Gas-discharge lamps are a family of artificial light sources that generate light by sending anelectric discharge through anionized gas, aplasma.
Typically, such lamps use anoble gas (argon,neon,krypton, andxenon) or a mixture of these gases. Some include additional substances, such asmercury,sodium, andmetalhalides, which are vaporized during start-up to become part of the gas mixture.
Single-ended self-starting lamps are insulated with amica disc and contained in aborosilicate glass gas discharge tube (arc tube) and a metal cap.[1][2] They include thesodium-vapor lamp that is used in gas-discharge lamps in some street lighting.[3][4][1][2]
In operation, some of the electrons are forced to leave theatoms of the gas near theanode by theelectric field applied between the two electrodes, leaving these atoms positivelyionized. The free electrons thus released flow to the anode, while thecations thus formed are accelerated by the electric field and flow towards thecathode.
The ions typically cover only a very short distance before colliding with neutral gas atoms, which give the ions their electrons. The atoms which lost an electron during the collisions ionize and speed toward the cathode while the ions which gained an electron during the collisions return to alower energy state, releasing energy in the form ofphotons. Light of a characteristic frequency is thus emitted. In this way, electrons are relayed through the gas from the cathode to the anode.
The color of the light produced depends on theemission spectra of the atoms making up the gas, as well as the pressure of the gas,current density, and other variables. Gas discharge lamps can produce a wide range of colors. Some lamps produceultraviolet radiation which is converted to visible light by afluorescent coating on the inside of the lamp's glass surface. Thefluorescent lamp is perhaps the best known gas-discharge lamp.
Compared toincandescent lamps, gas-discharge lamps offer higherefficiency,[5][6] but are more complicated to manufacture and most exhibitnegative resistance, causing the resistance in the plasma to decrease as the current flow increases. Therefore, they usually require auxiliary electronic equipment such asballasts to control current flow through the gas, preventing current runaway (arc flash).
Some gas-discharge lamps also have a perceivable start-up time to achieve their full light output. Still, owing to their greater efficiency, gas-discharge lamps were preferred overincandescent lights in many lighting applications, until recent improvements inLED lamp technology.
The history of gas-discharge lamps began in 1675 when the French astronomerJean Picard observed that the empty space in his mercurybarometer glowed as the mercury jiggled while he was carrying the barometer.[7] Investigators, includingFrancis Hauksbee, tried to determine the cause of the phenomenon. Hauksbee first demonstrated a gas-discharge lamp in 1705.[8] He showed that an evacuated or partially evacuated glass globe, in which he placed a small amount of mercury, while charged by static electricity could produce a light bright enough to read by. The phenomenon of electric arc was first described byVasily V. Petrov in 1802.[9][10][11] In 1809, SirHumphry Davy demonstrated theelectric arc at theRoyal Institution of Great Britain.[12][13] Since then, discharge light sources have been researched because they create light from electricity considerably more efficiently thanincandescent light bulbs.
The father of the low-pressure gas discharge tube was German glassblowerHeinrich Geissler, who beginning in 1857 constructed colorful artisticcold cathode tubes with different gases in them which glowed with many different colors, calledGeissler tubes. It was found that inert gases such as thenoble gases neon, argon, krypton or xenon, as well ascarbon dioxide worked well in tubes. This technology was commercialized by the French engineerGeorges Claude in 1910 and becameneon lighting, used inneon signs.
The introduction of the metal vapor lamp, including various metals within the discharge tube, was a later advance. The heat of the gas discharge vaporizes some of the metal and the discharge is then produced almost exclusively by the metal vapor. The usual metals aresodium andmercury owing to their visible spectrum emission.
One hundred years of research later led to lamps without electrodes which are instead energized by microwave or radio-frequency sources. In addition, light sources of much lower output have been created, extending the applications of discharge lighting to home or indoor use.
Ruhmkorff lamps were an early form of portable electric lamp, named afterHeinrich Daniel Ruhmkorff and first used in the 1860s. The lamp consisted of aGeissler tube that was excited by a battery-powered Ruhmkorffinduction coil; an early transformer capable of converting DC currents of low voltage into rapid high-voltage pulses. Initially the lamp generated white light by using a Geissler tube filled with carbon dioxide. However, the carbon dioxide tended to break down. Hence in later lamps, the Geissler tube was filled with nitrogen (which generated red light), and the clear glass was replaced withuranium glass (which fluoresced with a green light).[14]
Intended for use in the potentially explosive environment of mining, as well as oxygen-free environments like diving or for a heatless lamp for possible use in surgery, the lamp was actually developed both by Alphonse Dumas, an engineer at the iron mines ofSaint-Priest and of Lac, nearPrivas, in the department ofArdèche, France, and by Dr Camille Benoît, a medical doctor in Privas.[15] In 1864, the French Academy of Sciences awarded Dumas and Benoît a prize of 1,000 francs for their invention.[16] The lamps, cutting-edge technology in their time, gained fame after being described in several ofJules Verne's science-fiction novels.[17]
Each gas, depending on its atomic structure emits radiation of certain wavelengths, itsemission spectrum, which determines the color of the light from the lamp. As a way of evaluating the ability of a light source to reproduce the colors of various objects being lit by the source, theInternational Commission on Illumination (CIE) introduced thecolor rendering index (CRI). Some gas-discharge lamps have a relatively low CRI, which means colors they illuminate appear substantially different from how they do under sunlight or other high-CRI illumination.
