Lambert's cosine law

In telecommunication, Lambert's cosine law is the mathematical statement that the radiance of certain idealized optical sources is directly proportional to the cosine of the angle--with respect to the direction of maximum radiance--from which the source is viewed.

Note: Lambert's cosine law may also apply to certain idealized diffuse reflectors. Synonyms cosine emission law, Lambert's emission law.

Source: from Federal Standard 1037C