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Light tubes (also known as solar pipes, tubular skylights or sun tunnels^[1]) are structures that transmit or distribute natural or artificial light for the purpose of illumination and are examples of optical waveguides.

In their application to daylighting, they are also often called tubular daylighting devices, sun pipes, sun scopes, or daylight pipes. They can be divided into two broad categories: hollow structures that contain the light with reflective surfaces; and transparent solids that contain the light by total internal reflection. Principles of nonimaging optics govern the flow of light through them.^[2]

Types

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IR light tubes

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Manufacturing custom designed infrared light pipes, hollow waveguides and homogenizers is non-trivial. This is because these are tubes lined with a highly polished infrared reflective coating of gold, which can be applied thick enough to permit these tubes to be used in highly corrosive atmospheres. Carbon black can be applied to certain parts of light pipes to absorb IR light (see photonics). This is done to limit IR light to only certain areas of the pipe.

While most light pipes are produced with a round cross-section, light pipes are not limited to this geometry. Square and hexagonal cross-sections are used in special applications. Hexagonal pipes tend to produce the most homogenized type of IR Light. The pipes do not need to be straight. Bends in the pipe have little effect on efficiency.

Light tube with reflective material

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A light tube installed in the subterranean train station at Potsdamer Platz, Berlin

Capturing sunlight above ground

Distributing sunlight below ground

The first commercial reflector systems were patented and marketed in the 1850s by Paul Emile Chappuis in London, utilizing various forms of angled mirror designs. Chappuis Ltd’s reflectors were in continuous production until the factory was destroyed in 1943.^[3] The concept was rediscovered and patented in 1986 by Solatube International of Australia.^[4] This system has been marketed for widespread residential and commercial use. Other daylighting products are on the market under various generic names, such as “SunScope”, “solar pipe”, “light pipe”, “light tube”, and “tubular skylight”.

A tube lined with highly reflective material leads the light rays through a building, starting from an entrance-point located on its roof or one of its outer walls. A light tube is not intended for imaging (in contrast to a periscope, for example); thus image distortions pose no problem and are in many ways encouraged due to the reduction of “directional” light.

The entrance point usually comprises a dome (cupola), which has the function of collecting and reflecting as much sunlight as possible into the tube. Many units also have directional “collectors”, “reflectors”, or even Fresnel lens devices that assist in collecting additional directional light down the tube.

In 1994, the Windows and Daylighting Group at Lawrence Berkeley National Laboratory (LBNL) developed a series of horizontal light pipe prototypes to increase daylight illuminance at distances of 4.6-9.1 m, to improve the uniformity of daylight distribution and luminance gradient across the room under variable sun and sky conditions throughout the year. The light pipes were designed to passively transport daylighting through relatively small inlet glazing areas by reflecting sunlight to depths greater than conventional sidelight windows or skylights.^[5][6]

A set-up in which a laser cut acrylic panel is arranged to redirect sunlight into a horizontally or vertically orientated mirrored pipe, combined with a light spreading system with a triangular arrangement of laser cut panels that spread the light into the room, was developed at the Queensland University of Technology in Brisbane.^[7] In 2003, Veronica Garcia Hansen, Ken Yeang, and Ian Edmonds were awarded the Far East Economic Review Innovation Award in bronze for this development.^[8][9]

Light transmission efficiency is greatest if the tube is short and straight. In longer, angled, or flexible tubes, part of the light intensity is lost. To minimize losses, a high reflectivity of the tube lining is crucial; manufacturers claim reflectivities of their materials, in the visible range, of up to almost 99.5 percent.^[10][11]

At the end point (the point of use), a diffuser spreads the light into the room.

The first full-scale passive horizontal light pipes were built at the Daylight Lab at Texas A&M University, where the annual daylight performance was thoroughly evaluated in a 360 degree rotating 6 m wide by 10 m deep room. The pipe is coated with a 99.3% specular reflective film and the distribution element at the end of the light pipe consists of a 4.6 m long diffusing radial film with an 87% visible transmittance. The light pipe introduces consistently illuminance levels ranging between 300 and 2,500 lux throughout the year at distances between 7.6 m to 10 m.^[12]

To further optimize the use of solar light, a heliostat can be installed which tracks the movement of the sun, thereby directing sunlight into the light tube at all times of the day as far as the surroundings’ limitations allow, possibly with additional mirrors or other reflective elements that influence the light path. The heliostat can be set to capture moonlight at night.

