Albedo (/ælˈbiːdoʊ/) is the "whiteness" of a surface. It is a reflection coefficient, and has a value of less than one. Albedo is derived from Latin albedo "whiteness" (or reflected sunlight) in turn from albus "white", is the diffuse reflectivity or reflecting power of a surface.
The albedo of a surface is the ratio of radiation reflected from the surface to the incident radiation. Its dimensionless nature lets it be expressed as a percentage and is measured on a scale from zero (no reflection) of a perfectly black surface to 1 for perfect reflection of a white surface. Because albedo is the ratio of all reflected radiation to incident radiation, it will include both the diffuse and specular radiation reflected from an object. It is, however, common to assume a surface reflects in either a totally specular manner or a totally diffuse manner, as this can simplify calculations.
Albedo depends on the wavelength of the radiation. When quoted unqualified, it usually refers to some appropriate average across the spectrum of visible light. In general, the albedo depends on the directional distribution of incident radiation, except for the extreme case of Lambertian surfaces, which scatter radiation in all directions according to a cosine function and therefore have an albedo that is independent of the incident distribution. In practice, a bidirectional reflectance distribution function (BRDF) may be required to accurately characterize the scattering properties of a surface, but albedo is very useful as a first approximation.
The albedo is an important concept in climatology, astronomy, and calculating reflectivity of surfaces in Leadership in Energy and Environmental Design (LEED) sustainable-rating systems for buildings. The average overall albedo of Earth, its planetary albedo, is 30 to 35% because of cloud cover, but widely varies locally across the surface because of different geological and environmental features.
The term was introduced into optics by Johann Heinrich Lambert in his 1760 work Photometria....LESS