suggest that one type of RAL designation is directly related to CIELAB specifications expressed in terms of h,L*,C* (in that order). There is free software from GretagMacbeth to convert between Munsell and CIELAB designations. It appears that you only need to convert the L*a*b* values from the GretagMacbeth program to L*C*h (a rectangular to cylindrical coordinate conversion that can be found in any book on colorimetry) and then round them to integers to convert to RAL specifications.  (Back to top)

I am developing a visual program to pick color harmonies of a color based on RGB values. Are there equations that could help me determine color harmonies? (244)
Color harmonies typically are not specified in rigorous mathematical terms, but rather through general rules based on color appearance. There are also a number of theories and techniques for color harmony that make it difficult to specify equations. However, there are colorimetric spaces, and approximations thereof, that do a good job of predicting color appearance and one of these would be a good basis for deriving color harmony computations. The most accurate and complex would be to use the CIECAM02 color appearance model. However, that is probably more complex than warranted for this question. As a simpler approach, I would suggest transforming the RGB values into a simple HVC (hue-value-chroma) or HLS (hue-lightness-saturation) space as is often done in computer graphics. You could then define your color harmonies in that space. There is a lot of information on the internet on color harmonies. This page gives some general overview of the concepts. This one provides computations of harmonious colors for various RGBs, but doesn't specify how "harmony" is computed. It does, however, give code examples for transformations between various color spaces (like RGB to HVC). Lastly, several places mentioned this book as a good reference and there are several other links on that amazon.com page.  (Back to top)

Would you explain what ΔE is and how is it measured? (243)
ΔE refers to a quantitative measure of color difference that is designed to correlate with our perception of the magnitude of color difference between two stimuli. There are a number of different ΔE formulas that each compute this quantity in a slightly different way. The reason there are more than one is that the prediction of perceived color difference is an imperfect science. There are a lot of physical and perceptual variables involved and research is ongoing to develop improved color difference equations. That said, there are several that are commonly used with significant success. The simplest is the CIELAB ΔE*ab, which is simply the Euclidean distance between the color coordinates of the two stimuli in the CIELAB color space. The coordinates are L* (lightness), a* (redness-greenness), and b* (yellowness-blueness). More complicated color difference equations typically start with these CIELAB coordinates, but are based on more complex distance formulas intended to better predict human perception. The measurements of the stimuli required are either spectral reflectance/transmittance of materials or the spectral power distributions of light sources which are then converted into CIE XYZ tristimulus values and then CIELAB coordinates computationally. A good overview of the development and use of these color difference equations can be found in Berns' introductory text, Principles of Color Technology, 3rd Ed. MCSL also has several related resources available.  (Back to top)

What is L-star? (242)
In the CIE system of colorimetry, the Y tristimulus value represents the luminance factor of a sample. This number is intended to correlate with our perception of how light or dark the sample appears, referred to as lightness. Our perception of lightness also depends on our state of light adaptation, which is how our visual system adjusts to various illumination conditions (like automatic exposure on a camera), and is nonlinearly related to the energy in the stimulus (represented by Y). Thus, the L* equation was derived to account for those two factors. It accounts for adaptation by dividing the sample luminance factor by the luminance factor for the illuminant (or a perfect white) and it accounts for the compressive nonlinearity in our visual response through the cube-root and other constants in the equation. Therefore, L* is a computed colorimetric quantity that is intended to linearly correlate with our perception of the lightness of a stimulus. You can find more details on the L* in any modern textbook on colorimetry such as Principles of Color Technology, 3rd Ed. by Berns. L* is also one of the three dimensions of the CIELAB color space where the other two, a* and b*, represent the hue and chroma of the simulus.

The L* equation is
L* = 116(Y/Yn)^(1/3) - 16
where Y is the luminance factor of the stimulus and Yn is the luminance factor for the illuminant (or perfect white).  (Back to top)

