Editing Binoculars (section)

== Optical coatings ==
{{Main|Optical coating}}
[[File:DFRBinoculars.jpg|thumb|Binoculars with red-colored multicoatings. Sometimes they are fraudulently advertised as "infrared" binoculars.]]
Because a typical binocular has 6 to 10 optical elements <ref>{{cite book |first1=Robert Bruce |last1=Thompson |first2=Barbara Fritchman |last2=Thompson |url=https://books.google.com/books?id=piwP9HXtpvUC&pg=PA35 |title=Astronomy Hacks: O'Reilly Series |isbn=9780596100605 |publisher=O'Reilly Media, Inc. |date=2005 |page=35 |access-date=2016-10-10 |archive-date=2016-12-27 |archive-url=https://web.archive.org/web/20161227103325/https://books.google.com/books?id=piwP9HXtpvUC&pg=PA35 |url-status=live }}</ref> with special characteristics and up to 20 atmosphere-to-glass surfaces, binocular manufacturers use different types of [[optical coating]]s for technical reasons and to improve the image they produce.
Lens and prism optical coatings on binoculars can increase light transmission, minimize detrimental reflections and interference effects, optimize beneficial reflections, repel water and grease and even protect the lens from scratches. Modern optical coatings are composed of a combination of very thin layers of materials such as oxides, metals, or rare earth materials. The performance of an optical coating is dependent on the number of layers, manipulating their exact thickness and composition, and the refractive index difference between them.<ref>{{Cite web |url=https://www.edmundoptics.com/knowledge-center/application-notes/lasers/an-introduction-to-optical-coatings/ |title=An Introduction to Optical Coatings |access-date=2022-10-02 |archive-date=2022-10-02 |archive-url=https://web.archive.org/web/20221002151839/https://www.edmundoptics.com/knowledge-center/application-notes/lasers/an-introduction-to-optical-coatings/ |url-status=live }}</ref> These coatings have become a key technology in the field of optics and manufacturers often have their own designations for their optical coatings. The various lens and prism optical coatings used in high-quality 21st century binoculars, when added together, can total about 200 (often superimposed) coating layers.<ref>{{Cite web |url=https://birdsatfirstsight.com/binocular-lens-and-prism-coatings/#Coatings_For_Prisms |title=Binocular Lens and Prism Coatings |date=19 April 2022 |access-date=2022-09-20 |archive-date=2022-09-20 |archive-url=https://web.archive.org/web/20220920192949/https://birdsatfirstsight.com/binocular-lens-and-prism-coatings/#Coatings_For_Prisms |url-status=live }}</ref>

===Anti-reflective===
{{Main|Anti-reflective coating}}

[[File:Optical-coating-2.svg|thumb|A quarter-wavelength (λ) thick anti-reflection coating, which leads to destructive interference]]

