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 Ω, divided by the solid angle and by the orthogonally projected area of the element in a plane normal to the given beam direction, dS⊥ = dS cos θ**Notes: **

*Source: *

PAC, 2007,*79*, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 408 (https://doi.org/10.1351/pac200779030293)

- Mathematical definition: \[L = \frac{\text{d}^{2}P}{\text{d}\Omega \, \text{d}S_{\perp }} = \frac{\text{d}^{2}P}{\text{d}\Omega \, \text{d}S\, \text{cos}\,\theta}\] for a
__divergent__beam propagating in an elementary cone of the solid @A00346@ \(\varOmega\) containing the direction \(\theta\). SI unit is \(\text{W m}^{-2}\ \text{sr}^{-1}\). - For a
__parallel__beam it is the @R05046-2@, \(P\), 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: \(\text{d}P/(\text{d}S\, \text{cos}\,\theta)\). If the @R05046-1@ is constant over the surface area considered, \(P/(S\, \text{cos}\,\theta)\). SI unit is \(\text{W m}^{-2}\). - Equivalent to \(L = \int_{\lambda}L_{\lambda}\, \text{d}\lambda\), where \(L_{\lambda}\) is the @S05824@ at @W06659@ \(\lambda\).

PAC, 2007,