Total radiant power, , incident from all directions onto a small sphere divided by the cross-sectional area of that sphere. SI unit is .
- Mathematical definition: . If the radiant power is constant over the area , . Equivalent definition: , with the solid angle of each beam passing through the given point on the surface and the radiance of the beam at that point.
- Fluence rate is identical to spherical irradiance and reduces to irradiance, , for a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 340