radiant intensity, I

Radiant power, P, at all wavelengths per solid angle, Ω. The radiant power 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 angle of the cone. SI unit is W sr −1.
  1. Mathematical definition: I = d P / d Ω. If the radiant power is constant over the solid angle considered, I = P / Ω.
  2. Equivalent to I = ∫ λ I λ ⁢ d λ, where I λ is the spectral radiant intensity at wavelength λ.
  3. It is not recommended to abbreviate this term to just intensity because it is confusing.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 410