Optical coatings depend for their performance on interference effects coupled with the optical properties of their materials. Interference effects depend on path differences that are essentially inversely proportional to wavelength. While it is fairly straightforward with interference to achieve the properties required over a limited spectral range, extended spectral ranges are much more difficult. This is especially true of dielectric extended zone high reflectance coatings. The normal quarterwave stack reflector has a quite limited range, rather less than the visible region. To extend it we normally add further quarterwave stacks or introduce a tapered structure. Although the reflectance thus achieved can be high over much wider regions there is a range of unintended consequences in design problems, surface phase, scattering, absorption, surface figure, sensitivity to contamination and short pulse effects, all of which are important in the application of such coatings. This paper will discuss these effects, which, although they may sometimes appear mysterious, are understandable and predictable.
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