When light enters glass from air or from glass into air, about 5% is reflected. Objectives, Prisms and Eyepieces for Each Tube of Binoculars
Together, there are generally 10 to 16 surfaces in contact with air. If these surfaces are not treated in any way, the incident light will lost about 50% by reflection. To reduce this unwanted reflection, modern refractor telescopes are coated with single or multiple layers of antireflection coatings on each optical surface. The material of the multilayer AR coating is magnesium fluoride. The single-layer anti-reflection coating only has the best anti-reflection effect for a specific wavelength of light, and the anti-reflection effect for other wavelengths is slightly worse.
It can reduce the reflection of light to 1.5% per surface, such as for all surfaces of binoculars, the transmittance of light can exceed 80%. Good multilayer film per the reflectivity of light from each surface is only about 0.25%, and if it is used on all surfaces of binoculars, the transmittance of light can reach 90-95%.
In general, a single layer of antireflection coating for visual telescopes is best at 5500 angstroms of yellow-green light, because the human eye is most sensitive to this light.Blue and red light far from this wavelength are more reflective, so we see that the lenses coated with a single layer are blue-violet or red.
The film of Multi-coated mirrors is light green or dark purple, and the single-layer film that is too thick will also appear green. It has been found that some foreign manufacturers use this as multilayer film, but it reflects much more light than a true multilayer film.
Some people will feel that large-diameter binoculars are fine even if the coating quality is poor, and its aperture can make up for the loss of light.
In fact, it is not. The reflected light from the various surfaces inside the binoculars will form stray light, reduce the contrast of the scene, and make the image blurry. This phenomenon is especially obvious when viewing objects in the shadows in sunlight. Objects that disappear into the dazzling brilliance of poorly-coated binoculars are easy to see with well-coated binoculars.
According to different quality, anti-reflection coatings can be divided into the following types, which are generally marked on the obvious position of the mirror body.
Coated Optics: Since no legitimate manufacturer sells completely uncoated binoculars, this is actually a low-level Anti-reflection coating. It only means that at least one surface is coated with a single layer of anti-reflection coating, usually the outer surface of the two objectives and two eyepieces, while The inner lens and prism are not coated.
FullyCoated: All lenses and prisms are coated with a single layer, but may not be coated if optical plastic lenses are used in eyepieces.
Multi-Coated: At least one surface is coated with a multi-layer film, other surfaces may be coated with a single-layer film, or may not be coated at all,
Usually only the outer surfaces of the objective and eyepiece are coated with multilayer coatings.
FullyMulti-Coated: All surfaces are AR-coated, some manufacturers have multiple layers on all surfaces film, while others are only coated with multi-layer film on part of the surface, and the other surface is still coated with a single-layer film.
The quality of the AR coating is closely related to the imaging quality of the binoculars. Be careful when purchasing.