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    Light Bulb Background Information


    The incandescent light bulb is a source of artificial light that works by incandescence.

    Incandescence is the emission of light (visible electromagnetic radiation) from a hot body due to its temperature. The term derives from the verb incandesce, to glow white.


    The incandescent light bulb (also spelled lightbulb) or incandescent lamp is a source of artificial light that works by incandescence. An electrical 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. The operating principle of an incandescent lamp is similar to that of blackbody radiation.

    Incandescent bulbs are also called electric lamps, a term originally applied to the original arc lamps. In the theater, television, and motion-picture industries, the incandescent lamps used in lighting instruments are sometimes referred to as globes, and light globe is also used widely in Australia to refer to incandescent lamps generally.

    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, some car headlamps and electric flashlights, and for decorative and advertising lighting.

    Some applications of the incandescent bulb make use of the heat generated, such as incubators (for hatching eggs), brooding boxes for young poultry, heat lights for reptile tanks, 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 (compact) fluorescent lights, high-intensity discharge lamps, LEDs, and other devices, which give more visible light for the same amount of electrical energy input. Some jurisdictions are attempting to ban the use of incandescent lightbulbs in favor of more energy-efficient lighting.

    History of the light bulb

    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 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."

    Early pre-commercial research

    In 1802 Humphry Davy had what was then the most powerful 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 until Thomas Edison's creation of the first practical incandescent lamp in 1879. In 1809, Davy created the first arc lamp by making a small but blinding electrical connection between two charcoal rods connected to a 2000 cell battery. Demonstrated to the Royal Institution 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, 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 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 powdered charcoal heated between two platinum wires contained within a vacuum bulb.

    In 1845 American John Wellington 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 Alexander Nikolayevich 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, the German-American inventor Heinrich Göbel claimed he had 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. Lewis Latimer demonstrated that the bulbs Göbel had purportedly built in the 1850s had actually been built much later, and found the glassblower who had constructed the fraudulent exhibits for Göbel. In a patent interference suit in 1893, the judge ruled that Göbel's claim was "extremely improbable."


    Joseph Wilson Swan (1828–1914) was a physicist and druggist (chemist) born in Sunderland, United Kingdom. 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, Swan developed a method of processing that avoided the early bulb blackening in 1878. This received a British Patent No 8 in 1880. On 18th 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 17th January 1879. It was also shown to 700 who attended a meeting of the Literary and Philosophical Society of Newcastle on 3rd February 1879. 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, and in the early 1880s he had started his company. See also

    In North America, 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 rods held between electrodes in glass cylinders filled with nitrogen. Woodward and Evans attempted to commercialize their lamp, but were unsuccessful. The ended up selling their patent (U.S. Patent 0,181,613 to Thomas Edison in 1879.

    Thomas Edison 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 to sell practical incandescent electric lamps, after Edison. 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. Maxim in October 1880 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 then 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 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 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. In the same year Franjo Hannaman, a Croatian from Zagreb, invented a tungsten (wolfram) filament lamp, which lasted longer and gave a brighter light than the carbon filament. 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. Marvin Pipkin, an American chemist, in 1924 patented a process for frosting the inside of lamp bulbs without weakening them, and in 1947 patented a process for coating the inside of lamps with silica. In 1936 the coiled-coil filament was introduced which further improved the efficiency of lamps.

    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


    Incandescent light bulbs consist of a glass enclosure (the envelope, or bulb) which is filled with an inert gas to reduce evaporation of the filament and reduce the required strength of the glass. Inside of the bulb is a filament of tungsten wire, through which an electrical current is passed. The current heats the filament to an extremely high temperature (typically 2000 K to 3300 K depending on the filament type, shape, size, and amount of current passed through). The heated filament emits light with a continuous spectrum. The useful part of the emitted energy is visible light, but also significant energy is given off in the in the near-infrared wavelengths.

    Incandescent light bulbs usually 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. 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 only used 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 electrical current to pass through the base to 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, also known as a 'coiled coil'. For a 60 watt 120-volt lamp, the uncoiled length of the filament is usually 22.8 inches or 580 mm and the filament diameter is 0.0018 inches (0.045 mm).

    An SEM image (75x) of a 60 W line voltage light bulb filament. In order to increase the filament length while keeping its physical size small, the filament takes the form of a coiled coil. By comparison, low voltage lamp filaments usually take the form of a single coil.

    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, and so ordinary incandescent light bulbs over about 25 watts in rating are now filled with nitrogen, argon, or krypton. However, a filament breaking in a gas-filled bulb can form 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 several hundred to 2000 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.

    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.

    In a halogen lamp uneven evaporation of the filament and darkening of the envelope is reduced by filling the lamp with a halogen gas at low pressure. These lamps can operate at a higher filament temperature without unacceptable loss of life, giving them a higher luminous efficiency.

    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.

