radiance, L

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⊥=dScosθ
Notes:
  1. Mathematical definition: L=d2PdΩdS⊥=d2PdΩdScosθ for a divergent beam propagating in an elementary cone of the solid angle Ω 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: dP/(dScosθ). If the radiant power is constant over the surface area considered, P/(Scosθ). SI unit is W m−2.
  3. Equivalent to L=∫λLλdλ, where Lλ is the spectral radiance at wavelength λ.
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 408 [Terms] [Paper]
Citation: 'radiance' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.R05037 RIS BibTex EndNote
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