Subject: BE/UTHD
Topic: Different types of light source
Classification of sources of lights:
ARTIFICIAL SOURCES OF LIGHTS
In a broader sense the different methods of producing light by electricity may be divided into the following three groups:
1. By temperature incandescence:
In this method, an electric current is passed through a filament of thin wire placed in vacuum or an inert gas. The current generates enough heat to raise the temperature of the filament to luminosity. Examples. Incandescent tungsten filament lamps; since their output depends on the temperature of their filaments, they are known as 'temperature radiators'.
2. By establishing an arc between two carbon electrodes:
In this case the source of light is the incandescent electrode.
Examples. Carbon arc lamp: flame arc lamp, magnetic arc lamp.
3. Discharge lamps:
In these lamps, gas or vapour is made luminous by electric discharge through them. The colour and intensity of light i.e., candle-power emitted depends on the nature of the gas or vapour only.
These lamps are luminiscent-light lamps and do not depend on temperature for higher efficiencies. In this respect, they differ radically from incandescent lamps whose efficiency is dependent on temperature.
Examples. Mercury vapour lamp, sodium vapour lamp, neon-gas lamp and fluorescent lamp.
INCANDESCENT LAMPS
The incandescent or filament type lamp consists of a glass globe completely evacuated and a fine wire known as filament within it. The glass globe is evacuated to prevent the oxidation and convection currents of the filament and also to prevent the temperature being lowered by radiation.
Materials commonly used materials for incandescent lamps: The materials used for making filaments of incandescent lamps are carbon, tantalum and tungsten.
1. Carbon:
(i) Resistivity, p = 1000 to 7000 μ ohm cm
(ii) Temperature coefficient, α = -0.0002 to 0.0008
(iii) Melting point = 3500°C
(iv) Density = 1.7 to 3.5
To prevent the blackening of the bulb, the working temperature is 1800
. The commercial efficiency of filament lamp is about 4.5 lumen app.
2. Tantalum:
p =124 ohm cm.
(ii) α=0.0036
(ii) Melting point (MP)=2906°C.
(iv) Density= 16.6.
• The efficiency is low such as 2 lumens per watt. So it is not used much now-a-days.
3. Tungsten
(i)p=5.6 micro ohms cm.
(ii)α =0.0045
(iii) M.P.=3400°C (iv) Density 19.3.
The efficiency, when worked at 2000'C in an evacuated bulb is 18 lumens per watt.
This metal is most widely used for the purpose.
To prepare filament, pure tungsten powder is pressed in steel mould for small bars .Mechanical strength of bars is improved by heating electrically nearly to the melting point. These bars are then hammered at red heat and drawn into filaments. To improve efficiency, the bulb is filled with an inert gas argon with a small percentage of nitrogen. To reduce the convection currents, produced by the gas molecules in the bulb, the filament is wound into a close spiral and suspended horizontally in the form of a circular arc.
The efficiency of the gas filled "coiled coil tungsten filament is about 30 lumens per watt
This is due to high working temperature of 2500°C.
The ideal material for the filament of the incandescent lamps in one which has the following properties:
1. High melting point
2. Low vapour pressure.
Fig-(i)
3. High resistivity.
4. Low temperature coefficient.
5. Ductility.
6. Sufficient mechanical strength to withstand vibrations during use.
Fig shows the construction of modern coiled coil- filled filament lamp.
.The lamp has a wreath filament i.e. a coiled filament arranged in the form of a
wreath on radial supports.
Aging effects. With the passage of time the light output of an incandescent lamp decreases due to the following two reasons: u) Evaporation of the filament tends to cause the bulb to blacken.
(ii) Evaporation makes the filament slowly decrease in diameter, which means that the resistance of the filament increases. Therefore, an old filament draws less current and operates at a lower temperature, which reduces its light output, Consequently the efficiency of the lamp (lumensoutput/watt input) also decreases with the passage of time.. Fig. 1.34 and 1.35 shows a coiled filament and coiled-coil filament respectively.
Fig -(iii) Fig- (iv)
The total depreciation of the light output is roughly 15 perce. over the useful life range. Effects of voltage variation. When an incandescent lamp is subjected to voltages different from normal voltage, its operating characteristics are affected.
The efficiency of a lamp (lumens/watt) increases with the increase in voltage owing to increase
in temperature and is proportional to the square of the voltage.
