irradiance (at a point of a surface), \(E\)

https://doi.org/10.1351/goldbook.I03254
@[email protected], \(P\), 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 \(\text{W m}^{-2}\).
Notes:
  1. Mathematical definition: \(E = \frac{\text{d}P}{\text{d}S}\). If the @[email protected] is constant over the surface area considered, \(E = \frac{P}{S}\).
  2. Alternative definition: Integral, taken over the hemisphere @[email protected] from the given point, of the expression \(L\, \text{cos}\,\theta\,\text{d}\varOmega\), where \(L\) is the @[email protected] at the given point in the various directions of the incident elementary beams of solid @[email protected] \(\varOmega\) and \(\theta\) is the @[email protected] 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, @[email protected] (\(E_{o}\)) is an equivalent term.
  4. \(E = \int_{\lambda}E_{\lambda}\, \text{d}\lambda\) where \(E_{\lambda}\) is the @[email protected] at @[email protected] \(\lambda\).
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 357 [Terms] [Paper]