Incandescent light bulb

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"Light bulb" redirects here. For similar devices, see Lamp (electrical component).

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. 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 (the "envelope") 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.

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 (one or more contacts at the tip, one at the shell), a bayonet base (one or more contacts on the base, shell used as a contact or only used as a mechanical support), 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 nonuniform 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 lightbulb. [link]

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 tradeoffs involved in setting a lamp life specification.

Glass bulb (or "envelope") Low pressure inert gas Tungsten filament Contact wire (goes to foot) Contact wire (goes to base) Support wires Glass mount/support Base contact wire Screw threads Insulation Electrical foot contact

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 (the first to market the device successfully). Some sources indicate that Heinrich Göbel built the first functional bulbs three decades earlier. He later challenged Edison's patent while living in the United States, but his legal "interference case" was overruled in court. 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
ImageSize = width:280 height:750 PlotArea = width: 240 height:700 left:40 bottom:25 DateFormat = yyyy Period = from:1801 till:1910 TimeAxis = orientation:vertical order:reverse ScaleMajor = unit:year increment:25 start:1801 ScaleMinor = unit:year increment:5 start:1801 PlotData= at:1910 text:"William David Coolidge -\nTungsten filament" at:1882 text:"Lewis Latimer - Better filament production" at:1880 text:"Thomas Edison - Long lasting filament" at:1875 text:"Warren & Evans - Gas filled ""globe"" at:1873 text:"Joseph Wilson Swan - Carbon fiber filament" at:1854 text:"Heinrich Göbel -\nCarbonised bamboo filament" at:1841 text:"Frederick de Moleyns -\nPowdered charcoal filament" at:1840 text:"Warren De la Rue -\nVacuum tube enclosure" at:1809 text:"Sir Humphry Davy -\nCarbon arc ""Davy lamp"" at:1801 text:"Sir Humphry Davy - Platinum filament"

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. In 1809 he 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.

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 publically demonstrated the first perfected incandescent lightbulbs on his estate in Blois, France. His lightbulbs are on permanent display in the museum of the Chateau of Blois.

In 1854, the German inventor Heinrich Göbel developed the first 'modern' light bulb: a carbonized bamboo filament, in a vacuum bottle to prevent oxidation. In the following five years he developed what many call the first practical light bulb.

Joseph Wilson Swan (1828–1914) was a physicist and chemist born in Sunderland, England. In 1850 he began working with carbonized 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 utilized 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 commercialize 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 (the first successful test was on October 21 1879; it lasted 13.5 hours). Edison continued to improve this design and by 1880 had the patent for a lamp that could last over 1200 hours using a carbonized 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 (later known as Ediswan, which was then incorporated into Thorn Lighting Ltd). 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 (1987, 115-117) 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 centralized 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." (Hughes 1977, 9)

It is an interesting side note that one of Edison's successful marketing ploys is still with us today. In trying to get people to adopt his lighting system, he designed it to be compatible with the fixtures already present in households. That is why Edison designed light bulbs that screwed in; they needed to fit into existing gaslamp fixtures -- which which had to screw together to provide an airtight seal.

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. (Some of Edison's experiments to stop this blackening led to the invention of the electronic vacuum tube.) 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 (1873–1975) 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. This type of incandescent lamp creates an equilibrium reaction in which the tungsten filament that evaporates when giving off light is chemically 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 (as e.g. by means of optical coating) 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 (hard glass blocks UV without need of dopants).

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 (IRC). 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 neighboring 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 (about 20% efficiency), 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 (see the section on halogen lamps above). 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 (or lamp base) 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 (E26), 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. (E27 also common in Australia and the UK.)

In each designation, the E stands for Edison, who created the screw-base lamp, and the number is the diameter in millimeters. (This is even true in North America, where designations for the actual bulb glass diameter are in eighths of an inch.) 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 (MES) in North America, E27 in Europe
• mogul: E39 North America, E40 in Europe).
• There is also a rare "admedium" size (E29), and a very miniature size (E5) 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 (such as a 100/200/300-watt three-way) 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 (push-twist) 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 (or GY4). Some common sizes include G4 (4 mm), G6.35 (6.35 mm), G8 (8 mm), GY8.6 (8.6 mm), G9 (9 mm), and GY9.5 (9.5 mm). The second letter (or lack thereof) 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 (such as 82), 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 (W)	Output (lm)	Efficiency (lm/W)
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 (and in fact against electrical codes) 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 cost of electricity in the United States ranges from $0.07 to $0.13 per kilowatt-hour (kWh).)

The following shows how to calculate total cost of electricity for using an incandescent light bulb vs. a compact fluorescent light bulb. (Also note that 1 kWh = 1000 Wh).

