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Pt.2: The Incandescent Light Electric Lighting The development of the electric light took many years and took researchers down many false trails along the way. This month, we look at the early research and describe the different types of incandescent lamps. By JULIAN EDGAR The incandescent lamp is the oldest electric light source still in general use. Early attempts at constructing electric incandescent lights were made in the 1840s and Joseph Swan exper­ imented with carbon-filament evacuated-glass incandescent lights in the 1860s. However, it was Thomas 18  Silicon Chip Edison who made real pro­gress in the years from 1878. Edison understood that for the electric lamp to be success­ ful, he needed to do more than just invent a viable lamp. The organisation of the electricity supply infrastructure was vital to the success of electric light and Edison decided to model much of his approach on the methods used by the gas industry. This meant that he would call his electric lights “burners”, that each “burner” would have a power similar to a standard gas lamp, that each light needed to be independently operable (ie, wired in parallel), and that each consumer’s usage would be recorded on a meter to be read monthly. It was this “big picture” approach that gave Edison a sub­stantial advantage over competitors such as Joseph Swan. Edison’s work on the electric light bulb initially set off in the wrong direction, based as it was on the use of platinum filaments. Platinum was expensive and the temperature at which it becomes incandescent is very close to its melting point. However, he soon rediscovered Swan’s idea of using carbonised fibres, initially thread and then later bamboo. By October 1879, Edison had developed a carbon filament that had a resistance of 140Ω and which would burn for 13 hours. Having convinced himself that somewhere in the world there exist­ed the ideal bamboo for the manufacture of carbonised filaments, Edison despatched agents to Japan, China, the West Indies and Central America. Even the upper reaches of the Amazon were scoured for the best bamboo. All attempts were ultimately unsuc­cessful. Electric lamps using carbonised filaments were the mainstay behind the early commercial success of electric lights but the output of such lamps was relatively low. In 1883, a squirted-cellulose filament was adopted, giving a small but useful in­crease in luminous efficiency. This filament was initially made by forcing a solution of nitrocellulose in acetic acid through a die. This was coagulated in alcohol and the continuous thread that was formed was washed and then de-nitrated with ammonium sulphide. The thread was then carbonised. Incidentally, the research on making filaments in this way later led to the discov­ery of artificial textiles early this century. Even though carbonised filaments had an efficacy of just 1.68 lm/W (general purpose incandescent lamps of today have an efficacy of 8-21.5 lm/W), production was approaching 100,000 lamps per annum by the end of 1882 in England alone. But although the search for a better filament material proved difficult, the characteristics needed of such a material were easy to define: (1) it had to be an electrical conductor with a very high melting point; (2) it had to be relatively cheap; and (3) it had to be relatively easy to work into filamentary form. In 1898, a major breakthrough came with the development of a process for making filaments from osmium. But osmium had a number of disadvantages: it was expensive, its low electrical resistance meant that the lamps could not be run at voltages higher than 44V and up to one metre of wire needed to be coiled within a single lamp! Although the use of Glass-blown lamps use cheap soda-lime glass. Amongst many other types, they are available with an internal reflector (left) and with a pearl finish (right). Pearl lamps use a glass bulb which has been internally etched with acid. osmium persisted for about another decade (sometimes in alloys with other metals), it was eventually overtaken by other metals. Its name lives on, however, in the brand name “Osram”, the trademark of the company which first used osmium. Tungsten filaments The next filamentary material that was tried was tantalum. It was cheaper than osmium and had a higher resistance. However, it was tungsten that really made the electric light a practical proposition. In 1904, two Viennese researchers developed a process for forming tungsten into filaments. The process consisted of evaporating the liquid from a tungsten colloid and then passing a high current through the honeycomb material that had formed. This fused the honeycomb into a pure metal wire. The first tungsten-filament lights appeared on the market in September 1906. While these developments were taking place in Germany and Austria, General Electric in the US developed the General Elec­tric Metallised (GEM) lamp. This used a metal-coated carbon filament. However, it had a lower efficiency than the new metal filament lamps and so was doomed to commercial failure. Early tungsten filaments were fragile and costly. The lamps were packed in cotton wadding for shipment but there was still much filament breakage. This problem was eventually overcome in the period from 19061910 by General Electric scientist Dr Year of Introduction Type Of Filament Initial Efficacy (lm/W) Useful Life (hr) 1881 1.68 600 1884 Carbonised thread of bamboo Squir ted cellulose 3.4 400 1898 Osmium 5.5 1000 1902 Tantalum GEM (metallised carbon) Non-ductile tungsten Ductile tungsten 5 250-700 4 800 7.85 800 10 1000 1904 1904 1910 Fig.1: the sequence of incandescent filament development. (Moralee, D; The Electric Lamp Business in Electronics & Power). December 