Principle of operation of the fluorescent lamp
Fluorescent lamps — the second in the world on prevalence the light source, and they win in Japan even first place. Annually in the world more than one billion fluorescent lamps are produced.
The first samples of fluorescent lamps of modern type were shown by the American firm General Electric at the World Fair in New York in 1938.
In 70 years of existence they strongly entered our life, and it is already difficult to provide some large shop or office at which there would be no lamp with fluorescent lamps.
The fluorescent lamp is the typical digit light source of low pressure in which the discharge occurs in mix of vapors of mercury and inert gas, most often argon. The device of the lamp is shown in fig. 1.
The flask of the lamp is always the cylinder 1 of glass with the outside diameter 38, 26, 16 or 12 of mm. The cylinder can be direct or curved in the form of the ring, the letter U or more of the difficult figure. In the edge ends of the cylinder glass legs 2 on which electrodes 3 are from the inside mounted are hermetically sealed. Electrodes on the design are similar to the coiled-coil body of heat of glow lamps and also are made of the tungsten wire. In some types of lamps electrodes are made in the form of the trispirala, that is filched from the double helix. From the outer side electrodes are soldered to pins 4 socles 5. In direct and U-shaped lamps only two types of socles are used: G5 and G13 (digits 5 and 13 specify distance between pins in mm).
As well as in glow lamps, from flasks of fluorescent lamps air is carefully pumped out through the exhaust tube 6 which is sealed in one of legs. After pumping the volume of the flask is filled with inert gas 7 and mercury in the form of the small drop 8 (the mass of mercury in one lamp usually about 30 mg) or in the form of so-called amalgam, that is mercury alloy with bismuth, indiy and other metals is entered into it.
The layer of the activating substance is always applied on coiled-coil or trispiralny electrodes of lamps — it is normal mix of oxides of barium, strontium, calcium, sometimes with small additive of thorium.
If tension bigger, than ignition tension is attached to the lamp, then in it between electrodes there is the electric discharge which current is surely limited to any external elements. Though the flask is filled with inert gas, at it there are always mercury vapors which amount is defined by temperature of the coldest point of the flask. Atoms of mercury are excited and ionized in discharge much easier, than noble-gas atoms therefore also current through the lamp, and its luminescence are defined by mercury.
In mercury discharges of low pressure the share of visible radiation does not exceed 2% of discharge power, and light return of mercury discharge — only 5-7 lm/W. But more than a half of the power allocated in discharge turns into invisible ultra-violet radiation with lengths of waves of 254 and 185 nanometers. From physics it is known: the radiation wavelength is shorter, the this radiation has bigger energy. By means of the special substances called by phosphors it is possible to turn one radiation into another, and, on the law of energy conservation, "new" radiation can be only "less vigorous", than primary. Therefore ultra-violet radiation can be turned in seen by means of phosphors, and seen in ultra-violet — it is impossible.
All cylindrical part of the flask is from the inside covered with the lamina of such phosphor 9 which turns the ultra-violet radiation of atoms of mercury into visible. In the majority of modern fluorescent lamps as the phosphor it is used галофосфат calcium with additives of antimony and manganese (as specialists, "activated by antimony and manganese" speak). At radiation of such phosphor ultra-violet radiation it begins to shine this world of different shades. The phosphor radiation range — continuous with two maxima, about 480 and 580 nanometers (fig. 2).
The first maximum is defined by availability of antimony, the second — manganese. Changing the ratio of these substances (activators), it is possible to receive this world of different color shades, from warm to day. As phosphors turn more than a half of power of discharge into visible light, their luminescence determines lighting parameters of lamps.
In the 70th years of the past century began to do lamps not with one phosphor, and with three, having radiation maxima in blue, green and red areas of the range (450, 540 and 610 nanometers). These phosphors were created originally for kinescopes of color television where with their help it was succeeded to receive quite acceptable reproduction of flowers. The combination of three phosphors allowed and to achieve in lamps considerably the best color rendition at simultaneous increase in light return, than when using the galofosfat of calcium. However new phosphors are much more expensive old as in them connections of rare-earth elements are used: europium, cerium and terbium. Therefore in the majority of fluorescent lamps phosphors on the basis of the galofosfat of calcium are still applied.
