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a gas-discharge light source whose light flux comes mainly from the luminescence of phosphors under the action of the ultraviolet radiation of the discharge; the visible luminescence of the discharge does not exceed a few percent. Fluorescent lamps are widely used for general lighting; their luminous efficiency and service life are several times greater than those of incandescent lamps used for the same purpose.
The most widely used type of fluorescent light source is the mercury lamp, which is a glass tube with a layer of phosphor deposited on its inner surface. Spiral tungsten electrodes enter the tube through its end faces. To increase the emissivity, an oxide suspension made from carbonates or peroxides of alkaline earths is deposited on the electrodes. A drop of mercury and an inert gas (such as argon or neon) are introduced into the lamp; the gas enhances the service life of the lamp and improves the conditions for excitation of mercury atoms. If a fluorescent lamp is connected to an alternating current (AC) source, an electric current of the order of tenths of an ampere begins to flow between the electrodes, exciting luminescence of the mercury atoms. The mercury vapor pressure in a fluorescent lamp depends on the wall temperature of the lamp and at the normal operating temperature of 40°C is about 0.13-1.3 newtons per sq m (10-2-10-3 mm of mercury). Such a low pressure assures an intensive discharge radiation in the ultraviolet spectral region (mainly with wavelengths λ = 184.9 and 253.7 nanometers [nm]), which excites the luminescence of the phosphor layer of the lamp.
The most widely used phosphor is calcium halophosphate activated by antimony and manganese. By changing the proportions of activators, phosphors of various types—and, consequently, lamps of various chromaticities—may be produced. In the USSR, the maximum luminous efficiency (75-80 lumens per watt) is exhibited by white light lamps. The luminous efficiency of cold white light lamps is about 65 lumens per watt. Daylight lamps are used to provide maximum color fidelity of illuminated objects. Lamps with a diffuse-reflective layer (reflector lamps) have a reduced total light flux, but their light intensity in the direction of reflection of the layer is almost twice as great. The service life of these types of lamps exceeds 10,000 hr. The power is 4-200 W; length, 136-2,440 mm. The lamps may be straight, U-shaped, W-shaped, annular, panel-type, or candle-shaped.
Fluorescent lamps containing amalgams of indium, cadmium, and other elements are widely used. The lower mercury vapor pressure over the amalgam makes possible an increase in the temperature range of optimum luminous efficiency to 60°C, rather than 18°-25°C for pure mercury.
If the ambient temperature is increased above the permissible limit (25°C for mercury and 60°C for an amalgam), the wall temperature and the vapor pressure increase and the light flux is reduced. An even sharper reduction of light flux is observed if the temperature (and hence the vapor pressure of mercury) is decreased (see Figure 1). Concurrently, the starting of the lamps deteriorates drastically, making impossible their use at temperatures below 0°C unless heating fixtures are used. As a result mercuryless fluorescent lamps with a low-pressure discharge in inert gases are of interest. In this case the phosphor is excited by radiation with wavelengths X = 58.4-147.0 nm. Since the gas pressure in mercuryless fluorescent lamps is virtually independent of the ambient temperature, their luminous characteristics also remain constant.
The luminous efficiency of a fluorescent lamp increases with increasing tube length through a decrease in the proportion of the anode-cathode losses in the total light flux. Fluorescent lamps are characterized by low surface brightness and by pulsation of the light flux when the lamp is operated on alternating current (the stroboscopic effect). The pulsations may be reduced by balanced connection of the lamp to the three phases of the power supply. The service life of lamps is limited by deactivation and cathode sputtering. Voltage fluctuations of the power supply and frequent switching of the lamps on and off reduce the service life. Luminous efficiency is reduced as the lamp burns.
As a gas-discharge device, a fluorescent lamp has a falling current-voltage characteristic, which makes necessary the use of inductive or capacitive start-control devices. The cathodes must be heated to increase the thermal emission and thereby assure ignition of the lamp. This may be achieved by connecting the cathodes to the circuit in series with the start-control devices by means of a starter (starter circuits) or filament transformer (starterless circuits).
Fluorescent lamps are widely used as light sources. White-light and cold white-light lamps are used for general lighting; warm white-light lamps, for lighting areas rich in white and pink tones; blue-light reflector-type lamps, for electrophotographic copying and reproducing apparatus; and lamps made of uviol glass, which are partly transparent to ultraviolet radiation, for prophylactic radiation therapy.
Fluorescent lamps are produced on mechanized flow assembly lines with an output of 700-800 units per hr. In the USSR and abroad work is aimed at increasing the luminous efficiency of the lamps to 80 lumens per watt and the service life to 12,000-15,000 hr with a switching frequency of eight times per day (instead of the current four times per day) and at the development of facilities to produce 2,000-3,000 lamps per hr.
REFERENCESFabrikant, V. A. “Fizika i tekhnika liuminestsentnykh lamp.” Uspekhi fizicheskikh nauk, 1945, vol. 27, no. 2.
Vavilov, S. I. O “teplom” i “kholodnom” svete. Moscow, 1956.
Fedorov, V. V. “Novoe v fizike i tekhnike liuminestsentnykh lamp.” Svetotekhnika, 1966, nos. 9-10.
Fedorov, V. V. Proizvodstvo liuminestsentnykh lamp. 2nd ed. Moscow, 1968.
V. V. FEDOROV