Incandescent light bulb

"Light bulb" redirects here. For other types, see Lamp (electrical component).

An incandescent light bulb

The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric light that works by incandescence (a general term for heat-driven light emissions which includes the simple case of black body radiation). An electric current passes through a thin filament, heating it until it produces light. The enclosing glass bulb prevents the oxygen in air from reaching the hot filament, which otherwise would be destroyed rapidly by oxidation. Incandescent bulbs are also sometimes called electric lamps, a term also applied to the original arc lamps.

Incandescent bulbs are made in a wide range of sizes and voltages, from 1.5 volts to about 300 volts. They require no external regulating equipment and have a low manufacturing cost, and work well on either alternating current or direct current. As a result the incandescent lamp is widely used in household and commercial lighting, for portable lighting, such as table lamps, car headlamps, flashlights, and for decorative and advertising lighting.

Some applications of the incandescent bulb make use of the heat generated, such as incubators, brooding boxes for poultry, heat lights for reptile tanks, infrared heating for industrial heating and drying processes, and the Easy-Bake Oven toy. In cold weather the heat shed by incandescent lamps contributes to building heating, but in hot climates lamp losses increase the energy used by air conditioning systems.

Incandescent light bulbs are gradually being replaced in many applications by other types of electric light such as (compact) fluorescent lamps, high-intensity discharge lamps, light-emitting diodes (LEDs), and other devices. These newer technologies give more visible light for the same amount of electrical energy input, and so generate much less heat. Some jurisdictions, such as the European Union are in the process of phasing-out the use of incandescent light bulbs in favor of more energy-efficient lighting.

History of the light bulb

Original carbon-filament bulb from Thomas Edison

In addressing the question "Who invented the incandescent lamp?" historians Robert Friedel and Paul Israel list 22 inventors of incandescent lamps prior to Joseph Wilson Swan and Thomas Edison. They conclude that Edison's version was able to outstrip the others because of a combination of three factors: an effective incandescent material, a higher vacuum than others were able to achieve (by use of the Sprengel pump) and a high resistance lamp that made power distribution from a centralized source economically viable.

Due to its significance in altering our lifestyle, light bulbs are often used metaphorically to refer to a great idea or the ingenuity of humanity in general.

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."

Early evolution of the light bulb

Early pre-commercial research

In 1802, Humphry Davy had what was then the most powerful electrical battery in the world at the Royal Institution of Great Britain. In that year, he created the first incandescent light by passing the current through a thin strip of platinum, chosen because the metal had an extremely high melting point. It was not bright enough nor did it last long enough to be practical, but it was the precedent behind the efforts of scores of experimenters over the next 75 years. In 1809, Davy also created the first arc lamp by making a small but blinding electrical connection between two carbon charcoal rods connected to a 2000-cell battery; it was demonstrated to the Royal Institution in 1810.

Over the first three-quarters of the 19th century many experimenters worked with various combinations of platinum or iridium wires, carbon rods, and evacuated or semi-evacuated enclosures. Many of these devices were demonstrated and some were patented.

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, British scientist Warren de la Rue enclosed a coiled platinum filament 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 fewer gas molecules to react with the platinum, improving its longevity. Although 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 platinum wires contained within a vacuum bulb.

In 1845, American John W. Starr acquired a patent for his incandescent light bulb involving the use of carbon filaments. He died shortly after obtaining the patent. Aside from the information contained in the patent itself, little else is known about him.

In 1851, Jean Eugène Robert-Houdin publicly demonstrated 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 1872 A. N. Lodygin invented an incandescent light bulb. In 1874 he obtained a patent for his invention.

In a suit filed by rivals seeking to get around Edison's lightbulb patent, German-American inventor Heinrich Göbel claimed he developed the first light bulb in 1854: a carbonized 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. Despite a successful recreation of his lamp in 1882, Lewis Latimer demonstrated that the bulbs which Göbel had purportedly built in the 1850s, had actually been built much later, and found the glassblower who had constructed the fraudulent exhibits. In a patent interference suit in 1893, the judge ruled Göbel's claim "extremely improbable".

In North America, parallel developments were also taking place. On July 24, 1874 a Canadian patent was filed by a Toronto medical electrician named Henry Woodward and a colleague Mathew Evans. They built their lamps with different sizes and shapes of carbon rods held between electrodes in glass cylinders filled with nitrogen. Woodward and Evans attempted to commercialize their lamp, but were unsuccessful. They ended up selling their patent (U.S. patent 0,181,613) to Thomas Edison in 1879 .

Commercialization

Carbon filament lamp (E27 socket, 220 volts, approx. 30 watts, left side: running with 100 volts)

Joseph Wilson Swan (1828–1914) was a British physicist and chemist. 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.

