The simplest shading model is the faceted model. You compute only one intensity (color shade) per polygon. The light vector, L is computed as the vector between any point on the polygon, e.g. a vertex, and the light source position. Generally only the two term illumination model (ambient plus diffuse reflection) is used with faceted shading. Faceted shading is simple and quick but not very realistic, the facets are easily seen since there may be sharp color discontinuities at the polygon edges.
A Pixar Shutterbug example image with faceted shading.
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