The lifetime of a fluorescent lamp is limited primarily by the electron-emitting material on the electrodes and the phosphor. The electro-emissive material is consumed in a number of ways when the tube is used. First, the "dark space," a region of high electric field found near a cathode, accelerates ions towards the electrode, and the resulting bombardment removes the material. This effect can be alleviated by operating at high frequencies, since the bombardment is reduced as explained above. A specially shaped cathode can also be used to reduce the electric field across the dark space, and thus reduce impact erosion during normal operation. Second, the electro-emissive material suffers excess erosion when the discharge is struck due to the short-lived, high electric fields. Modern electronic control circuitry can prevent misfiring and striking the discharge when the electrodes are cold and thus reduce this erosion. The use of electronic starters can double the lifetime of a tube. The induction lamp, a commercial version of which was introduced by GE in 1994, contains no electrodes, and the discharge current is induced by a radio-frequency discharge. Since there is no erosion problem, the induction lamp has the capability of lasting for up to 60,000 hours, many times longer than standard fluorescent lamps.
The phosphor in fluorescent lamps has a finite lifetime. The older halophosphates, which were widely used before the introduction of triphosphors, exhibit a drop of fluorescent light output of 30-50% over a period of 8,000 hours. Triphosphors, however, only demonstrate a drop of 10-20% over 8,000 hours, thus extending the useful lifetime of the tube.
Cayles, M.A., and A.M. Martin. Lamps and Lighting. London: Edwin Arnold, 1983.
White, Julian. "Green Lights." Physics World (October 1994).
Iain A. McIntyre
Science EncyclopediaScience & Philosophy: Ferroelectric materials to Form and matterFluorescent Light - Construction And Operation, Starting And Running The Discharge, Ac Operation, Phosphors And Color, Lifetime