With the help of Charles Stearn, an expert on vacuum pumps, in 1878 Swan developed a method of processing that avoided the early bulb blackening. This received a British Patent No 8 in 1880. On 18 December 1878 a lamp using a slender carbon rod was shown at a meeting of the Newcastle Chemical Society, and Swan gave a working demonstration at their meeting on 17 January 1879. It was also shown to 700 who attended a meeting of the Literary and Philosophical Society of Newcastle on 3 February 1879. These lamps used a carbon rod from an arc lamp rather than a slender filament. Thus they had low resistance and required very large conductors to supply the necessary current, so they were not commercially practical, although they did furnish a demonstration of the possibilities of incandescent lighting with relatively high vacuum, a carbon conductor, and platinum lead-in wires. Besides requiring too much current for a central station electric system to be practical, they had a very short lifetime. Swan turned his attention to producing a better carbon filament and the means of attaching its ends. He devised a method of treating cotton to produce 'parchmentised thread' and obtained British Patent 4933 in 1880. From this year he began installing light bulbs in homes and landmarks in England. His house was the first in the world to be lit by a lightbulb and so the first house in the world to be lit by Hydro Electric power. In the early 1880s he had started his company.

Thomas Edison (1847–1931) began serious research into developing a practical incandescent lamp in 1878. Edison filed his first patent application for "Improvement In Electric Lights" on October 14, 1878 (U.S. patent 0,214,636). After many experiments with platinum and other metal filaments, Edison returned to a carbon filament. The first successful test was on October 22, 1879, and lasted 13.5 hours. Edison continued to improve this design and by Nov 4, 1879, filed for a U.S. patent (granted as U.S. patent 0,223,898 on Jan 27, 1880) for an electric lamp using "a carbon filament or strip coiled and connected ... to platina contact wires." Although the patent described several ways of creating the carbon filament including using "cotton and linen thread, wood splints, papers coiled in various ways," it was not until several months after the patent was granted that Edison and his team discovered that a carbonized bamboo filament could last over 1200 hours.

Hiram S. Maxim started a lightbulb company in 1878 to exploit his patents and those of William Sawyer. His United States Electric Lighting Company was the second company, after Edison, to sell practical incandescent electric lamps. They made their first commercial installation of incandescent lamps at the Mercantile Safe Deposit Company in New York City in the fall of 1880, about six months after the Edison incandescent lamps had been installed on the steamer Columbia. In October 1880, Maxim patented a method of coating carbon filaments with hydrocarbons to extend their life. Lewis Latimer, his employee at the time, developed an improved method of heat-treating them which reduced breakage and allowed them to be molded into novel shapes, such as the characteristic "M" shape of Maxim filaments. On January 17, 1882, 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. Latimer patented other improvements such as a better way of attaching filaments to their wire supports.

In Britain, the Edison and Swan companies merged into the Edison and Swan United Electric Company (later known as Ediswan, which was ultimately incorporated into Thorn Lighting Ltd). Edison was initially against this combination, but after Swan sued him and won, Edison 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 did, in order to protect Edison's patents from claims against them in the United States. In 1881, the Savoy Theatre became the first public building in the world to be lit entirely by electric lights.

U.S. patent 0,223,898 by Thomas Edison for an improved electric lamp, January 27, 1880

In 1882, the first recorded set of miniature incandescent lamps for lighting a Christmas tree was installed. These did not become common in homes for many years.

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 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 1898 he patented the osmium lamp and started marketing it in 1902, the first commercial metal filament incandescent lamp.

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 metal-coated carbon filament that would not blacken the inside of a light bulb.

On December 13, 1904, Hungarian Sándor Just and Croatian Franjo Hanaman were granted a Hungarian patent (No. 34541) for a tungsten filament lamp in Budapest, which lasted longer and gave a brighter light than the carbon filament. Tungsten filament lamps were first marketed by the Hungarian company Tungsram in 1905, so this type is often called Tungsram-bulbs in many European countries.

In 1906, the General Electric Company patented a method of making tungsten filaments for use in incandescent light bulbs. Sintered tungsten 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.

In 1913 Irving Langmuir found that filling a lamp with inert gas instead of a vacuum resulted in twice the luminous efficacy and reduction of bulb blackening. In 1924, Marvin Pipkin, an American chemist, patented a process for frosting the inside of lamp bulbs without weakening them, and in 1947 he patented a process for coating the inside of lamps with silica.

In 1930, Hungarian Imre Bródy filled lamps with krypton gas in lieu of argon. He used krypton and/or xenon filling of bulbs. Since the new gas was expensive, he developed a process with his colleagues to obtain krypton from air. Production of krypton filled lamps based on his invention started at Ajka in 1937, in a factory co-designed by Polányi and Hungarian-born physicist Egon Orowan.

