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# Spectral power distribution

In color science, the power per unit area per unit wavelength of a radiant object. Mathematically, one may write:

$M_\lambda=\frac{\partial^2\Phi}{\partial A\partial\lambda}\approx\frac{\Phi}{A \Delta\lambda}$

where M(λ) is the spectral exitance (or emittance) of the source (SI units: watt meter–3); Φ 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 λ 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.

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.[1]

## References

1. ^ Günter Wyszecki and W S Stiles, Color Science: Concepts and Methods; Quantitative Data and Formulae, second edition. New York: Wiley, 1982.