${\displaystyle H_{\text{p}}=\frac{\text{d}{N}_{\text{p}}}{\text{d}A}}$

where N_p is photon number and A is the area on which the photons are incident

Note 1 to entry: The corresponding radiometric quantity is "radiant exposure". The corresponding photometric quantity is "luminous exposure".

Note 2 to entry: The photon exposure is expressed in metre to the power minus two (m⁻²).

Note 3 to entry: This entry was numbered 845-01-44 in IEC 60050-845:1987.