Fluorescent Light

The fluorescent lamp is formed from a sealed, hollow glass tube which is straight, although other shapes can also be used. The tube contains a low pressure mixture of noble gas and mercury vapor through which an AC electrical discharge is run, has electrodes located at either end, and has a coating of an inorganic phosphor on the inside surface. Each electrode acts as cathode and anode during one complete period of the AC discharge and is coated with a material of low work-function, such as barium oxide, which, when heated, acts as a source of electrons to feed the electrical discharge. Other electrons are created in the discharge by impact ionization of the gas mixture. The gas mixture uses a noble gas, usually krypton, to act as a buffer in the discharge. On excitation by electrons in the discharge, the mercury atoms emit light, mostly at a wavelength of 254 nm which is in the deep ultraviolet (UV). This UV light reaches the phosphor coating on the walls of the tube where it is absorbed and re-emitted at a longer wavelength in the visible. The visible light passing out of the glass envelope is used for illumination. The color of the emitted light is determined by the phosphor and is a particularly important characteristic of the lamp.