Gas-discharge lamps are a family of artificial light sources that generate light by sending an electric discharge through an ionized gas, a plasma.
Typically, such lamps use a noble gas (argon, neon, krypton, and xenon) or a mixture of these gases. Some include additional substances, such as mercury, sodium, and metal halides, which are vaporized during start-up to become part of the gas mixture.
Single-ended self-starting lamps are insulated with a mica disc and contained in a borosilicate glass gas discharge tube (arc tube) and a metal cap. [1] [2] They include the sodium-vapor lamp that is the gas-discharge lamp in street lighting. [3] [4] [1] [2]
In operation, some of the electrons are forced to leave the atoms of the gas near the anode by the electric field applied between the two electrodes, leaving these atoms positively ionized. The free electrons thus released flow to the anode, while the cations thus formed are accelerated by the electric field and flow towards the cathode.
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 a lower energy state, releasing energy in the form of photons. 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 the emission 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 produce ultraviolet radiation which is converted to visible light by a fluorescent coating on the inside of the lamp's glass surface. The fluorescent lamp is perhaps the best known gas-discharge lamp.
Compared to incandescent lamps, gas-discharge lamps offer higher efficiency, [5] [6] but are more complicated to manufacture and most exhibit negative resistance, causing the resistance in the plasma to decrease as the current flow increases. Therefore, they usually require auxiliary electronic equipment such as ballasts 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 over incandescent lights in many lighting applications, until recent improvements in LED lamp technology. [ citation needed ]
The history of gas-discharge lamps began in 1675 when the French astronomer Jean Picard observed that the empty space in his mercury barometer glowed as the mercury jiggled while he was carrying the barometer. [7] Investigators, including Francis 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 by Vasily V. Petrov in 1802. [9] [10] [11] In 1809, Sir Humphry Davy demonstrated the electric arc at the Royal Institution of Great Britain. [12] [13] Since then, discharge light sources have been researched because they create light from electricity considerably more efficiently than incandescent light bulbs.
The father of the low-pressure gas discharge tube was German glassblower Heinrich Geissler, who beginning in 1857 constructed colorful artistic cold cathode tubes with different gases in them which glowed with many different colors, called Geissler tubes. It was found that inert gases such as the noble gases neon, argon, krypton or xenon, as well as carbon dioxide worked well in tubes. This technology was commercialized by the French engineer Georges Claude in 1910 and became neon lighting, used in neon 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 are sodium and mercury 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 after Heinrich Daniel Ruhmkorff and first used in the 1860s. The lamp consisted of a Geissler tube that was excited by a battery-powered Ruhmkorff induction 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 with uranium 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 of Saint-Priest and of Lac, near Privas, in the department of Ardèche, 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 of Jules Verne's science-fiction novels. [17]
Each gas, depending on its atomic structure emits radiation of certain wavelengths, its emission 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, the International Commission on Illumination (CIE) introduced the color 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 to orange; under some conditions may be gray, blue, or green-blue. | Used by artists for special-purpose lighting. | ||
Neon | Red-orange | Intense light. Used frequently in neon signs and neon lamps. | ||
Argon | Violet to pale lavender blue | Often used together with mercury vapor. | ||
Krypton | Gray off-white to green. At high peak currents, bright blue-white. | Used by artists for special-purpose lighting. | ||
Xenon | Gray or blue-gray dim white. At high peak currents, very bright green-blue. | Used in flashlamp, xenon HID headlamps, and xenon arc lamps. | ||
Nitrogen | Similar to argon but duller, more pink; at high peak currents bright blue-white. | used in the Moore lamp (historically) | ||
Oxygen | Violet to lavender, dimmer than argon | |||
Hydrogen | Lavender at low currents, pink to magenta over 10 mA | |||
Water vapor | Similar to hydrogen, dimmer | |||
Carbon dioxide | Blue-white to pink, at lower currents brighter than xenon | Used in carbon dioxide laser, the Moore lamp (historically). | ||
Carbon Monoxide | Similar to carbon dioxide. | |||
Methane | Magenta, but more purple and pink. | |||
Chlorine | Lime or chartreuse. | used in the Halogen lamp (historically) | ||
Fluorine | Mustard or ivory. | used in the Halogen lamp (historically) | ||
Ammonia | Fuchsia, but more purple. | |||
Ozone | Indigo or navy, similar to oxygen | |||
Mercury vapor | Light blue, intense ultraviolet | Ultraviolet not shown on this spectral image. Used in combination with phosphors used to generate many colors of light. Widely used in mercury-vapor lamps and fluorescent tubes. | ||
Sodium vapor (low pressure) | Bright orange-yellow | Widely used in sodium-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 knocks electrons out of the electrodes by thermionic emission, which helps maintain the arc. In many types the electrodes consist of electrical 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 (the striking voltage) must be applied to ionize the gas, so these lamps require higher voltage to start.