Electrodes in fluorescent lamps perform functions of sources and receivers of electrons and ions due to which electric current through discharge gap proceeds. In order that electrons began to pass with electrodes into discharge gap (as speak, to start thermoemission of electrons), electrodes have to be heated to temperature of 1100 - 1200 degrees Celsius. At such temperature tungsten shines in very weak cherry color, its evaporation is not enough. But for increase in quantity of the taking-off electrons at electrodes the layer of the activating substance which is considerable less термостойко, than tungsten is put, and during the work this layer is gradually sprayed from electrodes and settles on flask walls. Usually process of spraying of the activating covering of electrodes defines service life of lamps.
For achievement of the greatest efficiency of discharge, that is for the greatest exit of ultra-violet radiation of mercury, it is necessary to maintain the certain temperature of the flask. Diameter of the flask gets out of this requirement. In all lamps approximately identical current density — the current size divided into flask sectional area is provided. Therefore lamps of different power in flasks of one diameter, as a rule, work at equal rated currents. Voltage drop on the lamp is directly proportional to its length. And as power is equal to the work of current on tension, with the identical diameter of flasks and the power of lamps is directly proportional to length. At the 36 most mass lamps (40) W length is equal to 1210 mm, at the 18 lamps (20) W — 604 mm.
Big length of lamps constantly forced to look for ways of its reduction. Simple reduction of length and achievement of the necessary capacities due to increase in current of discharge is irrational as at the same time flask temperature increases that leads to pressure increment of vapors of mercury and decrease in light return of lamps. Therefore creators of lamps tried to reduce their dimensions due to change of the form: the long cylindrical flask was bent in half (U-shaped lamps) or in the ring (ring lamps). In the USSR in the 50th years did the 30 W U-shaped lamps in the flask with a diameter of 26 mm and 8 W in the flask with a diameter of 14 mm.
However it is cardinal it was succeeded to solve the problem of reduction of dimensions of lamps only in the 80th years when began to use the phosphors assuming heavy electric loads that allowed to reduce diameter of flasks considerably. Flasks began to do of glass tubes with the outside diameter of 12 mm and to repeatedly bend them, reducing thereby the total length of lamps. There were so-called compact fluorescent lamps. By the principle of work and the internal device compact lamps do not differ from normal linear lamps.
In the mid-nineties in the world market there was the new generation of fluorescent lamps in advertizing and technical literature called by "T5 series" (in Germany — T16). At these lamps the outside diameter of the flask is reduced up to 16 mm (or 5/8 inches, from here and the name T5). By the principle of work they also do not differ from normal linear lamps. One very important change is made to the design of lamps: the phosphor is from the inside covered with the thin protective film, transparent both for ultra-violet, and for visible radiation. The film protects the phosphor from hit of mercury particles on it, the activating covering and tungsten from electrodes thanks to what "poisoning" of the phosphor is excluded and high stability of the luminous flux during service life is provided. Also the composition of the filling gas and the design of electrodes are changed that made impossible work of such lamps in old schemes of inclusion. Besides. for the first time since 1938 lengths of lamps were changed so that the sizes of lamps with them corresponded to the sizes of standard modules of false ceilings very fashionable now.
Fluorescent lamps, especially the last generation, in flasks with a diameter of 16 mm, considerably exceed glow lamps on light return and service life. The values of these parameters reached today are equal 104 lm/W and 40000 hours.
However fluorescent lamps have also the set of shortcomings which need to be known and considered at the choice of light sources:
- Big dimensions of lamps often do not allow to redistribute the luminous flux as necessary.
- Unlike glow lamps, the luminous flux of fluorescent lamps strongly depends on surrounding temperature.
- Lamps contain mercury — very poisonous metal that does them ecologically dangerous.
- The luminous flux of lamps is established not right after inclusion, and after a while, depending on the design of the lamp, the surrounding temperature and lamps. At some types of lamps into which mercury is entered in the form of amalgam this time can reach 10-15 minutes.
- Depth of pulsations of the luminous flux is much higher, than at glow lamps, especially at lamps with rare-earth phosphors. It complicates use of lamps in many production rooms and, besides, has an adverse effect on health of the people working at such lighting.
As it was told above, fluorescent lamps, as well as all gas-discharge devices, demand for inclusion in network of use of additional devices.Top