By 1964, improvements in efficiency and production of incandescent lamps had reduced the cost of providing a given quantity of light by a factor of thirty, compared with the cost at introduction of Edison's lighting system

Cartels

Main article: Phoebus cartel

Between 1924 and 1939 the international market for incandescent light bulbs was controlled by the Phoebus cartel, which dictated wholesale prices and whose members controlled most of the world market for lamps.

Efficiency comparisons

Xenon Halogen Lamp (105 W) in the standard E27 bulb housing, used for direct replacement.

Approximately 90% of the power consumed by an incandescent light bulb is emitted as heat, rather than as visible light. Many light sources, such as the fluorescent lamp and the light-emitting diode offer higher efficiency, and some have for convenience been designed to be retrofitted in the already existing fixtures.

Luminous efficacy of a light source is a ratio of the visible light energy emitted ( the luminous flux) to the total power input to the lamp. It is measured in lumens per watt (lm/W). The maximum efficacy possible is 683 lm/W for monochromatic green light at 555 nanometres wavelength, the peak sensitivity of the human eye. (The lumen is defined in terms of the power of light as perceived by the human eye, not in terms of the absolute power output by the light.) For white light, the maximum luminous efficacy is around 240 lumens/watt. 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.

A closely related idea is the luminous efficacy of radiant energy, also measured in lumens/watt. Not all wavelengths of visible electromagnetic energy are equally effective at stimulating the human eye; the luminous efficacy of radiant energy is a measure of how well the distribution of energy matches the perception of the eye.

The chart below lists values of overall luminous efficacy and efficiency for several types of general service, 120 volt, 1000-hour lifespan 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.

Type	Overall luminous efficiency	Overall luminous efficacy (lm/W)
40 W tungsten incandescent	1.9%	12.6
60 W tungsten incandescent	2.1%	14.5
100 W tungsten incandescent	2.6%	17.5
glass 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
ideal monochromatic 555 nm (green) source	100%	683

Unfortunately, the spectrum emitted by a blackbody radiator does not match the sensitivity characteristics of the human eye. Tungsten filaments radiate mostly infrared radiation at temperatures where they remain solid (below 3683 kelvins / 3410°C / 6,170°F). 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 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. An upper limit for incandescent lamp luminous efficacy is around 52 lumens per watt, the theoretical value emitted by tungsten at its melting point.

For a given quantity of light, an incandescent light bulb produces more heat (and consumes more power) than a fluorescent lamp. Incandescent lamps' heat output increases load on air conditioning in the summer, but the heat from lighting can contribute to building heating in cold weather.

High-quality halogen incandescent lamps have higher efficacy, which will allow a 60 W bulb to provide nearly as much light as a non-halogen 100 W. Also, a lower-wattage halogen lamp can be designed to produce the same amount of light as a 60 W non-halogen lamp, but with much longer life.

Alternatives to standard incandescent lamps for general lighting purposes include:

• Fluorescent lamps, and Compact fluorescent lamps
• High-intensity discharge lamps
• LED lamps

None of these devices rely on incandescence to produce light. Instead, all these devices produce light by the transition of electrons from one energy level to another. These mechanisms produce discrete spectral lines and so are not associated with the broad "tail" of invisible infrared emissions produced by incandescent emitters, which is energy not usable for illumination. By careful selection of which electron energy level transitions are used, the spectrum emitted can be tuned to either mimic the appearance of incandescent sources or else produce different color temperatures of white for visible light.

Efforts to improve efficiency

Due to the measures noted above, there have been recent efforts to improve the efficiency of incandescents. For example the consumer lighting division of General Electric announced that they are working on a "high efficiency incandescent" (HEI) lamp, which they claim could ultimately be as much as four times more efficient than current incandescents, although their initial production goal is to be approximately two times more efficient.

The U.S. Department of Energy is also currently developing a filament lamp at Sandia National Laboratories with improved efficiency from 5% to 60%.

Prompted by U.S. legislation mandating increased bulb efficiency by 2012, new "hybrid" incandescent bulbs have been introduced by Philips. The "Halogena Energy Saver" incandescent is 30 percent more efficient than traditional designs, using a special chamber to reflect formerly-wasted heat back to the filament to provide additional lighting power.

