Number of photons (quanta of radiation, $N_{\text{p}}$) per time interval (photon flux), $q_{\text{p}}$, leaving or passing through a small transparent element of surface in a given direction from the source about the solid angle $\Omega$, divided by the solid angle and by the orthogonally projected area of the element in a plane normal to the given beam direction, $\text{d}{S}_{\perp }=\text{d}Scos\theta$, with $\theta$ the angle between the normal to the surface and the direction of the beam. Equivalent definition: Integral taken over the hemisphere visible from the given point, of the expression $L_{\text{p}}cos\theta \text{d}\Omega$, with $L_{\text{p}}$ 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.