Fluorescent Light
Fluorescent Light
Starting and running the discharge
Fluorescent light is the most common type of electrical light found in the United States; it is used for practically all commercial lighting, i.e., offices, factories, stores and schools, and it is estimated that there are 1.5 billion fluorescent lamps in use nationwide. Fluorescent lighting is popular due to its high efficacy—it produces between three to five times more light than an incandescent lamp consuming the same electrical power. The main reason for this is that the fluorescent lamp employs a phosphor which converts the non-visible light produced by the lamp into visible light, whereas a large fraction of the output from the incandescent lamp is infra-red light which escapes as heat.
Although the fluorescent lamp was first demonstrated by Antoine Becquerel (1852–1908), in the late 1800s, it was not commercially available until 1938 with the introduction of phosphors which could withstand the rigors of operation for a reasonable length of time. Since then improvements have been made in all aspects of the lamp: electrodes, phosphors, gas mixtures, and control circuitry. These improvements are particularly important simply because there are so many fluorescent lamps in use. Over its lifetime, a standard fluorescent lamp consumes as much electricity as is generated by a barrel of oil: the importance of even small increases in efficacy become apparent when one considers that even a 10% increase will result in savings of approximately 40 million barrels a year in the United States alone.
Construction and operation
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 nanometers (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.
Starting and running the discharge
Unlike the electrical circuit for an incandescent lamp, which contains only a switch, the control circuit for a fluorescent lamp must do two things. It must first provide a high voltage spike to strike the discharge, and it must thereafter control the current and voltage once the discharge is stable. The latter is important because the discharge itself is unstable and will terminate if the current is not controlled externally.
There are several types of starter circuits which all do two things. They supply a large current to the electrodes in order to produce electrons via thermioemission (the electrons “boil off” as the electrodes heat up) and they supply a high voltage to strike the discharge. Typical examples of these include the switch start, instant-start, and rapid start. The switch start has the advantage of being actively controlled and therefore avoids the misfirings which can have the deleterious effect of removing the coating on the electrodes and thus shorten the tube’s life.
The switch is initially closed, thus shorting the electrodes and allowing a large current to flow which heats the electrodes to their operating temperature. After a short time (1 to 2 seconds), the switch is opened. The large voltage spike created by the sudden reduction of current through the ballast (an inductor) then strikes the discharge and the lamp lights up. The capacitor reduces the reactance of the inductive ballast.
The switch used to be an argon glow tube with a bimetallic electrode, but this function has been replaced in recent years with solid state circuitry which can be actively controlled.
AC operation
Fluorescent lamps are usually operated with an AC discharge whose frequency is set by the power supply-60 hertz (Hz) in the United States. However, it has been found that the tube has a higher efficacy if it is operated at a high frequency, for example 20-30 kHz. The reason for this increase in power is that there is less time between field reversals for the ions to collide with the electrodes, and so the rate of energy loss through electrode collision is reduced. Operation at high frequency requires a transistorized ballast, which has the added advantage that the lamp can be dimmed, unlike low frequency lamps where the current and voltage to the tube are fixed and the tube cannot be dimmed.
Phosphors and color
The phosphor converts the UV output from the mercury discharge into visible light via fluorescence. The mix of color emitted depends on the chemical compounds used in the phosphor. Many compounds produce what is perceived as a white light, which may indeed be a broad emission centered around 590 nm, as in the case of the so-called cool white and warm white halophosphates (the warm contains more red than the cool). However, recent developments in phosphors for television tubes have resulted in the introduction of the “triphosphor,” which is a mixture of three different phosphor components emitting in the blue, green, and red. The light from a triphosphor tube distorts an object’s perceived color less than that of a halophosphate tube, and changing the mix of the three components allows the lighting engineer to adapt the output of the lamp to suit certain specific purposes, for instance to better match the lighting within a building to the activities of its occupants.
Lifetime
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.
See also Electric circuit; Incandescent light.
KEY TERMS
Efficacy —The ratio of light output from a lamp divided by the electrical power driving the lamp.
Phosphor —An inorganic compound which emits visible light when illuminated by ultraviolet light.
Thermionic emission —The emission of electrons from the surface of a material when the material’s temperature is raised.
Resources
BOOKS
Cayles, M.A., and A.M. Martin. Lamps and Lighting. London: Edwin Arnold, 1983.
PERIODICALS
White, Julian. “Green Lights.” Physics World (October 1994).
Iain A. McIntyre