Equivalent definition. Integral, taken over the hemisphere visible from the given point, of the expression L_p • cos θ · dΩ where L_p is the photon radiance at the given point in the various directions of the incident elementary beams of solid angle dΩ, and θ is the angle between any of these beams and the normal to the surface at the given point

${\displaystyle E_{\text{p}}=\frac{\text{d}{\Phi }_{\text{p}}}{\text{d}A}={\displaystyle {\int }_{2\pi \text{sr}}{L}_{\text{p}}\cdot \cos \theta \cdot \text{d}\Omega }}$

unit: s^-1 • m^-2