[[Anti-reflective_coating#Interference_coatings|Anti-reflective interference coating]]s reduce light lost at every optical surface through [[reflection (physics)|reflection]] at each surface. Reducing reflection via anti-reflective coatings also reduces the amount of "lost" light present inside the binocular which would otherwise make the image appear hazy (low contrast). A pair of binoculars with good optical coatings may yield a brighter image than uncoated binoculars with a larger objective lens, on account of superior light transmission through the assembly. The first transparent interference-based coating ''Transparentbelag (T)'' used by Zeiss was invented in 1935 by [[Olexander Smakula]].<ref>{{Cite web |url=http://www.zeiss.com/corporate/en_de/history/company%20history/at-a-glance/at-a-glance-milestones.html#1895-_-1945 |title=History of Camera Lenses from Carl Zeiss — 1935 — Alexander Smakula develops anti-reflection coating |access-date=2022-04-03 |archive-date=2016-10-08 |archive-url=https://web.archive.org/web/20161008215703/http://www.zeiss.com/corporate/en_de/history/company%20history/at-a-glance/at-a-glance-milestones.html#1895-_-1945 |url-status=live }}</ref> A classic lens-coating material is [[magnesium fluoride]], which reduces reflected light from about 4% to 1.5%. At 16 atmosphere to optical glass surfaces passes, a 4% reflection loss theoretically means a 52% light transmission ({{math|0.96<sup>16</sup>}} = 0.520) and a 1.5% reflection loss a much better 78.5% light transmission ({{math|0.985<sup>16</sup>}} = 0.785). Reflection can be further reduced over a wider range of wavelengths and angles by using several superimposed layers with different refractive indices. The anti-reflective multi-coating ''Transparentbelag* (T*)'' used by Zeiss in the late 1970s consisted of six superimposed layers. In general, the outer coating layers have slightly lower index of refraction values and the layer thickness is adapted to the range of wavelengths in the [[visible spectrum]] to promote optimal [[destructive interference]] via reflection in the beams reflected from the interfaces, and constructive interference in the corresponding transmitted beams. There is no simple formula for the optimal layer thickness for a given choice of materials. These parameters are therefore determined with the help of simulation programs. Determined by the optical properties of the lenses used and intended primary use of the binoculars, different coatings are preferred, to optimize light transmission dictated by the human eye [[luminous efficiency function]] variance. Maximal light transmission around [[wavelength]]s of 555&nbsp;nm ([[green]]) is important for obtaining optimal [[photopic vision]] using the eye [[cone cell]]s for observation in well-lit conditions. Maximal light transmission around wavelengths of 498&nbsp;nm ([[cyan]]) is important for obtaining optimal [[scotopic vision]] using the eye [[rod cell]]s for observation in low light conditions. As a result, effective modern anti-reflective lens coatings consist of complex multi-layers and reflect only 0.25% or less to yield an image with maximum brightness and natural colors.<ref>{{Cite web |url=https://www.edmundoptics.com/knowledge-center/application-notes/lasers/anti-reflection-coatings/ |title=Anti-Reflection (AR) Coatings |access-date=2022-10-02 |archive-date=2022-10-02 |archive-url=https://web.archive.org/web/20221002153232/https://www.edmundoptics.com/knowledge-center/application-notes/lasers/anti-reflection-coatings/ |url-status=live }}</ref> These allow high-quality 21st century binoculars to practically achieve at the eye lens or ocular lens measured over 90% light transmission values in low light conditions. Depending on the coating, the character of the image seen in the binoculars under normal daylight can either look "warmer" or "colder" and appear either with higher or lower contrast. Subject to the application, the coating is also optimized for maximum color fidelity through the [[visible spectrum]], for example in the case of lenses specially designed for bird watching.<ref>{{Cite web |url=https://blogs.zeiss.com/sports-optics/hunting/en/zeiss-t-coating/ |title=ZEISS T* Coating |date=13 July 2020 |access-date=2022-04-04 |archive-date=2022-05-20 |archive-url=https://web.archive.org/web/20220520154130/https://blogs.zeiss.com/sports-optics/hunting/en/zeiss-t-coating/ |url-status=live }}</ref><ref>{{Cite web |url=https://www.photoartfromscience.com/single-post/camera-lens-anti-reflection-coatings-magic-explained |title=Camera Lens Anti-Reflection Coatings: Magic Explained |date=4 March 2022 |access-date=2022-05-07 |archive-date=2022-09-09 |archive-url=https://web.archive.org/web/20220909045406/https://www.photoartfromscience.com/single-post/camera-lens-anti-reflection-coatings-magic-explained |url-status=live }}</ref><ref>{{cite web
 |url=http://www.smecc.org/ziess.htm
 |title=Carl Zeiss – A History of a Most Respected Name in Optics
 |publisher=[[Southwest Museum of Engineering, Communications and Computation]]
 |year=2007
 |access-date=2022-05-07
 |archive-date=2017-06-27
 |archive-url=https://web.archive.org/web/20170627194608/http://smecc.org/ziess.htm
 |url-status=live
 }}</ref>
A common application technique is physical [[vapor deposition]] of one or more superimposed anti-reflective coating layer(s) which includes [[evaporative deposition]], making it a complex production process.<ref>{{Cite web |url=https://www.photonics.com/Articles/Vapor_Deposition_Method_Suits_Coating_Curved/a63473 |title=Vapor Deposition Method Suits Coating Curved Optics by Evan Craves |access-date=2022-09-27 |archive-date=2022-09-27 |archive-url=https://web.archive.org/web/20220927133418/https://www.photonics.com/Articles/Vapor_Deposition_Method_Suits_Coating_Curved/a63473 |url-status=live }}</ref>

===Phase correction===
[[File:p-belag.png|thumb|Beam path at the roof edge (cross-section); the P-coating layer is on both roof surfaces]]

