The following points highlight the top eight types of lamps. The types are:- 1. Arc Lamps 2. Carbon Arc Lamp 3. Incandescent Lamps 4. Carbon Filament Lamps 5. Tungsten Filament Lamps 6. Gaseous Discharge Lamps 7. Metal Halide Lamp 8. Neon Lamp.
Type # 1. Arc Lamps:
In an arc lamp electric current is made to flow through two electrodes. When two rods touch it produces hot arc resulting in light due to vapourisation of some of carbon. There are various types of arc lamps such as carbon-arc lamp and flame arc lamp.
Type # 2. Carbon Arc Lamp:
Carbon arc lamps are still used in cinema projector, search light and flash cameras. When two carbon rods are placed end to end and connected to dc supply of 45 V, the current flows through them. If they are slightly pulled, an arc will be formed between two carbon rods and white light will be produced. The arc is maintained by transfer of carbon particles from one rod to another rod.
The rate of burning of positive rod is twice of that of negative rod that is why the X-section of the positive rod is twice of that of the negative one. A resistance R is used for stabilising the arc. The whole assembly is put into a sealed tube with addition of some gas. Different gases produce different colours. Neon gives reddish light, argon gives bluish-white light and helium gives pinkish light. The efficiency is 12 lumens/watt.
Type # 3. Incandescent Lamps or Filament Lamp:
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When any material whose melting point is very high is heated to very high temperatures, it starts radiating electromagnetic waves, which are having heat and light energy. This phenomenon is called incandescence if heating temperature is in the range of 4000 to 6000°C absolute and light emission is mostly in red light. It is also called filament lamp.
The advantages of incandescent lamps are given below:
i. It has low initial cost
ii. Brightness is easily controlled
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iii. High quality of colour
iv. Ambient temperature does not affect their working.
Type # 4. Carbon Filament Lamps:
The carbon filament lamps were used in earlier days and consist of a fine filament of carbon suitably mounted in an evacuated bulb. The vacuum is necessary in order to expel any oxygen which will necessarily reduce the life of the filament.
A temperature of about 1700°C is possible before the carbon starts vapourising at an excessive rate. Since, it is the maximum temperature which increases efficiency. So carbon filament lamps are less efficient. Somewhat higher temperatures are possible with gas filled lamps. The melting point of carbon is 3600°C.
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The carbon filament lamps suffered from a number of disadvantages:
(i) Carbon filament lamps cannot be operated at higher temperature, so their efficiency is low. It starts vapourising at about 1800°C.
(ii) Even at operating temperatures, the carbon would vapourise steadily. This not only reduces life of the filament but also impairs its efficiency as vapourised carbon would settle down on the inside of bulb and blackening it.
(iii) Carbon being a brittle material, it is difficult and expensive to stretch filaments.
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(iv) The filament of carbon is brittle and would break easily. So a search was made for another material and later on tungsten filament lamps were developed. The average power consumption of carbon filament lamp is 4 watts/C.P. against 1.2 watts/C.P. in case of tungsten filament lamp.
Type # 5. Tungsten Filament Lamps:
In this case the lamp filament is made of tungsten metal. Again there are two types, the vacuum lamp and the gas filled lamp. In case of vacuum lamp of filament temperature of about 2000°C is possible (compared to 1700°C of carbon) so a higher efficiency is obtained. The glass lamp is evacuated to prevent oxidation of the filament and also to prevent the temperature being lowered by radiation.
But when the bulb is filled with an inert gas like Nitrogen and Argon, it is possible to raise temperature to about 2500°C thus further increasing the efficiency. Tungsten filaments have more life and are not brittle as carbon. The melting point of tungsten filament is 3400°C. Nitrogen is added to reduce the possibility of arcing.
The filament maybe made in two ways viz. stretch of zig-zag thread or coiling it. The zig-zag stretch filament is used in vacuum lamps of lower wattage and the coiled filament is used in Gas filled lamps. Due to presence of gas there is heat loss due to convection currents.
This loss depends upon the surface area of the filament. As such coiled-coil filament which take much less space than coiled filament, are used in gas filled lamps. The coiling reduces the exposed surface of filament to the gas and thus helps to attain it higher temperatures.
The filament is wound in the form of a close helix. A coiled filament is made by winding tungsten wire on a fine iron wire to form a spiral which is again wound on to a thick iron wire to form a coiled-coil. The iron is later on dissolved out by acid.
