A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. A fluorescent lamp converts electrical power into useful light much more efficiently than incandescent lamps. The luminous efficacy of a compact fluorescent light bulb is about 60 lumens per watt, four times the efficacy of a typical incandescent bulb. For conventional tube fluorescent lamps the fixture is more costly because it requires a ballast to regulate the current through the lamp, but the lower energy cost typically offsets the higher initial cost.