Schlieren photography

Schlieren photography is a process for photographing fluid flow. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects.

The process works by allowing normally unobservable changes in a fluid's refractive index to be seen,^[1] and is accomplished by viewing the deflections that occur when light is refracted by a moving fluid. Because changes to flow rate directly impact the refractive index of a fluid, one can therefore photograph a fluid's flow rate (as well as other changes to density, temperature, and pressure) by viewing changes to its refractive index.^[2]

Using the Schlieren photography process, other unobservable fluid changes can also be seen, such as convection currents, and the standing waves used in acoustic levitation.^[1]