Abstract

A technique for producing adherent and well-protected front surface Ag mirrors with an evaporated Al₂O₃ + SiO_x overcoat and an evaporated Al₂O₃ underlayer is described. A thin Al₂O3 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 Å for the Al₂O₃ layer and between 1000 Å and 2000 Å 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 pronounced restrahlen 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 λ = 8.1 μm and 45° incidence for the Ag + Al₂O₃ + SiO_x coating.

© 1975 Optical Society of America

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