irradiance (at a point of a surface), E

https://dx.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 m −2.
Notes:
  1. Mathematical definition: E = d P d S. If the radiant power is constant over the surface area considered, E = P S.
  2. Alternative definition: Integral, taken over the hemisphere visible from the given point, of the expression L cos ⁡ θ d Ω, where L is the radiance at the given point in the various directions of the incident elementary beams of solid angleΩ and θ is the angle between any of the beams and the normal to the surface at the given point.
    E = ∫ 2 π L cos ⁡ θ ⁢ d Ω
  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 (E o) is an equivalent term.
  4. E = ∫ λ E λ ⁢ d λ where E λ is the spectral irradiance at wavelengthλ.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 357