Schlieren are optical inhomogeneities in transparent material not visible to the human eye. Schlieren physics developed out of the need to produce high quality lenses void of these inhomogeneities. These inhomogeneities are localized differences in optical path length that cause light deviation. This light deviation is converted to shadow in a schlieren system.

History

Schlieren were first observed by Robert Hooke¹ 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 August Toepler. Toepler's original system ² was designed to detect schlieren in glass used to make lenses. In the conventional schlieren system ³, 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: where f is the focal length of the lens, d_o is the distance from the object to the lens and 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. A portrait, claimed by historian Lisa Jardine to be of Robert Hooke Robert Hooke, FRS (July 18, 1635 - March 3, 1703), one of the greatest experimental scientists of the seventeenth century, played an important role in the scientific revolution. ... Events March 4 - Start of the Second Anglo-Dutch War. ... Convex lens converging light rays A convex lens is a lens that is curved outward (convex): the ends are narrow and the middle is wide. ... August Toepler (1836 - 1912) was a German physicist known for his experiments in electrostatics. ... This article refers to the material. ... A lens can be considered a thin lens if d << f. ...

Schlieren Flow Visualization

Schlieren flow visualization is based on the deflection of light by a refractive index gradient. The index gradient is directly related to flow density gradient. The deflected light is compared to undeflected light at a viewing screen. The undisturbed light is partially blocked by a knife edge. The light that is deflected toward or away from the the knife edge produces a shadow pattern depending upon whether it was previously blocked or unblocked. This shadow pattern is a light-intensity representation of the expansions (low density regions) and compressions (high density regions) which characterize flow. The refractive index of a material is the factor by which the phase velocity of electromagnetic radiation is slowed relative to vacuum. ...

External Links

Non-intrusive optical diagnostic experiments for high-speed flow generator flowfield characterization
Flow Visualization System for a Pulse Detonation Engine

• see also Schlieren photography

Schlieren photography is a visual process that is used to photograph the flow of air (or other compressible fluids) around objects. ...

References

¹Hooke, R., "Of a New Property in the Air," Micrographia Observation, LVIII,217-219, London(1665).
²Toepler, A., Beobachtungen nach einer neuen optischen Methode,"Maximillan Cohen und Sohn, Bonn (1864).
³Rienitz, J., "Schlieren Experiments 300 years ago," Nature[London]254, 293-295 (March 27, 1975).