The photon flow, , incident on an infinitesimal element of surface containing the point under consideration divided by the area () of that element (, simplified expression: when the photon flow is constant over the surface considered). The SI unit is . Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI unit then being . For a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings photon fluence rate () is an equivalent term.