Electric Lighting PDF

2. LIGHT SOURCES 2.0 INCANDESCENT LAMPS The incandescent lamp is the oldest electric light source still in general use,. It is also the most varied as regards types. It can be found in almost any application, especially where comparatively small light packages are required and where...

2. LIGHT SOURCES 2.0 INCANDESCENT LAMPS The incandescent lamp is the oldest electric light source still in general use,. It is also the most varied as regards types. It can be found in almost any application, especially where comparatively small light packages are required and where simplicity and compactness are favoured. The incandescent lamp produces its light by the electrical heating of a wire (the filament), usually tungsten filament to such a high temperature that radiation in the visible region of the spectrum is emitted. Performance characteristics Energy balance Fig. 1 shows the energy balance in an incandescent Lamp. An incandescent lamp operates at about 2800K and emits radiation throughout the visible spectrum with a bias towards the higher wavelengths. The outer glass envelope is filled typically with a mixture of nitrogen and argon whose function is to limit the evaporation of tungsten and also to prevent arcing across the filament. The luminous efficacy of practical tungsten incandescent lamps is always considerably low since most of the radiation is in the infra red range of wavelengths. For example, for modern GLS lamps with a rated operating life of 1000 hours it varies between 8 and 21.5 Lm/W. Colour appearance and colour rendering The normal incandescent lamp with its low colour temperature of around 2800 K, is 7 generally described as having an excellent colour appearance. The radiation emitted by the lamp covers the entire visible spectrum which means that its colour rendering ability, with its Ra of 100, is second to none. switching Frequent switching is not normally detrimental to lamp life. It is only when the filament has become critically thin through age that the mechanical strain caused by the rapid temperature change as a result of switching will be sufficient to cause its breakdown. Dimming Normal incandescent lamps can be dimmed without restriction. A dimmed lamp will have a lower filament temperature, which results in a lower colour temperature, a lower luminous efficacy and a longer operating life. Thus, the advantage of a longer life is at the cost of efficacy and it is generally better, in a situation where a lamp is almost continually dimmed, to use one of a lower wattage rating. Below 50% of the nominal operating voltage the light output of an incandescent lamp is negligible, but energy consumption is nevertheless still appreciable. It is strongly recommended therefore, that dimmers be used in such a way that switch the lamp off at the point of 50% of voltage Effect of voltage variations Any variation in voltage applied to an incandescent lamp causes a change in its operating characteristics. For example a 5% over voltage reduces the lamp life substantially 2.0.1 Tungsten Halogen lamps Tungsten Halogen incandescent lamp is a comparatively recent development. Although originally designed for use in specialized applications such as flood lighting, studio lighting and projection lighting, it has rapidly penetrated many other areas of lighting application once smaller lumen packages became available. The high temperature of the filament in a normal incandescent lamp causes tungsten particles to evaporate off and condense on the bulb wall, resulting in blackening of the glass bulb and loss of tungsten material in the filament. Tungsten Halogen lamps have a halogen 8 (eg. iodine, chlorine, bromine) added to the normal gas filling and work on the principle of-a halogen regenerative cycle to prevent blackening. The evaporated tungsten combines with the halogen to form a tungsten halogen compound. This stays in the form of a gas and does not condense at the bulb surface since the temperature of the bulb is high enough (250° C) to prevent condensation. When this gas comes near to the incandescent filament it is broken down by the high temperature into tungsten which is redeposited on the filament, and into the halogen which continues its role in the regenerative cycle. The main point of difference with a normal incandescent lamp, apart from the halogen additive already mentioned, concerns the bulb. Because the bulb temperature must be high, halogen lamps are much smaller than normal incandescent lamps The tubular envelope is made of a special quartz glass, which is resistant to the high temperatures needed for the halogen cycle to function. The Tungsten halogen lamp operates with an internal pressure above atmospheric pressure. Since their introduction in 1960 tungsten halogen lamps have made inroads into almost all applications where normal incandescent lamps used to be employed. Advantages of tungsten halogen lamps over normal incandescent lamps are; a much longer life time, a higher luminous efficacy compactness, a higher colour temperature and a little or no light depreciation with age. Performance characteristics Luminous efficacy The tungsten halogen lamp is characterized by its high efficacy (some 10% higher than that of a comparable normal incandescent ) and its almost perfectly maintained light output throughout life. Colour appearance and colour rendering Tungsten halogen lamps for normal lighting purposes have a colour temperature of between 3000° K and 3400°K. The source can therefore be described as providing a whiter light, with a correspondingly cooler colour appearance, than that given by normal incandescent. 