These lamps are about 20% more efficient as compared to the single coil lamp. The average power consumption of tungsten filament lamp is about 1.2 watts/c.p. The efficiency of coiled coil lamp is about 12 lumens/watt against only 10 lumens/watt for a coiled lamp.
The material used for filament must have the following properties:
1. It must have high melting point, low vapour pressure, high resistivity and low temperature coefficient.
2. It must be ductile and very strong mechanically to withstand vibrations during normal use.
These lamps, (coiled coil) have an efficiency of about 15 Lumens/watt. The type and colour used for the glass cover produce a vital effect on the quality of light emitted from the filament lamp. A milky glass cover gives a diffused light output which is almost like day light or natural light.
The bluish glass cover gives the effect of moonlight which is pleasant only in hot summer nights. The green, yellow and red glass covers are employed for the purposes of signaling in road crossing and railways. The normal working life of a lamp may be taken as 1000 working hours.
Type # 6. Gaseous Discharge Lamps:
A tungsten filament gas filled lamp suffers from two disadvantages; low efficiency (12 lumens per watt in case of a 100 w lamp) and coloured light (yellowish-white). The gas discharge lamps have been developed to overcome these shortcomings.
Discharge lamp works on the phenomenon of electric discharge through vapour or gas. The colour of the light obtained depends upon the nature of the gas or vapour used.
Discharge Phenomenon:
If two electrodes are sealed at two ends of a glass tube containing a vapour or gas under low pressure and a potential is applied across them, the electrons are emitted from cathode and travel towards the anode (i.e. +ve terminal). The velocity of electrons depends upon the potential applied. The kinetic energy of electrons is measured in electron volts. (1 eV = 1.602 x 10–19 Joules) While travelling towards the anode, the electrons come across the atoms of free gas introduced in the tube.
Discharge lamps are of two types:
(i) Those which give the light of the same colour as produced by the discharge through the gas or vapour such as sodium vapour and mercury vapour lamps and also neon gas lamps.
(ii) Those which use the phenomenon of fluorescence and are known as fluorescent lamps. In these lamps the discharge through the vapour produces ultra-violet rays which cause fluorescence in certain materials called phosphors. The fluorescent lamps absorb invisible ultra-violet rays and radiate visible rays.
Drawbacks of gaseous discharge lamps:
(i) Low power factor
(ii) High initial cost
(iii) Takes time to come to full brilliancy
(iv) Can be used only in a particular position
(v) Starting is complicated
(vi) Light output fluctuates at twice the supply frequency hence produces stroboscopic effect.
Type # 7. Metal Halide Lamp:
This is similar in construction to high pressure mercury vapour lamps, but in addition to mercury, number of metal halides normally iodides are added which add to emission of radiations of intermediate wavelengths which are normally not emitted in ordinary HPMV lamp.
This reduces the colour distortion. The efficiency is 75 – 100 lumen/watt. These are available upto 2 kW. A separate ignition device is required in addition to choke. These are also called mercury iodide lamp. Such lamps are suitable for application in the field of flood-lighting and public lighting.
Type # 8. Neon Lamp:
It consists of a glass bulb filled with neon gas with a small percentage of helium. Pure iron electrodes are placed closed together so that lamp can be made to work on low voltage such as 110 volts ac or 150 volts dc. For use on ac, the electrodes are of equal size. On dc the gas glow near the negative electrode, therefore negative electrode is made of larger size.
A series resistance of 2000 Ω is placed inside the bulb in order to reduced fluctuations in current due to varying potential difference. On ac both the electrodes glow with equal intensity. This lamp gives orange pink coloured light. The efficiency of neon lamp lies between 15 – 40 lumens/ watt.
The power consumption is of 5 watts. These lamps are used as indicator lamp and night lamps. The size of lamp is similar to an ordinary incandescent lamp. On dc supply the negative electrode glows only and so the polarity of dc supply can be checked with this lamp.
Neon Tubes:
These tubes are used for advertising and decoration purpose. The neon tube, which is used in varying length and can be bent into any desired shape during manufacturing. In order to obtain different colour different gases are filled. For red colour Argon gas is used and yellow colour helium gas is used.
They are available in 10, 20 and 30 mm diameter and working voltage is about 500 V to 1000 V per metre of the tube length. They are operated at a very low temperature of about 180°C. For getting high voltage step up transformers are employed.
Capacitor ‘C’ is connected to for p.f. improvement connections in the shape of letter M are shown in Fig. 2.26.