The various relationships are:
· Lumens output α (V)3.55
● Power consumption α(V)1.55
· Luminous efficiency - (V)²
· Life α(V-13 for vacuum lamps;
α(V)-14 for gas-filled lamps.
Fig. (v) shows the variation in power consumption, lumens output, efficiency and life of
incandescent lamps with the variation in voltages.
Advantages of incandescent lamps:
1. Direct operation on standard distribution voltage.
2. Operating power factor unity.
3. Good radiation characteristics in the luminous range.
4. No effect of surrounding air temperature.
5. Availability in various shapes and shades.
Note::…… & Math
HALOGEN LAMP:
A halogen lamp also called tungsten halogen, quartz-halogen, and quartz iodine lamp is an incandescent lamp consisting of a tungsten filament sealed in a compact transparent envelope that is filled with a mixture of an inert gas and a small amount of a halogen , such as iodine or bromine .
The combination of the halogen gas and the tungsten filament produces a halogen cycle chemical reaction, which redeposits evaporated tungsten on the filament, increasing its life and maintaining the clarity of the envelope. This allows the filament to operate at a higher temperature than a standard incandescent lamp of similar power and operating life; this also produces light with higher luminous efficiency and colour temperature .
The small size of halogen lamps permits their use in compact optical systems for projectors and illumination. The small glass envelope may be enclosed in a much larger outer glass bulb, which has a lower temperature, protects the inner bulb from contamination, and makes the bulb mechanically more similar to a conventional lamp.
Halogen lamp- As the life and efficiency of an incandescent lamp falls off with use-partly due to slow evaporation of the filament and partly due to black deposit formed on the inside of the bulb, the addition of a small amount of halogen vapour to the filling gas restores part of the evapoated tungsten vapour back to the filament by means of a chemical reaction i.e., there is a sort of regenerative cycle.
Following are the advantages of halogen lamps:
(i) Long life-2000 hours.
(ii) High operating temperature with increased luminous efficiency varying from 22 to 33 lumens/watt.
(iii) No blackening of lamp, hence no depreciation of lumens output. These lamps, being manufactured in sizes upto 5 kW, are suitable for outdoor illumination of buildings, playing fields, large gardens fountains, car parks etc.
Sodium Vapour Lamp
Construction and working of a sodium vapour lamp are described below.
Construction:
This type of lamp is of low luminosity, so the length of this lamp is large. To get the required length it is made in the form of a U-tube. Two oxide-coated electrodes are sealed with the ends. The tube contains a little sodium and neon gas. The U-tube is enclosed in a double-walled vacuum fil to keep the temperature within working range.
Fig. shows the connection diagram. Capacitor is connected to improve the power factor
which will become low by using poor regulation transformer.
Working:
Before the lamp starts working, the sodium is in the form of a solid, deposited on the sides of the tube walls. In the beginning when the switch is on, it operates as a low-pressure neon lamp with pink colour. The lamp gets warmed, sodium is vaporised and it radiates yellow light and then, after sometime, about 10-15 minutes, the lamp starts giving full light.
** In order to start the discharge lamp, a striking voltage of 380 V is required for 40 W lamp and 450 V for 100 W lamp.
These voltages are obtained from a high reactance transformer or auto transformer. At no load the voltage is very high which falls down as the lamp starts giving light, since the regulation of transformer is poor.
The lamp fails to operate when
(i) the filament breaks or burns out,
(ii) the cathode stops to emit electrons,
(iii) the sodium particles may concentrate on one side of the tube, and
(iv) (iv) the lamp is blackened owing to sodium vapour action on the glass, in which case the output will be reduced.
· Efficiency: Efficiency of a sodium vapour lamp under practical conditions is about 40-50 lumen watt.
· Ratings: lamps are manufactured in 45, 60, 85 and 140 watts .
· Life: The average life is about 3000 hours and is not affected by voltage variations.
** At the end of this period the light output will be reduced by 15 percent due to aging.
Application:
This type of lamp is mainly used for highway and general outdoor lighting where colour discrimination is not required. This lamp should be hung vertical otherwise sodium will blacken the inside of the tube.
High Pressure Mercury Vapour Lamp
Construction:
It consists of two bulbs-an arc-tube containing the electric discharge and outer bulb which protects the arc-tube from changes in temperature. The inner tube or arc-tube is made of quartz (or hard glass) and the outer bulb of hard glass.