Measures to prohibit use

Main article: Phase-out of incandescent light bulbs

Due to the higher energy usage of incandescent light bulbs in comparison to more energy efficient alternatives, such as compact fluorescent lamps and LED lamps, many governments have introduced measures to prohibit their use. Brazil and Venezuela started to phase them out in 2005, and other nations are planning scheduled phase-outs: Australia, Ireland and Switzerland in 2009; Argentina, Italy, Russia and the United Kingdom by 2011; Canada in 2012; the European Union by September 2012. and the U.S. between 2012 and 2014

Construction

Incandescent light bulbs consist of a glass enclosure (the envelope, or bulb) with a filament of tungsten wire inside the bulb, through which an electric current is passed. Contact wires and a base with two (or more) conductors provide electrical connections to the filament. Incandescent light bulbs usually contain a stem or glass mount anchored to the bulb's base which allows the electrical contacts to run through the envelope without gas/air leaks. Small wires embedded in the stem in turn support the filament and/or its lead wires. The bulb is filled with an inert gas such as argon to reduce evaporation of the filament.

An electrical current heats the filament to typically 2000 K to 3300 K (about 3100–5400°F), well below tungsten's melting point of 3695 K (6192°F). Filament temperatures depend on the filament type, shape, size, and amount of current drawn. The heated filament emits light that approximates a continuous spectrum. The useful part of the emitted energy is visible light, but most energy is given off as heat in the near-infrared wavelengths.

Three-way light bulbs have two filaments and three conducting contacts in their bases. The filaments share a common ground, and can be lit separately or together. Common wattages include 30–70–100, 50–100–150, and 100–200–300, with the first two numbers referring to the individual filaments, and the third giving the combined wattage.

While most light bulbs have clear or frosted glass, other kinds are also produced, including the various colors used for Christmas tree lights and other decorative lighting. Neodymium-containing glass is sometimes used to provide a more natural-appearing light.

Outline of Glass bulb Low pressure inert gas (argon, neon, nitrogen) Tungsten filament Contact wire (goes out of stem) Contact wire (goes into stem) Support wires Stem (glass mount) Contact wire (goes out of stem) Cap (sleeve) Insulation (vitrite) Electrical contact

Many arrangements of electrical contacts are used. Large lamps may have a screw base (one or more contacts at the tip, one at the shell) or a bayonet base (one or more contacts on the base, shell used as a contact or used only as a mechanical support). Some tubular lamps have an electrical contact at either end. Miniature lamps may have a wedge base and wire contacts, and some automotive and special purpose lamps have screw terminals for connection to wires. Contacts in the lamp socket allow the electric current to pass through the base to the filament. Power ratings for incandescent light bulbs range from about 0.1 watt to about 10,000 watts.

Filament

The first successful light bulb filaments were made of carbon (from carbonized paper or bamboo). Early carbon filaments had a negative temperature coefficient of resistance - as they got hotter, their electrical resistance decreased. This made the lamp sensitive to fluctuations in the power supply, since a small increase of voltage would cause the filament to heat up, reducing its resistance and causing it to draw even more power and heat even further. In the "flashing" process, carbon filaments were heated by current passing through them, while in an evacuated vessel containing hydrocarbon (gasoline) vapor. The carbon deposited by this treatment improved the uniformity and strength of filaments, and their efficiency. A metallized or graphitized filament was first heated in a high-temperature oven before flashing and lamp assembly; this transformed the carbon into graphite, which further strengthened and smoothed the filament, and as a byproduct had the advantage of changing the lamp to a positive temperature coefficient like a metallic conductor. This helped stabilize power consumption, temperature and light output against minor variations in supply voltage.

In 1902 the Siemens company developed a tantalum lamp filament. These lamps were more efficient than even graphitized carbon filaments and could operate at higher temperatures. Since the metal had a lower resistivity than carbon, the tantalum lamp filament was quite long and required multiple internal supports. The metal filament had the property of gradually shortening in use; the filaments were installed with large loops which tightened in use. This made lamps in use for several hundred hours quite fragile. Metal filaments had the property of breaking and re-welding, though this would usually decrease resistance and shorten the life of the filament. General Electric bought the rights to use tanalum filaments and produced them in the United States until 1913.

From 1898 to around 1905 osmium was also used as a lamp filament in Europe, but the metal was so expensive that used broken lamps could be returned for part credit. It could not be made for 110 V or 220 V so several lamps were wired in series for use on standard voltage circuits.

In 1906 the tungsten filament was introduced, which is still used. Tungsten metal was initially not available in a form that allowed it to be drawn into fine wires. By 1910, a process was developed by W. D. Coolidge at General Electric for production of a ductile form of tungsten. The process required pressing chemically produced tungsten powder into bars, then several steps of sintering, swaging, and then wire drawing. It was found that very pure tungsten formed filaments that sagged in use, and that a very small "doping" treatment with potassium, silicon, and aluminum oxides at the level of a few hundred parts per million, greatly improved the life and durability of the tungsten filaments.

