The comparison between traditional black light lamp design and fluorescent lamps only has a few important changes. Fluorescent lamps generate light by passing an electric current through a tube filled with inert gas and a small amount of mercury. When electrified, mercury atoms emit energy in the form of visible photons. They emit some visible photons, but most of them are emitted in the ultraviolet wavelength range. The wavelength of ultraviolet light is too short to be seen. Fluorescent lamps must convert this energy into visible light. To achieve this goal, a phosphorus coating was applied on the outside of the tube.