After
Johann Heinrich Lambert, from
Photometria published in 1760, the
Lambertian diffuse lighting model describes a simple non shiny surface where any incident light is reflected evenly in all directions after modulation by the surface's reflectivity. Naturally any incident light energy is spread over the area upon which it falls and this gives rise to the realization that the apparent brightness of a point on an illuminated surface viewed from any angle is therefore the cosine of the angle between the
surface normal and the
vector pointing towards the illuminating light source times the intensity of the incident light at that distance and location from the light modulated by the
albedo of the surface. The calculation of incident energy is commonly known as
Lambert's cosine law.
It should be noted that the part of this model that assumes all light reflected from a surface is reflected evenly in all directions is a very approximate equation when compared to how real surfaces reflect light and is only even approximately correct for very smooth matte surfaces. In some forms it remains popular for diffuse lighting calculations, particularly in 3D computer graphics.
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