To improve the efficiency of the lamp, the filament usually consists of coils of fine wire, also known as a 'coiled coil.' For a 60-watt 120-volt lamp, the uncoiled length of the tungsten filament is usually 22.8 inches or 580 mm , and the filament diameter is 0.0018 inches (0.045 mm). The advantage of the coiled coil is that evaporation of the tungsten filament is at the rate of a tungsten cylinder having a diameter equal to that of the coiled coil. Due to the coils creating gaps, such a filament has a lower surface area than the perceived surface area of the filament, and so evaporation is reduced. If the filament is then run hotter to bring back evaporation to the same rate, the resulting filament is a more efficient light source.

There are several different shapes of filament used in lamps, with differing characteristics. Manufacturers designate the types with codes such as C-6, CC-6, C-2V, CC-2V, C-8, CC-88, C-2F, CC-2F, C-Bar, C-Bar-6, C-8I, C-2R, CC-2R, and Axial. Template:Fixbunching

Filament of a 200 watt incandescent lightbulb highly magnified

Filament of a 50 watt incandescent lightbulb in the SEM in stereoscopic / anaglyph mode, magnification 50x

Filament of a 50 watt incandescent lightbulb in the SEM in stereoscopic / anaglyph mode, magnification 500x.

Electrical filaments are also used in hot cathodes of fluorescent lamps and vacuum tubes as a source of electrons or in vacuum tubes to heat an electron-emitting electrode.

Reducing filament evaporation

One of the problems of the standard electric light bulb is evaporation of the filament. Small variations in resistivity along the filament cause "hot spots" to form at points of higher resistivity ; a variation of diameter of only 1% will cause a 25% reduction in service life. The hot spots evaporate faster than the rest of the filament, increasing resistance at that point—a positive feedback which ends in the familiar tiny gap in an otherwise healthy-looking filament. Irving Langmuir found that an inert gas, instead of vacuum, would retard evaporation. General service incandescent light bulbs over about 25 watts in rating are now filled with a mixture of mostly argon and some nitrogen, or sometimes krypton. Xenon gas, much more expensive, is used occasionally in small bulbs, such as those for flashlights. Since a filament breaking in a gas-filled bulb can form an electric arc which may spread between the terminals and draw very heavy current, intentionally thin lead-in wires or more elaborate protection devices are therefore often used as fuses built into the light bulb. More nitrogen is used in higher-voltage lamps to reduce the possibility of arcing.

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 several hundred to 2,000 hours for lamps used for general illumination. Theatrical, photographic, and projection lamps may have a useful life of only a few hours, trading life expectancy for high output in a compact form. Long-life general service lamps have lower efficiency but are used where the cost of changing the lamp is high compared to the value of energy used.

Filament notching describes another phenomenon that limits the life of lamps. Lamps operated on direct current develop random stair-step irregularities on the filament surface, reducing the cross section and further increasing heat and evaporation of tungsten at these points. In small lamps operated on direct current, lifespan may be cut in half compared to AC operation. Different alloys of tungsten and rhenium can be used to counteract the effect.

If a light bulb envelope leaks, the hot tungsten filament reacts with air, yielding an aerosol of brown tungsten nitride, brown tungsten dioxide, violet-blue tungsten pentoxide, and yellow tungsten trioxide which then deposits on the nearby surfaces or the bulb interior.

Bulb blackening

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. An incandescent lamp which gives 93% or less of its initial light output at 75% of its rated life is regarded as unsatisfactory, when tested according to IEC Publication 60064. Light loss is due to filament evaporation and bulb blackening. Study of the problem of bulb blackening lead to the discovery of the Edison effect, thermionic emission and invention of the vacuum tube.

A very small amount of water vapor inside a light bulb can significantly affect lamp darkening. Water vapor dissociates into hydrogen and oxygen at the hot filament. The oxygen attacks the tungsten metal, and the resulting tungsten oxide particles travel to cooler parts of the lamp. Hydrogen from water vapor reduces the oxide, reforming water vapor and continuing this water cycle. The equivalent of a drop of water distributed over 500,000 lamps will significantly increase darkening. Small amounts of substances such as zirconium are placed within the lamp as a getter to react with any oxygen that may bake out of the lamp components during operation.

Some old, high-powered lamps used in theater, 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.

Halogen lamps

Close-up of a tungsten filament inside a halogen lamp. The two ring-shaped structures left and right are filament supports.

A halogen lamp is an incandescent lamp in which a tungsten filament is sealed into a compact transparent envelope filled with an inert gas and a small amount of halogen such as iodine or bromine. The halogen cycle increases the lifetime of the bulb and prevents its darkening by redepositing tungsten from the inside of the bulb back onto the filament. The halogen lamp can operate its filament at a higher temperature than a standard gas filled lamp of similar power without loss of operating life.

