Term generally applied to the time-integrated radiation
incident from all
upward directions on a small sphere divided by the cross-sectional area of that sphere.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 338 (https://doi.org/10.1351/pac200779030293)