https://doi.org/10.1351/goldbook.FT07376
Total radiant power, \(P\), incident from all directions onto a small sphere divided by the cross-sectional area of that sphere. SI unit is \(\rm{W m}^{-2}\).
Notes:
- Mathematical definition: \(E_{\rm{o}} = \frac{\rm{d}P}{\rm{d}S} = \frac{\rm{d}H_{\rm{o}}}{\rm{d}t}\). If the radiant power is constant over the area \(S\), \(E_{\rm{o}} = \frac{P}{S}\). Equivalent definition: \(E_{\rm{o}} = \int_{4\pi}L\rm{d}\varOmega\), with \(\varOmega\) the solid angle of each beam passing through the given point on the surface and \(L\) the radiance of the beam at that point.
- Fluence rate is identical to spherical irradiance and reduces to irradiance, \(E\), for a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings.