Editing Binoculars (section)

=== 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.