| Gas | Color | Spectrum | Notes | Image |
|---|---|---|---|---|
| Helium | White toorange; under some conditions may begray,blue, orgreen-blue. |  | Used by artists for special-purpose lighting. |  |
| Neon | Red-orange |  | Intense light. Used frequently inneon signs andneon lamps. |  |
| Argon | Violet topale lavender blue |  | Often used together withmercury vapor. |  |
| Krypton | Grayoff-white togreen. At high peak currents,bright blue-white. |  | Used by artists for special-purpose lighting. |  |
| Xenon | Gray orblue-gray dim white. At high peak currents,very bright green-blue. |  | Used inflashlamp,xenon HID headlamps, andxenon arc lamps. |  |
| Nitrogen | Similar toargon but duller, morepink; at high peak currentsbright blue-white. |  | used in theMoore lamp (historically) |  |
| Oxygen | Violet tolavender, dimmer thanargon |  |  | |
| Hydrogen | Lavender at low currents,pink tomagenta over 10 mA |  |  | |
| Water vapor | Similar tohydrogen, dimmer | |||
| Carbon dioxide | Blue-white topink, at lower currents brighter thanxenon | Used incarbon dioxide laser, theMoore lamp (historically). |  | |
| Carbon Monoxide | Similar tocarbon dioxide. | |||
| Methane | Magenta, but morepurple andpink. | |||
| Chlorine | Lime orchartreuse. | used in theHalogen lamp (historically) | ||
| Fluorine | Mustard orivory. | used in theHalogen lamp (historically) | ||
| Ammonia | Fuchsia, but morepurple. | |||
| Ozone | Indigo ornavy, similar tooxygen | |||
| Mercury vapor | Light blue, intenseultraviolet |  | Ultraviolet not shown on this spectral image. Used in combination withphosphors used to generate many colors of light. Widely used inmercury-vapor lamps andfluorescent tubes. |  |
| Sodium vapor (low pressure) | Bright orange-yellow |  | Widely used insodium-vapor lamps. |  |
Lamps are divided into families based on the pressure of gas, and whether or not the cathode is heated.Hot cathode lamps have electrodes that operate at a high temperature and are heated by the arc current in the lamp. The heat knockselectrons out of the electrodes bythermionic emission, which helps maintain the arc. In many types the electrodes consist ofelectrical filaments made of fine wire, which are heated by a separate current at startup, to get the arc started.Cold cathode lamps have electrodes that operate at room temperature. To start conduction in the lamp a high enough voltage (thestriking voltage) must be applied to ionize the gas, so these lamps require higher voltage to start.
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Low-pressure lamps have working pressure much less than atmospheric pressure. For example, commonfluorescent lamps operate at a pressure of about 0.3% of atmospheric pressure.
Fluorescent lamps, a heated-cathode lamp, the most common lamp in office lighting and many other applications, produces up to 100lumens perwatt
Neon lighting, a widely used form of cold-cathode specialty lighting consisting of long tubes filled with various gases at low pressure excited by high voltages, used as advertising inneon signs.
Low pressuresodium lamps, the most efficient gas-discharge lamp type, producing up to 200 lumens per watt, but at the expense of very poorcolor rendering. The almostmonochromatic yellow light is only acceptable for street lighting and similar applications.
A small discharge lamp containing abi-metallicswitch is used tostart a fluorescent lamp. In this case the heat of the discharge is used to actuate the switch; the starter is contained in an opaque enclosure and the small light output is not used.
Continuous glow lamps are produced for special applications where the electrodes may be cut in the shape of alphanumeric characters and figural shapes.[18]
A flicker light bulb, flicker flame light bulb or flicker glow lamp is a gas-discharge lamp which produces light byionizing agas, usuallyneon mixed withhelium and a small amount ofnitrogen gas, by an electric current passing through two flame shapedelectrode screens coated with partially decomposedbarium azide. The ionized gas moves randomly between the two electrodes which produces a flickering effect, often marketed as suggestive of a candle flame (see image).[19]
High-pressure lamps have a discharge that takes place in gas under slightly less to greater than atmospheric pressure. For example, a high pressure sodium lamp has an arc tube under 100 to 200torr pressure, about 14% to 28% of atmospheric pressure; some automotive HID headlamps have up to 50bar or fifty times atmospheric pressure.
Metal halide lamps produce almost white light, and attain 100 lumen per watt light output. Applications include indoor lighting of high buildings, parking lots, shops, sport terrains.
High pressure sodium lamps, producing up to 150 lumens per watt produce a broader light spectrum than the low pressure sodium lamps. Also used for street lighting, and for artificialphotoassimilation for growing plants
High pressuremercury-vapor lamps are the oldest high pressure lamp type and have been replaced in most applications by metal halide and the high pressure sodium lamps. They require a shorter arc length.
A high-intensity discharge (HID) lamp is a type ofelectrical lamp which produces light by means of an electric arc betweentungstenelectrodes housed inside a translucent or transparentfused quartz or fusedalumina arc tube. Compared to other lamp types, relatively high arc power exists for the arc length. Examples of HID lamps includemercury-vapor lamps,metal halide lamps,ceramic discharge metal halide lamps,sodium vapor lamps andxenon arc lamps
HID lamps are typically used when high levels of light and energy efficiency are desired.
TheXenon flash lamp produces a single flash of light in the millisecond-microsecond range and is commonly used in film,photography and theatrical lighting. Particularly robust versions of this lamp, known asstrobe lights, can produce long sequences of flashes, allowing for thestroboscopic examination of motion. This has found use in the study of mechanical motion, in medicine and in the lighting of dance halls.
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