Encyclopedia
The
incandescent light bulb or
incandescent lamp is a source of artificial
light that works by
incandescence. An electric current passes through a thin
filament, heating it and causing it to emit light. The enclosing glass bulb prevents the oxygen in air from reaching the hot filament, which would be otherwise rapidly destroyed by oxidation.
Incandescent bulbs are also called
electric lamps, extending the use of a term applied to the original
arc lamps, and in
Australia and
South Africa they are also called
light globes or more commonly
light bulbs.
A benefit of the incandescent bulb is that they can be produced for a wide range of
voltages, from a few volts to several hundred volts.
Because of their relatively poor
luminous efficacy, incandescent light bulbs are gradually being replaced in many applications by
fluorescent lights,
high-intensity discharge lamps,
LEDs, and other devices.
Operation
The incandescent light bulb consists of a
glass enclosure which either contains a
vacuum or is filled with a low-pressure noble gas.
Irving Langmuir found that filling the bulb with an inert gas reduces evaporation of the filament and reduces the required strength of the glass. Inside of the bulb is a
filament of tungsten wire, through which an electrical current is passed. As the electrons that travel through the filament bump into the atoms, some of the electrons in the atom may become
excited. This means they temporarily boost its energy level and raise to higher orbit. When they fall back, energy is released as
photons, a photon being the particle form of
light. A great deal of lower-energy electromagnetic radiation is released as
infrared as well—more, in fact, than is released as visible light. This manifests as heat. The common lightbulb, though it does not perfectly conform to the expected light emission spectrum, can nonetheless be considered as a simple
blackbody emitter.
Incandescent light bulbs usually also contain a glass mount on the inside, which supports the filament and allows the electrical contacts to run through the envelope without gas/air leaks. Many arrangements of electrical contacts are used, such as a screw base , a bayonet base , and for some lamps an electrical contact at either end of a tubular lamp. Contacts in the lamp socket allow the electrical current to pass through the filament. Power ratings range from about 0.1 watt to about 10,000 watts.
To improve the efficacy of the lamp, the filament usually consists of coils of fine wire. For a 60 watt 120-volt lamp, the length of the filament is usually 2 meters or 6.5 feet.
One of the major problems of the standard electric light bulb is evaporation of the filament. The inevitable variations in resistivity along the filament cause non-uniform heating, with "hot spots" forming at points of higher resistivity. Thinning by evaporation increases resistivity. But hot spots evaporate faster, increasing their resistivity faster—a positive feedback which ends in the familiar tiny gap in an otherwise healthy-looking filament.
Irving Langmuir suggested that an inert gas, instead of vacuum, would retard evaporation and still avoid combustion, and so ordinary incandescent light bulbs are now filled with
nitrogen,
argon, or
krypton. However, a filament breaking in a gas-filled bulb can pull an
electric arc, which may spread between the terminals and cause very heavy current flow; intentionally thin lead-in wires or more elaborate protection devices are therefore often used as fuses built into the light bulb.
During ordinary operation, the tungsten of the filament evaporates; hotter, more-efficient filaments evaporate faster. Because of this, the lifetime of a filament lamp is a trade-off between efficiency and longevity. The trade-off is typically set to provide a lifetime of 750-1000 hours for ordinary lamps. See the section below,
Voltage, light output, and life, for a discussion of the trade-offs involved in setting a lamp life specification.
In a conventional lamp, the evaporated tungsten eventually condenses on the inner surface of the glass envelope, darkening it. For bulbs that contain a vacuum, the darkening is uniform across the entire surface of the envelope. When a filling of inert gas is used, the evaporated tungsten is carried in the thermal convection currents of the gas, depositing preferentially on the uppermost part of the envelope and blackening just that portion of the envelope.
