${\displaystyle I_{\text{p}}=\frac{\text{d}{\Phi }_{\text{p}}}{\text{d}\Omega }}$

where Φ_p is the photon flux emitted in the given direction, and Ω is the solid angle containing that direction

Note 1 to entry: The distribution of the photon intensities as a function of the direction of emission, e.g. given by the polar angles (ϑ, φ), is used to determine the photon flux, Φ_p, within a certain solid angle, Ω, of a source:

${\Phi }_{\text{p}}={\displaystyle \underset{\Omega }{\iint }{I}_{\text{p}}\left(\vartheta ,\phi \right)\text{sin}\vartheta \text{ d}\phi \text{ d}\vartheta }$.

Note 2 to entry: The corresponding radiometric quantity is "radiant intensity". The corresponding photometric quantity is "luminous intensity".

Note 3 to entry: The photon intensity is expressed in second to the power minus one times steradian to the power minus one (s⁻¹·sr⁻¹).

Note 4 to entry: This entry was numbered 845-01-32 in IEC 60050-845:1987.