Term: irradiance (at a point of a surface), \(E\) https://doi.org/10.1351/goldbook.I03254 Definition: 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: 0) Mathematical definition: E = dP/dS. If the @R05046@ is constant over the surface area considered, E = P/S. 1) 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. \[E = \int_{2\pi}L\, \text{cos}\,\theta\, \text{d}\varOmega\] 2) 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. 3) E = ∫(λ)Eλ.dλ where Eλ is the @S05817@ at @W06659@ λ. Related Terms: 1) radiant power (http://doi.org/10.1351/goldbook.R05046). 2) visible (http://doi.org/10.1351/goldbook.VT07496). 3) radiance (http://doi.org/10.1351/goldbook.R05037). 4) angle (http://doi.org/10.1351/goldbook.A00346). 5) fluence rate (http://doi.org/10.1351/goldbook.FT07376). 6) spectral irradiance (http://doi.org/10.1351/goldbook.S05817). 7) wavelength (http://doi.org/10.1351/goldbook.W06659). 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) Citation: 'irradiance (at a point of a surface), \(E\)' 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 License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.