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Emissivity

1. The ratio of (a) the power radiated by a substance to (b) the power radiated by a blackbody at the same temperature. Note 1: Emissivity is a function of wavelength and temperature. Note 2: Power units and area units for both bodies must be the same or be fractional, i.e., be normalized. 2. The ratio of (a) the radiant emittance of, i.e., the radiated flux