Generalized ray-tracing computer programs, which simulate the propagation of light through three-dimensional models of optical systems, have their origin in the 1960s. Progress in generalized ray-tracing software has proceeded on three fronts since then, in the fields of optical design and analysis, the radiation transfer part of the thermal analysis problem, and in photorealistic computer graphics rendering. These three fields have evolved largely independently, though they have much in common: computer representation of three-dimensional geometry, computation of ray-surface intersections, propagation of optical flux, and modeling of the interaction of light with matter including the use of BRDF and BTDF to model surface scattering. The size of the computer graphics industry dwarfs the others, as measured by the number of workers in each field and the volume of published literature. Only recently have ideas from the computer graphics industry been utilized in the optical analysis field.