Bidirectional reflectance and specularity of twelve spacecraft thermal control materials

Abstract

Many spacecraft external payloads are sensitive to focused solar heating caused by specular reflections from exterior thermal control surfaces. This paper presents a method of calculating specularity using bidirectional reflectance distribution function (BRDF) input. Definitions are presented for directional and hemispherical specularity as a function of the conical half-angle surrounding the specular ray. Measured BRDF data are presented for commonly used thermal control materials including plastic films and paints. Angles of incidence range from 5 to 78 deg and data are taken both in and out of the plane of incidence. Most measurements are made at a wavelength of 0.488 /urn, though measurements for two white paints are also made at 1.06, 3.39, and 10.63 ftm. The BRDF values are found to increase with both angle of incidence and wavelength. A numerical code is described that integrates BRDF data to yield both directional and hemispherical specularity as a function of cone half-angle around the specular ray. Specularity results are presented for all 12 materials studied.