Some old, high-powered lamps used in theatre, projection, searchlight, and lighthouse service with heavy, sturdy filaments contained loose tungsten powder within the envelope. From time to time, the operator would remove the bulb and shake it, allowing the tungsten powder to scrub off most of the tungsten that had condensed on the interior of the envelope, removing the blackening and brightening the lamp again.
When a light bulb envelope breaks while the lamp is on or if air leaks into the envelope, the hot tungsten filament reacts with the air, yielding an aerosol of brown tungsten nitride, brown tungsten dioxide, blue-violet tungsten pentoxide, and yellow tungsten trioxide which then deposits on the nearby surfaces or the bulb interior.
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- Glass bulb
- Low pressure inert gas
- Tungsten filament
- Contact wire
- Contact wire
- Support wires
- Glass mount/support
- Base contact wire
- Screw threads
- Insulation
- Electrical foot contact
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History of the light bulb
While conversion of electrical energy to light was demonstrated in laboratories as early as 1801, it took more than 100 years for the modern form of electric light bulb to be developed, with the contributions of many inventors.
The invention of the light bulb is usually attributed in Britain to
Joseph Wilson Swan and in the United States to
Thomas Alva Edison .
Alexander Nikolayevich Lodygin independently developed an incandescent light bulb in 1874. Many others also had a hand in the development of a practical device for the production of electric light.
| Early evolution of the light bulb |
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In 1801 Sir
Humphry Davy, an
English chemist, made
platinum strips glow by passing an electric current through them, but the strips
evaporated too quickly to make a useful light source. The problem of the filament burning out after a few minutes, and the low resistance and high current draw made incandescent lamps a failure in practical terms until the developments by Edison and Swan in the 1870's.
In 1809 Davy created the first
arc lamp by creating a small but blinding electrical connection between two
charcoal rods connected to a battery. Demonstrated to the Royal Institution of Great Britain in 1810, the invention came to be known as the
Arc lamp.
In 1835 James Bowman Lindsay demonstrated a constant electric light at a public meeting in Dundee, Scotland. He stated that he could "read a book at a distance of one and a half feet". However, having perfected the device to his own satisfaction, he turned to the problem of wireless telegraphy and did not develop the electric light any further. His claims are not well documented.
In 1840, a British scientist Warren de la Rue enclosed a platinum
coil in a
vacuum tube and passed an
electric current through it. The design was based on the concept that the high melting point of platinum would allow it to operate at high temperatures and that the evacuated chamber would contain less gas molecules to react with the platinum, improving its longevity. Although it was an efficient design, the cost of the platinum made it impractical for commercial use.
In 1841 Frederick de Moleyns of England was granted the first patent for an incandescent lamp, with a design using powdered
charcoal heated between two platinum wires contained within a vacuum bulb.
In 1851
Robert Houdin publicly demonstrated an incandescent light bulbs on his estate in Blois, France. His light bulbs are on permanent display in the museum of the Chateau of Blois.
In 1893, the German inventor
Heinrich Göbel claimed he had developed the first light bulb in 1854: a carbonised
bamboo filament, in a vacuum bottle to prevent oxidation, and that in the following five years he developed what many call the first practical light bulb. In a patent interference suit in 1893, the judge ruled that his claim was "extremely improbable."
Joseph Wilson Swan was a physicist and chemist born in Sunderland, England. In 1850 he began working with carbonised paper filaments in an evacuated glass bulb. By 1860 he was able to demonstrate a working device but the lack of a good vacuum and an adequate supply of electricity resulted in a short lifetime for the bulb and an inefficient source of light. By the mid-1870s better pumps became available, and Swan returned to his experiments. Swan received a
British patent for his device in 1878. Swan reported success to the Newcastle Chemical Society, and at a lecture in
Newcastle in February 1873 he demonstrated a working lamp that utilised a
carbon fiber filament, but by 1877 he had turned to slender rods of
carbon. The most significant feature of Swan's lamp was that there was little residual
oxygen in the vacuum tube to ignite the filament, thus allowing the filament to glow almost white-hot without catching fire. From this year he began installing light bulbs in homes and landmarks in England, and by the early 1880s he had started his own company.
Across the
Atlantic, parallel developments were also taking place. On July 24 1874 a
Canadian patent was filed for the Woodward and Evans Light by a
Toronto medical electrician named Henry Woodward and a colleague
Mathew Evans. They built their lamps with different sizes and shapes of
carbon filaments held between electrodes in glass globes filled with
nitrogen. Woodward and Evans attempted to commercialise their bulb, but were unsuccessful. Nonetheless, Thomas Edison considered their approach sufficiently promising that he bought the rights to both their Canadian and US patents before embarking on his own light bulb development program.
After many experiments with platinum and other metal filaments, Edison returned to a
carbon filament . Edison continued to improve this design and by 1880 had the patent for a lamp that could last over 1200 hours using a carbonised bamboo filament. Edison and his team did not find this commercially viable filament until more than 6 months after Edison filed the patent application.
In January 1882, Lewis Latimer received a patent for the "Process of Manufacturing Carbons", an improved method for the production of light bulb filaments which was purchased by the United States Electric Light Company.
In Britain, the Edison and Swan companies merged into the Edison and Swan United Electric Company . Edison was initially against this combination, but was eventually forced to cooperate, and the merger was made. Eventually, Edison acquired all of Swan's interest in the company. Swan sold his United States patent rights to the Brush Electric Company in June 1882. Swan later wrote that Edison had a greater claim to the light than he, in order to protect Edison's patents from claims against them in the US.
The
United States Patent Office gave a ruling October 8, 1883 that Edison's patents were based on the prior art of William Sawyer and were invalid. Litigation continued for a number of years. Eventually on October 6, 1889, a judge ruled that Edison's electric light improvement claim for "a filament of carbon of high resistance" was valid.
In addressing the question "Who invented the incandescent lamp?" historians Robert Friedel and Paul Israel list 22 inventors of incandescent lamps prior to Swan and Edison. They conclude that Edison's version was able to outstrip the others because of a combination of factors: an effective incandescent material, a higher vacuum than others were able to achieve and a high resistance lamp that made power distribution from a centralised source economically viable. Another historian, Thomas Hughes, has attributed Edison's success to the fact that he invented an entire, integrated system of electric lighting. "The lamp was a small component in his system of electric lighting, and no more critical to its effective functioning than the Edison Jumbo generator, the Edison main and feeder, and the parallel-distribution system. Other inventors with generators and incandescent lamps, and with comparable ingenuity and excellence, have long been forgotten because their creators did not preside over their introduction in a system of lighting."
In the 1890s the Austrian inventor
Carl Auer von Welsbach worked on
metal-filament mantles, first with
platinum wiring, and then osmium, and produced an operative version in 1898.
In 1897, German physicist and chemist
Walther Nernst developed the
Nernst lamp, a form of incandescent lamp that used a ceramic globar and did not require enclosure in a vacuum or inert gas. Twice as efficient as carbon filament lamps, Nernst lamps were briefly popular until overtaken by lamps using metal filaments.
In 1903, Willis Whitnew invented a filament that would not blacken the inside of a light bulb. It was a metal-coated carbon filament. In 1906, the
General Electric Company was the first to patent a method of making tungsten filaments for use in incandescent light bulbs. The filaments were costly, but by 1910 William David Coolidge had invented an improved method of making tungsten filaments. The tungsten filament outlasted all other types of filaments and Coolidge made the costs practical.