Why might the published measurements for Munsell neutral scale differ? For example, the reflectance values for the 1929 "Munsell Book of Color" are different in the 1933 Munsell et al article as compared to the 1940 Glenn article. (241)
I took a look at the two sets of numbers and I cannot come up with a definite answer. Historically, reflectance values were measured relative to smoked magnesium oxide which has a reflectance factor of about 0.98. That would make an older reported value of 1.0 (or 100%) map to a modern measured value of 0.98 (or 98%). That was my first thought for your data, however, the two sets don't differ by a constant factor and the discrepancy is in the wrong direction for this explanation. The differences also do not suggest any other simple error in the measurements. There are several factors that could contribute to what you see in the data. These include: measurement of different samples, degradation of the samples (fading or becoming soiled), differences in the instrumentation (geometry, measured area, accuracy/precision, calibration). I would suspect that these differences are caused by differences in the measurement techniques and instrumentation, particularly in that time period. For example, it is quite likely that the 1933 measurement were completed with a visual instrument (perhaps relative to some arbitrary white) and that the 1940 measurements were completed with an early photo-electric spectrophotometer.  (Back to top)

What is the method for selecting the best combination of CMYK inks for a cmyk ink jet printer? What is the best way to determine the CMY values in order to get as large color gamut printer as possible? (240)
Generally, this is accomplished through simulation using a mathematical model of the printing procedure. One would start with candidate spectral reflectance properties of inks (or model them as simple functions) and then manipulate those while optimizing the volume of the color gamut expressed in a space such as CIELAB. More recently, researchers have been following similar procedures to optimize the selection of greater numbers of inks for 6- or 8-ink printers. There has been a lot published on this topic at the IS&T/SID Color Imaging Conference over the years (see www.imaging.org) with one very recent example that includes numerous references to previous literature coming from our laboratory (Y. Chen, R.S. Berns, and L.A. Taplin, Extending Printing Color Gamut by Optimizing the Spectral Reflectance of Inks, IS&T/SID 12th Color Imaging Conference, Scottsdale, 163-169 (2004).). An abstract can be found at www.imaging.org/store/epub.cfm?abstrid=32139.  (Back to top)

I do not have access to a spectrophotometer to measure paint reflectances. Where can I obtain the reflectance data for determining the Kubelka-Munk coefficients? (239)
This is a difficult question. If you are interested in just working with the Kubelka-Munk theory in general then you can essentially make up reflectance values to use in your code. Another alternative would be to search the internet for reflectance databases. For example, our website www.cis.rit.edu/mcsl/online/cie.php has spectral reflectances for the Macbeth Color Checker. Unfortunately those sources won't give you information about colorants at various concentrations that you would need to rigorously test a Kubelka-Munk implementation. If you are interested in a specific system (like a brand of paints), you might try making a request to the manufacturer for some nominal spectral reflectance data. Another alternative is to look for specification sheets from colorant manufacturers, who sometimes include reflectance data. Unfortunately, the only way to truly build and test a colorant formulation system is to have ready access to a spectrophotometer of your own. You might also refer to this web page regarding textile dying www.techexchange.com/thelibrary/colormatching.html.  (Back to top)

I want to calibrate my monitor by eye. Are there color charts available for download that can help me do this? I require skin tone photo, color and gray scale patches. (238)
This is a difficult question because an accurate monitor calibration/characterization requires both the display of desired values and typically their instrumental measurement to associate those values with the proper colors. A number of schemes have been devised to approximate this process visually. Adjusting your monitor with a test image requires an image that looks "good" or "correct" on an ideal monitor. The question then becomes, "what is the ideal monitor?" There is no single answer for that. The image you describe is much like the Kodak Q60 target that is used for calibrating scanners. So, if you could find a digital version of that that looks good on an ideal monitor, you could use it for visual adjustments. As an alternative, a Google search turned up this website that has some tips on visual and instrumental monitor setup and also includes links to images that might fulfill your needs. Since these images are designed to be "good," visual adjustment of your monitor for those images should at least bring you into the same general range as others in the field.  (Back to top)

What is the established color difference tolerance for the plastics industry? (237)
There are not generally accepted standards for specific industries that suggests a single ΔE value as a standard. The tolerance is usually agreed upon between buyer and seller for the particular colors and applications in question. There are several issues that preclude use of such a simple tolerance. The first is the specification of the color difference equation to be used. Choice of a different equation (e.g. CIE94, CIE2000, CMC, or CIELAB) will result in differences in the perceptual magnitudes and characteristics of the stated Delta-E tolerance. The second is the dependency on color. For example, a CIELAB ΔE of 1.5 for a gray color is perceptually different from the same ΔE for a yellow color. This is part of what the newer equations like CIE94 and CIE2000 attempt to address. The differences between the various color difference equations and techniques for establishing meaningful tolerances with them are described in Chapter 4 of Roy Berns' book, Principles of Color Technology, 3rd Ed.  (Back to top)

What is a color management system? (236)
Color management systems address problems associated with color reproduction within modern desktop environments. Since computers communicate only through the use of binary numbers, color peripheral devices must relate between numbers and colors. This unnatural association is at the root of a host of quality problems found in a color reproduction system. Unless some digital intervention takes place between the input side of the color reproduction process and the output side, many factors conspire to make it likely that some number of colors will be reproduced poorly. Color management systems provide a level of intelligent digital intervention to attempt to create a superior level of matched colors.

