Editing Radiance (section)

==Conservation of basic radiance==
Radiance of a surface is related to [[étendue]] by
:<math>L_{\mathrm{e},\Omega} = n^2 \frac{\partial \Phi_\mathrm{e}}{\partial G},</math>
where
*''n'' is the [[refractive index]] in which that surface is immersed;
*''G'' is the étendue of the light beam.

As the light travels through an ideal optical system, both the étendue and the radiant flux are conserved. Therefore, ''basic radiance'' defined by<ref>William Ross McCluney, ''Introduction to Radiometry and Photometry'', Artech House, Boston, MA, 1994 {{ISBN|978-0890066782}}</ref>
:<math>L_{\mathrm{e},\Omega}^* = \frac{L_{\mathrm{e},\Omega}}{n^2}</math>
is also conserved. In real systems, the étendue may increase (for example due to scattering) or the radiant flux may decrease (for example due to absorption) and, therefore, basic radiance may decrease. However, étendue may not decrease and radiant flux may not increase and, therefore, basic radiance may not increase.