The energy or amount of photons absorbed per unit area or unit volume by an irradiated object during a particular exposure time. In medicine and in some other research areas (e.g. photopolymerization and water handling through irradiation) dose is used in the sense of fluence, i.e. the energy or amount of photons per unit area or unit volume received by an irradiated object during a particular exposure time. The SI units are or and or , respectively. For special purposes it must be appropriately qualified, e.g. absorbed, maximum permissible, mean lethal.