Schlieren photography is a visual process that is used to photograph the flow of fluids of varying density. Invented by the German

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 physicist August Toepler

August Toepler

August Joseph Ignaz Toepler was a German physicist known for his experiments in electrostatics. In 1864 he applied Foucault's knife-edge test for telescope mirrors to the analysis of fluid flow and the shock wave. He named this new method schlieren photography, for which he is justifiably famous...

 in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects. Its role is changing due to the increasing use of computational fluid dynamics

Computational fluid dynamics

Computational fluid dynamics, usually abbreviated as CFD, is a branch of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. Computers are used to perform the calculations required to simulate the interaction of liquids and gases with...

, which reduces the need for all such experimental fluid flow measurement techniques.
The basic optical schlieren system uses light from a single collimated source shining on, or from behind, a target object.