Optical fiber

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Optical fibers can also be used for daylighting. A solar lighting system based on plastic optical fibers was in development at Oak Ridge National Laboratory in 2004.^[13][14] The system was installed at the American Museum of Science and Energy, Tennessee, USA, in 2005,^[15] and brought to market the same year by the company Sunlight Direct.^[16][17] However, this system was taken off the market in 2009.

In view of the usually small diameter of the fibers, an efficient daylighting set-up requires a parabolic collector to track the sun and concentrate its light. Optical fibers intended for light transport need to propagate as much light as possible within the core; in contrast, optical fibers intended for light distribution are designed to let part of the light leak through their cladding.^[18]

Optical fibers are also used in the Bjork system sold by Parans Solar Lighting AB.^[19][20] The optic fibers in this system are made of PMMA (PolyMethyl MethAcrylate) and sheathed with Megolon, a halogen-free thermoplastic resin. A system such as this, however, is quite expensive.^[21]

The Parans system^[22] consists of three parts. A collector, fiber optic cables, and luminaires spreading the light indoors. One or more collectors are placed on or near the building in a place where they will have good access to direct sunlight. The collector consists of lenses mounted in aluminum profiles with a covering glass as protection. These lenses concentrate sunlight down in the fiber optic cables.

The collectors are modular, which means they come with either 4,6,8,12 or 20 cables depending on the need. Every cable can have an individual length. The fiber optic cables transport the natural light 100 meters (30 floors) in and through the property while retaining both a high level of light quality and light intensity. Examples of implementations are Kastrup Airport, University of Arizona and Stockholm University.

A similar system, but using optical fibers of glass, had earlier been under study in Japan.^[23]

Corning Inc. makes Fibrance Light-Diffusing Fiber. Fibrance works by shining a laser through a light-diffusing fiber optic cable. The cable gives off a lighted glow.^[24]

Optical fibers are used in fiberscopes for imaging applications.

Transparent hollow light guides

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A prism light guide was developed in 1981 by Lorne Whitehead, a physics professor at the University of British Columbia,^[25][26] and has been used in solar lighting for both the transport and distribution of light.^[27][28] A large solar pipe based on the same principle was set up in the narrow courtyard of a 14-floor building of a Washington, D.C. law firm in 2001,^{[29][30][31][32][33]} and a similar proposal has been made for London.^[34] A further system has been installed in Berlin.^[35]

The 3M company developed a system based on optical lighting film^[36] and developed the 3M light pipe,^[37] which is a light guide designed to distribute light uniformly over its length, with a thin film incorporating microscopic prisms,^[26] which has been marketed in connection with artificial light sources, e.g. sulfur lamps.

In contrast to an optical fiber which has a solid core, a prism light guide leads the light through air and is therefore referred to as a hollow light guide.

The project ARTHELIO,^[38][39] partially funded by the European Commission, was an investigation in years 1998 to 2000 into a system for adaptive mixing of solar and artificial light, and which includes a sulfur lamp, a heliostat, and hollow light guides for light transport and distribution.

Disney has experimented with using 3D printing to print internal light guides for illuminated toys.^[40]

Fluorescence based system

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In a system developed by Fluorosolar and the University of Technology, Sydney, two fluorescent polymer layers in a flat panel capture short wave sunlight, particularly ultraviolet light, generating red and green light, respectively, which is guided into the interior of a building. There, the red and green light is mixed with artificial blue light to yield white light, without infrared or ultraviolet. This system, which collects light without requiring mobile parts such as a heliostat or a parabolic collector, is intended to transfer light to any place within a building. ^[41][42][43] By capturing ultraviolet, the system can be especially effective on bright but overcast days; this is since ultraviolet is diminished less by cloud cover than are the visible components of sunlight.

