Energy-saving light bulb switched off. This is a form of fluorescent lighting. The light bulb consists of three tubes. Each tube has an electrode at each end, an inner coating of phosphor and is filled with an inert gas, usually argon, and mercury vapour. When the light is switched on a stream of electrons flow through the gas from one electrode to the other, exciting the mercury atoms. As the mercury atoms move back to the unexcited state they give off ultraviolet photons. These hit the bulb's phosphor coating creating visible light. Fluorescent bulbs produce less heat than filament bulbs, therefore more of the electrical energy is converted to light, making them more efficient.