# Spectral power distribution

- Spectral power distribution
In color science, a **spectral power distribution** 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). [*cite book | title = Color Appearance Models | author = Mark D. Fairchild | isbn = 0470012161 | year = 2005 | publisher = John Wiley and Sons| url = http://books.google.com/books?id=8_TxzK2B-5MC&pg=PA56&dq=light+source+%22spectral+power+distribution%22&lr=&as_brr=3&ei=wgCJR6bHNpOMtAPpj6HQBQ&sig=dIlnEgPmXu9Sy3YpEVF6wy9m8tU#PPA57,M1 | *]

Mathematically, for the spectral power distribution of a radiant exitance or irradiance one may write:

: $M\_lambda=frac\{partial^2Phi\}\{partial\; Apartiallambda\}approxfrac\{Phi\}\{A\; Deltalambda\}$

where $M(lambda)$ is the spectral irradiance (or exitance) of the light (SI units: watt meter^{–3}); $Phi$ is the radiant flux of the source (SI units: watt); $A$ is the area over which the radiant flux is integrated (SI units: meter^{2}); and $lambda$ is the wavelength (SI unit: meter). (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 watt meter^{–2} nanometer^{–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 560 nanometers, coinciding with the peak of the eye's luminosity function.[cite book|first=Günter|last=Wyszecki|coauthors=Stiles, Walter Stanley|title=Color Science: Concepts and Methods; Quantitative Data and Formulae|edition=second edition|publisher=New York: Wiley|year=1982|isbn=978-0-471-39918-6] ]

**References**

**External links**

* [*http://www.gelighting.com/na/business_lighting/education_resources/learn_about_light/distribution_curves.htm Spectral Power Distribution Curves*] , GE Lighting.

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