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 \(\rm{W m}^{-2}\).
Notes:
- Mathematical definition: \(E = \frac{\rm{d}P}{\rm{d}S}\). If the radiant power is constant over the surface area considered, \(E = \frac{P}{S}\).
- 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. \[E = \int_{2\pi}L\, \rm{cos}\,\theta\, \rm{d}\varOmega\]
- 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.
- \(E = \int_{\lambda}E_{\lambda}\, \rm{d}\lambda\) where \(E_{\lambda}\) is the spectral irradiance at wavelength \(\lambda\).