The halogen lamp
One invention that addressed the problem of short lamp life was the
halogen lamp, also called the
tungsten-halogen lamp, the
quartz-halogen lamp or the
quartz-iodine lamp, wherein a tungsten filament is sealed into a small envelope filled with a
halogen gas such as iodine or bromine. In an ordinary incandescent lamp, the thickness of the filament may vary slightly. The resistance of the filament is higher at the thinner portions which causes the thin areas to be hotter than the thicker parts of the filament. The rate of tungsten evaporation will be higher at these points due to the increased temperature, causing the thin areas to become even thinner, creating a runaway effect until the filament fails. A tungsten-halogen lamp creates an equilibrium reaction in which the tungsten that evaporates when giving off light is preferentially re-deposited at the hot-spots, preventing the early failure of the lamp. This also allows halogen lamps to be run at higher temperatures which would cause unacceptably short lamp lifetimes in ordinary incandescent lamps, allowing for higher luminous efficacy, apparent brightness, and
whiter color temperature. Because the lamp must be very hot to create this reaction, the halogen lamp's envelope must be made of hard glass or fused quartz, instead of ordinary soft glass which would soften and flow too much at these temperatures.
The envelope material can be selected and modified to achieve whatever lamp characteristics are required. Halogen bulbs are widely used in automobile
headlamps, for example, and because headlamps often contain plastic parts, halogen headlamp bulbs' envelopes are made out of hard glass or quartz that has been 'doped' with additives to block most of the
UV output .
Conversely, some applications
require ultraviolet light, and in such cases, the lamp envelope is made out of undoped quartz. Thus, the lamp becomes a source of
UV-B light. Undoped quartz halogen lamps are used in some scientific, medical and dental instruments as a UV-B light source.
A typical halogen lamp is designed to run about 2000 hours, twice as long as a typical ordinary incandescent lamp.
Halogen infrared
A further development that has added to halogen lamp efficacy is an infrared-reflective coating . The quartz envelope is coated with a multi-layered
dichroic coating which allows visible light to be emitted while reflecting a portion of the
infrared radiation back onto the filament. Such lamps are called
halogen-infrared lamps, and they require less power than standard halogen lamps to produce any given light output. The efficiency increase can be as much as 40% when compared to its standard equivalent.
Safety
Because the halogen lamp operates at very high temperatures, it can pose
fire and burn hazards. Additionally, it is possible to get a
sunburn from excess exposure to the
UV light emitted by an undoped
quartz halogen lamp. To mitigate the negative effects of unintentional UV exposure, and to contain hot bulb fragments in the event of explosive bulb failure, manufacturers of lamps intended for general-purpose usage usually install UV-absorbing glass filters over or around the bulb. Alternatively, they may add a coating of UV inhibitors on the bulb envelope that effectively filters UV radiation. When this is done correctly, a halogen lamp with UV inhibitors will produce less UV than its standard incandescent counterpart.
Handling precautions
Any surface contamination, notably fingerprints, can damage the quartz envelope when it is heated, by causing the neighbouring quartz to change from its vitreous form into a weaker,
crystalline form which leaks gas. Consequently, quartz lamps should be handled without touching the clear quartz, either by using a clean paper towel or carefully holding the porcelain base. If the quartz is contaminated in any way, it must be thoroughly cleaned with rubbing alcohol and dried before use.
Applications and popularity
The incandescent lamp is still widely used in domestic applications, and is the basis of most portable lighting, such as table lamps, some car
headlamps and electric
flashlights. Halogen lamps have become more common in auto
headlamps and domestic situations, particularly where light is to be concentrated on a particular point. The
fluorescent light has, however, replaced many applications of the incandescent lamp with its superior life and energy efficiency.
LED lights are beginning to see increased home and auto use, replacing incandescent lamps.
Efficiency and alternatives
Approximately 95% of the power consumed by an incandescent light bulb is emitted as
heat, rather than as visible
light. An incandescent light bulb, with this ~5% efficiency, is about one quarter as efficient as a
fluorescent lamp , and produces about six times as much heat with the same amounts of light from both sources. One reason why incandescent lamps are unpopular in commercial spaces is that the heat output results in the need for more
air conditioning in the summer. Incandescent lamps can usually be replaced by self-ballasted
compact fluorescent light bulbs, which fit directly into standard sockets. This lets a 100 W incandescent lamp be replaced by a 23-watt fluorescent bulb, while still producing the same amount of light.
