Abstract: A wavelength based bidirectional reflectance function is developed for use in realistic image synthesis. A geodesic sphere is employed to represent the BRDF, and a novel data structure is used to store this description and to recall it for rendering purposes. A virtual goniospectrophotometer is implemented by using a Monte Carlo ray tracer to cast rays into a surface. An optics model that incorporates phase is used in the ray tracer to simulate interference effects. An adaptive subdivision technique is applied to elaborate the data structure from rays scattered into the hemisphere above the surface. The wavelength based BRDF and virtual goniospectrophotometer are utilized to analyze and make pictures of thin films, idealized pigmented materials, and pearlescent paints.