Incandescent arc lamps

A variation of the incandescent lamp did not use a hot wire filament, but instead used an arc struck on a spherical bead electrode to produce heat. The electrode then became incandescent, with the arc contributing little to the light produced. Such lamps were used for projection or illumination for scientific instruments such as microscopes. These arc lamps ran on relatively low voltages and incorporated tungsten filaments to start ionization within the envelope. They provided the intense concentrated light of an arc lamp but were easier to operate. Developed around 1915, these lamps were displaced by mercury and xenon arc lamps.

Electrical characteristics

Incandescent lamps are nearly pure resistive loads with a power factor of 1. This means the actual power consumed (in watts) and the apparent power (in volt-amperes) are equal. The actual resistance of the filament is temperature-dependent. The cold resistance of tungsten-filament lamps is about 1/15 the hot-filament resistance when the lamp is operating. For example, a 100-watt, 120-volt lamp has a resistance of 144 ohms when lit, but the cold resistance is much lower (about 9.5 ohms) . Since incandescent lamps are resistive loads, simple triac dimmers can be used to control brightness. Electrical contacts may carry a "T" rating symbol indicating that they are designed to control circuits with the high inrush current characteristic of tungsten lamps. For a 100-watt, 120 volt general-service lamp, the current stabilizes in about 0.10 seconds, and the lamp reaches 90% of its full brightness after about 0.13 seconds.

Power

Comparison of efficacy by power (120 Volt lamps)

Power (W)	Output (lm)	Efficacy (lm/W)
5	25	5
15	110	7.3
25	200	8.0
35	350	10.0
40	500	12.5
50	700	14.0
55	800	14.5
60	850	14.2
65	1,000	15.4
70	1,100	15.7
75	1,200	16.0
90	1,450	16.1
95	1,600	16.8
100	1,700	17.0
135	2,350	17.4
150	2,850	19.0
200	3,900	19.5
300	6,200	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. For two bulbs of the same voltage, type, color, and clarity, the higher-powered bulb gives more light.

The table 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.

Comparison of electricity cost

The kilowatt-hour is the usual unit of electrical energy purchase. The residential cost of electricity in the United States ranges from $0.07 to $0.18 per kilowatt-hour (kWh) in most states, but can be as high as $0.36 per kWh in certain areas such as Hawaii.

The desired product of any electric lighting system is light (lumens), not power (watts). To compare incandescent lamp operating cost with other light sources, the calculation must also consider the lumens produced by each lamp. For commercial and industrial lighting systems the comparison must also include the required illumination level, the capital cost of the lamp, the labor cost to replace lamps, the various depreciation factors for light output as the lamp ages, effect of lamp operation on heating and air conditioning systems, as well as the energy consumption.

Overall cost of lighting must also take into account light lost within the lamp holder fixture; internal reflectors and updated design of lighting fixtures can improve the amount of usable light delivered. Since human vision adapts to a wide range of light levels, a 10% or 20% decrease in lumens still may provide acceptable illumination, especially if the changeover is accompanied by cleaning of lighting equipment or improvements in fixtures.

Physical characteristics

Bulb shapes, sizes, and terms

Incandescent light bulbs come in a range of shapes and sizes. The names of the shapes may be slightly different in some regions. Many of these shapes have a designation consisting of one or more letters followed by one or more numbers, e.g. A55 or PAR38. The letters represent the shape of the bulb. The numbers represent the maximum diameter, either in eighths of an inch, or in millimetres, depending on the shape and the region. For example, 63 mm reflectors are designated R63, but in the U.S. they are known as R20 (2.5 inches). However, in both regions, a PAR38 reflector is known as PAR38.

Common shapes:

General Service

Light emitted in (nearly) all directions. Available either clear or frosted.

Types: General (A), Mushroom

High Wattage General Service

Lamps greater than 200 watts.

Types: Pear-shaped (PS)

Decorative

lamps used in chandeliers, etc.

Types: Candle (B), Twisted Candle, Bent-tip Candle (CA & BA), Flame (F), Fancy Round (P), Globe (G)

Reflector (R)

Reflective coating inside the bulb directs light forward. Flood types (FL) spread light. Spot types (SP) concentrate the light. Reflector (R) bulbs put approximately double the amount of light (foot-candles) on the front central area as General Service (A) of same wattage.

Types: Standard Reflector (R), Elliptical Reflector (ER), Crown Silvered

Parabolic Aluminized Reflector (PAR)

Parabolic Aluminized Reflector (PAR) bulbs control light more precisely. They produce about four times the concentrated light intensity of General Service (A), and are used in recessed and track lighting. Weatherproof casings are available for outdoor spot and flood fixtures.

