Abstract: One way to overcome the limitations imposed by analytical models of reflection is to use discretely sampled reflectance data directly. Through either empirical measurement or simulation, a bidirectional reflectance distribution function (BRDF) is sampled and stored as a table of numbers. The generality of these tabulated BRDFs is useful for generating realistic images, but the inevitable inaccuracy associated with the data gathering process can lead to a BRDF that is much more general than it needs to be, or that lacks certain physical properties. In this paper we propose measures for several properties of tabulated BRDFs: reciprocity, energy conservation, isotropy, and separability. Techniques to transform tabulated BRDFs to match one or more of these properties are also described. These transformations allow compression of the BRDF data, elimination of noise, improved computation time in some rendering tasks, and improved compliance with physical laws.