9  The tungsten halogen lamp, with its Ra of 100, provides excellent colour rendering. 2.1 TUBULAR FLUORESCENT LAMPS The fluorescent lamp is a low pressure mercury discharge lamp in which light is produced predominantly by fluorescent powders activated by the ultraviolet energy of the discharge. The lamp, generally is in the form of a long tubular bulb with an electrode sealed into each end contains mercury vapour at low pressure with a small amount of inert gas for starting and arc regulation. The inner surface of the bulb is coated with a fluorescent powder or phosphor, the composition of which determines the quantity and colour of the light emitted. In the fluorescent lamps the electrical discharge produced are mainly in the wave lengths of 253 nm and 185 nm, both in the ultraviolet region of the optical radiation. The phosphor coating absorbs the ultraviolet light and re-radiates them into visible part of the spectrum. Initially halophosphates were used as phosphor coatings to make white lamps, but research in phosphor development has lead to development of narrow band phosphors, which separately emit the red, blue and green primary colors (Tri-phosphors). The combination of these emissions then create white light with colour rendering indexRa between 60 to 80 The earlier tubular fluorescents were the T 12 type, whose tube diameter was 1 ½ inches. This was followed by the T 8 version whose diameter is 1 inch. 2.1.1 Compact Fluorescent lamps These have been developed for use in those applications mainly as a replacement for incandescent lamps. The compact fluorescent lamps (CFL) have very thin fluorescent tubes in various shapes. Some of the types are Twin tube, Quad tube and spiral type lamps. They have an electronic ballast of the basic type inbuilt in them which ensures quick start and higher efficiency. They combine high efficacy and better colour rendering characteristics than the normal tubular fluorescent lamps with low energy consumption and longer life (typically 6000 to 8000 hours as against 1000 hours of a GLS lamp). Their colour rendering index Ra varies between 60 to 80 depending on the type and quality of lamp 10 Performance characteristics Energy balance Fig. 2 shows the energy balance of a 36/40 W (Normal T12 or T8) fluorescent lamp. Light output versus ambient temperature The light output of the lamp reaches a maximum at an ambient temperature of 25°C in still air. At ambient temperature below 15°C the output rapidly decreases, while at temperatures above the optimum also the light output decreases, but at a slower rate. Luminous efficacy The luminous efficacy of a fluorescent lamp expressed in terms of system efficacy is influenced by the type of circuitry and the components used. Two factors influencing the luminous efficacy of the lamp itself, besides the phosphors used are the ambient temperature and the frequency of the supply voltage. The luminous efficacy is hence substantially improved by use of high frequency electronic ballast. Influence of temperature Just as with the light output, the luminous efficacy of the tubular fluorescent lamp decreases if the ambient temperature is above or below the optimum value. However, since the power dissipated by the lamp also decreases rapidly with increase in temperature, the luminous efficacy will in fact fall off less rapidly than the luminous flux. 11 Influence of supply frequency Operation on a high frequency supply will result in an increase of the luminous efficacy of about 10%. This is one of the reasons for employing high frequency electronic ballasts. Colour characteristics Tubular fluorescent lamps are available in various combinations of colour appearance and colour rendering index to suit a wide range of applications. Performance comparison - Tubular fluorescent lamps (36/40 W) Colour Colour Colour Group Temp.(K) Rendering Index (Ra) 2900 51 2700 85 Warm - white 3000 85 2700 95 3000 95 White 3500 57 4100 63 Cool - white 4000 70 4000 85 3800 95 Day light 5300 98 Cool - 6200 72 Day light 6500 85 6500 97 Depreciation During the life of a fluorescent lamp the luminous flux decreases. After 6000 hours it will be between 70 and 90% of the initial value. The main cause of depreciation is that the fluorescent powders slowly become less effective. When mixtures of different fluorescent powders are used, it may sometimes happen that older lamp will show a slight discoloration compared with new ones. A secondary cause of depreciation is the blackening of the tube wall (especially at its ends), by dispersed emitter material. Employing high frequency ballasts will result in less sputtering of the emitter material, which, in turn will give a lower depreciation rate. 12 2.1.2 Lamp circuits The fluorescent lamp has a negative resistance characteristic and so must be operated in conjunction with a current limiting device (or ballast) to prevent current runaway. The ballast, which has a positive resistance characteristic can be : a resistor a choke or inductor an electronic circuit. Each has its specific advantages and disadvantages, but they have all found practical application in some form. Inductive ballasts An inductor choke is the most widely used ballast for normal a.c. applications. In combination with a switch starter, it can also be made to produce the high voltage pulse needed to ignite the lamp. A practical choke/ ballast consists of a large number of windings of copper wire on a laminated iron core. Heat losses, occurring