Lambert’s cosine law says that the radiant intensity or luminous intensity observed from an ideal diffusely reflecting surface or ideal diffuse radiator is directly proportional to the cosine of the angle θ between the direction of the incident light and the surface normal.Lambert’s law states that the reflected energy from a small surface area in a particular direction is proportional to the cosine of the angle between that direction and the surface normal. Lambert’s law determines how much of the incoming light energy is reflected.