The photon irradiance, , integrated over the time of irradiation (, simplified expression: when the photon irradiance is constant over the time 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 is an equivalent term.