SPECTRAL POWER DISTRIBUTION

In color science, the power per unit area per unit wavelength of a radiant object. Mathematically, one may write:
:
where $M(lambda)$ is the spectral exitance (or emittance) of the source (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.
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]

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References

References

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