The first lighting model which accounts for diffuse surfaces was developed by Gouraud [7]. In 1975 Phong [16] proposed the first model in computer graphics able to deal with non-diffuse surfaces. In this model the color of a pixel is expressed as a linear combination of a diffuse part and a specular part. Blinn improved the physical correctness of this model and made it visually more satisfying. This so-called Blinn-Phong model [2] is commonly used for hardware accelerated lighting. These early models were ad hoc empirical models without any exact value of energy or intensity. The most important of physically more correct models is the one by Cook and Torrance [4], it is based on a Gaussian micro facet distribution. In addition to isotropic models, anisotropic models like the model from Banks [1] have also been proposed. As mentioned above, hardware based rendering methods usually use the Blinn-Phong model [2], because of its mathematical simplicity. However, Heidrich etal. [8] recently showed how to handle more complex BRDF models by factoring the Banks model and the Cook-Torrance model analytically, and storing the factors in texture maps. The original model can then be reconstructed using texture mapping. Kautz etal. [9] and McCool etal. [13] reparameterized BRDFs and then decomposed them numerically. Again the original model is reconstructed using texture mapping.