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

https://doi.org/10.1351/goldbook.I03254
@R05046@, \(P\), 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 @R05046@ is constant over the surface area considered, \(E = \frac{P}{S}\).
  2. Alternative definition: Integral, taken over the hemisphere @VT07496@ from the given point, of the expression \(L\, cos\,\theta\,\text{d}\varOmega\), where \(L\) is the @R05037@ at the given point in the various directions of the incident elementary beams of solid @A00346@ \(\varOmega\) and \(\theta\) is the @A00346@ between any of the beams and the normal to the surface at the given point. \[E = \int_{2\pi}L\, cos\,\theta\, \text{d}\varOmega\]
  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@ (\(E_{o}\)) is an equivalent term.
  4. \(E = \int_{\lambda}E_{\lambda}\, \text{d}\lambda\) where \(E_{\lambda}\) is the @S05817@ at @W06659@ \(\lambda\).
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 357 [Terms] [Paper]