In binoculars with [[Roof prism#Phase correction|roof prisms]] the light path is split into two paths that reflect on either side of the roof prism ridge. One half of the light reflects from roof surface 1 to roof surface 2. The other half of the light reflects from roof surface 2 to roof surface 1. If the roof faces are uncoated, the mechanism of reflection is [[Total internal reflection|Total Internal Reflection]] (TIR).  In TIR, light polarized in the plane of incidence (p-polarized) and light polarized orthogonal to the plane of incidence (s-polarized) experience different phase shifts.  As a consequence, linearly polarized light emerges from a roof prism elliptically polarized.  Furthermore, the state of elliptical polarization of the two paths through the prism is different.  When the two paths recombine on the retina (or a detector) there is [[Wave interference|interference]] between light from the two paths causing a distortion of the [[Point spread function|Point Spread Function]] and a deterioration of the image.  Resolution and contrast significantly suffer. These unwanted interference effects can be suppressed by [[Chemical vapor deposition|vapor depositing]] a special [[dielectric coating]] known as a ''phase-correction coating'' or ''P-coating'' on the roof surfaces of the roof prism. To approximately correct a roof prism for polychromatic light several phase-correction coating layers are superimposed, since every layer is wavelength and [[Angle of incidence (optics)|angle of incidence]] specific.<ref>Paul Maurer: Phase Compensation of Total Internal Reflection. In: Journal of the Optical Society of America. Band 56, Nr. 9, 1. September 1966, S. 1219–1221, doi:10.1364/JOSA.56.001219</ref> 
The ''P-coating'' was developed in 1988 by Adolf Weyrauch at [[Carl Zeiss (company)|Carl Zeiss]].<ref name="Weyrauch">{{Cite web |url=https://www.juelich-bonn.com/jForum/file.php?9,file=1967,filename=P-Belag_Weyrauch.pdf |title=A. Weyrauch, B. Dörband: ''P-Coating: Improved imaging in binoculars through phase-corrected roof prisms.'' In: ''Deutsche Optikerzeitung.'' No. 4, 1988<!--Page?-->. |access-date=2022-09-24 |archive-date=2022-09-24 |archive-url=https://web.archive.org/web/20220924181649/https://www.juelich-bonn.com/jForum/file.php?9,file=1967,filename=P-Belag_Weyrauch.pdf |url-status=live }}</ref>
Other manufacturers followed soon, and since then phase-correction coatings are used across the board in medium and high-quality  roof prism binoculars. This coating suppresses the difference in phase shift between s- and p- polarization so both paths have the same polarization and no interference degrades the image.<ref>{{Cite web |url=https://skyandtelescope.org/astronomy-resources/astronomy-questions-answers/why-do-the-best-roof-prism-binoculars-need-a-phase-correction-coating/ |title=Why do the best roof-prism binoculars need a phase-correction coating? |date=24 July 2006 |access-date=2022-05-20 |archive-date=2022-05-23 |archive-url=https://web.archive.org/web/20220523052958/https://skyandtelescope.org/astronomy-resources/astronomy-questions-answers/why-do-the-best-roof-prism-binoculars-need-a-phase-correction-coating/ |url-status=live }}</ref> In this way, since the 1990s, roof prism binoculars have also achieved resolution values that were previously only achievable with Porro prisms.<ref>Konrad Seil: Progress in binocular design. In: SPIE Proceedings. Band 1533, 1991, S. 48–60, doi:10.1117/12.48843</ref> The presence of a phase-correction coating can be checked on unopened binoculars using two polarization filters.<ref name="Weyrauch" /> Dielectric phase-correction prism coatings are applied in a vacuum chamber with maybe thirty or more different superimposed vapor coating layers deposits, making it a complex production process.

Binoculars using either a [[Schmidt–Pechan prism|Schmidt–Pechan roof prism]], [[Abbe–Koenig prism|Abbe–Koenig roof prism]] or an [[Uppendahl prism|Uppendahl roof prism]] benefit from phase coatings that compensate for a loss of resolution and contrast caused by the [[interference (physics)|interference effects]] that occur in untreated roof prisms. [[Porro prism]] and [[Perger prism]] binoculars do not split beams and therefore they do not require any phase coatings.

