The rate of photon fluence. Four times the ratio of the photon flow, , incident on a small, transparent, imaginary spherical volume element containing the point under consideration divided by the surface of that sphere, . (, simplified expression: when the photon flow is constant over the solid angle 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 . It reduces to photon irradiance for a parallel and normally incident beam not scattered or reflected by the target or its suroundings.