Color management systems consist of a collection of data structures, algorithms, protocols and methodologies. The industry de facto standard for color management has been specified by the International Color Consortium (ICC) ICC defines a source device as the input device which delivers an image and a destination device as the output device which renders an image. The color properties of these devices are encapsulated in an ICC data structure known as a device profile. In an ICC workflow, a Color Management Module (CMM) invokes appropriate algorithms for transforming an image delivered by a source device so that it will be rendered with appropriate colors on a destination device. Parameters for the transformation algorithms are derived by a CMM through reference to two profiles - one associated with the source device and one associated with the destination device.  (Back to top)

How should a color lab be designed? What colors should the walls and furniture be, so as not to influence the perception of color? (235)
There probably is no single correct answer to this question, but the general best practice is to have the walls and furniture a neutral gray to minimize the impact on the color judgements. A preferred wall paint would be similar to the gray inside a standard light booth. This is typically a neutral with a CIELAB L* of approximately 70. To avoid having a completely gray lab, it is sometimes pleasing to have some texture in the countertops or tabletops (like a granite texture) and black furniture can also be used. You also need to make sure you are using appropriate illumination for your application. Sometimes it is also helpful to paint the wall behind an observer black to minimize any reflection of ambient light from behind the observer onto the color stimuli being evaluated. Lastly, rooms were color measurements are made are often painted completely black to minimize any stray light bouncing around the room and contaminating the measurements.  (Back to top)

Can you help me convert between RGB or CMYK and Munsell, CIELAB, or CIEXYZ? (234)
There is a freeware program for Munsell conversions (runs on Windows systems only) available from GretagMacbeth (the company that now sells Munsell products). Among other things, it converts from Munsell notations to XYZ, CIELAB, RGB, and CMYK. It can be found at www.gretagmacbeth.com.  (Back to top)

My two eyes see color differently. What causes this? (233)
You have posed a very interesting question. Fortunately it has a rather simple explanation. There is an area on your retina called the fovea where you have the greatest concentration of cone photoreceptors and therefore the best visual acuity and color vision. When you look at an object, you move your head and eyes such that the image of that object falls on the fovea. The fovea is protected by a yellow filter called the macula. This yellow filter protects this important part of your retina from accidental high-intensity exposure to short-wavelength light (blue and ultraviolet light is partially absorbed by the yellow filter). It is very common for the density of this yellow filter to vary from person to person causing significant differences in color vision. While we don't normally notice a yellow tint to the world around us due to chromatic adaptation (our ability to adjust to overall changes in the color of the illumination), there are more subtle differences in vision that can be observed. It is also not uncommon for people to have differing amounts of macular pigmentation between the two eyes. An eye with greater density of macular pigment than will see colors slightly warmer (redder). This is nothing to be concerned about and usually goes unnoticed. You could probably find a light yellow filter that would equalize the perceptions and give you a rough idea of the differences between your two eyes.

Also, the lens in our eyes yellows as we age, providing another cause of differences in color vision between observers. However, it is not widely recognized that the lens yellowing would be significantly different from one eye to the other (although theoretically it could be and would be if, for example, an observer had cataract surgery on one eye). Refer to a book on visual perception for more details on the various eye structures mentioned above.  (Back to top)

What can you tell me about the blend mode algorithms? (232)
The specific algorithms in a commercial software package like Adobe Photoshop are usually proprietary. However, a little investigation with various images can usually be used to figure out what the program is doing, if not the actual algorithmic implementation. There is an open source program called GIMP that has similar functionality. There is an explanation of the blend mode algorithms in GIMP and a more generic and detailed explanation of blend modes with reference to Adobe Photoshop 6.0.  (Back to top)