The arc-tube contains a small amount of mercury and argon gas. In addition to two main electrodes, an auxiliary starting electrode connected through a high resistance (about 50 k) is also provided. The main electrodes consist of tungsten coils with electron-emitting coating or elements of thorium metal.
Working:
When the supply is switched on, initial discharge for the few seconds is established in the argon gas be tween the auxiliary starting electrode and the neighbouring main electrode and then in argon between the two main electrodes. The heat produced due to this discharge through the gas is sufficient to vaporise mercury. Consequently, pressure inside the are-tube increases to about one to two atmospheres and p.d, across the main electrodes grows from about 20 to 150 V, the operation taking about 5 to 7 minutes. During this time, discharge is established through the mercury vapours which emit greenish-blue light.
** The choke is provided to limit the current to a safe - value. This choke lowers the power factor, so a capacitor C is connected across the circuit to improve the power factor.
Efficiency: The efficiency of this type of lamp is 30-40 lumens/ watt.
Ratings: These lamps are manufactured in 250 W and 400 W ratings for use on 200-250 V A.C. supply mains.
Applications: These lamps are used for general industrial light ing, railway yards, ports, work areas, shopping centres etc. where greenish-blue colour ligh
Fluorescent Tube (lamp) or low pressure mercury vapour lamp :
Fluorescent lighting has a great advantage over other light sources in many applications. The tubes can be obtained in a variety of lengths, with illumination in a variety of colours.It is possible to achieve quite high lighting intensities without excessive temperatures rise and owing to the nature of light sources, the danger of glare is minimised.
The efficiency of fluorescent lamp is about 40 lumens/watt about three times the efficiency of a equivalent tungsten filament lamp.
Construction:
Fig : Fluorescent Tube (lamp)
• Fluorescent Tube (lamp) is a low pressure mercury vapour lamp. Due to low pressure, the lamp is in the form of a long tube, coated inside with phosphor.
The tube contains a small amount of mercury and a small quantity of argon gas at a pressure of 2.5 mm of mercury.
At each end of the tube the electrodes are of spiral form made of tungsten and coated with an electron emitting materia
• A choke is connected in series with the tube filament. It provides a voltage impulse for starting the lamp and acts as a ballast later on when the lamp is running. The filament is connected to a starter switch which is small cathode glow lamp with bimetal strip at the electrodes.
Parts of a fluorescent lamp are shown in Fig
Working:
When the starter is cold, the electrodes are open. When supply is given, full voltage acts on the starter. A glow discharge is set up in the starter which warms the electrodes and causes the bimetal strip to bend and touch the electrodes. The circuit becomes a complete series. Current flows and causes emission of free electrons from filaments. At the same. time voltage at the starter falls to zero and the bimetal strip cools down. The electrodes of the starter switch then open and interrupt the current in the circuit. Its effort is to induce high voltage surge of about 1000 volts in the choke. This voltage produces the flow of electrons between the lamp electrodes and the lamp lights up immediately. Then starting contacts, are left open.
. In order to improve the power factor, usually a condenser of 4 µF capacity is connected across the supply.
Startless fluorescent lamp:
Fig. Startless fluorescent lamp
Above circuit shows a startless fluorescent lamp circuit which does not require the use of a starter switch and is commercially known as 'instant start' or 'quick-start'. In this case, the normal starter is replaced by a filament heating transformer whose secondaries SS heat up the lamp electrodes A and B to incandescence in a fraction of a second. This combination of preheating and application of full supply voltage across lamp electrodes is sufficient to start ionization in the neighbourhood of the lectrodes which further spreads to the whole tube. To ensure satisfactory starting an earthed strip E is used.
This startless method claims the following advantages:
1. Almost instantaneous starting.
2 No flickering and no false starts.
3. Larap life is lengthened.
Stroboscopic effect of fluorescent lamps:
"Stroboscopie (or flickering) effect" produced by fluorescent lamps is due to the periodic fluctuations in the light output of a lamp caused by the cyclic variations of the current on A.C. circuits. This phenomenon creates multiple-image appearance of moving objects and makes the movement appear jerky.
-This effect is more pronounced at lower frequencies. –
The frequency of such flickers is twice the supply frequency.
-This effect is reduced to some extent due to after-glow, as the fluorescent powder used in the tube is slightly phosphorescent.
