Marco Tomat wrote:
"I have been understanding they work by interference and then colours must be
seen. If one cannot see them, that means the coating cannot too.
Of course there are many ways to skin the 7-MC-coat of the cat ;-) but every
layer works on a different wavelenght, that is a different colour."
No, William is correct. First the colours of thin films you refer to will only
be seen when the film is thicker than 1/4 wavelength.
A single 1/4 wavelength film of refractive index which is the square root of
the glass the lens is made of will match the light to the lens at one
wavelength. The effect however is quite broad and helps across the spectrum.
This is the basis of single coating. Interestingly it was discovered because
early photographers found that older lenses which had developed a 'bloom' were
sharper than newly ground lenses. The bloom was a naturally forming
antireflection coating.
A multicoated lens uses a stack of these layers, all of slightly different
thickness and refractive index. Its design is directly analogous to designing
electrical filters such as are found in TV's, radios etc.
The ideal coating has no reflection at any wavelength. However there is always
a compromise between the number of layers, the thicknesses and the choice of
materials (i.e. the refractive indices, and hardness) of the layers. So all
multicoatings tend to reflect some colour. As William points out, a MC lens
still reflects less than a SC lens.
Chris Barrett
Malvern
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