The integral of the amount of all photons (quanta) which traverse a small, transparent, imaginary spherical target, divided by the cross-sectional area of this target. The photon fluence rate, , integrated over the duration of the irradiation (, simplified expression: when is constant over the time considered). Photons per unit area (). 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 .