This type of coatings are found on roof prisms of binocular and are there to increase light reflectivity - As the light is incident at a glass–air boundary with an angle less than the critical angle, total internal reflection does not occur at that surface. To solve this problem, a mirror coating is used on those surfaces.
To quantify typically an aluminium mirror coating is used that has a reflectivity of 87% to 93%) or a silver mirror coating (reflectivity of 95% to 98%) is used.
The light transmission of the prism can be improved by using a dielectric coating rather than a metallic mirror coating. This causes the prism surfaces to act as a dielectric mirror. The dielectric 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.