120V Sizes:PAR 16, 20, 30, 38, 56 and 64

230V Sizes:Par 38, 56 and 64

Available in numerous spot and flood beam spreads. Like all light bulbs, the number represents the diameter of the bulb in 1/8s of an inch. Therefore, a PAR 16 is 2" in diameter, a PAR 20 is 2.5" in diameter, PAR 30 is 3.75" and a PAR 38 is 4.75" in diameter.

Multifaceted Reflector (MR)

HIR

"HIR" is a GE designation and means that the bulb has a special coating that reflects infrared back onto the filament. Therefore, less heat escapes, so the filament burns hotter and more efficiently. Osram designation for similar coating is "IRC".

Bulb materials

The glass bulb of a general service lamp can reach temperatures between 200 and 260 degrees Celsius (400 to 550 degrees Fahrenheit). Lamps intended for high power operation or used for heating purposes will have envelopes made of hard glass or fused quartz.

Lamp bases

See also: Automotive lamp types

Most domestic and industrial light bulbs have a metal fitting (or lamp base) compatible with standard sockets. The lamp base must carry current to the lamp, provide physical support, and resist heat. Lamp bases may be secured to the bulb with a cement, or by mechanical crimping to indentations molded into the glass bulb. Some miniature lamps have no metal bases at all and have only wire leads molded into the bulb. General Electric introduced standard base 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.

Screw thread

Main article: Edison screw

40 watt light bulbs with standard E10, E14 and E27 Edison screw base

In each designation, the E stands for Edison, who created the screw-base lamp, and the number is the diameter of the screw base in millimeters. (This is even true in North America, where designations for the bulb glass diameter are in eighths of an inch.) There are four common sizes of screw-in sockets used for line-voltage lamps:

• candelabra: E12 North America, E10 & E11 in Europe
• intermediate: E17 North America, E14 (SmallES) in Europe
• medium or standard: E26 (MES) in North America, E27 (ES) in Europe
• mogul: E39 North America, E40 (GoliathES) in Europe.

Other screw thread sizes exist.

The large E39 is used on street lights, and high-wattage lamps (such as a 100/200/300- Watt three-way) and many non-incandescent high-intensity discharge bulbs. Medium Edison screw (MES) bulbs for 12 V are also produced for recreational vehicles. Large outdoor Christmas lights use an intermediate base, as do some desk lamps and many microwave ovens. Formerly Emergency exit signs also tended to use the intermediate base (but U.S. and Canadian rules now require more energy-efficient lamps). A medium screw base should not carry more than 25 amperes current; this may limit the practical rating of low-voltage lamps.

Screw bases suffer from two disadvantages. Firstly, as they only have a single central contact, the metal screw itself forms one of the contacts for the circuit. If the lighting system is not correctly wired, the metal screw can become live, presenting a hazard to anyone attempting to change the bulb. Secondly, if the bulb unscrews in the socket for any reason it can lose contact with the central contact and stop working until it is screwed in tighter. This is not always a disadvantage, however: it provides a method for turning a (single) bulb off without using a wall or fixture switch.

Bayonet

Main article: Bayonet mount

The double-contact bayonet cap on an incandescent bulb

Bulbs with a bayonet mount (push-twist) base for use with sockets having spring-loaded base plates, are produced in similar sizes and are given a B, BA or BY designation. These are common in 12-volt automobile lighting worldwide.

BC or B22 or B22d or double-contact bayonet light bulbs are used for most 220–240 V mains lamps in the UK, Ireland, Cyprus, Australia, India, Pakistan, New Zealand, and various other parts of the British Commonwealth (but not Canada). B22 bulbs and bulb holders are also readily available in France, though fittings using this type are no longer widely sold. A smaller version, the B15 or SBC, is sometimes used for candle bulbs for chandeliers. A miniature bayonet is used in North America for appliances such as sewing machines and vacuum cleaners.

An advantage of bayonet bulbs is that they are less likely than screw bulbs to work loose. A disadvantage is that the base is more complex and deteriorates more rapidly. The sprung contacts may become stiff or the insulation may start to crumble, particularly when high-wattage bulbs are used.

Pin base

Main article: Bipin

A pin base has two contact pins on the underside of the bulb. These are given a G or GY designation, with the number being the center-to-center 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 lamps come in a tube, with blades or dimples at either end.

Special lamp bases

Miniature lamps used for some automotive lamps or decorative lamps have wedge-bases 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.

There are also special bases for projectors and stage lighting instruments. Projector lamps, in particular, may run on unusual voltages (such as 82V), perhaps intended as a vendor lock-in or to optimize light output for a particular optical system.

Lamps intended for use in optical systems (such as film projectors, microscope illuminators, or theatrical lighting instruments) have bases with alignment features so that the filament is positioned accurately within the optical system. A screw-base lamp may have a random orientation of the filament when the lamp is installed in the socket.