=== Metallic mirror ===
{{Main|Mirror}}
In binoculars with Schmidt–Pechan or Uppendahl roof prisms, mirror coatings are added to some surfaces of the roof prism because the light is incident at one of the prism's glass-air boundaries at an angle less than the [[Critical angle (optics)|critical angle]] so [[total internal reflection]] does not occur. Without a mirror coating most of that light would be lost. Roof prism aluminum mirror coating ([[reflectivity]] of 87% to 93%) or silver mirror coating (reflectivity of 95% to 98%) is used.<ref>{{Cite web |url=https://www.edmundoptics.com/knowledge-center/application-notes/optics/metallic-mirror-coatings/ |title=Metallic Mirror Coatings |access-date=2022-10-02 |archive-date=2022-10-02 |archive-url=https://web.archive.org/web/20221002151653/https://www.edmundoptics.com/knowledge-center/application-notes/optics/metallic-mirror-coatings/ |url-status=live }}</ref><ref>{{Cite web |url=https://www.edmundoptics.com/knowledge-center/application-notes/optics/highly-reflective-coatings/ |title=Highly Reflective Coatings |access-date=2022-10-02 |archive-date=2022-10-02 |archive-url=https://web.archive.org/web/20221002151649/https://www.edmundoptics.com/knowledge-center/application-notes/optics/highly-reflective-coatings/ |url-status=live }}</ref>

In older designs silver mirror coatings were used but these coatings oxidized and lost reflectivity over time in unsealed binoculars. Aluminum mirror coatings were used in later unsealed designs because they did not tarnish even though they have a lower reflectivity than silver. Using vacuum-vaporization technology, modern designs use either aluminum, enhanced aluminum (consisting of aluminum overcoated with a multilayer dielectric film) or silver.<ref>{{Cite web |url=https://imaging.nikon.com/lineup/sportoptics/how_to/guide/binoculars/technologies/technologies_07.htm |title=Coating on roof (Dach) prism |access-date=2022-10-02 |archive-date=2022-10-02 |archive-url=https://web.archive.org/web/20221002093130/https://imaging.nikon.com/lineup/sportoptics/how_to/guide/binoculars/technologies/technologies_07.htm |url-status=live }}</ref> Silver is used in modern high-quality designs which are sealed and filled with nitrogen or argon to provide an inert atmosphere so that the silver mirror coating does not tarnish.<ref>{{cite web |url=http://www.zbirding.info/Truth/prisms/prisms.htm |title=www.zbirding.info |publisher=www.zbirding.info |access-date=2009-11-03 |archive-url=https://web.archive.org/web/20090527011313/http://www.zbirding.info/Truth/prisms/prisms.htm |archive-date=2009-05-27 |url-status=dead }}</ref>

[[Porro prism]] and [[Perger prism]] binoculars and roof prism binoculars using the [[Abbe–Koenig prism|Abbe–Koenig roof prism configuration]] do not use mirror coatings because these prisms reflect with 100% reflectivity using [[total internal reflection]] in the prism rather than requiring a (metallic) mirror coating.

=== Dielectric mirror ===
{{Main|Dielectric mirror}}

[[image:Dielectric mirror diagram.svg|thumb|right|Diagram of a dielectric mirror. Thin layers with a high refractive index ''n''<sub>1</sub> are interleaved with thicker layers with a lower refractive index ''n''<sub>2</sub>. The path lengths ''l''<sub>A</sub> and ''l''<sub>B</sub> differ by exactly one wavelength, which leads to constructive interference.]]