What are color vision deficiencies? How common are they? (231)
Color vision deficiencies result from either a lack of one or more cone types, or cones that behave somewhat differently from average. Those lacking long wavelength cone pigment suffer from protanopia. A related condition is anomalous protanopia, or protanomaly. Here, long wavelength cones are present, but their sensitivity is shifted spectrally to shorter wavelengths, so they interpret certain stimuli differently than normal observers. Similarly, deuteranopia (deuteranomaly) is the lack (spectral shift to longer wavelengths) of the middle wavelength cone pigment, and tritanopia is the lack of short wavelength cones (tritanomaly is incomplete tritanopis). It is important to remember than none of these conditions should be referred to as true color blindness. This is not simply a politically correct statement. In fact, those suffering from any of these conditions do experience color, but not in the sense that a color "normal" observer does.

Other less-common deficiencies are rod monochromacy and cone monochromacy. With rod monochromacy, there are no cones present, only rods. Persons suffering from this are truly color blind. With cone monochromacy, a person has only one cone type. For more details on these and other vision deficiencies, see reference 3.

From reference 2, percentages of the more common deficiencies:
 Type Male % Female % Protanopia 1.0 0.02 Deuteranopia 1.1 0.01 Tritanopia 0.002 0.001 Cone monochromatism ~0 ~0 Rod monochromatism 0.003 0.002 Protanomaly 1.0 0.02 Deuteranomaly 4.9 0.38 Tritanomaly ~0 ~0 Totals 8.0 0.4
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What is color? (230)
Color is that characteristic of a visible object or light source by which an observer may distinguish differences between two structure-free fields of the same size and shape, such as may be caused by differences in the spectral composition of the light concerned in the observation.[ref 1, p 723] In other words, color is that perception by which we can tell two objects apart, when they have otherwise similar attributes of shape, size, texture, etc.

OK, that's the textbook answer. This is admittedly unsatisfying, because color is an inherently subjective experience. Color only exists in our minds, and putting a scientific definition together of no easy task. The usual definition, given above, is really a circular argument. It amounts to: "Color is that attribute of an object leftover when you eliminate all attributes except color." So, if an two objects look different, but have the same size, shape, texture, etc., then the way you are telling them apart is their color. Still not satisfied? Here's another answer for you.  (Back to top)

How do we see in color? (229)
In the retina of our eye are photoreceptors that are sensitive to light. When light is absorbed by the photoreceptors, the light energy is converted into electrical and chemical signals that the neurons in our eye and brain process. There are two kinds of photoreceptors in the retina: rods and cones. Rods mediate vision at lower levels of illumination. Cones mediate vision at higher levels of illumination. There are three types of cones with each type differentially sensitive to a different region of the visible spectrum. They are known as the Short-wavelength sensitive cones, the Middle-wavelength sensitive cones and the Long-wavelength sensitive cones. Sometimes they are referred to as R-, G-, and B-cones but these are misnomers based on the colors in the spectrum. For example, very short wavelength light can uniquely stimulate the S-cones but the sensation associated with this light stimulation has a reddish and bluish component. Fundamentally our color vision derives from comparisons between the amount of light being absorbed by each cone type. Our visual system compares the outputs of the cone types to process color. In addition, color appearance is influenced by the ratios of cone excitations in surrounding regions and by the overall levels of cone excitation caused by the prevailing illumination. These comparisons occur at different stages of processing that start in the retina and continue to the cerebral cortex of the brain.  (Back to top)

What can you tell me about automotive paint pigments? (228)
MCSL doesn't do too much work on the actual formulation of pigments and other colorants. However, we do a lot of research on the use of colorants in various applications and the perception of the resulting colored materials. As you suggest, there are a wide variety of materials used as colorants in the automotive industry ranging from simple inorganic compounds to complex pearlescent or metallic "effects" colorants. To get more information on specific colorants and applications it is probably best to contact pigment manufacturers directly. A first step might be the Color Pigments Manufacturers Association. Also automotive paint manufacturers provide some information on their processes. For example, some can be found at DuPont Automotive. Also, a couple of books you might look at are "High Performance Pigments" by Smith and "The Pigment Handbook" by Lewis. The ultimate reference for all types of colorants is the "Colour Index International."  (Back to top)

