Employing geometrical optics and assuming Lambert reflection, a general method is developed for calculating the diffusely reflected light from a class of surfaces known as quadrics. The analysis is applied to several well-known nondegenerate as well as degenerate quadric surfaces for cases which are considered to be convex or concave with respect to the directions of incident light and observer vectors. For several examples from the convex category, the results are exact and agree with those previously obtained with other methods of development. The effects of shadowing are included for a few concave-surface geometries, but to the extent that diffraction around edges and multiple reflections are omitted, the results are approximate.
KOLMAN W. BRAND and FRANK A. SPAGNOLO, "Lambert Diffuse Reflection from General Quadric Surfaces," J. Opt. Soc. Am. 57, 452-456 (1967)