Coating, anti-reflection and dispersion
In the lens (I)
If you observe with attention the surface of lens objectives of different quality or photographic filters you will see colored reflections or glare. These effects can tell us something about the coating used to protect them again parasitic light.
When light hits the front or rear surface of a lens a small part is reflected causing glare and ghost images. For thin optics or filters a second reflection can appear when the incoming ray hits the second surface. A fraction of the light is still reflected, reducing accordingly the amount of light reaching the detector. With dozen of air-to-glass surfaces to pass through this process could go to infinity until the light is completely extinct. Therefore opticians have thought about a mechanism to suppress that undesirable phenomenon.
We have explained in a previous report about eyepieces that the brightness of an image is very important and has a direct impact on its sharpness and contrast. Manufacturers take a special attention to reduce these effects in order to improve the image quality. They speak in terms of coatings, anti-reflection and glasses of wide dispersion. What do they mean and what are their effects on optics performance ?
Many objectives or eyepieces are qualified of "full coated" (FC) or protected by an "anti-reflection coating" while others are "multicoated" (MC) or "fully multicoated" (FMC)... All these terms are not similar. They represent characteristics of specific protections.
The simpliest protection against light scattering and parasitic reflections consists of covering each optical surface with a thin film of anti-reflection coating in order to reduce the light loss. As we do to design interferometrical filters, the technique take advantage of the wave nature of the light. In particular the phenomenon of phase opposition of light and the dependence of the reflectivity on index of refraction. By covering a surface with a single coating 1/4λ thick opticians can eliminate reflection rising at that specific wavelength as the two rays beams are 180 degrees out of phase. Usually a single coated lens optimizes the antireflection for the green light (550 nm) in the middle of the visible spectrum. Thus using multi-layers coatings we can reduce the light loss over different parts of the visible spectrum.
This coating is in fact a thin film of scratch-resistant antireflection material like magnesium fluoride, silicon or titanium dioxide (MgF2, SiO2, TiO2). It is applied with an electron beam on one or more air-to-glass surfaces placed in a vacuum chamber.
Theoretically an uncoated lens can loose about 4 % of light transmission per surface in reflection and scattering. This factor in exponentially multiplied when adding more lenses. So in an eyepiece using 6 lenses the light transmission decreases to (0.96)6 = 78 %.
A single layer of MgF2 anti-reflection coating can reduce the loss to 1.3 % and a multilayers composed of different coatings to as low as 0.25 %. The light transmission of our 6 lenses eyepiece reaches now 98.5 %. This explain why multicoatings provide the highest level of light transmission and yield images with the best contrast.
In theory at least, lens coatings range in quality as follows :
- Coated or single-coated : one air-to-glass is coated with a single layer of antireflection material
- Fully coated : all air-to-glass are single-coated
- Multicoated : at least one air-to-glass surface has been coated with multiple layers, the rest are single-coated or not coated at all.
- Fully multicoated : all air-to-glass surfaces are covered with multiple coatings.
So "multicoated" eyepieces like the Celestron Ultima's serie or Plössl's enhanced show a positive difference against a standard "full coated" Celestron Plössl.
The same for an achromatic lens. The Megrez 80 SD refractor from William-Optics was protected with an MC coating. It is now discontinued and replaced with the Megrez II 80 ED which lenses are FMC. According to the manufacturer the internal reflexions are reduced up to 40 %.
Looking closely a fully multicoated eyepiece (without disassembled it) we must see one dark green or purple reflection for each air-to-glass surface. In a Tele Vue Plössl for example there are four such reflections (4 lenses). Those optics reflect a blue-amber (single coated) or a green-purple (multicoated) light.
If you don't see all reflections this is because the cemented interface surface appears brighter than the air-to-glass multicoating. If you see a reflection having the same color as the incoming light then this lens is not coated.
Note that infrared filters (Schott RG665, Kodak W89B, Hoya R72, etc.) are not coated because the protection is only efficient in the visible spectrum. There are coatings adapted to near IR between 600 and 900 nm but they are very expensive and there is no demand from the general public for such products.
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