Schlieren photography

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.

[1] The classical implementation of an optical schlieren system uses light from a single collimated source shining on, or from behind, a target object.

As a result, one can place a camera after the knife edge such that the image of the object will exhibit intensity variations due to the deflections of the rays.

The result is a set of lighter and darker patches corresponding to positive and negative fluid density gradients in the direction normal to the knife edge.

If the fluid flow is uniform, the image will be steady, but any turbulence will cause scintillation, the shimmering effect that can be seen over heated surfaces on a hot day.

Because of this focus requirement, they tend to be used for large-scale applications where both the schlieren object and the background are distant (typically beyond the hyperfocal distance of the optical system).

In some cases, the background may be provided by the experimenter, such as a random speckle pattern or sharp line, but naturally occurring features such as landscapes or bright light sources such as the sun and moon can also be used.

[9] The USSR once produced a number of sophisticated schlieren systems based on the Maksutov telescope principle, many of which still survive in the former Soviet Union and China.

Thus schlieren photography can be used in many engineering problems such as heat transfer, leak detection, study of boundary layer detachment, and characterization of optics.