irradiance (at a point of a surface), \(E\)
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.
  1. Mathematical definition: E = dP/dS. If the @[email protected] is constant over the surface area considered, E = P/S.
  2. Alternative definition: Integral, taken over the hemisphere @[email protected] from the given point, of the expression L.cosθ.dΩ, where L is the @[email protected] at the given point in the various directions of the incident elementary beams of solid @[email protected] Ω and θ 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] (Eo) is an equivalent term.
  4. E = ∫(λ)Eλ.dλ where Eλ is the @[email protected] at @[email protected] λ.
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 357 (