R = DIFFUSE(Nx,Ny,Nz,S) returns the reflectance for a surface with
normal vector components [Nx,Ny,Nz]. S is a three vector that
defines the direction to the light source. S can also be a two vector
S = [Az,El] specifying the direction with azimuth and elevation.
Lambert's Law: R = cos(theta) where theta is the angle between the
surface normal and light source.
See also SPECULAR , SURFNORM , SURFL .