Number of photons (quanta of radiation, $N_{\text{p}}$) per time interval (photon flux), $q_{\text{p}}$, 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 ${\text{m}}^{-2}{\text{s}}^{-1}$. Equivalent definition: Integral, taken over the hemisphere visible from the given point, of the expression $L_{\text{p}}\text{cos}\theta \text{d}\Omega$ the photon radiance at the given point in the various directions of the incident beam of solid angle $\Omega$ and $\theta$ the angle between any of these beams and the normal to the surface at the given point.