Since the discovery that Al overcoated with MgF(2) or LiF produces high reflectances to wavelengths as short as 1150 A and 1000 A, respectively, these coatings have been used extensively in vacuum ultraviolet instruments in the wavelength region where their reflectance is high. If the instrument is intended to cover wavelengths shorter than the two given above, usually either Pt or Ir is used, with a loss of speed at the longer wavelengths. This paper presents reflectance data showing that fluoride-overcoated Al can be useful to wavelengths as short as 500 A. Measurements were made from 1600 A to about 300 A at normal, 35 degrees , and 85 degrees angles of incidence, angles used in normal, Seya, and grazing incidence spectrometers, respectively. These measurements show that from the boundary of the high reflectance region to 500 A, the reflectance at normal and 35 degrees depends on the thickness of the fluoride coating and can be as high as 24% at 800 A for a MgF(2) thickness of 150 A. For shorter wavelengths, the reflectance shows a decreasing thickness dependence and at 304 A is very low-about 1%. At grazing incidence, the reflectance shows some thickness dependence from 1500 A to about 1000 A, but toward shorter wavelengths the dependence disappears and the reflectance increases slowly to about 80% at 500 A. In addition to the reflectance measurements, polarization effects are discussed.

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A technique for producing adherent and well-protected front surface Ag mirrors with an evaporated Al(2)O(3) + SiO(x) overcoat and an evaporated Al(2)O(3) underlayer is described. A thin Al(2)O(3) layer promotes adhesion between the Ag film and its substrate. The optimum thickness of each outer layer necessary to give good adhesion and protection to the Ag surface with minimal loss in reflectance due to ir absorption was found to be about 300 A for the Al(2)O(3) layer and between 1000 A and 2000 A for the SiO(x) film. Ag surfaces coated in this fashion retained a normal incidence reflectance in excess of 95% over the wavelength region from 450 nm to the far ir, even when exposed to harsh sulfide and humidity environments. Calculations demonstrate the advantage of using protected front surface Ag in comparison to more durable metal reflectors, such as Al or Rh, in terms of high reflectance and low visible region polarization. However, a very pronouncedrestrahlen reflectance and absorption effect in the thicker SiO(x) film is responsible for a calculated drop in reflectance from 98.5% to about 65% at lambda = 8.1 mum and 45 degrees incidence for the Ag + Al(2)O(3) + SiO(x) coating.

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