Chris,
Thanks for the edifying trip into AR coating technology. Very interesting.
Art
chris@ion-dreams.com wrote:
>>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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