Anti Reflective Coating
Anti Reflective Coating – A chemical coating on the surfaces of the binocular lens elements and prisms will produce images that are sharper, less hazy, and higher contrast by reducing the amount of light that is randomly scattered within the binocular. There can be as many as ten lenses and prisms between a binoculars’ objective and your eye, and each element has two surfaces that can reflect back and lose about 4% of the light that strikes its surface. In order to minimize this light loss, an anti reflective coating of magnesium fluoride is vacuum-deposited on these surfaces (one-fourth of a wavelength of light or a few-millionths of an inch in thickness) to increase the light transmission through the surface. Although each anti reflective coating minimally reduces light loss (approximately 1.5% per surface or 0.5% with multi-coating), the overall effect is to increase light transmission by 50% or more.
The following types of anti reflective coating are available:
- Coated lens usually consists of a single layer of an anti reflective coating on at least one lens surface, usually the first and last lens surface that can be seen.
- Fully coated lens consists of a single layer of an anti reflective coating on all air-to-glass lens surfaces.
- Multicoated lens consists of multiple layers of an anti reflective coating (containing zirconium oxide, cryolite, zinc sulfide, and other arcane materials) on at least one lens surface, usually the first and last lens surface that can be seen. This type of anti reflective coating will produce approximately an order of magnitude brighter image than a fully coated lens.
- Fully multicoated lens consist of multiple layers of an anti reflective coating on all air-to-glass lens surfaces and prisms which produces the brightest image of any anti reflective coating system. If you can afford it, go with fully multicoated lens, as this is the best choice in optical quality. Even though fully multicoated scopes tend to be expensive, if you are ever hunting or shooting in low light conditions like dawn or dusk, you will appreciate its benefits.
With a visual inspection, you can check the anti reflective coating quality used on the binocular by examining the color of the image of a fluorescent light reflected in them. A magnesium fluoride anti reflective coating of the correct thickness gives a purple/violet tint to the largest reflected image (the one on the outer lens surface), although the tint can range from pale blue to magenta, depending on the type of glass used in the lens. A magnesium fluoride anti reflective coating that is too thin yields a pink reflection, while a anti reflective coating that is too thick looks green. The smaller reflections (from the internal surfaces of the lenses) are generally off-white and tinted a faint violet, amber, or green. Multicoated lens are usually green in color, although the reflected image can be any of a number of tints, even red and amber, depending on the type of glass and the materials used for the anti reflective coating.