Radiant energy, , incident from all upward directions on an small sphere divided by the cross-sectional area of that sphere. SI unit is .
- Equivalent definition: Irradiance, integrated over the time of irradiation.
- Mathematical definition: If is constant over the area, . If is constant over the time interval, .
- This term refers to a beam not scattered or reflected by the target or its surroundings. For a beam incident from all directions fluence (,) is an equivalent term.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 409