Albedo used in astronomy are the (V-band) geometric albedo (measuring brightness when illumination comes from directly behind the observer) and the Bond albedo (measuring total proportion of electromagnetic energy reflected). Their values can differ significantly, which is a common source of confusion. In detailed studies, the directional reflectance properties of astronomical bodies are often expressed in terms of the five Hapke parameters which semi-empirically describe the variation of albedo with phase angle, including a characterization of the opposition effect of regolith surfaces. The correlation between astronomical (geometric) albedo, absolute magnitude and diameter is: formula_7, where formula_8 is the astronomical albedo, formula_9
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.