An equation used in

computer graphics that tells us how to
compute the intensity of a point. Typically, the illumination equation depends the normal to the surface (N) and the direction to the light source (L). It is the basis of most of the lighting systems used in

real-time rendering as well as

ray tracers. The simplest version of the illumination equation is based on

diffuse reflection. It is:

I = I_{l}k_{d}N.L

i.e. the intensity of this point (I) is the intensity of the light source (I_{l}) times the coefficient of diffuse reflection of the surface (k_{d}) -- basically the colour of the surface, times the dot product of the normal to the surface and the direction to the light source -- this basically computes the cosine of the angle between the normal to the surface and the light source. The above is called Lambert's Law.