Office ventilation, Schlieren image. Air blown by a ventilation fan (lower left) and the heated air rising from the man and the computer monitor, are shown by Schlieren photography, a way of viewing density changes in transparent materials. These changes (here due to heat and turbulence) cause light passing through the air to bend (refract). The imaging method alters the colour or brightness of this refracted light. This study of air flow is improving ventilation design to remove pollutants. 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.