Freezer, Schlieren image. Shopper removing an item from a chest freezer. The cold air, seen coiling below the item, has been revealed by Schlieren photography, a way of visualizing density changes in transparent materials. The density change (here caused by cold and convection turbulence) causes light passing through the air to bend (refract). The imaging method alters the colour or brightness of this refracted light. The method is being used to ensure efficient design of such freezers. Photographed by the world's only (as of 2003) full-scale Schlieren system, developed by Gary Settles, professor of mechanical engineering at Pennsylvania State University, USA.