fluence rate, E o

https://dx.doi.org/10.1351/goldbook.FT07376
Total radiant power, P, incident from all directions onto a small sphere divided by the cross-sectional area of that sphere. SI unit is W m −2.
Notes:
  1. Mathematical definition: E o = d P d S = d H o d t. If the radiant power is constant over the area S, E o = P S. Equivalent definition: E o = ∫ 4 π L ⁢ d Ω, with Ω the solid angle of each beam passing through the given point on the surface and L the radiance of the beam at that point.
  2. Fluence rate is identical to spherical irradiance and reduces to irradiance, E, for a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 340