Albedo formula_5 (also called blue-sky albedo) can then be given as: Directional-hemispherical reflectance is sometimes referred to as black-sky albedo and bi-hemispherical reflectance as white-sky albedo. These terms are important because they allow the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface. The albedos of planets, satellites and minor planets such as asteroids can be used to infer much about their properties. The study of albedos, their dependence on wavelength, lighting angle ("phase angle"), and variation in time comprises a major part of the astronomical field of photometry. For small
Bond albedo phase angles from the Earth, the only reliable data for measuring their Bond albedo comes from spacecraft. The Bond albedo ("A") is related to the geometric albedo ("p") by the expression where "q" is termed the "phase integral" and is given in terms of the directional scattered flux "I"("α") into phase angle "α" (averaged over all wavelengths and azimuthal angles) as The phase angle "α" is the angle between the source of the radiation (usually the Sun) and the observing direction, and varies from zero for light scattered back towards the source, to 180° for observations looking towards the source.