Editing Interference filter

{{Short description|Wavelength-selective optical filter}}
[[Image:Optical-dichrotic-filter-0.5inch.jpg|thumb|Band-pass interference filter for laser experiments]]
[[Image:Dichroic filters.jpg|thumb|right|300px|Dichroic filters]]
An '''interference filter''', '''dichroic filter''', or '''thin-film filter''' is an [[optical filter]] that [[Reflection (physics)|reflects]] some [[wavelength]]s (colors) of [[light]] and transmits others, with almost no [[absorption (optics)|absorption]] for all wavelengths of interest. An interference filter may be [[high-pass]], [[low-pass]], [[bandpass]], or band-rejection. They are used in scientific applications, as well as in [[Architectural lighting design|architectural]] and [[theatrical lighting]].

An interference filter consists of multiple thin layers of [[dielectric]] material having different refractive indices. There may also be metallic layers. Interference filters are [[wavelength]]-selective by virtue of the [[Interference (wave propagation)|interference]] effects that take place between the incident and reflected waves at the thin-film boundaries. The principle of operation is similar to a [[Fabry-Perot etalon]].

'''Dichroic mirrors''' and '''dichroic reflectors''' are the same type of device, but are characterized by the colors of light that they reflect, rather than the colors they pass. [[Dielectric mirror]]s operate on the same principle, but focus exclusively on reflection.

== Theory ==
Dichroic filters use the principle of [[thin-film interference]], and produce colors in the same way as oil films on water. When light strikes an oil film at an angle, some of the light is reflected from the top surface of the oil, and some is reflected from the bottom surface where it is in contact with the water. Because the light reflecting from the bottom travels a slightly longer path, some light wavelengths are reinforced by this delay, while others tend to be canceled, producing the colors seen. The color transmitted by the filter exhibits a blue shift with increasing angle of incidence, see [[Dielectric mirror]].

In a dichroic mirror or filter, instead of using an oil film to produce the [[interference (wave propagation)|interference]], alternating layers of [[optical coating]]s with different [[refractive index|refractive indices]] are built up upon a [[glass]] substrate. The interfaces between the layers of different refractive index produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. The layers are usually added by [[vacuum deposition]]. By controlling the thickness and number of the layers, the [[frequency]] of the [[passband]] of the filter can be tuned and made as wide or narrow as desired. Because unwanted wavelengths are reflected rather than absorbed, dichroic filters do not absorb this unwanted energy during operation and so do not become nearly as hot as the equivalent conventional filter (which attempts to absorb all energy except for that in the passband). (See [[Fabry–Pérot interferometer]] for a mathematical description of the effect.)

Where [[Visible spectrum|white light]] is being deliberately separated into various color bands (for example, within a color [[video projector]] or [[Professional video camera|color television camera]]), the similar [[dichroic prism]] is used instead. For cameras, however, it is now more common to have an absorption filter array to filter individual pixels on a single CCD array.

== Applications ==
[[Dichroism|Dichroic]] filters can filter light from a white light source to produce light that is [[Perception|perceived]] by humans to be highly [[saturation (color theory)|saturated]] in color. Such filters are popular in [[architecture|architectural]]<ref>{{Cite web |url=http://www.operaen.dk/site/OplevTeateret/Galleri/Scenerne/Scener%20og%20huse/Operaen.aspx?sc_lang=en |title=The Copenhagen Opera House |access-date=2009-09-04 |archive-url=https://web.archive.org/web/20090510004004/http://www.operaen.dk/site/OplevTeateret/Galleri/Scenerne/Scener%20og%20huse/Operaen.aspx?sc_lang=en |archive-date=2009-05-10 |url-status=dead }}</ref> and [[stage lighting|theatrical]] applications.

[[Image:nocb.png|thumb|right|IEC 60598 No Cool Beam symbol]]
Dichroic reflectors known as [[cold mirror]]s are commonly used behind a light source to reflect visible light forward while allowing the invisible [[infrared]] light to pass out of the rear of the fixture.  Such an arrangement allows intense illumination with less heating of the illuminated object.  Many [[quartz-halogen lamp]]s have an [[Multifaceted reflector|integrated dichroic reflector]] for this purpose, being originally designed for use in slide projectors to avoid melting the slides, but now widely used for interior home and commercial lighting. This improves whiteness by removing excess red; however, it poses a serious fire hazard if used in recessed or enclosed [[luminaire]]s by allowing [[infrared]] radiation into those luminaires. For these applications non-cool-beam ([[Aluminium|ALU]] or [[Silver]]back) lamps must be used. Recessed or enclosed [[luminaire]]s that are unsuitable for use with dichroic reflector lights can be identified by the [[International Electrotechnical Commission|IEC]] 60598 No Cool Beam symbol.

In [[fluorescence microscopy]], dichroic filters are used as [[beam splitter]]s to direct illumination of an excitation frequency toward the sample and then at an analyzer to reject that same excitation frequency but pass a particular emission frequency.

