No abstract

The single point diamond turning process is ideally suited to making aspheric optical elements for use in the infrared region.In this paper, applications of diamond turning to various materials and geometries for both refractive and reflective optical elements are considered, based upon direct manufacturing experience.

Light weight beryllium baffle materials have been fabricated by laying down thick, fine grained Be coatings using magnetron sputtering on formable Be ingot sheet material. Six different coating process variations were used: three different bias voltage settings (-200 V, -50 V and no bias) were applied to either water cooled or thermally isolated substrates. A 20% solution of HF acid in ethanol was used to etch the Be coatings in order to increase light absorption. Scanning electron microscopy revealed significant variations in surface morphology of the etched coatings. Acid etching increased the number of light traps in the surface, particularly those below one micrometer in feature size, thus increasing light absorption in the visible region. Specular reflectance measurements from 400 nm to 50 .im revealed that thick coatings laid down with a -50 V bias and etched for one minute produced diffuse, light absorbing baffle surfaces with reflectivities less that 5% out to 40 m.

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).