As computation speeds have increased dramatically over the last decade, we can now trace enough rays in a short enough time to use ray tracing to predict the performance of an illumination system. The biggest obstacle, however, to accurately model, and thus design, illumination optics is in developing an accurate source model3. In the past, sources were simplistically modeled as very basic geometrical shapes such as points, spheres, or cylinders. Some illumination design software now allows an engineer to create a more complex theoretical model of the source that could include multiple geometrical shapes to more closely approach real source properties. These models, however, are time consuming to create and still fall short of the goal to accurately model the system. Rather than to build up a source model based on a (combination of) geometric shape(s) with some assigned output distribution based on either measured or theoretical data, the authors will demonstrate a new technique for developing and applying source models based on careful, consistent and general measurements. The measurement system consists of a CCD camera mounted on a 2-axis goniometer that allows images of the source to be captured over variable polar and azimuth increments. These source models produce accurate results even when optical surfaces are placed near the source.