What do we know about research done on colors & their meanings? (227)
This is indeed a very interesting question and one that comes up often. It is certainly true that color alone can evoke strong emotional and cognitive responses. However, the systematic study of such responses is somewhat uncharted territory. This is largely because many (if not all) of these responses are learned and we all have different life experiences. A color that evokes a very negative response, or meaning, to me could evoke great joy in another observer. There are also almost certainly cultural differences in the meaning and interpretation of colors. That said, some research is done on the topic. One place to learn about it is in the journal "Color Research and Application". Another place you might look for some introductory information is the Color Association of the United States Color Association of the United States. A good overview reference on the topic is chapter 5 of the first edition "The Science of Color" published by the Optical Society of America. The title of the chapter is "Psychological Concepts: Perceptual and Affective Aspects of Color". Unfortunately that topic is not covered in the second edition of the book. Another interesting book with some prototypical psychological interpretations of various hues is "Color Bytes: Blending the Art and Science of Color" by Bourges.  (Back to top)

What does it mean to be color normal? (226)
A color normal individual is one whose color vision is not greatly different from the average person. This may seem obvious, but remember that the color vision deficiencies are, in most cases, not simply on/off conditions. There is a continuum of, for example, deuteranomalous people. Some will have only a slight shift in middle-wavelength cone sensitivity and others may have so large a shift that middle-wavelength cone behavior is no longer distinguishable from that of long-wavelength cones. A series of color-matching experiments can determine what type of deficiency a person might have. The end result is usually simply an understanding of approximately how far from average an individual might be. To state a cutoff between normal and anomalous vision assumes some criterion which is application specific. In other words, just how normal you are depends on just how normal you need to be for your task. If you are a taxi driver, you certainly need to determine the color a traffic signal. If you are an interior designer, you probably need somewhat better color vision to please your customers.  (Back to top)

What parts of the eye are important for color vision? (225)
 Click on the image for a larger viewUse by permission from reference 4. Most of the important parts of the eye are labeled on the diagram. The cornea and lens focus the image onto the retina. The retina is the part that actually detects incoming light. The iris adjusts in width to partially account for light levels. The fovea is the central focal point of the eye. (That is, when we look at something, we are casting its image onto the fovea.) The fovea is the are where we get most of the spatial detail and color in what we see. The optic nerve is a bundle of nerves which carries the visual information to the brain. Not shown is the macula, which is a filter over the fovea. It serves to limit the damage that might be cause to the fovea if we accidentally focus on intense light sources, such as the sun.
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How do we select names for colors? (224)
This question has been of interest not only to scientists who study color and the visual system but also to linguists and philosophers. The conventional wisdom used to be that culture and language determined our use of color names. This view began to change in 1969 when Berlin and Kay published a book that showed that there is a high degree of universality in the use of color terms across cultures and languages. Now many investigators believe that there is a physiological basis for the use of certain basic color terms and the parsing of color space into categorical regions denoted by these basic color terms (black, white, gray, red, green, blue, yellow, purple, orange, brown and pink). There is evidence from animal studies and studies with infants to support this categorical view of color.

However, people do use many more color words; hundreds of different terms have been catalogued. It seems however that the non-basic terms are used without the same generality and consistency as the basic terms. For example, cyan may have a specific meaning to a couple of printers working together in a print shop but the man on the street may have an altogether different notion of what cyan is. Yet everyone, within the limits of the homogeneity of normal color vision, will agree about the meaning of orange or pink.  (Back to top)

How are cones organized in our eyes? (223)
During the development of the embryo, part of the neural tube which develops into the central nervous system forms outcroppings that extend and develop into the retinas. Therefore the retina is considered part of the brain: a part that is easily accessible for study. The retina consists of 5 layers of cells and between each layer there are extensive interconnections in which visual processing takes place. There are about 125 times as many receptors in the retina as there are ganglion cells in the optic nerve, which connects the eye to the brain. This gives an indication of some of the processing that must go on. Because the retina is so accessible, much is know about the processing of color. The signals from different cone types are segregated in opponent pairs during this processing so that ganglion cells have specific receptive fields that are excited by one cone type in the center and inhibited by another in the surround. Although the effects this organization can be evidenced in psychophysical experiments that measure different aspects of visual function, our conscious experience of color does not relate well to this organization. It is at the later stages of processing in the brain in which our subjective experience of color is processed.  (Back to top)

