The assumption of diffuse emission and diffuse-specular reflection in gray enclosures is adequate for many applications encountered in engineering practice. This is especially true in the conceptual and preliminary design stages. However, final design and performance studies often require the use of bidirectional surface optical models restricted to a specified wavelength interval. In this chapter we develop the concept of the band-averaged spectral radiation distribution factor, which is the most general tool available for treating radiant exchange among surfaces, but first we learn through extended examples how to compose and use models to simulate the optical behavior of non-diffuse surfaces....