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((Short description|Astronomical spectral light processor))
((Refimprove|date=December 2009))
A '''Lyot filter''', named for its inventor [[Bernard Lyot]], is a type of [[filter (optics)|optical filter]] that uses [[birefringence]] to produce a narrow [[passband]] of transmitted [[wavelength]]s. Lyot filters are often used in [[astronomy]], particularly for [[sun|solar]] astronomy.

A Lyot filter<ref>((Cite journal|author=Lyot, B | title=Optical apparatus with wide field using interference of polarized light | journal=Acad. Sci. | volume=197 |year=1933))</ref> is made from one or more [[wave plate|birefringent plates]] (usually [[quartz]]), with (in multi-plate filters) each plate being half the thickness of the previous one. Because the plates are birefringent, the ''ordinary'' and ''extraordinary'' polarization components of a light beam experience a different [[refractive index]] and thus have a different [[phase velocity]]. Therefore, the polarization state of light with an arbitrary wavelength will in general be modified after a passage through the filter plate, and this causes a loss of optical power in a subsequent [[polarizer]]. For certain wavelengths, however, the [[optical path length]] difference is an integer multiple of the wavelength, so that the losses are very small. By rotating the plates, one can shift the wavelengths of the transmission peaks.  

Each stage of a Lyot filter is 1/2 the size of the preceding stage. The largest stage sets the bandwidth and the smallest stage sets the Free Spectral Range. If two of the second largest crystals are used the contrast of the desired line increases, and if the crystals are split in half and a 1/2 waveplate is placed in the center the field of view of the filter increases. The separation and narrowness of the transmission peaks depends on the number, thicknesses, and orientation of the plates.

If electrically tunable birefringent elements are inserted (liquid crystal or electrooptic), then an "Electrically Tunable Lyot Filter" is obtained. [[Liquid crystal tunable filter]]s allow analogue tuning of the transmitted wavelength by carefully adjusting the voltage over the liquid crystal cells. Often these filters are based on the original Lyot design, but many other designs exist in order to achieve either narrow or broad band transmission, polarization selectivity, etc.<ref>((cite journal|last=Beeckman|first=Jeroen |author2=Hui, Tian |author3=Vanbrabant, Pieter J. M. |author4=Zmijan, Robert |author5=Neyts, Kristiaan|title=Polarization Selective Wavelength Tunable Filter|journal=Molecular Crystals and Liquid Crystals|year=2009|volume=502|issue=1|pages=19–28|doi=10.1080/15421400902813626|bibcode=2009MCLC..502...19B |url=http://www.elis.ugent.be/web/ext/publ/pdf.jsp?nr=P109.373|citeseerx=10.1.1.159.2814 |s2cid=9235494 ))</ref>

Single and multi-plate Lyot filters are often used inside the [[optical cavity]] of [[laser]]s to allow tuning of the laser. In this case, [[Brewster's angle|Brewster]] losses from the plate and other intracavity elements are usually sufficient to produce the polarizing effect and no additional polarizers are required.  Although their mechanisms are different, [[modelocking]] lasers and Lyot-filter lasers both produce a comb of multiple wavelengths which can be placed on the [[International Telecommunication Union|ITU]] channel grid for Dense Wave Division Multiplexing [[Wavelength-division multiplexing|(DWDM)]] or used to give each suburban home its own return-signal laser wavelength in a [[passive optical network]] (PON) used to provide [[Fiber to the premises|FTTH]] (Fiber To The Home).

==See also==
* [[Lyot stop]]
* [[Lyot depolarizer]]
* [[Liquid crystal tunable filter]]

==References==
((Reflist))

((DEFAULTSORT:Lyot Filter))
[[Category:Optical filters]]


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