# 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:

: $M_lambda=frac\left\{partial^2Phi\right\}\left\{partial Apartiallambda\right\}approxfrac\left\{Phi\right\}\left\{A Deltalambda\right\}$

where $M\left(lambda\right)$ 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: meter2); 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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