DIRECTIONAL REFLECTANCE AND EMISSIVITY OF AN OPAQUE SURFACE
SYLVANIA ELECTRONIC SYSTEMS-WEST MOUNTAIN VIEW CA ELECTRONIC DEFENSE LABS
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Concepts, terminology, and symbols are presented for specifying and relating directional variations in reflectance and emissivity of an opaque surface element. Their relationship to more familiar concepts, including those of perfectly diffuse and specular reflectance, is given, and they are applied to illustrative examples. It is shown that, when the usual reciprocity relationship holds, the reflectance for a ray incident on an opaque surface element is related by Kirchhoffs Law to the emissivity of that element for a ray emitted along the same line in the opposite sense.