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
Incompressible surface compressing "S" along "D". A non-negative complexity measure on compact surfaces without 2-sphere components is "b"("S") − "χ"("S"), where "b"("S") is the zeroth Betti number (the number of connected components) and "χ"("S") is the Euler characteristic. When compressing a compressible surface, the Euler characteristic increases by two, while "b" might remain the same or increase by 1. Thus, every properly embedded compact surface without 2-sphere components is related to an incompressible surface through a sequence of compressions. Sometimes we drop the conditions that "S" be compressible or that "D" be anything more than a disk that intersects "S" only along