${\displaystyle E_{\text{p}}=\frac{\text{d}{\Phi }_{\text{p}}}{\text{d}A}}$

where Φ_p is photon flux and A is the area on which the photon flux is incident

Note 1 to entry: The corresponding radiometric quantity is "irradiance". The corresponding photometric quantity is "illuminance".

Note 2 to entry: The photon irradiance is expressed in second to the power minus one per square metre (s⁻¹·m⁻²).

Note 3 to entry: This entry was numbered 845-01-39 in IEC 60050-845:1987.