Properties and applications

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Solar and hybrid lighting systems

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Solar light pipes, compared to conventional skylights and other windows, offer better heat insulation properties and more flexibility for use in inner rooms, but less visual contact with the external environment.

In the context of seasonal affective disorder, it may be worth considering that an additional installation of light tubes increases the amount of natural daily light exposure. It could thus possibly contribute to residents´ or employees´ well-being while avoiding over-illumination effects.

Compared to artificial lights, light tubes have the advantage of providing natural light and of saving energy. The transmitted light varies over the day; should this not be desired, light tubes can be combined with artificial light in a hybrid set-up.^{[27][44][45][46]}

Some artificial light sources are marketed which have a spectrum similar to that of sunlight, at least in the human visible spectrum range,^[47][48][49] as well as low flicker.^[49] Their spectrum can be made to vary dynamically such as to mimic changes in natural light over the day. Manufacturers and vendors of such light sources claim that their products can provide the same or similar health effects as natural light.^[49][50][51] When considered as alternatives to solar light pipes, such products may have lower installation costs but do consume energy during use; therefore they may well be more wasteful in terms of overall energy resources and costs.

On a more practical note, light tubes do not require electric installations or insulation and are thus especially useful for indoor wet areas such as bathrooms and pools. From a more artistic point of view, recent developments, especially those pertaining to transparent light tubes, open new and interesting possibilities for architectural lighting design.^{[citation needed]}

Security applications

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Due to the relatively small size and high light output of sun pipes, they have an ideal application to security-oriented situations, such as prisons, police cells, and other locations where restricted access is required. Being of narrow diameter, and not largely affected by internal security grilles, this provides daylight to areas without providing electrical connections or escape access, and without allowing objects to be passed into a secure area.

In electronic devices

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Moulded plastic light tubes are commonly used in the electronics industry to direct illumination from LEDs on a circuit board to indicator symbols or buttons. These light tubes typically take on a highly complex shape that uses either gentle curving bends as in an optic fiber or has sharp prismatic folds which reflect off the angled corners. Multiple light tubes are often moulded from a single piece of plastic, permitting easy device assembly since the long thin light tubes are all part of a single rigid component that snaps into place.

Light tube indicators make electronics cheaper to manufacture since the old way would be to mount a tiny lamp into a small socket directly behind the spot to be illuminated. This often requires extensive hand labor for installation and wiring. Light tubes permit all lights to be mounted on a single flat circuit board, but illumination can be directed up, and away from the board wherever it is required.

An electric light, lamp, or light bulb is an electrical component that produces light. It is the most common form of artificial lighting. Lamps usually have a base made of ceramic, metal, glass, or plastic that secures them in the socket of a light fixture, which is also commonly referred to as a ‘lamp.’ The electrical connection to the socket may be made with a screw-thread base, two metal pins, two metal caps or a bayonet mount.

The three main categories of electric lights are incandescent lamps, which produce light by a filament heated white-hot by electric current, gas-discharge lamps, which produce light by means of an electric arc through a gas, such as fluorescent lamps, and LED lamps, which produce light by a flow of electrons across a band gap in a semiconductor.

The energy efficiency of electric lighting has significantly improved since the first demonstrations of arc lamps and incandescent light bulbs in the 19th century. Modern electric light sources come in a profusion of types and sizes adapted to many applications. Most modern electric lighting is powered by centrally generated electric power, but lighting may also be powered by mobile or standby electric generators or battery systems. Battery-powered light is often reserved for when and where stationary lights fail, often in the form of flashlights or electric lanterns, as well as in vehicles.