What spatial and temporal processing takes place in the eye? the brain? (222)
Subjectively, we experience many different qualitative aspects of vision, for example, form, color, motion, depth, etc. This information is all input into the visual system through the photoreceptors in the retina. Therefore the retina has to process the information to preserve all these different aspects of the visual stimulus. The term multiplexing is often used to describe the way the early visual system simultaneously sends this varied information to the brain. Specialized pathways are determined very early in visual processing. The second layer of retinal cells already has specialized neurons that process form by creating center-surround receptive fields. Different types of temporal response are seen in cells with either sustained or transient responses in the next layer of processing. Two streams of visual processing have been identified at the optic nerve level. One responsible for fine detail and color and the other responsible for detecting rapid temporal change with high sensitivity to changes in contrast. (In other animals, cells that respond to directional motion are already present in the retina.)

In the brain we see evidence of hierarchical and modular processing of visual information. At the early stages of cortical processing, cells exist that respond to stationary or moving edges and bars of light contrast, evidence of form processing. At later stages of cortical processing we find areas of the brain that seem to be specialized for the processing of specific visual attributes. For example area MT in the temporal lobe shows a specialization for motion detection. Not only do cells here respond best to moving stimuli with specific characteristics (direction and speed) but also studies with animals has demonstrated that artificially stimulating these cells can influence the perceptual judgments of motion. In monkeys, an area called V4 has been identified as being critical for the processing of color. It has been shown that humans with brain damage in the area homologous to this one lose color vision.  (Back to top)

Chromatic adaptation is the ability of the human visual system to adjust itself in response to varying illuminant conditions. In other words, we adapt to the color of the light source in order to better preserve the color of objects. For example, if viewed under incandescent light, white paper has a decidedly yellow cast. However, we have the ability to automatically account for the yellowish light, and we therefore see the paper as white. If you think about it, this makes a lot of sense. It would be a very confusing world if objects were changing color every time the light source changed. From an evolutionary point of view, we still need to know if the fruit is ripe whether it is morning, noon, or evening. Chromatic adaptation makes this possible.  (Back to top)

How do we characterize a person’s color vision? (220)
There are several test available, depending on what aspects of color vision you wish to focus on. The Farnsworth-Munsell 100 Hue Test presents the observer with 80 color disks and a few anchor points. The observer must order the disks between each anchor point. The color span the whole circle of hue space, and mistakes made are plotted on a polar graph, the angle corresponding to hue, and the distance from center increasing with error in disk placement. General trends, such as large errors in the red/orange region, can be mapped to specific vision deficiencies. Observers with poor performance throughout the hue circle are not necessarily color deficient, but they do lack good color discrimination.

Another popular test are psuedoisochromatic plates. The most common of these are Ishihara\'s Tests for Color-Blindness. These plates, common in grade school vision testing in the U.S., consist of dots of various colors. Plates contain a number or letter which is only visible to observers with the ability to distinguish between the various colors of the dots. This test is not designed to accurately predict specific vision deficiencies, but rather as a general screen for color vision defects.  (Back to top)

How many colors can we see? (219)
This is a very popular question and it is usually answered vaguely by, "Millions and millions!" However, it is better to ask the question in a more specific way to get a more comprehensive answer.

Many of us can select a setting for our computer monitors that displays millions of colors and we see an improvement in image quality with this setting. However, if you select the colors correctly you can reduce the number of colors to a couple of hundred or even fewer (depending on the image) without noticing a degradation in quality. This would indicate that we can’t see millions of color variations simultaneously.

One way to answer the question is to measure the ability of people to discriminate colors. Many researchers have investigated chromatic discrimination by varying the wavelength of two monochromatic lights until they are just noticeably different. Other studies have used the variability of color matching to gauge discriminability and yet others have directly measured threshold differences throughout color space. Using these measures we find that our visual systems can discriminate millions of colors.

In our laboratory we are interested also in larger than threshold color differences: the type of differences that would make you reject a certain touch-up paint because it’s not a close enough match to the color of the paint of your car. Even with such a metric there are close to a million discriminable colors on a computer monitor which only can reproduce a fraction of the colors we can see out in the real world.  (Back to top)

References

1. G. Wyszecki and W.S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae 2nd Ed., Wiley, New York, 1982.
2. R.W.G. Hunt, Measuring Colour 3rd Ed., Fountain Press, England, 1998.
3. R.M. Boynton, Human Color Vision, Special Limited Edition, Optical Society of America, Washington D.C., 1992.