(1) The ratio of reflected power to incident power. Note: In optics, frequently expressed as optical density or as a percent; in communication applications, generally expressed in dB. (2) A measure of the ratio of outgoing to incoming Radiation calculated by converting a radiometrically calibrated image to an innate characteristic of the target being observed. Calibrated at-satellite spectral radiance is converted to unitless reflectance by separating out the atmospheric component of the reflective band radiance and assuming that the target is a Lambertian reflector, re-radiating incident solar radiation equally in all directions. In general, reflectance is a function of Incident Angle of the energy, viewing angle of the sensor, spectral wavelength, and bandwidth, and the nature of the object. Also see planetary albedo, Bidirectional reflectance and atmospheric correction. (Source: Dr. John Barker)