## irradiance (at a point of a surface), $$E$$

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 m-2.
Notes:
1. Mathematical definition: E = dP/dS. If the @R05046@ is constant over the surface area considered, E = P/S.
2. Alternative definition: Integral, taken over the hemisphere @VT07496@ from the given point, of the expression L.cosθ.dΩ, where L is the @R05037@ at the given point in the various directions of the incident elementary beams of solid @A00346@ Ω and θ is the @A00346@ between any of the beams and the normal to the surface at the given point. $E = \int_{2\pi}L\, \text{cos}\,\theta\, \text{d}\varOmega$
3. This term refers to a beam not scattered or reflected by the target or its surroundings. For a beam incident from all directions, @FT07376@ (Eo) is an equivalent term.
4. E = ∫(λ)Eλ.dλ where Eλ is the @S05817@ at @W06659@ λ.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 357 (https://doi.org/10.1351/pac200779030293)