Editing Spectral power distribution (section)

==Relative SPD==
[[Image:Spectral Power Distributions.png|right|frame|Characteristic spectral power distributions (SPDs) for an [[Incandescent light bulb|incandescent lamp]] (left) and a [[fluorescent lamp]] (right). The horizontal axes are in [[nanometer]]s and the vertical axes show relative intensity in arbitrary units.]]

The ratio of spectral concentration (irradiance or exitance) at a given wavelength to the concentration of a reference wavelength provides the relative SPD.<ref name=meas/> This can be written as:
: <math>M_\mathrm{rel}(\lambda)=\frac{M(\lambda)}{M\left(\lambda_0\right)}</math>
For instance, 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]].<ref name=focal/><ref name="WyszeckiStiles">{{cite book | first=Günter | last=Wyszecki |author2=Stiles, Walter Stanley | title=Color Science: Concepts and Methods; Quantitative Data and Formulae | edition=second | publisher=New York: Wiley | date=1982 | isbn=978-0-471-39918-6}}</ref>