Two-layer anti-reflexion coating for low refractive index glass

Abstract

A method is described for producing durable two-layer anti-reflexion coatings on low refractive index (approximately 1.5) glasses. Practical difficulties in evaporating absorption free films of high index materials are avoided by reactively sputtering a bismuth oxide film which is then coated with an evaporated magnesium fluoride film. The initial oxide layer is prepared by sputtering bismuth in oxygen from sector-shaped cathodes placed under a rotary holder, and the magnesium fluoride is evaporated in sequence from a vapour source arranged under the edge of the rotary holder. To reduce the reflectance of a glass (n = 1.5) at a specified wavelength in the visible region, the bismuth oxide is deposited until the glass reflectance has risen to 8% and the magnesium fluoride evaporated until the reflectance falls to a minima (R < 0.1%). Curves are given for the reflectance of the two-layer coating in the visible region. A simple method of monitoring the wavelength of the reflectance minima is described. It is shown that the wear resistance of the two-layer coating is equal to that of a single-layer magnesium fluoride film and is improved by baking at 250° C.