Geometric albedo compares a body's full-phase brightness with that of a flat, perfectly diffusing reference disc of the same cross-section.
The orbital variation of a planet's reflected stellar light as the visible illuminated area changes. In a simple spherical model, , where and is the star-planet-observer angle. Exoplanet clouds can alter the geometric albedo and the angular scattering law; this observable differs from an exoplanet thermal phase curve.
The phase integral converts a monochromatic geometric albedo into the corresponding reflected power fraction. The bolometric Bond albedo is the incident-spectrum-weighted average of .
Bond albedo is the fraction of total incident power reflected in all directions and over all wavelengths.
This idealized surface follows Lambertian reflectance, with projected-area dependence governed by Lambert's cosine law.
A Lambertian surface has direction-independent radiance. A perfectly reflecting Lambertian patch receiving normal flux emits specific intensity into its outward hemisphere.
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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.