radiance, L

https://dx.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 angleΩ, divided by the solid angle and by the orthogonally projected area of the element in a plane normal to the given beam direction, d S ⊥ = d S cos ⁡ θ
Notes:
  1. Mathematical definition:
    L = d 2 P d Ω d S ⊥ = d 2 P d Ω d S cos ⁡ θ
    for a divergent beam propagating in an elementary cone of the solid angleΩ containing the direction θ. SI unit is W m −2 sr −1.
  2. 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, θ. Mathematical definition in this case: d P / ( d S cos ⁡ θ ) If the radiant power is constant over the surface area considered, P / S cos ⁡ θ. SI unit is W m −2.
  3. Equivalent to L = ∫ λ L λ ⁢ d λ, where L λ is the spectral radiance at wavelengthλ.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 408