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

where Φ_e is radiant flux and A is the area on which the radiant flux is incident

Note 1 to entry: The corresponding photometric quantity is "illuminance". The corresponding quantity for photons is "photon irradiance".

Note 2 to entry: The irradiance is expressed in watt per square metre (W·m⁻²).

Note 3 to entry: This entry was numbered 845-01-37 in IEC 60050-845:1987.