Geometric albedo is a measure of the reflectivity of a celestial body, such as a planet, moon, or asteroid, as observed from a specific geometrical configuration. Specifically, it defines the ratio of the brightness of the object when illuminated by a light source (usually the Sun) to the brightness of a flat, fully reflective surface (like a perfect diffuser) under the same illumination conditions.
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The geometric albedo measures a body's brightness at full phase relative to a perfectly reflecting, diffusely scattering flat disc of the same projected area, illuminated identically. It is therefore a directional brightness comparison. For an unresolved spherical planet at separation , the full-phase reflected contrast is under the usual distant-source approximation.
An optical exoplanet secondary eclipse can constrain this contrast after accounting for planetary thermal radiation. The geometric albedo differs from Bond albedo, which measures reflected power over every direction and wavelength. Strong backscattering can even make a directional geometric albedo exceed one without violating conservation of energy; total reflected power remains bounded by the incident power.