Bond albedo is a measure of the reflectivity of an astronomical body, specifically in the context of the entire hemisphere of that body. It quantifies the fraction of total incoming solar energy that is reflected back into space by that body, taking into account all wavelengths of light and all angles of incidence. This is different from the more commonly known geometrical albedo, which only considers light reflected at a specific angle, typically from a direct overhead view.
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The Bond albedo is the fraction of total incident radiant power that a body reflects, summed over outgoing directions and over the incident spectrum. The absorbed fraction is . It is a global energy-budget quantity, so it appears directly in the planetary equilibrium temperature and must lie between zero and one for a passive reflecting body.
A bright appearance from one direction does not determine Bond albedo. Geometric albedo measures full-phase brightness, and the planetary phase integral supplies the angular weighting needed to connect that brightness to reflected power at a given wavelength. The spectral weighting also depends on the illuminating star: the same reflecting material need not absorb the same fraction of light from hot and cool stars.