Thick, fine-grained beryllium optical coatings

Abstract

Fine grained sputter deposited beryllium coatings with thicknesses up to 290 micrometers have been laid down on six coupons of various substrate materials including S-200 grade beryllium, SiC and Ti6AI4V alloy. Each coated substrate was subsequently polished to a very low surface roughness (5 A(rms)) finish by one of two different polishing techniques. Bidirectional reflectance distribution function (BRDF) measurements were taken at 632.8 nm and at 10.6 pm wavelengths. BRDF values at 10.6 pm were measured as low as 9 X iO sr1 at 10° off-specular. Calculated power spectrum density (PSD) curves showed remarkably little or no anomalous scatter except for one coating laid down with no substrate bias and an uncooled substrate. The measured reflectances of the polished Be coatings at 10.6 pm is about 5% below that for the gold reference surface (i.e., =94% absolute reflectance).