Schlieren

Schlieren (from German; singular "Schliere", meaning "streak") are optical inhomogeneities in transparent material not necessarily visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses devoid of these inhomogeneities. These inhomogeneities are localized differences in optical path length that cause light deviation. This light deviation can produce localized brightening, darkening, or even color changes in an image, depending on which way the ray deviates.

Schlieren were first observed by Robert Hooke in 1665 using a large convex lens and two candles. One candle served as a light source. The warm air rising from the second candle provided the schliere. The conventional schlieren system is credited mostly to German physicist August Toepler, though Jean Bernard Léon Foucault invented the method in 1859 that Toepler improved upon. Toepler's original system was designed to detect schlieren in glass used to make lenses. In the conventional schlieren system, a point source is used to illuminate the test section containing the schliere. An image of this light is formed using a converging lens (also called a schlieren lens). This image is located at the conjugate distance to the lens according to the thin lens equation: ${\displaystyle {\frac {1}{f}}={\frac {1}{d_{o}}}+{\frac {1}{d_{i}}}}$ where ${\displaystyle f}$ is the focal length of the lens, ${\displaystyle d_{o}}$ is the distance from the object to the lens and ${\displaystyle d_{i}}$ is the distance from the image of the object to the lens. A knife edge at the point source-image location is positioned as to partially block some light from reaching the viewing screen. The illumination of the image is reduced uniformly. A second lens is used to image the test section to the viewing screen. The viewing screen is located a conjugate distance from the plane of the schliere.

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