Some [[LCD projector]]s use dichroic filters instead of prisms to split the white light from the lamp into the three colours before passing it through the three LCD units.

[[File:Dichroic filter from a DLP projector color wheel.jpg|thumb|Six-segment dichroic color wheel from a DLP projector. Segments transmit red, green and blue, and therefore reflect cyan, magenta, and yellow.]]
Older [[Digital Light Processing|DLP]] projectors typically transmit a white light source through a color wheel which uses dichroic filters to rapidly switch colors sent through the (monochrome) [[Digital micromirror device]]. Newer projectors may use laser or LED light sources to directly emit the desired light wavelengths.

They are used as [[laser harmonic separator]]s. They separate the various harmonic components of frequency doubled laser systems by selective spectral reflection and transmission.

Dichroic filters are also used to create [[Gobo (lighting)|gobos]] for high-power lighting products. Pictures are made by overlapping up to four colored dichroic filters.

Photographic enlarger [[color head]]s use dichroic filters to adjust the color balance in the print.

Artistic glass [[jewelry]] is occasionally fabricated to behave as a dichroic filter. Because the wavelength of light selected by the filter varies with the [[angle of incidence (optics)|angle of incidence]] of the light, such jewelry often has an [[Iridescence|iridescent]] effect, changing color as the (for example) [[earring]]s swing. Another interesting application of dichroic filters is [[spatial filter]]ing.<ref>[http://ol.osa.org/abstract.cfm?id=83357 Optics Letters<!-- Bot generated title -->]</ref>

With a technique licensed from [[Infitec]], [[Dolby Labs]] uses dichroic filters for screening [[stereoscopic|3D]] movies. The left lens of the Dolby 3D glasses transmits specific narrow bands of red, green and blue frequencies, while the right lens transmits a different set of red, green and blue frequencies. The projector uses matching filters to display the images meant for the left and right eyes.<ref>{{cite web|title=Dolby Stakes Its Claim in 3D Movie Tech|url=http://www.cnet.com.au/dolby-stakes-its-claim-in-3d-movie-tech-339282656.htm|archive-url=https://web.archive.org/web/20120224030713/http://www.cnet.com.au/dolby-stakes-its-claim-in-3d-movie-tech-339282656.htm|first=Stephen|last=Shankland|website=[[CNET]]|publisher=[[CBS Interactive]]|date=2007-10-09|archive-date=2012-02-24|access-date=2016-12-08}}</ref>

Long-pass dichroic filters applied to ordinary lighting can prevent it from attracting insects. In some cases, such filters can prevent attraction of other wildlife, reducing adverse environmental impact.<ref>{{cite journal |issn=1092-194X |journal=Florida Marine Research Institute Technical Report TR-2 |title=Understanding, Assessing, and Resolving Light-Pollution Problems on Sea Turtle Nesting Beaches |first1=Blair E. |last1=Witherington |first2=R. Erik |last2=Martin |publisher=Florida Fish and Wildlife Conservation Commission |edition=3rd |year=2003 |page=23 |url=http://www.fws.gov/caribbean/es/PDF/Library%20Items/LightingManual-Florida.pdf }}</ref>

== Advantages ==

Dichroic filters have a much longer life than conventional filters; the color is intrinsic in the construction of the hard microscopic layers and cannot "bleach out" over the lifetime of the filter (unlike for example, gel filters). They can be fabricated to pass any [[passband]] frequency and block a selected amount of the [[stopband]] frequencies. Because light in the stopband is reflected rather than absorbed, there is much less heating of the dichroic filter than with conventional filters. Dichroics are capable of achieving extremely high [[laser damage threshold]]s, and are used for all the mirrors on the world's most powerful laser, the [[National Ignition Facility]].

== See also ==
* [[Color gel]]
* [[Dielectric mirror]]
* [[Filter (optics)]]
* [[Holographic Versatile Disc]]
* [[Thin-film interference]]
* [[Thin-film optics]]

==References==
{{Reflist}}

==Additional sources==
*{{FS1037C}}
*M. Bass, ''Handbook of Optics'' (2nd ed.) pp.&nbsp;42.89-42.90 (1995)

== Further reading ==
* {{cite book|title=Thin-Film Optical Filters|year=2010|last=MacLeod|first=H. Angus|edition=4th|publisher=Taylor & Francis|isbn=978-1420073027}}
* {{cite journal|doi=10.1364/OL.30.000914|title=Thin-film spatial filters|year=2005|last1=Moreno|first1=Ivan|journal=Optics Letters|volume=30|pages=914–6|pmid=15865397|last2=Araiza|first2=JJ|last3=Avendano-Alejo|first3=M|issue=8|bibcode = 2005OptL...30..914M }}

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[[Category:Optical filters]]
[[Category:Interference]]