Chris, Thanks for the edifying trip into AR coating technology. Very interesting.
Art [EMAIL PROTECTED] 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 > > http://www.svs.co.uk > > ---------------------------------------------------------------------------- > ------------ > ---------------------------------------------------------------------------------------- Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body
