Lambert's cosine law

Lambert's cosine law states that illumination on a surface varies with the cosine of the angle between incoming light and the surface normal. For an ideal diffuse emitter, radiance is direction-independent, while observed intensity varies with viewing angle.

Surface normal: A vector perpendicular to a surface at a specified point. The law measures the angle between light or viewing direction and this reference direction.
Lambertian reflectance model: A simple image-formation model that represents matte surfaces with direction-independent reflected radiance. It converts surface orientation and illumination into predicted image brightness.
Specular reflection: Reflection in which light leaves a surface in a direction determined by the mirror-reflection rule. Unlike ideal diffuse reflection, its appearance depends strongly on viewing direction.
Johann Heinrich Lambert: An eighteenth-century Swiss polymath whose work included optics, mathematics, and astronomy. He formulated the cosine relation for illumination and diffuse surfaces.
Cosine projection law: The rule that a projected area equals the original area multiplied by the cosine of its tilt angle. It accounts geometrically for the reduced area presented to oblique light.
Phong reflection model: A local illumination model combining ambient, diffuse, and specular reflection terms. Its diffuse term uses cosine dependence, while its specular term adds directional highlights.
Bidirectional reflectance distribution function: A function describing how a surface reflects light between incoming and outgoing directions. It generalizes Lambertian behavior to surfaces with directional reflection.
Photometria: Johann Heinrich Lambert's 1760 treatise on the measurement and theory of light. It presented foundational quantitative work on photometry and diffuse reflection.
Lambertian reflectance: An ideal diffuse-reflection model in which radiance is independent of viewing direction. It describes the diffuse-surface behavior associated with the law.
Radiosity: A method for computing diffuse light exchange among surfaces in a scene. Its surface-to-surface energy calculations use cosine-weighted exchange.

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