in radiometry, photometry, and color science, a spectral power distribution (spd) measurement describes the power per unit area per unit wavelength of an illumination (radiant exitance). more generally, the term "spectral power distribution" can refer to the concentration, as a function of wavelength, of any radiometric or photometric quantity (e.g. radiant energy, radiant flux, radiant intensity, radiance, irradiance, radiant exitance, radiosity, luminance, luminous flux, luminous intensity, illuminance, luminous emittance).