Spectral flux density (SFD) is a quantitative measure used in various fields, particularly in physics and astronomy, to describe the amount of energy or power emitted by a source of radiation at different wavelengths or frequencies per unit area. In more general terms, it represents how the intensity of a signal varies over frequency or wavelength.
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Spectral flux density is radiant power received per unit area and per unit wavelength or frequency. The two conventions satisfy under , hence when the arguments refer to the same photons. Integrating over a photometric passband gives its measured flux.