Figure 2.

Typical large scale Schlieren imaging set-up (A), showing the spherical cross-sectional mirror, the test area, the light-emitting diode (LED) light source, the knife-edge that discriminates refracted from un-refracted light rays, and the camera that captures the schlieren image. Light from the LED is refracted through the exhaled air of a human subject in the test area, since that air is at a different temperature than the surroundings, creating Schlieren images such as (B), coughing and (C), coughing whilst wearing a surgical mask.