## Wikipedia - تشعيع (ar) 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]