This effect is very troublesome in the following cases:
(1) When an operator has to move object very quickly particularly those having polished finish. These objects would appear to move with jerky motion which over a long period would produce visual fatigue;
(2) In the case of rotating machines whose frequency of rotation happens to be a multiple of flicker frequency, the machines appear to decrease in speed of rotation or be stationary. Sometimes machines may even seem to rotate in the opposite direction.
The stroboscopic effect can be minimized as follows:
1. By using three lamps on the separate phases of a 3-phase supply.
2. By using a 'twin lamp' circuit on a single-phase supply, one of the chokes having a capacitor series with it and the lamp.
3. By operating the lamp from a high frequency supply (obviously, stroboscopic effect will
entirely disappear on D.C. supply).
Fluorescent lamp for D.C. supply:
Fig shows the connections of a fluorescent tube with D.C. mains. For making a fluorescent lamp to work on D.C. supply a resistance is connected in addition to the usual choke.
Connections of a fluorescent tube with D.C. mains.
Fig- Connections of a fluorescent lamp with DC mains
When the tube works for some time, the positive end of the tube gets blackened due to migration of ionized mercury vapour to the negative end. To decrease this effect, a change over switch is used. Generally a rotary switch if placed ensures the polarity reversal, every time the tube is switched on. The use of a resistance results in increased power consumption and lesser efficiency. But there is no stroboscopic effect.
In D.C. operation of fluorescent tube there is no problem of power factor correction and stroboscopic effect.
Its demerits are:
(i) Low efficiency due to power loss in ballast series-resistance.
(ii) Increased cost of the ballast resistance and reversing switch.
(iii) Less life of the tube (about 80 percent of that with A.C. operation).
Now-a-days with D.C. supplies inverters using solid state circuitry are used for use in buses, aircraft etc.
Useful fluorescent lamp life:
-The normal life of a fluorescent lamp is 7500 hours.
-The active life may vary from 5000 to 10000 hours depending upon the operating conditions.
-Light output is reduced by 15-20 per cent after 4000 hours operation and it is, therefore, advisable to replace the fluorescent lamp after 4000-5000 hours burning on economical grounds.
Performance curves:
Fig. 1.45 shows the performance curves of fluorescent lamps,
. In case of fluorescent lamps, the effect of voltage variation is less marked as compared to the incandescent lamp. However, their life and performance are adversely affected both by low and high voltages
With increased voltage there is a greater heating of electrodes and they lose emissive material by evaporation, while with reduced voltage, the current reduces causing sputtering at the electrodes shortening their life.
The fluorescent lamps give the best performance at 20-25°C temperature. It decreases rapidly when a lamp is operated at a lower temperature or is exposed to cold with drafts. For operation at high temperature, the fittings with provision for air circulation should be employed.
Merits and demerits of fluorescent-lamps:
Merits:
(i) High luminous efficiency.
(ii) Long life.
(iii) Low running cost.
(iv) Low glare level.
(v) Less heat output.
Demerits:
(i) Stroboscopic effect.
(ii) Small wattage requiring large number of fittings.
(iii) Magnetic hum associated with choke causing disturbance.
Neon Lamp
Neon lamps belong to cold-cathode category.
The electrodes are in the form of iron shells and are coated on the inside.
The colour of light emitted is red.
If the helium gas is used in place of neon, pinkish white light is obtained. Helium and neon through coloured glass tubing produce a variety of effects.
Fig shows a circuit for a neon lamp. The transformer has a high leakage reactance which stabilizes the are in the lamp. A capacitor is used for power factor improvement. High voltage is used for starting.
Efficiency: The efficiency of neon lamp lies between 15-40 lumens/watt. •
Applications: These lamps are used as indicator lamps, night lamps for determination of polarity of
D.C. mains and in larger sizes on neon tubes for the purpose of "advertising".
Neon tube:
The neon tube which is used in varying lengths upto about 8 meters, may be bent into almost any desired shape during manufacture. It consists of a length of glass tubing containing two electrodes, normally cylindrical in shape, of iron, steel, or copper.
The tubes are mounted either on a wooden frame or a metal base. These are matched with step-up transformers by connecting suitable tappings for the rated current. Connections between letters are made by nickel wires, the glass tubings being slipped over them.
. The power factor of neon tubes is quite low and is improved by using capacitors. The capacitors can, however, be placed on the low voltage side of the transformer.
Reference: UTHD by J.B.Gupta , UTHD by R.K. Rajput
Subject: Building Electrification (BE/EDEC)
Topic: Illumination
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