An Inexpensive Schlieren Video Technique
The video shows hot air around a soldering iron.
"Inexpensive schlieren video technique using sensor dead space as a grid,"
Robert B. Teese and Matthew M. Waters,Optical Engineering, Volume 43, Issue 11, pp. 2501-2502, November 2004.
Matt Waters published a short book about the method.
Links for more information about schlieren photography: