In color science and radiometry, a spectral power distribution (SPD) describes the power per unit area per unit wavelength of an illumination (radiant exitance), or more generally, the per-wavelength contribution to any radiometric quantity (radiant energy, radiant flux, radiant intensity, radiance, irradiance, radiant exitance, or radiosity).
Mathematically, for the spectral power distribution of a radiant exitance or irradiance one may write:
where is the spectral irradiance (or exitance) of the light (SI units: W/m3 = kg/(m·s3)); is the radiant flux of the source (SI unit: watt, W); is the area over which the radiant flux is integrated (SI unit: square meter, m2); and is the wavelength (SI unit: meter, m). (Note that it is more convenient to express the wavelength of light in terms of nanometers; spectral exitance would then be expressed in units of W·m−2·nm−1.) The approximation is valid when the area and wavelength interval are small.
Read more about Spectral Power Distribution: Relative SPD
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