<P> A fluorescent lamp, or fluorescent tube, is a low - pressure mercury - vapor gas - discharge lamp that uses fluorescence to produce visible light . An electric current in the gas excites mercury vapor, which produces short - wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow . A fluorescent lamp converts electrical energy into useful light much more efficiently than incandescent lamps . The typical luminous efficacy of fluorescent lighting systems is 50--100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output . </P> <P> Fluorescent lamp fixtures are more costly than incandescent lamps because they require a ballast to regulate the current through the lamp, but the lower energy cost typically offsets the higher initial cost . Compact fluorescent lamps are now available in the same popular sizes as incandescents and are used as an energy - saving alternative in homes . </P>

What type of gas is in fluorescent light bulbs