radiant intensity, \(I\)

Also contains definition of: intensity
https://doi.org/10.1351/goldbook.R05045
@R05046-1@, \(P\), at all wavelengths per solid @A00346@, \(\varOmega\). The @R05046@ emitted in a given direction by a source or an element of the source in a small cone containing the given direction divided by the solid @A00346@ of the cone. SI unit is \(\text{W sr}^{-1}\).
Notes:
  1. Mathematical definition: \(I = \text{d}P/\text{d}\varOmega\). If the @R05046-2@ is constant over the solid @A00346@ considered, \(I = P/\varOmega\).
  2. Equivalent to \(I = \int_{\lambda}I_{\lambda}\, \text{d}\lambda\), where \(I_{\lambda}\) is the @S05827@ at @W06659@ \(\lambda\).
  3. It is not recommended to abbreviate this term to just intensity because it is confusing.
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 410 [Terms] [Paper]