History

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Before electric lighting became common in the early 20th century, people used candles, gas lights, oil lamps, and fires.^[1] In 1799–1800, Alessandro Volta created the voltaic pile, the first electric battery. Current from these batteries could heat copper wire to incandescence. Vasily Vladimirovich Petrov developed the first persistent electric arc in 1802, and English chemist Humphry Davy gave a practical demonstration of an arc light in 1806.^[2] It took more than a century of continuous and incremental improvement, including numerous designs, patents, and resulting intellectual property disputes, to get from these early experiments to commercially produced incandescent light bulbs in the 1920s.^[3][4]

In 1840, Warren de la Rue enclosed a platinum coil in a vacuum tube and passed an electric current through it, thus creating one of the world’s first electric light bulbs.^[5][6][7] 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 it was an efficient design, the cost of the platinum made it impractical for commercial use.^[8]

William Greener, an English inventor, made significant contributions to early electric lighting with his lamp in 1846 (patent specification 11076), laying the groundwork for future innovations such as those by Thomas Edison.

The late 1870s and 1880s were marked by intense competition and innovation, with inventors like Joseph Swan in the UK and Thomas Edison in the US independently developing functional incandescent lamps. Swan’s bulbs, based on designs by William Staite, were successful, but the filaments were too thick. Edison worked to create bulbs with thinner filaments, leading to a better design.^[9] The rivalry between Swan and Edison eventually led to a merger, forming the Edison and Swan Electric Light Company. By the early twentieth century these had completely replaced arc lamps.^[10][1]

The turn of the century saw further improvements in bulb longevity and efficiency, notably with the introduction of the tungsten filament by William D. Coolidge, who applied for a patent in 1912.^[11] This innovation became a standard for incandescent bulbs for many years.

In 1910, Georges Claude introduced the first neon light, paving the way for neon signs which would become ubiquitous in advertising.^[12][13][14]

In 1934, Arthur Compton, a renowned physicist and GE consultant, reported to the GE lamp department on successful experiments with fluorescent lighting at General Electric Co., Ltd. in Great Britain (unrelated to General Electric in the United States). Stimulated by this report, and with all of the key elements available, a team led by George E. Inman built a prototype fluorescent lamp in 1934 at General Electric‘s Nela Park (Ohio) engineering laboratory. This was not a trivial exercise; as noted by Arthur A. Bright, “A great deal of experimentation had to be done on lamp sizes and shapes, cathode construction, gas pressures of both argon and mercury vapor, colors of fluorescent powders, methods of attaching them to the inside of the tube, and other details of the lamp and its auxiliaries before the new device was ready for the public.”^[15]

The first practical LED arrived in 1962.^[16]

U.S. transition to LED bulbs

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In the United States, incandescent light bulbs including halogen bulbs stopped being sold as of August 1, 2023,^{[needs update]} because they do not meet minimum lumens per watt performance metrics established by the U.S. Department of Energy.^{[17][needs update]} Compact fluorescent bulbs are also banned despite their lumens per watt performance because of their toxic mercury that can be released into the home if broken and widespread problems with proper disposal of mercury-containing bulbs.

Types

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Incandescent

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Main article: Incandescent light bulb

In its modern form, the incandescent light bulb consists of a coiled filament of tungsten sealed in a globular glass chamber, either a vacuum or full of an inert gas such as argon. When an electric current is connected, the tungsten is heated to 2,000 to 3,300 K (1,730 to 3,030 °C; 3,140 to 5,480 °F) and glows, emitting light that approximates a continuous spectrum.

Incandescent bulbs are highly inefficient, in that just 2–5% of the energy consumed is emitted as visible, usable light. The remaining 95% is lost as heat.^[18] In warmer climates, the emitted heat must then be removed, putting additional pressure on ventilation or air conditioning systems.^[19] In colder weather, the heat byproduct has some value, and has been successfully harnessed for warming in devices such as heat lamps. Incandescent bulbs are nonetheless being phased out in favor of technologies like CFLs and LED bulbs in many countries due to their low energy efficiency. The European Commission estimated in 2012 that a complete ban on incandescent bulbs would contribute 5 to 10 billion euros to the economy and save 15 billion metric tonnes of carbon dioxide emissions.^[20]

Halogen

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Main article: Halogen lamp

Halogen lamps are usually much smaller than standard incandescent lamps, because for successful operation a bulb temperature over 200 °C is generally necessary. For this reason, most have a bulb of fused silica (quartz) or aluminosilicate glass. This is often sealed inside an additional layer of glass. The outer glass is a safety precaution, to reduce ultraviolet emission and to contain hot glass shards should the inner envelope explode during operation.^[21] Oily residue from fingerprints may cause a hot quartz envelope to shatter due to excessive heat buildup at the contamination site.^[22] The risk of burns or fire is also greater with bare bulbs, leading to their prohibition in some places, unless enclosed by the luminaire.

