https://doi.org/10.1351/goldbook.I03254
Radiant power
, P, of all wavelengths incident from all upward directions on a small element of surface containing the point under consideration divided by the area of the element. SI unit is W m2.
Notes:
  1. Mathematical definition: E=dPdS. If the
    radiant power
    is constant over the surface area considered, E=PS.
  2. Alternative definition: Integral, taken over the hemisphere
    visible
    from the given point, of the expression LcosθdΩ, where L is the
    radiance
    at the given point in the various directions of the incident elementary beams of solid
    angle
    Ω and θ is the
    angle
    between any of the beams and the normal to the surface at the given point. E=2πLcosθdΩ
  3. This term refers to a beam not scattered or reflected by the target or its surroundings. For a beam incident from all directions,
    fluence rate
    (Eo) is an equivalent term.
  4. E=λEλdλ where Eλ is the
    spectral irradiance
    at
    wavelength
    λ.
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 357 [Terms] [Paper]