Spectral power distribution

In color science 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:
 * $$M_\lambda=\frac{\partial^2\Phi}{\partial A\partial\lambda}\approx\frac{\Phi}{A \Delta\lambda}$$

where $$M(\lambda)$$ is the spectral irradiance (or exitance) of the light (SI units: W/m3 = kg/(m·s3)); $$\Phi$$ is the radiant flux of the source (SI unit: watt, W); $$A$$ is the area over which the radiant flux is integrated (SI unit: square meter, m2); and $$\lambda$$ 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.