Lambert's cosine law

Base for the rules of Area Foreshortening .

Wikipedia:

In optics, 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 between the direction of the incident light and the surface normal;

Lambert published it in:

@book{lambert1760photometria,
  title={Photometria sive de mensura et gradibus luminis, colorum et umbrae},
  author={Lambert, Johann Heinrich},
  year={1760},
  publisher={sumptibus vidvae E. Klett, typis CP Detleffsen}
}