1997  19 of the water vapour to pick up tungsten particles. However the nitrogen also cooled the filament which in turn reduced the light output. To overcome this problem, a longer coiled filament was used which had proportionally less heat loss. Tungsten incandescent lamps Tungsten halogen lamps use a small bulb so that the tem­perature of the lamp stays high. This is necessary if the evapo­rated tungsten is to be returned to the filament, prolonging its life and reducing bulb blackening. Wil­ liam Coolidge, who developed a process for converting crystalline tungsten into fibrous tungsten. Fibrous tungsten is very ductile (it can be drawn into wire) and has five times the tensile strength of steel. wall. The addition of inert gases such as nitrogen was tried and it was found that this reduced evaporation significantly. The nitrogen formed a blanket around the filament, retarding evaporation and reducing the ability Vacuum pump Because of oxidation, the presence of air within a bulb leads to an extremely short filament life. The early lamp devel­opers had enormous difficulties in evacuating the inside of the bulb but the invention of a vacuum pump in the late 1860s by German Herman Sprengel helped solve this problem. Edison used Sprengel’s pump to evacuate his lamp, noting that it was neces­sary to continue evacuating the bulb as the filament grew hot. This is because residual gases are released from both the fila­ment and the glass bulb as the temperature rises. However, even with a better vac­ uum, tungsten filaments evaporated rapidly, blackening the inside of the bulb and reduc­ing the light output. General Electric scientist Dr Irving Lang­muir discovered that even minute amounts of water vapour (as little as 10 parts per million) inside the bulb greatly increased the amount of tungsten deposited on the bulb 20  Silicon Chip 1 2 3 Fig.2: the principal parts of an incandescent lamp. (1) cap; (2) bulb; (3) filament. (de Boer, J; Interior Lighting). The principal parts of a modern incandescent lamp are shown in Fig.2. The filament consists of coiled ductile tungsten, with some lamps using a “coiled-coil”. A coiled filament presents a smaller effective surface area to the fill gas, thereby reducing heat loss by convection and conduction. The filament is supported by a glass stem, the lead-in wires and by support wires. The lead-in wires on general-purpose lamps are normally in three parts: (1) the upper part to which the filament is pinched or sometimes welded; (2) the central part which forms a vacuum-tight seal with the lead-glass of the stem; and (3) the lower part which often has a reduced melting point so that it acts as a built-in fuse. The wires supporting the filament are often made of molybdenum, as this metal is resilient, displays no affinity for tungsten and reduces heat loss. A glass bulb is necessary to prevent oxygen from coming into contact with the filament. This bulb is filled with argon or an argon and nitrogen mixture. The gas pressure in a general service lamp is about 0.9 atmospheres, rising to about 1.5 at­mospheres when the lamp is operating. The bulbs of most lamps are made from soda-lime glass, the cheapest glass available. These have a maximum bulb temperature rating of 375°C. For lamps that must withstand higher temperatures or temperature shocks, more resistant glasses are used, including pure fused silica for lamps that must meet the highest standards. The inside of the bulb can be treated in various ways to achieve a special effect. For example, it can be frosted to give a pearl lamp by etching the inside of the glass with acid. Anoth­ er treatment known as “opalising” involves coating the inside of the bulb with a mixture of finely powdered silica and titanium dioxide. Clear and pearl lamps have the same Fig.3: the effect of voltage variation on life, luminous effica­cy, power dissipation and luminous flux of an incandescent lamp. (Julian, W; Lighting: Basic Concepts). efficacy, while opalised lamps have 4-8% lower efficacy. Reflector bulbs of the PAR-type (PAR stands for parabolic reflector) are moulded in two pieces from tough, heat-resistant glass. Part of the inside of the bulb has a reflective coating applied to it – usually vaporised silver or aluminium. Because the internal reflector is not subjected to any damage, corrosion or contamination, cleaning is never necessary and a high light output is maintained. Glass-blown bulb reflector lamps (ie, bulbs formed by glass blowing) are available with the reflector at either end of the bulb. They are cheaper than PAR reflector bulbs and have a lower luminous intensity than PAR bulbs of the same power. An enormous range of decorative lamps is also available. Candle-shaped lamps, coloured lamps, box-shaped lamps and so on are widely used. The energy balance of a typical 100-watt general service lamp is shown in Fig.3. Of the 100W of power input, just 5W of visible radiation is produced. Most of the rest is produced as infrared radiation. Infrared radiation from the filament makes up 61W while the bulb produces a further 22W, giving a total in­frared output of 83W. Convection and conduction losses make up the remaining 12W. Theoretically, an incandescent Fig.4: the energy balance of a typical 100 watt general service lamp. Of the 100 watts power input, just 5 watts of visible radiation is produced (source: Philips Lighting Manual). A PAR floodlight is made in two pieces and uses toughened glass to withstand the sudden temperature shocks that occur when it is exposed to rain. Vaporised silver or aluminium is used to form the internal reflector. lamp operating at the melt­ing point of tungsten (3380°C) and having no convection or conduc­ t ion losses could produce a luminous efficacy of 53lm/W. Lamps with