Binoculars with red-colored multicoatings
Because a typical binocular has 6 to 10 optical elements with special characteristics and up to 16 air-to-glass surfaces, binocular manufacturers use different types of optical coatings for technical reasons and to improve the image they produce.
Main article: Anti-reflective coating
Anti-reflective coatings reduce light lost at every optical surface through reflection at each surface. Reducing reflection via anti-reflective coatings also reduces the amount of "lost" light present inside the binocular which can 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. A classic lens-coating material is magnesium fluoride, which reduces reflected light from 5% to 1%. Modern lens coatings consist of complex multi-layers and reflect only 0.25% or less to yield an image with maximum brightness and natural colors.
Phase correction coatings
In binoculars with 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. This causes the light to become partially polarized (due to a phenomenon called Brewster's angle). During subsequent reflections the direction of this polarization vector is changed but it is changed differently for each path in a manner similar to a Foucault pendulum. When the light following the two paths is recombined the polarization vectors of each path do not coincide. The angle between the two polarization vectors is called the phase shift, or the geometric phase, or the Berry phase. This interference between the two paths with different geometric phase results in a varying intensity distribution in the image reducing apparent contrast and resolution compared to a porro prism erecting system. These unwanted interference effects can be suppressed by vapor depositing a special dielectric coating known as a phase-correction coating or P-coating on the roof surfaces of the roof prism. This coating corrects for the difference in geometric phase between the two paths so both have effectively the same phase shift and no interference degrades the image.
Binoculars using either a Schmidt–Pechan roof prism or an Abbe–Koenig roof prism benefit from phase coatings. Porro prism binoculars do not split beams and therefore they do not require any phase coatings.
Metallic mirror coatings
Main article: Mirror
In binoculars with Schmidt–Pechan 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 so total internal reflection does not occur. Without a mirror coating most of that light would be lost. Schmidt–Pechan roof prism aluminum mirror coating (reflectivity of 87% to 93%) or silver mirror coating (reflectivity of 95% to 98%) is used.
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. Modern designs use either aluminum or silver. Silver is used in modern high-quality designs which are sealed and filled with a nitrogen or argon inert atmosphere so that the silver mirror coating does not tarnish.
Porro prism binoculars and roof prism binoculars using the Abbe–Koenig roof prism typically do not use mirror coatings because these prisms reflect with 100% reflectivity using total internal reflection in the prism.
Dielectric mirror coatings
Main article: Dielectric mirror
Dielectric coatings are used in Schmidt–Pechan roof prisms to cause the prism surfaces to act as a dielectric mirror. The non-metallic dielectric reflective coating is formed from several multilayers of alternating high and low refractive index materials deposited on the roof prism's reflective surfaces. Each single multilayer reflects a narrow band of light frequencies so several multilayers, each tuned to a different color, are required to reflect white light. This multi-multilayer coating increases reflectivity from the prism surfaces by acting as a distributed Bragg reflector. A well-designed dielectric coating can provide a reflectivity of more than 99% across the visible light spectrum. This reflectivity is much improved compared to either an aluminium mirror coating (87% to 93%) or silver mirror coating (95% to 98%).
Porro prism binoculars and roof prism binoculars using the Abbe–Koenig roof prism do not use dielectric coatings because these prisms reflect with very high reflectivity using total internal reflection in the prism rather than requiring a mirror coating.
Terms used to describe coatings
Special reflective coatings on large naval binoculars
For 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.
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.