Editing Spectral power distribution (section)

== Physics ==

Mathematically, for the spectral power distribution of a radiant exitance or irradiance one may write:
: <math>M(\lambda)=\frac{\partial^2\Phi}{\partial A\,\partial\lambda}\approx\frac{\Phi}{A\,\Delta\lambda}</math>
where ''M''(''λ'') is the [[spectral irradiance]] (or exitance) of the light ([[SI]] units: [[watt|W]]/m<sup>2</sup> = [[kilogram|kg]]·m<sup>−1</sup>·[[second|s]]<sup>−3</sup>); ''Φ'' 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, m<sup>2</sup>); and ''λ'' is the wavelength (SI unit: meter, m). (Note that it is more convenient to express the wavelength of light in terms of [[nanometer]]s; spectral exitance would then be expressed in units of W·m<sup>−2</sup>·nm<sup>−1</sup><!--= W·m<sup>−9</sup> = W/mm<sup>3</sup> = W/μL-->.) The approximation is valid when the area and wavelength interval are small.<ref name=des>{{cite book | title = Radiometric System Design | author = Clair L. Wyatt | isbn = 0029488001 | date = 1987 | publisher = New York: Macmillan }}</ref>