放射強度

${\displaystyle \varPhi (\varSigma )=\int _{\omega (\varSigma )}I\,d\omega }$

${\displaystyle E={\frac {d\varPhi }{d\varSigma }}={\frac {d\varPhi }{d\omega }}{\frac {d\omega }{d\varSigma }}={\frac {I\cos \theta _{\text{inc}}}{r^{2}}}}$

${\displaystyle \cos \theta _{\text{inc}}=-{\hat {\boldsymbol {r}}}\cdot {\boldsymbol {n}}_{\varSigma }}$

${\displaystyle I=\int _{\Delta S}L\cos \theta \,dS=\cos \theta \int _{\Delta S}L\,dS}$

${\displaystyle \cos \theta ={\hat {\boldsymbol {r}}}\cdot {\boldsymbol {n}}}$

${\displaystyle I(\theta )=I_{n}\cos \theta }$