Device in which a gas conducting an electric current emits visible light. It is usually a glass tube from which virtually all the air has been removed (so that it “contains” a near vacuum), with electrodes at each end. When a high-voltage current is passed between the electrodes, the few remaining gas atoms in the tube (or some deliberately introduced ones) ionize and emit colored light as they conduct the current along the tube. The light originates as electrons change energy levels in the ionized atoms.
By coating the inside of the tube with a phosphor, invisible emitted radiation (such as ultraviolet light) can produce visible light; this is the principle of the fluorescent lamp.