Tubular lamps such as R7S-75 for halogen lamp tubes, in this case a 7 mm diameter socket with 75 mm tube length.

Voltage, light output, and lifetime

See also: Lamp rerating

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 near the rated voltage of the lamp:

• Light output is approximately proportional to V ^3.4
• Power consumption is approximately proportional to V ^1.6
• Lifetime is approximately proportional to V ⁻¹⁶
• Color temperature is approximately proportional to V ^0.42

This means that a 5% reduction in operating voltage will more than double the life of the bulb, at the expense of reducing its light output by about 20%. This may be a very acceptable trade off for a light bulb that is in a difficult-to-access location (for example, traffic lights or fixtures hung from high ceilings). "Long-life" bulbs take advantage of this tradeoff. Since the value of the electric power they consume is much more than the value of the lamp, general service lamps emphasize efficiency over long operating life. The objective is to minimize the cost of light, not the cost of lamps.

The relationships above are valid for only a few percent change of voltage around rated conditions, but they do indicate that a lamp operated at much lower than rated voltage could last for hundreds of times longer than at rated conditions, albeit with greatly reduced light output. The Centennial Light is a light bulb which is accepted by the Guinness Book of World Records as having been burning almost continuously at a fire station in Livermore, California, 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 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, 3695 K (6192°F). A 50-hour-life projection bulb, for instance, is designed to operate only 50 °C (90 °F) below that melting point. Such a lamp may achieve up to 22 lumens/watt, compared with 17.5 for a 750-hour general service lamp.

Lamps designed for different voltages have different luminous efficacy. For example, a 100-watt, 120-volt lamp will produce about 17.1 lumens per watt. A lamp with the same rated lifetime but designed for 230 V would produce only around 12.8 lumens/watt, and a similar lamp designed for 30 volts (train lighting) would produce as much as 19.8 lumens/watt. This comes about because the lower voltage lamps have a correspondingly thicker filament than their thinner higher-voltage counterpart. Because they are thicker and thus made up of more material, they are able to be run hotter for the same lifetime before the filament evaporates.

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 general-service 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.

Very low voltages are inefficient since the lead wires would conduct too much heat away from the filament, so the practical lower limit for incandescent lamps is 1.5 volts. Very long filaments for high voltages are fragile, and lamp bases become more difficult to insulate, so lamps for illumination are not made with rated voltages over 300 V. Some infrared heating elements are made for higher voltages, but these use tubular bulbs with widely separated terminals.

Heat output

Incandescents convert approximately, 95%of the electricity into heat. This heat is sometimes desirable as a source of heating for a small space. During the winter, their use helps to heat homes and is more effective for this purpose than flourescents which generates much less heat. Specialized incandescents know as heat lamps are used to keep small animals warm. They are also commonly used in resturants and super markets to help keep hot foods warms.

Symbolism of Light Bulbs

Light bulb has become a classic symbol of human ingenuity. This is mainly due to it being Thomas Edison's best know innovation and the great advances it enabled for our lifestyle during an period when kerosene lamps are sooty and a fire hazard. Light bulb's symbolisms are probably most commonly found in United States where it was made practical by Edison.

Metaphors related to the light bulb includes:

• "Light up": Cheerfulness and outgoing personality

• "Bright idea": An "Eureka" moment when an idea is born.

• "Beacon of light/hope": Hope and justice is cast over everything by one individual, idea or object.

• "Burned out": Physical fatigue or exhaustion of new ideas.

See also

• Centennial Light
• Flash (photography) and discussion of flashbulbs
• Fluorescent lamp
• Gas-filled tube
• Lightbulb jokes
• Longest-lasting light bulbs
• Light-emitting diode (LED)
• Light fixture
• LED lamp
• List of light sources
• Luminous efficacy
• MR16 (a popular style of halogen lamp)
• Over-illumination
• Photometry (optics) Main Photometry/Radiometry article – explains technical terms
• Spectrometer
• Thomas Edison
• Xelogen

External links

• Museum of Electric Lamp Technology
• Howstuffworks - "How Light Bulbs Work"
• Light Source Spectra 60W-100W Incandescent light bulb spectra
• Halogen Light Bulbs – Base Identification
• Light Bulbs - Lamps and Tubes - Lamp Bases Explained
• Good examples of incandescent bulbs for keeping reptiles
• The Perfection of the Electric Light Bulb – background and experiment
• Incandescent Light Bulb's Bulb and Base Types Explained
• Examples of how color temperature of light bulbs affect our surroundings
• An brief article explaing the light bulb - "The Composition of the Light Bulb"
• The history of the light bulb
• BanTheBulb.Org energy efficiency campaign