radiant intensity, I

Also contains definition of: intensity

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.
Notes:
  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.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 410