Air flow, Schlieren image. Researcher holding his hand in a ventilation fan's air curtain. The turbulence is revealed by Schlieren photography, a way of visualizing density changes in transparent materials. The density change (here caused by the turbulence) causes light passing through the air to bend (refract). The imaging method alters the colour or brightness of this refracted light. Air curtains are used at entrances to reduce heat loss and drafts, and control dust and humidity levels. 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.