Quality halogen incandescents are closer to 9% efficiency, which will allow a 60 W bulb to provide nearly as much light as a non-halogen 100 W. Alternatively, the higher halogen lamp can be designed to produce the same amount of light as a 60 W non-halogen lamp, but with much longer life. However, small halogen lamps are often still high-power, causing them to get extremely hot. This is both because the heat is more concentrated on the smaller envelope surface, and because the surface is closer to the filament. This high temperature is essential to their long life . Left unprotected, these can cause fires much more easily than a regular incandescent, which may only scorch easily inflammable objects such as drapery. Most safety codes now require halogen bulbs to be protected by a grid or grille, or by the glass and metal housing of the fixture. Similarly, in some areas halogen bulbs over a certain power are banned from residential use.
Standard fittings
Most domestic and industrial light bulbs have a metal fitting compatible with standard threaded sockets. The most common types of fitting are:
- Candelabra screw base, used in nightlights and Christmas lights, and by some halogen bulbs.
- MES or medium Edison screw , used in the USA and Japan for most 120 and 100-volt lamps. A slight variant of this base, E27, is used in Europe and elsewhere in the world with 220-240V household voltage.
- BC or B22 or double-contact bayonet cap, used in Australia, Ireland, New Zealand and the UK for most 220–240V mains lamps and is used in the US for certain 120V lamps in appliances such as sewing machines and vacuum cleaners.
In each designation, the E stands for Edison, who created the screw-base lamp, and the number is the
diameter in millimeters. There are four standard sizes of screw-in sockets used for line-voltage lamps:
- candelabra: E12 North America, E10 & E11 in Europe
- intermediate: E17 North America, E14 in Europe
- medium or standard: E26 in North America, E27 in Europe
- mogul: E39 North America, E40 in Europe).
- There is also a rare "admedium" size , and a very miniature size generally used only for low-voltage applications such as with a battery.
The largest size is now only used in large
street lights, however a few high-wattage household lamps used this at one point. MES bulbs for 12 volts are also produced for
recreational vehicles. Large outdoor Christmas lights use an intermediate base, as do some desk lamps and many
microwave ovens. Emergency exit signs also tend to use the intermediate base.
Bulbs with a bayonet base, for use with sockets having spring-loaded base plates, are produced in similar sizes and are given a B or BA designation. These are also extremely common in 12-volt
automobile lighting worldwide, in addition to wedge-base ones which have a partial plastic or even completely glass base. In this case, the wires wrap around to the outside of the bulb, where they press against the contacts in the socket. Miniature Christmas bulbs use a plastic wedge base as well.
Halogen bulbs are available with a standard fitting, but also come with a pin base, with two contacts on the underside of the bulb. These are given a G or GY designation, with the number being the centre-to-centre distance in millimeters. For example, a 4 mm pin base would be indicated as G4 . Some common sizes include G4 , G6.35 , G8 , GY8.6 , G9 , and GY9.5 . The second letter indicates pin diameter. Some spotlights or
floodlights have pins that are broader at the tips, in order to lock into a socket with a twist. Other halogen bulbs come in a tube, with blades or dimples at either end.
Fluorescent tubes use a different set of pins, but self-
ballasted
compact fluorescents are available in both medium and candelabra-base bulbs, intended to replace incandescents.
There are also various odd fittings for projectors and
stage lighting instruments. Projectors, in particular, may run on odd voltages , perhaps intended as a vendor lock-in.
General Electric introduced standard fitting sizes for tungsten incandescent lamps under the
Mazda trademark in 1909. This standard was soon adopted across the United States, and the Mazda name was used by many manufacturers under license through 1945.
