1. the total radiant exposure required to elicit a given photobiological or photochemical response is independent of the duration over which the exposure occurs
2. equivalent definition: the exposure duration required to elicit a given photobiological or photochemical response varies as the reciprocal of the incident irradiance
Equivalent term: "reciprocity law"
NOTE This relationship generally holds only over a given limited range of exposure durations. Reciprocity failure can occur due to repair mechanisms, recombination effects over long periods, or photon saturation for extremely short periods.