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Binocular Coatings Lens coatings play a very important role in the overall performance of binoculars. Each time light enters or leaves a glass element about 5% of the light is lost due to reflections. Binoculars may have up to 16 air-to-glass surfaces, and you will have light loss at every surface.
In the early days before optical coatings were available, less than half the light would actually reach your eyes, the rest of the light scattered around inside the binoculars causing even more unsatisfactory images. Larger objective lenses were used to compensate for this light loss, however the size and weight increased to almost non-practical levels.
In the 1930s it was discovered that when glass was coated with magnesium fluoride more light would be transmitted. This single layer coating technology was found to reduce light loss to about 1.5% per optical surface. In todays Broadband Fully Multicoated lenses only about 0.25% or less of light is lost at each surface, and in the best binoculars over 95% of the light is actually transmitted to your eyes.
The coating process depends on applying a perfectly uniform layer of coating material only a few nanometres thick, if its too thick or too thin then it just doesnt work.
With roof prism binoculars another feature to look for is anti-phase-shifting coatings. Binoculars with this coating on the roof surfaces will deliver higher contrast images.
Types of coatings available
Coated Optics: One or more surfaces coated. This usually means a single layer of anti-reflection coating on some lens elements, usually the first and last, (the lenses you can see).
Fully Coated Optics: All air to glass surfaces coated, but any plastic lenses used may not be coated. Only a single layer of coating is usually applied to the surfaces.
Multi-Coated Optics: One or more surfaces are multi-layer coated. The multi layers are usually applied to the first and last lenses, while the others usually are single layer coated.
Fully Multi-Coated Optics: All air-to-glass surfaces are multi layer coated. This is the highest level of coating generally available and provides the best light transmittance and contrast.
What about (RC)- Ruby Coating? Ruby coatings RC are actually intended to reduce glare in bright light. When using expensive good quality ruby coated binoculars the coating actually does what it was designed for, to reduce glare.
Unfortunately the market is completely flooded with cheap RC ruby coated binoculars. These are sold as the latest and greatest in optical fads. At the end of the day, the ruby coatings in many of the cheap binoculars are used to disguise the binoculars poor optical quality.
Ruby coated binoculars should not be used for astronomy as they actually reduce the amount of light that is transmitted to your eyes and also can change the colour of the image, resulting in objects with a green cast.
How do coatings work? Coatings work by harnessing the wave nature of light in such a way as to cause destructive interference in the light waves reflecting off the surface. Its the same principle as two people attempting to sing a note. If they get it right the overall sound is louder (constructive interference), if they get it wrong then the sound could cancel out altogether (destructive interference).
In a similar manner if you can manipulate the light reflecting off two surfaces in such a way that the reflecting waves cancel each other out, most of the light energy that would have been reflected is forced to pass through. This is the basic idea behind lens coatings. Magnesium fluoride is the most common form of coating because of its ease of use and durability. The key factors for enhancing maximum light transmission is the refractive index of the coating used and the thickness of the coating.
Coated or fully coated lenses (single coating), work best for single wavelengths of light, some wavelengths are affected less, while others are not affected at all. With light being composed of many wavelengths (colour), you can quickly see the need for more broadband coatings that cover more wavelengths. This is why broadband multicoated lenses are used. Multiple coatings of different materials work effectively over a larger band of the colour spectrum.
Naturally for the user who demands the best light transmittance and contrast the broadband fully multicoated lenses will provide them with the best performance.
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