through the ohmic resistance of the windings and hysteresis in the core, depends upon the mechanical construction of the ballast and the diameter of the copper wire. Electronic Ballast Although more expensive, electronic ballasts offer important advantages over conventional choke ballasts, such as : Improved lamp and system efficacy no flicker or stroboscopic effects instantaneous starting without the need for a separate starter increased lamp life excellent light regulation possibilities Higher power factor less temperature increase (due to lower losses) no hum or other noise lower weight, especially for big lamp sizes can also be used on d.c. A popular approach is to rectify the current drawn from the mains supply and convert it into high frequency square wave signal in the range 20 KHZ to 100 KHZ. This square wave is filtered by appropriate harmonic filters depending upon the T.H.D ( Total harmonic distortion ) required. The improvement in luminous efficacy and characteristics of light depends on the quality of the ballast. A good quality electronic ballast will give rise to high power factor and low T.H.D. Ignition The internal resistance of a cold tubular fluorescent lamp is far too high for it to start automatically when the mains voltage is applied. Some sort of aid to starting is therefore, needed to ignite the lamp. Fluorescent lamp circuits can be divided into three groups as far as starting is concerned. Preheat starter circuits, preheat starter less circuits and cold start circuits. 13 Run up After ignition, it takes two to three minutes before the mercury vapour in the fluorescent lamp has reached its working pressure. During this period the luminious flux gradually increases to a maximum. However, as the initial flux is about 60% of the final value, this increase will not normally be noticeable. Reignition When the lamp is switched off, the vapour pressure drops so quickly that instantaneous reignition will seldom, if ever, pose any problems. Dimming Present day dimming equipment for fluorescent lamps is either of the thyristor (chopper) type or of the variable frequency type (HF electronic light regulation). 2.1.3 T5 Fluorescent lamp The latest development in the fluorescent lamp is the T5 lamp. This lamp has a diameter of only 5/8 “(15 mm) and has an inbuilt electronic ballast of high quality. The main advantage of T5 lamps are very high luminous efficacy between 85 to 90 lumens/watt for the 2’T5 tube and up to 104 Lumens/watt for the 28 W 4’ T5 tube. The T5 lamp also a very high power factor greater than 0.85 and colour rendering index Ra of 90 The T5 uses less quantity of mercury vapour in the tube. A coating of calcium nitrate on the inside surface prevents absorbtion of mercury by the walls of the tube thereby prolonging the life of the lamp. The narrow tube along with the powerful electronic ballast substantially improves the luminous efficacies to the range of 90 to 104 Lumens/watt. The life of T5 lamp is between 15000 to 18000 burning hours. Another salient feature of this lamp is its low lumen depreciation factor due to calcium nitrate coating. Even after 12000 burning hours the luminous efficacy depreciates by only 5%. A comparison of T5 tube with T12 and T8 fittings are shown below 2.2 HIGH PRESSURE MERCURY LAMPS In these lamps the discharge takes place in a quartz discharge tube containing small quantity of mercury and an inert gas filling, usually argon to aid starting. A part of the radiation from the discharge occurs in the visible region mainly in the blue and green region )of the spectrum as light, but a part is also emitted in the ultra violet region. By coating the 14 inner surface of the outer bulb in which the discharge tube is housed with a fluorescent powder that converts this ultraviolet radiation into visible radiation, the lamp will give more light than a similar uncoated version. Further the phosphor coating will considerably improve the lamps colour rendering properties. Working principle When examining the working of the high-pressure mercury lamp, three distinct phases have to be considered: Ignition, run-up and stabilisation. Ignition Ignition is achieved by means of an auxiliary or starting electrode placed very close to one main electrode and connected to the other through a high value (typically 25 k) resistor. When the lamp is switched on, a high voltage gradient occurs between the main and starting electrodes, ionising the gas in the region in the form of a glow discharge, the current being limited by the resistor. The glow discharge then spreads throughout the discharge tube under the influence of the electric field between the two main electrodes. When the glow discharge reaches the farthest electrode, the current increases considerably. As a result the main electrodes are heated until the emission is increased sufficiently to allow the glow discharge to change completely into an arc discharge, the auxiliary electrode playing no further role in the process by virtue of the high resistance in series with it. At this stage the lamp is operating as a low pressure discharge (similar to that in tubular fluorescent lamp). The discharge fills.the tube and has a blue appearance. Run up Ionisation of the inert gas having been accomplished, the lamp still does not burn in the required manner or give its full light output until the mercury present in the discharge tube is completely vaporised. This does not occur until a certain time, termed the run-up time has elapsed. As a result of the arc discharge in the inert gas, heat