Lambert's laws (J.H. Lambert):
Lambert's first law
The illuminance on a surface illuminated through light falling onto it perpendicularly from a point source is proportional to the inverse square of the distance among the surface and the source.
Lambert's second law
If the rays meet the surface at a particular angle, then the illuminance is proportional to the cosine of the angle along the normal.
Lambert's third law
The luminous intensity of light reduced exponentially through distance as it travels via an absorbing medium.