Those designed for 12- or 24-volt operation have compact filaments, useful for good optical control. Also, they have higher efficacies (lumens per watt) and longer lives than non-halogen types. The light output remains almost constant throughout their life.

Fluorescent

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Main article: Fluorescent lamp

Fluorescent lamps consist of a glass tube that contains mercury vapour or argon under low pressure. Electricity flowing through the tube causes the gases to give off ultraviolet energy. The inside of the tubes are coated with phosphors that give off visible light when struck by ultraviolet photons.^[23] They have much higher efficiency than incandescent lamps. For the same amount of light generated, they typically use around one-quarter to one-third the power of an incandescent. The typical luminous efficacy of fluorescent lighting systems is 50–100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output. Fluorescent lamp fixtures are more costly than incandescent lamps, because they require a ballast to regulate the current through the lamp, but the lower energy cost typically offsets the higher initial cost. Compact fluorescent lamps are available in the same popular sizes as incandescent lamps and are used as an energy-saving alternative in homes. Because they contain mercury, many fluorescent lamps are classified as hazardous waste. The United States Environmental Protection Agency recommends that fluorescent lamps be segregated from general waste for recycling or safe disposal, and some jurisdictions require recycling of them.^[24]

LED

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Main article: LED lamp

The solid-state light-emitting diode (LED) has been popular as an indicator light in consumer electronics and professional audio gear since the 1970s. In the 2000s, efficacy and output have risen to the point where LEDs are now being used in lighting applications such as car headlights^[25] and brake lights,^[25] in flashlights^[26] and bicycle lights,^[27] as well as in decorative applications, such as holiday lighting.^[28] Indicator LEDs are known for their extremely long life, up to 100,000 hours, but lighting LEDs are operated much less conservatively, and consequently have shorter lives. LED technology is useful for lighting designers, because of its low power consumption, low heat generation, instantaneous on/off control, and in the case of single color LEDs, continuity of color throughout the life of the diode and relatively low cost of manufacture.^[28] LED lifetime depends strongly on the temperature of the diode.^[29] Operating an LED lamp in conditions that increase the internal temperature can greatly shorten the lamp’s life. Some lasers have been adapted as an alternative to LEDs to provide highly focused illumination.^[30][31]

Carbon arc

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Main article: Arc lamp

Carbon arc lamps consist of two carbon rod electrodes in open air, supplied by a current-limiting ballast. The electric arc is struck by touching the rod tips then separating them. The ensuing arc produces a white-hot plasma between the rod tips. These lamps have higher efficacy than filament lamps, but the carbon rods are short-lived and require constant adjustment in use, as the intense heat of the arc erodes them.^[32] The lamps produce significant ultraviolet output, they require ventilation when used indoors, and due to their intensity they need protection from direct sight.

Invented by Humphry Davy around 1805, the carbon arc was the first practical electric light.^[33][34] It was used commercially beginning in the 1870s for large building and street lighting until it was superseded in the early 20th century by the incandescent light.^[33] Carbon arc lamps operate at high power and produce high intensity white light. They also are a point source of light. They remained in use in limited applications that required these properties, such as movie projectors, stage lighting, and searchlights, until after World War II.^[32]

Discharge

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Main article: Gas-discharge lamp

A discharge lamp has a glass or silica envelope containing two metal electrodes separated by a gas. Gases used include, neon, argon, xenon, sodium, metal halides, and mercury. The core operating principle is much the same as the carbon arc lamp, but the term “arc lamp” normally refers to carbon arc lamps, with more modern types of gas discharge lamp normally called discharge lamps. With some discharge lamps, very high voltage is used to strike the arc. This requires an electrical circuit called an igniter, which is part of the electrical ballast circuitry. After the arc is struck, the internal resistance of the lamp drops to a low level, and the ballast limits the current to the operating current. Without a ballast, excess current would flow, causing rapid destruction of the lamp.

