Schlrieren is the German word for ‘streaks’ and is used for a technique of fluid visualisation using optics to expose differences in density, and therefore refractive index.
19th century German physicist August Toepler developed the technique while inspecting glass blanks, but the technique is now commonly used to examine fluid, especially air, in motion.
For example, the heat shimmer seen on hot days is caused by light refracting as it passes through the varying density of hot and cold air, Toepler’s original application of the technique.
There are a number of different set-ups and techniques for obtaining schlieren images, depending on the application and the budget.
The most common involves placing a concave mirror and a razor blade in such a way that a point source of light is focused at the blade, and a sensor placed behind the blade to observe the returning light.
Any deviation of the light from its return path will be magnified and can be analyzed to show flaws in the lens or, more commonly, density changes due to fluid movement.
Background-oriented Schlieren is a technique that uses a known background and deviations from it are picked up using digital signal processing.
