輝度 (光学)

${\displaystyle L={\frac {1}{\cos \theta }}{\frac {dI}{dS}}}$

${\displaystyle \cos \theta ={\boldsymbol {n}}\cdot {\boldsymbol {k}}}$

${\displaystyle M={\frac {d\varPhi _{\text{src}}}{dS}}={\frac {d}{dS}}\int _{\text{ext}}I\,d\omega =\int _{\text{ext}}L\cos \theta \,d\omega }$

${\displaystyle M=\oint d\phi \int _{0}^{\pi /2}L(\theta ,\phi )\cos \theta \sin \theta \,d\theta }$

${\displaystyle M=\pi L}$