Perhaps the most significant factor impacting the accuracy of software for the design of automobile illumination optics is the proper characterization of the light source. Techniques used in the past have employed a simplified geometry to model the emitting element of the lamp along with an intensity distribution which had been measured at some fixed distance. This produced major inaccuracies in the prediction of the optical performance of complicated systems.The authors have been active for several years in developing improved techniques for lamp characterization. It is the intent of this paper to describe the aspects of lamp modeling which we have found to be important. These factors include the use of “far-field” and “near-field” photometry, the spatial variation of luminance over the lamp, and re-emission of radiation incident on the lamp.The most difficult light sources to model accurately are high intensity discharge sources. Methods will be discussed for the solid modeling of such light sources by the use of luminance and intensity data. Instrumentation for the collection of these data is desired.