Spectral power distribution
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.
Relative SPD 
Because the luminance of lighting fixtures and other light sources are handled separately, a spectral power distribution may be normalized in some manner, often to unity at 555 or 560 nanometers, coinciding with the peak of the eye's luminosity function.
- Mark D. Fairchild (2005). Color Appearance Models. John Wiley and Sons. ISBN 0-470-01216-1.
- Michael R. Peres (2007). The Focal Encyclopedia of Photography. Focal Press. ISBN 978-0-240-80740-9.
- Wyszecki, Günter; Stiles, Walter Stanley (1982). Color Science: Concepts and Methods; Quantitative Data and Formulae (second edition ed.). New York: Wiley. ISBN 978-0-471-39918-6.
- Spectral Power Distribution Curves, GE Lighting.
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