3 .0 RESEARCH AND ANALYSIS 3.1 REFLECTANCE OF VEHICLES The reflection of the various system components is best considered in two parts. That is the elements that could significantly contribute to the ground illumination are identified and their qualities are described. The second part describes the general formulation for determining ground illumination intensities for any given reflector as a function of the reflector characteristics . 3.1.1 Reflection Analysis Most of our surroundings consist of diffuse surfaces where the light is reflected over a broad range of angles, usually a full 2tt steradians. The angular dependence of the reflection can vary due to surface orientation qualities such as nap and reflections may be peaked in one direction or another with a normalized angular dependence 6(0^0) where 0^ and 0f are incident and reflected directions measured from the surface normal. A Lambertian surface has a diffuse reflection that varies as 2 cos 0f cos 0.. Reflected energy from a diffuse surface at the center of a hemisphere of radius R has an apparent brightness at the hemisphere that is diminished 2 by G/2ttR . For a spacecraft surface area A with diffuse reflectivity r^, the radiant flux above the atmosphere is proportional to r^ A H0. Atmospheric extinction and light scattered by any optical instruments further degrade the brightness and are denoted by the factor k. Combining these quantities gives the general expression for diffuse irradiance, Any surface that produces a discernible reflection Tniage is partially specular due to minute optically flat elements. Some surfaces like glass and polished metal are optically flat over large areas and their specular response is more noticeable due to bright sunlight glints. A representation of the
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