Combining the Illumination:
Combining all three components (diffuse illumination, diffuse reflection from a point source and specular reflection from a point source) gives us the following expression:
E = Ed + Esd + Ess
or written out in full :
E = R . Id + (R cos (i) + w (i) . cosn (s)) . I s
where, E is the total intensity of light seen reflected from the surface by the observer.