a typical rated operating life of 1000 hours have an effica­cy of between 8-21.5lm/W. The colour temperature of a typical incandescent lamp is 2800°K, which means that, compared with the Sun, it has a warm, yellow appearance. However, because the radiation emitted from such a lamp covers the entire visible spectrum, its colour ren­dering ability (Ra of 99-100) is excellent. Lamp life In line with popular belief, frequent switching on and off does reduce lamp life. There are two reasons for this: (1) the very high surge currents at switch-on (typically 10 times the December 1997  21 This 500W double-ended tungsten halogen lamp is designed for use in a domestic floodlight. The same type of lamp as above but here rough handling has brought the filament into contact with the glass, partially melting it. The filament has also broken! lamp rating) cause thermal stresses in the filament; and (2) these high surge currents have associated magnetic forces which can literally blow a weakened filament apart. Mains voltage variations also have a dramatic effect on lamp life. If the lamp is nominally rated at 240 volts, increas­ing the voltage to 250V approximately halves the life of the lamp! However, with that voltage increase, luminous flux rises by 20%, luminous efficacy by 8% and power dissipation by 10%. Fig.4 shows the relationship between these factors. Note that while normal incandes22  Silicon Chip cent lamps can be dimmed, a dimmed light has a lower colour temperature (it is redder than normal) and has a poorer luminous efficacy than an un­ dimmed lamp. In fact, where a lamp is continually dimmed, it is better to replace it with one of a lower wattage. Tungsten halogen lamps Tungsten filament lamps blacken because the high tempera­ture of the filament causes tungsten particles to evaporate off the filament and condense on the relatively cold bulb wall. It was not until 1958 that E. G. Fridrich and E. H. Wiley discovered that adding a halogen gas (originally iodine) to the normal gas filling could increase efficacy and significantly improve lumen maintenance (the lamp stayed brighter for longer). This happens because the added halogen combines with the evaporated tungsten to form a tungsten-halogen compound. Unlike tungsten vapour, the compound stays in the form of a gas if the temperature of the bulb remains above about 250°C. This gas is swept around inside the bulb by convection currents. When it comes near to the incandescent filament, it is broken down by the high temperature, with the tungsten redeposited on the filament and the halogen continuing its role in the regenerative cycle. It has even been suggested (tongue in cheek) that if each tungsten particle could be guided back to the exact spot from which it came, the filament life would be infinite! The operation of a tungsten-halogen bulb is critically dependent on the temperatures of the various parts of the lamp. As indicated, the quartz bulb must be kept above 250°C, while the hermetic seal between the quartz bulb and the molybdenum lead-in wire must be kept below 350°C. Above this temperature, the lead-in wire starts to oxidise, placing mechanical stress on the seal. Furthermore, if the coolest part of the filament is not kept above a critical temperature, corrosion of the filament wire will take place, reducing lamp life. To maintain a high enough wall temperature, the bulb must be smaller than a conventional incandescent lamp. In addition, the bulb is made of quartz or fused silica to withstand such a high temperature. The stronger bulb wall and smaller volume mean that the lamp can be operated at up to several atmospheres of internal gas pressure, thereby reducing the rate of filament evaporation and thus further prolonging the life of the lamp. And why must you never touch a tungsten halogen bulb? The reason is that any finger grease deposits left behind on the quartz envelope will cause the surface to develop fine cracks and this will eventually lead to high-temperature failure. Any con­ tamin­ ation should therefore be cleaned off with methylated spir­its before the lamp is used. Tungsten halogen lamps have several advantages over ordi­ n ary SILICON CHIP This advertisment is out of date and has been removed to prevent confusion. P.C.B. Makers ! If you need: •  P.C.B. High Speed Drill •  P.C.B. Guillotine •  P.C.B. Material – Negative or Positive acting •  Light Box – Single or Double Sided – Large or Small •  Etch Tank – Bubble or Circulating – Large or Small •  U.V. Sensitive film for Negatives •  Electronic Components and Small 12V halogen lamps are often used for spotlighting displays in shops. •  tungsten lamps. These include: (1) a much longer life – up to 3500 hours; (2) typically 10% greater luminous efficacy; (3) compactness; (4) a higher colour temperature of 2800-3200°K; and (5) little or no light depreciation with age. Tungsten halogen lights are available in both mains-powered and 12V forms. Mains lamps are generally of the tubular, double-ended type and are often used for domestic flood lighting. The low voltage types are generally sealed in an exterior parabolic reflector which uses either an aluminium or dichroic multifaceted surface. Dimming of tungsten halogen lights should be avoided be­ cause of the temperature-critical nature of their operation. If a tungsten halogen lamp is dimmed, severe bulb blackening will quickly occur and early filament failure is likely. In part 3 next month, we shall look SC at fluorescent lamps. •  Equipment for TAFEs, Colleges and Schools FREE ADVICE ON ANY OF OUR PRODUCTS FROM DEDICATED PEOPLE WITH HANDS-ON EXPERIENCE Prompt and Economical Delivery KALEX 40 Wallis Ave E. Ivanhoe 3079 Ph (03) 9497 3422 FAX (03) 9499 2381 •  ALL MAJOR CREDIT CARDS ACCEPTED December 1997  23