is generated resulting in a rapidly increasing temperature with the discharge tube. This causes the mercury to gradually vaporise increasing the vapour pressure and constricting the discharge to a narrow band along the tube's axis. With further increase in pressure the radiated energy is concentrated progressively toward the spectral lines of longer wavelengths and a small proportion of continuos radiation is introduced so that the light becomes whiter. Eventually, the arc attains a point of stabilisation at a vapour pressure in the range of 2 to 15 atmospheres and the lamp is said to have reached the point of local thermodynamic equilibrium. All the mercury is then vaporised and the discharge takes place in unsaturated mercury vapour, The run up time which is defined as the time needed from the moment of switch on for the lamp to reach 80% of its full light output is about four minutes. Stabilisation The high pressure mercury lamp like the vast majority of discharge lamps, has negative resistance characteristics and so cannot be operated on its own in the circuit without a suitable ballast to stabilise the current flow through it. Performance characteristics Energy Balance The energy balance in a high pressure mercury lamp 400 w is shown in fig. 3. 15 External influences Temperature The light output, lamp voltage and life of these lamps are not significantly affected by changes in ambient temperature. Mains voltage fluctuations The high pressure mercury lamp is often found in those countries where the power supplies on which they must operate are rather poor. Where other lamps, when run on such supplies, may be prone to premature failure or else are unable to function at all the mercury lamp is untroubled :n this respect. Burning position The high pressure mercury lamp has a universal burning position. 16 Reignition Once extinguished, the lamp will not restart until it has cooled sufficiently to lower the vapour pressure to the point where the arc will restrike with the voltage available. This time, termed the reignition (one-strike) time, is in the order of five minutes. 2.2.1 Blended light lamps The blended light lamp is derived from the conventional high pressure mercury lamp. The principal difference between the two is that whereas the latter is dependent on an external ballast to stabilise the lamp current, the blended light lamp has the ballast built in the form of a tungsten filament connected in series with the discharge tube. The light from the mercury discharge and that from the heated filament combine, or blend (hence the name) to give a lamp with operating characteristics that are different to those possessed by either a pure mercury lamp or an incandescent lamp. 2.2.2 Metal Halide lamps The lamps are similar in construction to the high pressure mercury lamp. The major difference between the two types is that the discharge tube of the former contains a number of metal halides in addition to the mercury. Some of the halides used include dysprosium, sodium, lithium and thallium. Some of the metal halide lamps also have in addition phosphor coatings to further improve the colour rendering property of light rendered. These halides are partly vaporised when the lamp reaches its normal operating temperature. The halide vapour is then dissociated in the hot central region of the arc into the halogen and the metal, with the vaporised metal radiating its appropriate spectrum. Energy balance The energy balance in a 400 W Metal Halide lamp is shown in fig.4. 17 The important characteristics of metal halide lamps are 1) Long life of 12000 to 15000 burning hours 2) Good colour rendering index Ra between 60 to 80 depending on the model and type 3) Various types of warm and cool type lamps are available with different co-related colour temperature. 4) Most suitable for high bay fittings , industrial medium and high bay fittings , area lighting and road lighting. The main disadvantages are 1) High strike and re-strike time of 10 to 15 minutes with traditional ballasts 2) At 40% life, the lumen depreciation is more than 30% 3) Lower colour rendering when the lamp ages Different colors are produced in metal halide lamps by using various arc tube shapes and metal halide salts. In new lamps these halides need to "burn-in" for approximately 100 hours before they reach their optimum color. This is why new lamps can sometimes be unstable or vary in color. 2.2.3 Uni-form pulse start ballasts Pulse start metal halide ballasts provide the proper combination of open circuit voltage and high voltage pulses to start the lamp. The pulse is provided by a specially designed ignitor, or starter, that is used in conjunction with the ballast. As soon as the ignitor senses that the lamp has started, it discontinues the pulsing operation. At this point, the ballast sustaining voltage must be sufficient to maintain lamp operation. High intensity Discharge (HID) lamp ignitors provide a brief, high voltage pulse or pulse train to breakdown the gas between the electrodes of an arc lamp. Pulses can range from several hundred volts to 5KV. Typical durations are in the µsec range. They are usually timed to coincide with the peak of OCV. If they are timed too early or too late, lamps may not start reliably. A positive feature of this system is that the lamp will hot restart in 3-4 minutes following a power interruption The purpose of HID ballasts are to 1) Provide voltage to breakdown the gas between the electrodes of arc lamps and initiate starting. 