    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, violet-blue tungsten pentoxide, and yellow tungsten trioxide which then deposits on the nearby surfaces or the bulb interior.

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

    Electrical Characteristics

    Incandescent lamps are nearly pure resistive loads which means they have 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 is about 1/15 the resistance when the lamp is lit. For example, a 100 watt, 120 volt lamp has a resistance of 144 Ω 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.


    Comparison of efficacy by wattage (120 Volt lamps)
    Power (W) Output (lm) Efficacy (lm/W)
    15 110 7.3
    25 200 8.0
    35 350 10.3
    40 500 12.5
    50 700 13.5
    55 800 14.2
    60 850 14.5
    65 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. For two bulbs of the same voltage, type, colour, 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

    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 normally ranges from $0.06 to $0.18 per kilowatt-hour (kWh), but can be as high as $0.23 per kWh in certain areas such as Hawaii.

    The average rated laboratory lifetime of incandescent light bulbs is about 750–1000 hours (usually defined as the time it takes half of a given set of lamps to fail under test conditions).

    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 may still 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

    • General Service (A)

    Light emitted in all directions. Available in either clear or frosted. Types: General (A), Globe (G), Decorative (D) (flame, teardrop and other shapes)

    • 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.

    • 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 (PAR) 16, 20, 30 and 38 bulbs: 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, and a PAR 38 is 4.75" in diameter.

    • Multifaceted Reflector (MR)
    • HIR

    "HIR" 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.

    Standard fittings

    Most domestic and industrial light bulbs have a metal fitting (or lamp base) compatible with standard sockets. 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.

    Screw thread

    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 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 (SmallES) in Europe
    • medium or standard: E26 (MES) in North America, E27 (ES) in Europe
    • mogul: E39 North America, E40 (GoliathES) in Europe.
    • There is also a rare "admedium" size (E29), incompatible with standard and used to frustrate thieves of bulbs used in public places; and a very miniature size (E5) generally used only for low-voltage applications such as with a battery.

    The largest size E39 is now only used in large street lights, although a few high-wattage household lamps (such as a 100/200/300-watt three-way) use it as do 300, 500, 750, 1000 and 1500 watt light bulbs. 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. Formerly Emergency exit signs also tended to use the intermediate base (but modern exit signs must use LEDs). A medium screw base should not carry more than 25 amperes current; this may limit the practical rating of low-voltage lamps.


    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 lamps 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. BC or B22 or B22d or double-contact bayonet cap, used in Australia, India, Ireland, New Zealand and the UK for most 220–240 V mains lamps. A miniature bayonet is used in North America for appliances such as sewing machines and vacuum cleaners.

    Pin base

    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 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 halogen bulbs come in a tube, with blades or dimples at either end.

    Special lamp bases

    There are also various odd fittings for projectors and stage lighting instruments. Projector lamps, in particular, may run on odd voltages (such as 82), 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

    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 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 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 trade off. Since the value of the electric power they consume is much more than the value of the lamp, general service lamps for illumination usually emphasize efficiency over long operating life.

    The relationships above are only valid for 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 trade-off 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 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 only produce around 12.8 lumens/watt, and a similar lamp designed for 30 volts (train lighting) would produce as much as 19.8 lumens/watt.

    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 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 is 1.5 volts. Very long filaments for high voltages are fragile, and lamp bases become more difficult to insulate so lamps with rated voltages over 300 V are not made.

    Luminous efficacy and efficiency

    Approximately 90%-95% of the power consumed by an incandescent light bulb is emitted as heat, rather than as visible light.

    For a given quantity of light, an incandescent light bulb, with 5% efficiency, produces more heat (and consumes more power) than a fluorescent lamp (with 7%-15% efficiency) 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.

    Quality halogen incandescent lamps are closer to 9% efficiency, which will allow a 60 W bulb to provide nearly as much light as a non-halogen 100 W. Also, the 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. Halogen lamps get hotter than regular incandescent lamps because the heat is concentrated on a smaller envelope surface, and because the surface is closer to the filament. This high temperature is essential to their long life. 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 to prevent ignition of draperies or flammable objects in contact with the lamp. Similarly, in some areas halogen bulbs over a certain power are banned from residential use.

    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. 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.

    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 white light source 35.5% 242.5
    ideal monochromatic 555 nm (green) source 100% 683

    A 100 W bulb for 120 V systems, produces 17.5 lumens per watt, compared to a theoretical "ideal" of 242.5 lumens per watt for white light. Unfortunately, tungsten filaments radiate mostly infrared radiation at temperatures where they remain solid (below 3683 kelvin). 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. An upper limit for incandescent lamp luminous efficacy is around 52 lumens per watt, the theoretical value emitted by tungsten at its melting point.

    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 useable for illumination. By careful selection of which electron energy level transitions are used, the spectrum emitted can be tuned to the spectrum most suitable for visible light.

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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