> To my (admittedly limited) knowledge, I believe AR coatings
> need to be on the front/rear surfaces of the lens to do much
> of their work. Some lenses also have internal lens groups AR
> coated, but I do not believe it can be sandwiched, because as
> I understand the optics, it is the glass to air surface and
> differences in defraction that causes the principle
> reflection to begin with.
AR coatings can be applied wherever there are interfaces (air/glass,
glass/air). You may find coatings on sandwiched elements, as unless the
elements are in a vacuum, then there will be an air gap (however small)
between the elements.
> I believe most AR coatings are actually a vaporized metallic
> material that is "coated" to the glass or plastic surface,
> but it is literally a couple of molecules or atoms thick.
> Certainly, some of them are vulnerable to harsh and caustic
> chemicals, like ammonia.
Indeed. The most common way is electron beam evaporation, where an electron
beam is rastered over a lump of the material required (generally an oxide).
The vapour then coats the glass elements, which are spinning, and often at
an elevated temperature. Sequential layers are deposited. Generally of two
materials with differing, but finely chosen refractive indices, such that
each layer cancels out the phase change of light on reflection. End result
is an AR coating.
For white light (wavelength ~ 525 nm), and using SiO2 as the low nf (1.46),
the single layer thickness would be (for a quarter wave stack): 89.9 nm.
Many elements are given a final layer of a fluoride. These coatings can
often be rubbed off if cleaned aggressively.
> I have seen (and owned) some pretty costly lenses and they
> have had AR coatings on the exposed "surfaces".
Aye - the cost often reflects the quality of tuning of the elements and
coatings. The company I work for makes systems to deposit such coatings onto
lenses and other substrates.
HTH
Chris
--
Chris Aitken BEng(Hons) MInstP
Projects & Development Engineer
Scientific Vacuum Systems Ltd
11/12 Weller Drive, Hogwood Lane Industrial Estate Finchampstead, Berkshire,
UK.
Tel: +44 (0)118 973 1946, Fax: +44 (0)118 973 1834
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