Dielectric coatings are used in [[Schmidt–Pechan prism|Schmidt–Pechan]] and [[Uppendahl prism|Uppendahl]] roof prisms to cause the prism surfaces to act as a [[dielectric mirror]]. This coating was introduced in 2004 in Zeiss Victory FL binoculars featuring Schmidt–Pechan prisms. Other manufacturers followed soon, and since then dielectric coatings are used across the board in medium and high-quality Schmidt–Pechan and Uppendahl roof prism binoculars. The non-metallic [[dielectric]] reflective coating is formed from several multilayers of alternating high and low [[refractive index]] materials deposited on a prism's reflective surfaces. The manufacturing techniques for dielectric mirrors are based on [[thin-film deposition]] methods. A common application technique is [[physical vapor deposition]] which includes [[evaporative deposition]] with maybe seventy or more different superimposed vapor coating layers deposits, making it a complex production process.<ref>{{Cite web |url=https://www.walter-schwab.com/lesenswert/05-optikbuch-jagd-und-beobachtung/ |title=Optik für Jagd und Naturbeobachtung, Carl Zeiss Sports Optics / Walter J. Schwab, 2. Ausgabe- Wetzlar - 2017, page 45 |access-date=2022-11-22 |archive-date=2022-11-22 |archive-url=https://web.archive.org/web/20221122114032/https://www.walter-schwab.com/lesenswert/05-optikbuch-jagd-und-beobachtung/ |url-status=live }}</ref> This multilayer coating increases reflectivity from the prism surfaces by acting as a [[distributed Bragg reflector]]. A well-designed multilayer dielectric coating can provide a [[reflectivity]] of over 99% across the [[visible spectrum|visible light spectrum]].<ref>{{cite journal |first1=ZenaE. |last1=Slaiby |first2=Saeed N. |last2=Turki |title=Study the reflectance of dielectric coating for the visiblespectrum |journal=International Journal of Emerging Trends & Technology in Computer Science |volume=3 |issue=6 |date=November–December 2014 |pages=1–4 |issn=2278-6856 |url=https://www.ijettcs.org/Volume3Issue6/IJETTCS-2014-10-30-4.pdf |access-date=2022-11-21 |archive-date=2022-11-28 |archive-url=https://web.archive.org/web/20221128114127/https://www.ijettcs.org/Volume3Issue6/IJETTCS-2014-10-30-4.pdf |url-status=live }}</ref> This reflectivity is an improvement compared to either an aluminium mirror coating or silver mirror coating.

Porro prism and Perger prism binoculars and roof prism binoculars using the Abbe–Koenig roof prism do not use dielectric coatings because these prisms reflect with 100% reflectivity using [[total internal reflection]] in the prism rather than requiring a (dielectric) mirror coating.

=== Terms ===

==== All binoculars ====
The presence of any coatings is typically denoted on binoculars by the following terms:
* ''coated optics'': one or more surfaces are anti-reflective coated with a single-layer coating.
* ''fully coated'': all air-to-glass surfaces are anti-reflective coated with a single-layer coating. Plastic lenses, however, if used, may not be coated.<ref>{{Cite web |title=Fully Coated vs. Fully Multi Coated - Binoculars |url=https://www.cloudynights.com/topic/277442-fully-coated-vs-fully-multi-coated/ |access-date=2022-09-28 |website=Cloudy Nights |language=en |archive-date=2022-09-28 |archive-url=https://web.archive.org/web/20220928021436/https://www.cloudynights.com/topic/277442-fully-coated-vs-fully-multi-coated/ |url-status=live }}</ref>
* ''multi-coated'': one or more surfaces have anti-reflective multi-layer coatings.
* ''fully multi-coated'': all air-to-glass surfaces are anti-reflective multi-layer coated.

The presence of optical high transmittance  [[Crown glass (optics)|crown glass]] offering relatively low [[refractive index]] (≈1.52) and low [[dispersion (optics)|dispersion]] (with [[Abbe number]]s around 60) is typically denoted on binoculars by the following terms:<ref>{{Cite web |url=https://binocularsky.com/binoc_minefield.php |title=The Minefield: Advertising Hype in Binoculars |access-date=2022-08-03 |archive-date=2022-08-03 |archive-url=https://web.archive.org/web/20220803182931/https://binocularsky.com/binoc_minefield.php |url-status=live }}</ref>
* BK7 ([[Schott AG|Schott]] designates it as 517642. The first three digits designate its refractive index [1.517] and the last three designate its Abbe number [64.2]. Its critical angle is 41.2°.)
* BaK4 (Schott designates it as 569560. The first three digits designate its refractive index [1.569] and the last three designate its Abbe number [56.0]. Its critical angle is 39.6°.)

==== Roof prisms only ====
* ''phase-coated'' or ''P-coating'': the roof prism has a phase-correcting coating
* ''aluminium-coated'': the roof prism mirrors are coated with an aluminium coating (the default if a mirror coating isn't mentioned).
* ''silver-coated'': the roof prism mirrors are coated with a silver coating
* ''dielectric-coated'': the roof prism mirrors are coated with a dielectric coating