Power
Power
| Output
| Efficiency
|
| 15 | 100 | 6.7 |
| 25 | 200 | 8.0 |
| 34 | 350 | 10.3 |
| 40 | 500 | 12.5 |
| 52 | 700 | 13.5 |
| 55 | 800 | 14.5 |
| 60 | 850 | 14.2 |
| 67 | 1000 | 15.0 |
| 70 | 1100 | 15.7 |
| 75 | 1200 | 16.0 |
| 90 | 1450 | 16.1 |
| 95 | 1600 | 16.8 |
| 100 | 1700 | 17.0 |
| 135 | 2350 | 17.4 |
| 150 | 2850 | 19.0 |
| 200 | 3900 | 19.5 |
| 300 | 6200 | 20.7 |
| | |
Incandescent light bulbs are usually marketed according to the
electrical power consumed. This is measured in watts and depends mainly on the
resistance of the filament, which in turn depends mainly on the filament's length, thickness and material. It is difficult for the average consumer to predict the light output of a bulb given the power consumed but it can be safely assumed, for two bulbs of the same type, that the higher-powered bulb is brighter.
Light output ratings are given in lumens, although most buyers do not check for this. Some manufacturers engage in deceptive advertising, such that the claimed "long" bulb life is achievable at normal household voltages, but the claimed light output is only attainable at a higher voltage which does not normally exist, such as 130 volts in the United States.
The table to the right shows the approximate typical output, in lumens, of standard incandescent light bulbs at various powers. Note that the lumen values for "soft white" bulbs will generally be slightly lower than for standard bulbs at the same power, while clear bulbs will usually emit a slightly brighter light than correspondingly-powered standard bulbs.
Also note that the 34, 52, 67, 90 and 135 watt bulbs in the chart are listed for use at 130 volts. Since it is impossible to get 130 volts from any normal mains, these typically run at a more realistic 115 volts in North America. By dropping the voltage by at least 10%, the current also drops by the same amount, reducing the actual wattage by about 20%. This in turn reduces the light output by even more than 20%, but also increases the bulb's service life a great deal, well over 20%. This is the concept of the "long-life bulb".
Comparison of electricity cost
A kilowatt-hour is a unit of
energy, and this is the unit in which
electricity is purchased.
The following shows how to calculate total cost of electricity for using an incandescent light bulb vs. a
compact fluorescent light bulb. .
The average lifetime of incandescent light bulbs is about 750–1000 hours. It would take at least 6-11 incandescent bulbs to last as long as one compact fluorescent, which have an average lifetime between 11,250 and 15,000 hours. This causes an additional total cost of using incandescent bulbs. Another additional cost may be incurred if the bulbs are not in a readily accessible location and special equipment and/or personnel are needed to replace it.
Voltage, light output, and lifetime
Incandescent lamps are very sensitive to changes in the supply voltage. These characteristics are of great practical and economic importance. For a supply voltage
V,
- Light output is approximately proportional to V3.4
- Power consumption is approximately proportional to V1.6
- Lifetime is approximately inversely proportional to V16
- Color temperature is approximately proportional to V0.42
This means that 5% reduction in operating voltage will double the life of the bulb, at the expense of reducing its light output by 20%. This may be a very acceptable tradeoff for a light bulb that is a difficult-to-access location . So-called "long-life" bulbs are simply bulbs that take advantage of this tradeoff.
According to the relationships above , operating a 100-watt, 1000-hour, 1700-lumen bulb at half voltage would extend its life to about 65,000,000 hours or over 7000 years – while reducing light output to 160 lumens, about the equivalent of a normal 15 watt bulb. The
Guinness Book of World Records, known until 2000 [i] as
The Guinness Book of Records is a referenc...
states that a
fire station in
Livermore, California has a light bulb that is said to have been burning continuously for over a century since 1901 . However, the bulb is powered by only 4 watts. A similar story can be told of a 40-watt bulb in
Texas which has been illuminated since September 21, 1908. It once resided in an opera house where notable celebrities stopped to take in its glow, but is now in an area
museum .
