 | This article needs additional citations for verification. (December 2009) |
In optics, Lambert's cosine law says that the observed radiant intensity or luminous intensity from an ideal diffusely reflecting surface or ideal diffuse radiator is directly proportional to the cosine of the angle θ between the observer's line of sight and the surface normal; I = I0 cos θ.[1][2] The law is also known as the cosine emission law[3] or Lambert's emission law. It is named after Johann Heinrich Lambert, from his Photometria, published in 1760.[4]
A surface which obeys Lambert's law is said to be Lambertian, and exhibits Lambertian reflectance. Such a surface has a constant radiance/luminance, regardless of the angle from which it is observed; a single human eye perceives such a surface as having a constant brightness, regardless of the angle from which the eye observes the surface. It has the same radiance because, although the emitted power from a given area element is reduced by the cosine of the emission angle, the solid angle, subtended by surface visible to the viewer, is reduced by the very same amount. Because the ratio between power and solid angle is constant, radiance (power per unit solid angle per unit projected source area) stays the same.
- ^ RCA Electro-Optics Handbook, p.18 ff
- ^ Modern Optical Engineering, Warren J. Smith, McGraw-Hill, p. 228, 256
- ^ Pedrotti & Pedrotti (1993). Introduction to Optics. Prentice Hall. ISBN 0135015456.
- ^ Lambert, Johann Heinrich (1760). Photometria, sive de mensura et gradibus luminis, colorum et umbrae. Eberhard Klett.