The rate of the Gibbs energy storage in an endothermic photochemical reaction divided by the incident irradiance.
See also: efficiency (of a step)
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2240 (https://doi.org/10.1351/pac199668122223)