In flood lamps used for
photographic lighting, the trade-off is made in the other direction. Compared to general service bulbs, for the same power, these bulbs produce far more light, and light at a higher color temperature, at the expense of greatly reduced life . The upper limit to the temperature at which metal incandescent bulbs can operate is the melting point of the metal. Tungsten is the metal with the highest melting point. A 50-hour-life projection bulb, for instance, is designed to operate only 50
°C below that melting point.
Lamps also vary in the number of support wires used for the tungsten filament. Each additional support wire makes the filament mechanically stronger, but removes heat from the filament, creating another trade-off between efficiency and long life. Many modern 120 volt lamps use no additional support wires, but lamps designed for "rough service" often have several support wires and lamps designed for "vibration service" may have as many as five. Lamps designed for low voltages generally have filaments made of much heavier wire and do not require any additional support wires.
Luminous efficacy and efficiency
A light can waste power by emitting too much light outside of the
visible spectrum. Only visible light is useful for illumination, and some wavelengths are perceived as brighter than others. Taking this into account,
luminous efficacy is a ratio of the useful power emitted to the total radiant flux . It is measured in lumens per watt . The maximum efficacy possible is 683 lm/W. Luminous
efficiency is the ratio of the luminous
efficacy to this maximum possible value. It is expressed as a number between 0 and 1, or as a percentage. However, the term
luminous efficiency is often used for both quantities.
Two related measures are the
overall luminous efficacy and
overall luminous efficiency, which divide by the total power input rather than the total radiant flux. This takes into account more ways that energy might be wasted and so they are never greater than the standard luminous efficacy and efficiency. The term "luminous efficiency" is often misused, and in practice can refer to any of these four measures.
The chart below lists values of overall luminous efficacy and efficiency for several types of incandescent bulb, and several idealised light sources. A similar chart in the article on
luminous efficacy compares a broader array of light sources to one another.
| Type | Overall luminous efficiency | Overall luminous efficacy |
|---|
| 40 W tungsten incandescent | 1.9% | 12.6 |
| 60 W tungsten incandescent | 2.1% | 14.5 00 W tungsten incandescent | 2.6% | 17.5 lass halogen | 2.3% | 16 |
| quartz halogen | 3.5% | 24 |
| high-temperature incandescent | 5.1% | 35 |
| ideal black-body radiator at 4000 K | 7.0% | 47.5 |
| ideal black-body radiator at 7000 K | 14% | 95 deal white light source | 35.5% | 242.5 deal monochromatic 555 nm source | 100% | 683 |
Thus a typical 100 W bulb for 120 V systems, with a rated light output of 1750 lumens, has an overall efficacy of 17.5 lumens per watt, compared to an "ideal" of 242.5 lumens per watt for one type of white light. Unfortunately, tungsten filaments radiate mostly infrared radiation at temperatures where they remain solid . Donald L. Klipstein explains it this way: "An ideal thermal radiator produces visible light most efficiently at temperatures around 6300 °C . Even at this high temperature, a lot of the radiation is either infrared or ultraviolet, and the theoretical luminous efficiency [sic] is 95 lumens per watt."e also
Notes
External links, references, resources
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- Edward J. Covington's
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- Kruger, Anton, ""?
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- Little "Virtual Museum of Electric Lamps", from Spain.
- Friedel, Robert, and Paul Israel. 1987. Edison's electric light: biography of an invention. New Brunswick, New Jersey: Rutgers University Press.
- Hughes, Thomas P. 1977. Edison's method. In Technology at the Turning Point, edited by W. B. Pickett. San Francisco: San Francisco Press Inc., 5-22.
- Hughes, Thomas P. 2004. American Genesis: A Century of Invention and Technological Enthusiasm 1870-1970. 2nd ed. Chicago: The University of Chicago Press.
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