Some lamp types contain a small amount of neon, which permits striking at normal running voltage with no external ignition circuitry. Low-pressure sodium lamps operate this way. The simplest ballasts are just an inductor, and are chosen where cost is the deciding factor, such as street lighting. More advanced electronic ballasts may be designed to maintain constant light output over the life of the lamp, may drive the lamp with a square wave to maintain completely flicker-free output, and shut down in the event of certain faults.

The most efficient source of electric light is the low-pressure sodium lamp. It produces, for all practical purposes, a monochromatic orange-yellow light, which gives a similarly monochromatic perception of any illuminated scene. For this reason, it is generally reserved for outdoor public lighting applications. Low-pressure sodium lights are favoured for public lighting by astronomers, since the light pollution that they generate can be easily filtered, contrary to broadband or continuous spectra.

Characteristics

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Form factor

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Main articles: Incandescent light bulb § Bulb shapes, and Lightbulb socket

Many lamp units, or light bulbs, are specified in standardized shape codes and socket names. Incandescent bulbs and their retrofit replacements are often specified as “A19/A60 E26/E27″, a common size for those kinds of light bulbs. In this example, the “A” parameters describe the bulb size and shape within the A-series light bulb while the “E” parameters describe the Edison screw base size and thread characteristics.^[35]

Comparison parameters

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Common comparison parameters include:^[36]

• Luminous flux (in lumens)
• Energy consumption (in watts)
• Luminous efficacy (in lumens per watt)
• Color temperature (in kelvins)

Less common parameters include color rendering index (CRI).

Life expectancy

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Life expectancy for many types of lamp is defined as the number of hours of operation at which 50% of them fail, that is the median life of the lamps. Production tolerances as low as 1% can create a variance of 25% in lamp life, so in general some lamps will fail well before the rated life expectancy, and some will last much longer. For LEDs, lamp life is defined as the operation time at which 50% of lamps have experienced a 70% decrease in light output. In the 1900s the Phoebus cartel formed in an attempt to reduce the life of electric light bulbs, an example of planned obsolescence.^[37][38]

Some types of lamp are also sensitive to switching cycles. Rooms with frequent switching, such as bathrooms, can expect much shorter lamp life than what is printed on the box. Compact fluorescent lamps are particularly sensitive to switching cycles.^[39]

Uses

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The total amount of artificial light (especially from street light) is sufficient for cities to be easily visible at night from the air, and from space. External lighting grew at a rate of 3–6 percent for the later half of the 20th century and is the major source of light pollution^[40] that burdens astronomers^[41] and others with 80% of the world’s population living in areas with night time light pollution.^[42] Light pollution has been shown to have a negative effect on some wildlife.^[40][43]

Electric lamps can be used as heat sources, for example in incubators, as infrared lamps in fast food restaurants and toys such as the Kenner Easy-Bake Oven.^[44]

Lamps can also be used for light therapy to deal with such issues as vitamin D deficiency,^[45] skin conditions such as acne^[46][47] and dermatitis,^[48] skin cancers,^[49] and seasonal affective disorder.^[50][51][52] Lamps which emit a specific frequency of blue light are also used to treat neonatal jaundice^[53] with the treatment which was initially undertaken in hospitals being able to be conducted at home.^[54][55]

Electric lamps can also be used as a grow light to aid in plant growth^[56] especially in indoor hydroponics and aquatic plants with recent research into the most effective types of light for plant growth.^[57]

Due to their nonlinear resistance characteristics, tungsten filament lamps have long been used as fast-acting thermistors in electronic circuits. Popular uses have included:

• Stabilization of sine wave oscillators
• Protection of tweeters in loudspeaker enclosures; excess current that is too high for the tweeter illuminates the light rather than destroying the tweeter.
• Automatic volume control in telephones