Pictured: Don Winans of Boller and Chivens, on right
A Boller and Chivens instrument for viewing the Schlieren Effect during wind tunnel tests of various objects or assemblies such as models of air craft. This allows studying how shock waves are formed, which in turn can lead to more efficient designs. Schlieren (plural for the German word “schliere”) are optical inhomogeneities in transparent materials. In supersonic flow, air experiences large changes in density and therefore its index of refraction. Changes in index of refraction essentially turn the shock waves into lenses, bending the light. Toepler’s (1864) Schlieren “system” separates the light bent by the shock waves from the background light, allowing the shockwaves to be viewed as a combination of bright and dark areas.