https://doi.org/10.1351/goldbook.R05037
Radiant power, \(P\), leaving or passing through a small transparent element of surface in a given direction from the source about the solid angle \(\varOmega\), divided by the solid angle and by the orthogonally projected area of the element in a plane normal to the given beam direction, \(\rm{d}S_{\perp } = \rm{d}S\, \rm{cos}\,\theta\)
Notes:
- Mathematical definition: \[L = \frac{\rm{d}^{2}P}{\rm{d}\Omega \, \rm{d}S_{\perp }} = \frac{\rm{d}^{2}P}{\rm{d}\Omega \, \rm{d}S\, \rm{cos}\,\theta}\] for a divergent beam propagating in an elementary cone of the solid angle \(\varOmega\) containing the direction \(\theta\). SI unit is \(\rm{W m}^{-2}\ \rm{sr}^{-1}\).
- For a parallel beam it is the radiant power, \(P\), of all wavelengths leaving or passing through a small element of surface in a given direction from the source divided by the orthogonally projected area of the element in a plane normal to the given direction of the beam, \(\theta\). Mathematical definition in this case: \(\rm{d}P/(\rm{d}S\, \rm{cos}\,\theta)\). If the @R05046-1@ is constant over the surface area considered, \(P/(S\, \rm{cos}\,\theta)\). SI unit is \(\rm{W m}^{-2}\).
- Equivalent to \(L = \int_{\lambda}L_{\lambda}\, \rm{d}\lambda\), where \(L_{\lambda}\) is the spectral radiance at wavelength \(\lambda\).