2) Provide voltage and current to heat the electrodes to allow a low voltage, high current arc mode to develop (referred to as glow-to-arc transition, GAT). 3) Provide enough current to heat and evaporate the light emitting components after an arc has been established. 4) Provide enough sustaining voltage to maintain the arc during warm-up and operation. 5) Set lamp current once all the evaporable materials have reached thermal equilibrium. 2.3.1 High pressure sodium lamps Physically high-pressure sodium lamp is characterized by high pressure in the tube. which is responsible for, the properties of light emitted. As a consequence of the neon/argon gas filling within the discharge tube, the lamp output is initially a red glow slowly becoming monochromatic The discharge tube in a high-pressure sodium lamp contains an excess of sodium to give saturated vapour conditions when the lamp is running. Mercury is also present to provide a buffer gas and xenon is included to facilitate ignition and limit heat conduction 18 from discharge and to tube wall. The discharge tube is housed in an evacuated protective glass envelope. They are available with luminious efficacies upto 120 Lumen/watt.They can be used for railway yards where colour rendering is not important. The sodium vapour lamps are now being replaced by T5 lamps and metal halide lamps in railway stations, important railway yards, street lighting, colony lighting etc due to poor colour rendering index Ra of 23 Ignition and run up The high pressure sodium lamp must be ignited by a high voltage pulse, typically 1.8 KV to 5 KV depending on lamp type and wattage. Once ignition has taken place, the build up to normal operating pressure and full light output takes several minutes. Re-ignition Should the lamp extinguish because of an interruption to the mains supply, the lamp will first have to cool down enough for the vapour pressure in the discharge tube to be such that the sodium atoms can again be ionised by the ignition pulse, and this takes about one minute. 2.4 LED Lamps Like a normal P-N junction diode the LED consists of a chip of semi-conducting material doped with impurities. When voltage is impressed across the junction, current flows from the p-side or anode to the n-side or cathode but not in the reverse direction. Charge carriers, electrons and holes flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of photon. The wavelength of the light emitted, and therefore its color depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used should have a direct band gap with energies corresponding to near – infrared, visible or near ultraviolet light. A diagram indicating the working of LED is as follows. LED development began with infrared devices made with Gallium Arsenide. Advances in material science have made possible the production of devices with ever shorter 19 wavelengths, producing light in a variety of colors. LED’s are usually built on an n-type substrate, with an elecrode attached to the p-type layer deposited on its surface. P-type substrates are also used but less common. Blue LEDs are based on the wide band gap semiconductors Gallium nitride (GaN) and Indium Gallium Nitride (InGaN ). One way to produce white light is to use individual LEDs that emit three primary colors red, green and blue and then mix all the colors to produce white light. This method is involved with electro-optical design to control blending and diffusion of different colours. The other method is to use phosphor material coating to convert the ultraviolet or blue light of LED to broad spectrum white light similar to fluorescent lamp. The phosphor based LED method involves coating an LED of one colour (mostly the blue LED made of InGaN) with phosphorof different colours to produce white light. A fraction of the blue light is re-radiated by the phosphor coating to light of longer wave lengths. When several phosphor layers of different colours are applied, the emitted spectrum is broadened effectively increasing the CRI value of the LED. The phosphor method is the most popular technique for manufacture of high intensity white LEDs. One of the key advantages of LED lighting is its high efficiency as measured by light output per watt. Research has made it possible to commercially produce LEDs with efficiency of about 130 lumens per watt which is better than that of any other light source. It should be noted that high power LEDs are necessary for practical general lighting applications. High power (one watt) LEDs have been developed for practical general lighting operations. LEDs have several advantages over other light sources. The main advantages are : 1) Long burning life of more than 60,000 burning hours 2) High luminous efficacy of 120 lumens per watt 3) Wide variety of color LEDs have been developed which are suitable for canopy and façade lighting However when power LEDs are used suitable design of thermal management should be made so that the junction temperature increase is kept within specified limits. The LED should have a proper electronic driver circuit to drive the LEDs with constant current. If it is not driven by constant current the light output will depreciate when the LED burns for more time and will not give the desired performance. Currently the price of LED luminaries are still high. Recently street light fittings of 36 watt and 60 watt have been developed by reputed manufacturers which have good potential for street and colony lighting. Continuous research is in progress in various countries in the field of LED lighting and the future for LED lighting is indeed very bright. 20