Low-pressure lamps have working pressure much less than atmospheric pressure. For example, common fluorescent 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 100 lumens per watt
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 in neon signs.
Low pressure sodium lamps, the most efficient gas-discharge lamp type, producing up to 200 lumens per watt, but at the expense of very poor color rendering. The almost monochromatic yellow light is only acceptable for street lighting and similar applications.
A small discharge lamp containing a bi-metallic switch is used to start 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 by ionizing a gas, usually neon mixed with helium and a small amount of nitrogen gas, by an electric current passing through two flame shaped electrode screens coated with partially decomposed barium 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 200 torr pressure, about 14% to 28% of atmospheric pressure; some automotive HID headlamps have up to 50 bar 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 artificial photoassimilation for growing plants
High pressure mercury-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 of electrical lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. Compared to other lamp types, relatively high arc power exists for the arc length. Examples of HID lamps include mercury-vapor lamps, metal halide lamps, ceramic discharge metal halide lamps, sodium vapor lamps and xenon arc lamps
HID lamps are typically used when high levels of light and energy efficiency are desired.
The Xenon 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 as strobe lights, can produce long sequences of flashes, allowing for the stroboscopic examination of motion. This has found use in the study of mechanical motion, in medicine and in the lighting of dance halls.
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 which secures the lamp in the socket of a light fixture, which is often called a "lamp" as well. The electrical connection to the socket may be made with a screw-thread base, two metal pins, two metal caps or a bayonet mount.
Artificial lighting technology began to be developed tens of thousands of years ago and continues to be refined in the present day.
A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, to produce ultraviolet and make a phosphor coating in the lamp glow. Fluorescent lamps convert electrical energy into useful light much more efficiently than incandescent lamps, but are less efficient than most LED lamps. The typical luminous efficacy of fluorescent lamps is 50–100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output.
A cold cathode is a cathode that is not electrically heated by a filament. A cathode may be considered "cold" if it emits more electrons than can be supplied by thermionic emission alone. It is used in gas-discharge lamps, such as neon lamps, discharge tubes, and some types of vacuum tube. The other type of cathode is a hot cathode, which is heated by electric current passing through a filament. A cold cathode does not necessarily operate at a low temperature: it is often heated to its operating temperature by other methods, such as the current passing from the cathode into the gas.
A neon lamp is a miniature gas-discharge lamp. The lamp typically consists of a small glass capsule that contains a mixture of neon and other gases at a low pressure and two electrodes. When sufficient voltage is applied and sufficient current is supplied between the electrodes, the lamp produces an orange glow discharge. The glowing portion in the lamp is a thin region near the cathode; the larger and much longer neon signs are also glow discharges, but they use the positive column which is not present in the ordinary neon lamp. Neon glow lamps were widely used as indicator lamps in the displays of electronic instruments and appliances. They are still sometimes used for their electrical simplicity in high-voltage circuits.
In the signage industry, neon signs are electric signs lighted by long luminous gas-discharge tubes that contain rarefied neon or other gases. They are the most common use for neon lighting, which was first demonstrated in a modern form in December 1910 by Georges Claude at the Paris Motor Show. While they are used worldwide, neon signs were popular in the United States from about the 1920s to 1950s. The installations in Times Square, many originally designed by Douglas Leigh, were famed, and there were nearly 2,000 small shops producing neon signs by 1940. In addition to signage, neon lighting is used frequently by artists and architects, and in plasma display panels and televisions. The signage industry has declined in the past several decades, and cities are now concerned with preserving and restoring their antique neon signs.
A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube; these include fluorescent lamps, metal-halide lamps, sodium-vapor lamps, and neon lights. Specialized gas-filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices.
A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light at a characteristic wavelength near 589 nm.
