Geometrical theory of radiating surfaces

Geometrical theory of radiating surfaces

Year: 1907

Theoretical photometry assumes two general laws of radiation. (1) The inverse square law, which on the assumption of the rectilinear propagation of light is merely a statement of a geometrical property, and (2) Lambert's cosine law. The latter is an empirical law, based primarily on the observation that a uniformly bright sphere, when viewed at a distance, appears as a uniformly bright disc. No satisfactory experiments have yet been made to test the law in its application to glowing surfaces. It apparently follows from Kirchhoff's law that it can only be rigorously correct for a black body. The inverse square law is true for a point or a spherical source only, and the cosine law is always assumed as applicable primarily to the infinitesimal elements of a surface. Errors of considerable