Electricity Cost (for 800-900 lumens at a rate of Electricity Cost (for 800-900 lumens at a rate of Electricity Cost (for 800-900 lumens at a rate of Electricity Cost (for 800-900 lumens at a rate of Electricity Cost (for 800-900 lumens at a rate of Electricity Cost (for 800-900 lumens at a rate of Electricity Cost (for 800-900 lumens at a rate of \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10 per kWh) [\left( 60 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ ] [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ .20 ] .10 per kWh) [\left( 60 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ ] [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ .20 ] .10 per kWh) [\left( 60 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ ] [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ .20 ] .10 per kWh) [\left( 60 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ ] [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ .20 ] .10 per kWh) [\left( 60 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ ] [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ .20 ] .10 per kWh) [\left( 60 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ ] [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ .20 ] .10 per kWh)
	[\left( 60 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ ]
	[\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $48 ] |-valign=middle | || [\ \left( 14 \ watts \right) \times \left( 8000 \ hours \right) \times \left( \frac \right) = \ $11.20 ] |}.10} \right) = \ .20 ]

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 (potential) cost may be incurred if the bulbs are not in a readily accessible location and special equipment (e.g., cherry picker) 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 V^3.4
• Power consumption is approximately proportional to V^1.6
• Lifetime is approximately inversely proportional to V¹⁶
• Color temperature is approximately proportional to V^0.42

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 (for example, traffic lights or fixtures hung from high ceilings). So-called "long-life" bulbs are simply bulbs that take advantage of this tradeoff.

According to the relationships above (which are probably not accurate for such extreme departures from nominal ratings), 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 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 (presumably apart from power outages). 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 [link].

In flood lamps used for photographic lighting, the tradeoff is made in the other direction. Compared to general service bulbs, for the same power, these bulbs produce far more light, and (more importantly) light at a higher color temperature, at the expense of greatly reduced life (which may be as short as 2 hours for a type P1 lamp). 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 (90 °F) 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 tradeoff 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 (for example, 12 volts) 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 (power). It is measured in lumens per watt (lm/W). 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[link]. 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 idealized light sources. A similar chart in the article on luminous efficacy compares a broader array of light sources to one another.

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 (below 3683 kelvins). Donald L. Klipstein explains it this way: "An ideal thermal radiator produces visible light most efficiently at temperatures around 6300 °C (6600 K or 11,500 °F). 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." No known material can be used as a filament at this ideal temperature, which is hotter than the sun's surface.

See also

• Fluorescent light
• Lightbulb jokes
• Light-emitting diode (LED)
• List of light fixture manufacturers in the United States
• List of light sources
• Over-illumination
• Thomas Edison

Notes

External links, references, resources

Wikimedia Commons has media related to:

[Special]

• [Howstuffworks - "How Light Bulbs Work"]
• [Energy Efficient Light Bulbs]
• [Livermore bulb and North Fort Worth Historical Society bulb article]
• [Livermore Bulb Webcam - light bulb on since 1901]
• [How lightbulbs are made]
• Edward J. Covington's [Early Incandescent Lamps]
• [Great Internet Light Bulb Book]
• [Technical Information on Lamps]
• Kruger, Anton, "[When Can LEDs Replace Incandescent Lamps]"?
• [Kilokat's Antique Light Bulb Site, photos-history-discussion]
• [Energy Saving Light Bulb Caculator]
• [Worth Knowing?! — Technology: Bulb]
• [The Great Internet Light Bulb Book, Part I, by Donald L. Klipstein]
• http://www.mis-bombillas.com/ Little "Virtual Museum of Electric Lamps", from Spain.
• [Did Thomas Edison really invent the light bulb?]
• 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.
• [Lighting Efficiency Information & some general tips on lighting your home.]
• ["Operation removes lightbulb from anus"], Thursday, June 29, 2006 7:34 a.m. ET, Reuters.com

L i g h t i n g   and   L a m p s
Incandescent:	Incandescent light bulb>Conventional - Halogen - Parabolic Aluminized Reflector (PAR)
Fluorescent:	Compact fluorescent lamp>Compact Fluorescent (CFL) - Linear fluorescent - Induction lamp
Gas discharge:	High-intensity discharge lamp>High-Intensity Discharge (HID) - Mercury-vapor - Metal-halide - Neon - Sodium vapor
Electric arc:	Arc lamp - Hydrargyrum medium-arc iodide>HMI - Xenon arc - Yablochkov candle
Combustion:	Carbide lamp>Acetylene/Carbide - Candle - Gas lighting - Kerosene lamp - Limelight - Oil lamp - Safety lamp
Other types:	Sulfur lamp - Light-emitting diode>Light-Emitting Diode (LED) - Optical fiber - Plasma

 

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