radiant intensity, I

Radiant power, $P$ , at all wavelengths per solid angle, $Ω$ . The radiant poweremitted 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:

Mathematical definition: $I = d P / d Ω$ . If the radiant power is constant over the solid angle considered, $I = P / Ω$ .
Equivalent to $I = ∫ λ I λ ⁢ d λ$ , where $I λ$ is the spectral radiant intensity at wavelength $λ$ .
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

radiant intensity, $I$

Also contains definition of: intensity