A plasma ball, plasma globe, or plasma lamp is a clear glass container filled with noble gases, usually a mixture of neon, krypton, and xenon, that has a high-voltage electrode in the center of the container. When voltage is applied, a plasma is formed within the container. Plasma filaments extend from the inner electrode to the outer glass insulator, giving the appearance of multiple constant beams of colored light. Plasma balls were popular as novelty items in the 1980s.
High-intensity discharge lamps are a type of electrical gas-discharge lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. This tube is filled with noble gas and often also contains suitable metal or metal salts. The noble gas enables the arc's initial strike. Once the arc is started, it heats and evaporates the metallic admixture. Its presence in the arc plasma greatly increases the intensity of visible light produced by the arc for a given power input, as the metals have many emission spectral lines in the visible part of the spectrum. High-intensity discharge lamps are a type of arc lamp.
A Geissler tube is a precursor to modern gas discharge tubes, demonstrating the principles of electrical glow discharge, akin to contemporary neon lights, and central to the discovery of the electron. This device was developed in 1857 by Heinrich Geissler, a German physicist and glassblower. A Geissler tube is composed of a sealed glass cylinder of various shapes, which is partially evacuated and equipped with a metal electrode at each end. It contains rarefied gases—such as neon or argon, air, mercury vapor, or other conductive substances, and sometimes ionizable minerals or metals like sodium. When a high voltage is applied between the electrodes, there is an electric current through the tube, causing gas molecules to ionize by shedding electrons. The free electrons reunite with the ions and the resulting energic atoms emit light via fluorescence, with the emitted color characteristic of the contained material.
A mercury-vapor lamp is a gas-discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger soda lime or borosilicate glass bulb. The outer bulb may be clear or coated with a phosphor; in either case, the outer bulb provides thermal insulation, protection from the ultraviolet radiation the light produces, and a convenient mounting for the fused quartz arc tube.
An electric arc is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma, which may produce visible light. An arc discharge is initiated either by thermionic emission or by field emission. After initiation, the arc relies on thermionic emission of electrons from the electrodes supporting the arc. An arc discharge is characterized by a lower voltage than a glow discharge. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp".
A metal-halide lamp is an electrical lamp that produces light by an electric arc through a gaseous mixture of vaporized mercury and metal halides. It is a type of high-intensity discharge (HID) gas discharge lamp. Developed in the 1960s, they are similar to mercury vapor lamps, but contain additional metal halide compounds in the quartz arc tube, which improve the efficiency and color rendition of the light. The most common metal halide compound used is sodium iodide. Once the arc tube reaches its running temperature, the sodium dissociates from the iodine, adding orange and reds to the lamp's spectrum from the sodium D line as the metal ionizes. As a result, metal-halide lamps have high luminous efficacy of around 75–100 lumens per watt, which is about twice that of mercury vapor lights and 3 to 5 times that of incandescent lights and produce an intense white light. Lamp life is 6,000 to 15,000 hours. As one of the most efficient sources of high CRI white light, metal halides as of 2005 were the fastest growing segment of the lighting industry. They are used for wide area overhead lighting of commercial, industrial, and public places, such as parking lots, sports arenas, factories, and retail stores, as well as residential security lighting, automotive headlamps and indoor cannabis grow operations.
The induction lamp, electrodeless lamp, or electrodeless induction lamp is a gas-discharge lamp in which an electric or magnetic field transfers the power required to generate light from outside the lamp envelope to the gas inside. This is in contrast to a typical gas discharge lamp that uses internal electrodes connected to the power supply by conductors that pass through the lamp envelope. Eliminating the internal electrodes provides two advantages:
Hydrargyrum medium-arc iodide (HMI) is the trademark name of Osram's brand of metal-halide gas discharge medium arc-length lamp, made specifically for film and entertainment applications. Hydrargyrum comes from the Greek name for the element mercury.
A xenon arc lamp is a highly specialized type of gas discharge lamp, an electric light that produces light by passing electricity through ionized xenon gas at high pressure. It produces a bright white light to simulate sunlight, with applications in movie projectors in theaters, in searchlights, and for specialized uses in industry and research. For example, Xenon arc lamps and mercury lamps are the two most common lamps used in wide-field fluorescence microscopes.
An electrical ballast is a device placed in series with a load to limit the amount of current in an electrical circuit.
Electric discharge in gases occurs when electric current flows through a gaseous medium due to ionization of the gas. Depending on several factors, the discharge may radiate visible light. The properties of electric discharges in gases are studied in connection with design of lighting sources and in the design of high voltage electrical equipment.