放射照度

${\displaystyle E=\lim _{\Delta S\to 0}{\frac {\Phi (\Delta S)}{\Delta S}}={\frac {d\Phi }{dS}}}$

${\displaystyle \Phi (S)=\int _{S}E\,dS}$

${\displaystyle E_{\theta }=E_{\text{n}}\cos \theta }$

${\displaystyle E=\int _{0}^{\infty }E_{\lambda }\,d\lambda }$