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=dPdS. If the radiant power is constant over the surface area considered, E=PS.
  2. Alternative definition: Integral, taken over the hemisphere visible from the given point, of the expression LcosθdΩ, where L is the radiance at the given point in the various directions of the incident elementary beams of solid angle Ω and θ is the angle between any of the beams and the normal to the surface at the given point. E=∫2πLcosθdΩ
  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 (Eo) is an equivalent term.
  4. E=∫λEλdλ where Eλ is the spectral irradiance at wavelength λ.
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 357 [Terms] [Paper]
Citation: 'irradiance (at a point of a surface)' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.I03254 RIS BibTex EndNote
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Last revised: February 24, 2014