Background-oriented schlieren technique

Background-oriented schlieren (BOS) is a novel technique for flow visualization of density gradients in fluids using the Gladstone–Dale relation between density and refractive index of the fluid.

Shock waves around a T-38 aircraft visualized with background-oriented Schlieren photography

BOS simplifies the visualization process by eliminating the need for the use of expensive mirrors, lasers and knife-edges. In its simplest form, BOS makes use of simple background patterns of the form of a randomly generated dot-pattern, an inexpensive strobe light source and a high speed digital camera.

In its initial stages of implementation, it is mostly being used as a qualitative visualization method. Further research into developing the technique will enable quantitative visualization of fluid flows, in various applications including cryogenic flows, supersonic and hypersonic flows, biomedical device flow visualization etc.

Bright light sources such as the sun and moon can also be used to visualize shock wave patterns around aircraft, a technique named Background Oriented Schlieren using Celestial Objects.[1]