Title: irradiance
Long Title: IUPAC Gold Book - irradiance
DOI: 10.1351/goldbook.I03254
Status: current
Index: quantity

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 \(\rm{W m}^{-2}\).

Notes
1) Mathematical definition: \(E = \frac{\rm{d}P}{\rm{d}S}\). If the radiant power is constant over the surface area considered, \(E = \frac{P}{S}\).
2) Alternative definition: Integral, taken over the hemisphere visible from the given point, of the expression \(L\, \rm{cos}\,\theta\,\rm{d}\varOmega\), where \(L\) is the radiance at the given point in the various directions of the incident elementary beams of solid angle \(\varOmega\) and \(\theta\) is the angle between any of the beams and the normal to the surface at the given point. \[E = \int_{2\pi}L\, \rm{cos}\,\theta\, \rm{d}\varOmega\]
3) This term refers to a beam not scattered or reflected by the target or its surroundings. For a beam incident from all directions, fluence rate (\(E_{o}\)) is an equivalent term.
4) \(E = \int_{\lambda}E_{\lambda}\, \rm{d}\lambda\) where \(E_{\lambda}\) is the spectral irradiance at wavelength \(\lambda\).

Related Terms
- Radiant power: https://goldbook.iupac.org//terms/view/R05046
- angle: https://goldbook.iupac.org//terms/view/A00346
- fluence rate: https://goldbook.iupac.org//terms/view/FT07376
- radiance: https://goldbook.iupac.org//terms/view/R05037
- spectral irradiance: https://goldbook.iupac.org//terms/view/S05817
- visible: https://goldbook.iupac.org//terms/view/VT07496
- wavelength: https://goldbook.iupac.org//terms/view/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)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/I03254/html
- json: https://goldbook.iupac.org/terms/view/I03254/json
- xml: https://goldbook.iupac.org/terms/view/I03254/xml

Citation: Citation: 'irradiance' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.I03254
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Accessed: 2026-06-07T01:07:22+00:00