View source for Schlieren

{{Short description|Optical inhomogeneities in transparent media}}
{{For|the town|Schlieren, Switzerland}}
[[File:Schlieren video of a handgun.oggtheora.ogv|thumb|This video demonstrates a single-pass high-speed schlieren system capturing the transitional ballistic sequence of a handgun.]]
[[File:schlieren-color-candle-plume.jpg|thumb|Colored schlieren image of the thermal plume from a burning candle, disturbed by a breeze from the right]]

'''Schlieren''' ({{IPAc-en|ˈ|ʃ|l|ɪər|ən}} {{respell|SHLEER|ən}}; {{IPA|de|ˈʃliːʁn̩|lang|De-Schlieren.ogg}}, {{langnf|de||streaks}}) are optical inhomogeneities in [[Transparency and translucency|transparent]] [[optical medium|media]] that are not necessarily visible to the [[human eye]]. Schlieren physics developed out of the need to produce high-quality [[lens]]es devoid of such inhomogeneities. These inhomogeneities are localized differences in [[optical path length]] that cause deviations of light [[ray (optics)|rays]], especially by [[refraction]]. This light deviation can produce localized brightening, darkening, or even [[dispersion (optics)|color changes]] in an image, depending on the directions the rays deviate.

== History ==
Schlieren were first observed by [[Robert Hooke]]<ref name="Hooke">Hooke, R. (1665), "Of a New Property in the Air", ''[[Micrographia]]'', Observation LVIII, pp. 217–219, London.</ref> in 1665 using a large [[Lens (optics)#Types of simple lenses|concave 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 [[Léon Foucault|Jean Bernard Léon Foucault]] invented the method in 1859 that Toepler improved upon{{Citation needed|date=February 2025}}.  Toepler's original system<ref name="Toepler">Toepler, A. (1864), ''Beobachtungen nach einer neuen optischen Methode'', Maximillan Cohen und Sohn, Bonn.</ref> was designed to detect schlieren in [[glass]] used to make lenses.  In the conventional schlieren system,<ref name="Rienitz">{{cite journal|doi=10.1038/254293a0|title=Schlieren experiment 300 years ago|year=1975|last1=Rienitz|first1=J.|journal=Nature|volume=254|issue=5498|pages=293–295|bibcode=1975Natur.254..293R|s2cid=4288641}}</ref> 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:
<math> \frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i}</math>
where <math>f</math> is the focal length of the lens, <math>d_o</math> is the distance from the object to the lens and <math> d_i </math>  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.

== Schlieren flow visualization ==
[[File:Inlet shock waves at Mach 2.jpg|thumb|right|Schlieren flow visualization of a [[Lockheed SR-71]] [[Pratt & Whitney J58]] engine inlet at [[Mach number|Mach]] 2]]

Schlieren flow visualization is based on the deflection of light by a [[refractive index]] gradient<ref name="Settles">Settles, G. S. (2001), ''Schlieren and shadowgraph techniques: Visualizing phenomena in transparent media,'' Berlin:Springer-Verlag.{{ISBN|978-3540661559}}</ref>  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 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 the flow.

== Schlieren displays ==
The schlieren effect is often used in [[video projector]] technologies. The basic idea is some device, such as a [[liquid crystal]] light valve, is used to produce schlieren distortions in a controlled manner and these are projected on a screen to produce the desired image. Projection display systems such as the now-obsolete [[Eidophor]] and [[Talaria projector|Talaria]] have used variations of this approach as far back as 1940.<ref>Brennesholtz, M.S. and Stupp, E.H. (2008), ''Projection Displays'', John Wiley & Sons, p. 259 ff. {{ISBN|978-0-470-51803-8}}</ref>

== See also ==
* [[Background-oriented schlieren technique]]
* [[Laser schlieren deflectometry]]
* [[Mach–Zehnder interferometer]]
* [[Moire deflectometry]]
* [[Schlieren imaging]]
* [[Schlieren photography]]
* [[Shadowgraph]]
* [[Synthetic schlieren]]

== References ==
<references/>

== External links ==
{{Commons category|Schlieren imaging}}
{{wiktionary|schlieren}}
*[[doi:10.1007/s00193-007-0097-7|Background oriented schlieren for flow visualisation in hypersonic impulse facilities]]
*[http://espace.library.uq.edu.au/eserv.php?pid=UQ:8810&dsID=SreekanthRughuna.pdf Visualisation of supersonic flows in shock tunnels using Background Oriented Schlieren (BOS) technique]
*[http://sciencehack.com/videos/view/_gKNhGbsEf4 Video on Schlieren photographs]
*[https://www.youtube.com/watch?v=fJIe0kTbdGc Schlieren on YouTube]

[[Category:Optics]]
[[Category:Articles containing video clips]]

[[ja:シュリーレン現象]]