An ambiguous term used in connection with both gaseous-discharge devices and photoelectric cells or tubes. In gaseous-conduction tubes it refers to the region of operation known as the Townsend discharge. The name is derived from the fact that photons produced in the gas do not play an important part in the production of ionization. The initial ionization arises from independent effects such as cosmic rays, radioactivity, or thermionic emission. When applied to photoelectric devices, the term applies to background current. This is current which may be present as the result of thermionic emission or other effects when there is no light incident on the photosensitive cathode.