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 ∂"D". If "D" were to bound a disk inside "S" (which is always the case if "S" is incompressible, for example), then compressing "S" along "D" would result in a disjoint union of a sphere and a surface homeomorphic to "S". The resulting surface without the sphere might or might not be isotopic to "S". There is also an algebraic version of incompressibility. Suppose formula_4 is a proper embedding of a compact surface in a 3-manifold. Then "S" is "π"-injective (or algebraically incompressible) if the induced map on fundamental groups is injective. In general, every "π"-injective surface is incompressible, but