A source of radiant energy excited by a supply of electricity which creates a current of ionized gas between electrodes in an enclosure that contains the arc while permitting transmission of the radiant energy. Gaseous-discharge lamps or vapor lamps are given various names relating to the element responsible for the majority of the radiation (mercury, sodium metal-halide, xenon), to the physical attribute of the lamp (short-arc, high-pressure), or, in the case of fluorescent lamps, to the way a phosphor on the bulb wall fluoresces as a result of the lamp's low-pressure mercury-vapor excitation.