Illumination models can be either empirical or physically based . An emprical model results from the fitting of a function to observed data. Physically based models, on the other hand, attempt to simulate the interactions between light and various materials. The first Illumination models that we will discuss are largely empircal. Basically they are simplified formulas which generate results that fit both the underlying physics and our experiences.
Light is the most important ingredent in any illumination model.
However, most of the light that we recieve is not directly from the light source. Instead it is reflected off of intermediate surfaces. Therefore, information describing both the light sources and the relective properties of materials are needed to establish any illumination model.