Gas-discharge lamp
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Gas discharge lamps are a family of artificial light sources that generate light by sending an electric current through a special gas. Depending on the gas, this either generates light directly (as in most car park and roadway lighting), or the current generates ultraviolet light, 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.
Gas discharge lamps offer long life and high light efficiency, but are more complicated to manufacture, and they require electronics to provide the correct current flow through the gas.
History
Francis Hauksbee first demonstrated a gas discharge lamp in 1705. He showed that an evacuated or partially evacuated glass globe, while charged by static electricity could produce a light bright enough to read by. Sir Humphry Davy demonstrated in 1802 the first electric arc at the Royal Institution of Great Britain. Since then, discharge light sources have been researched because they create light from electicity considerably more efficiently than incandescent light bulbs.Later it was discovered that the arc discharge could be optimized by using an inert gas instead of air as a medium. Therefore noble gases neon, argon, krypton or xenon were used, as well as carbon dioxide historically.
The introduction of the metal vapour lamp, including various metals within the discharge tube, was a later advance. The heat of the gas discharge vaporized some of the metal and the discharge is then produced almost exclusively by the metal vapour. The usual metals are sodium and mercury owing to their high vapour pressures that increase efficiency of 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.
Color
Each gas, depending on its atomic structure emits certain wavelengths which translates in different colors of 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. Some of gas discharge lamps exhibit indexes below 100 which means that the colors appear completely different from, for instance with sun-light illumination. Some people are uncounsciously aware of this phenomenon and when buying clothes, they try to illuminate them with sun-light in order to know the "real" color.
| Gas | Color | Notes | Image |
|---|---|---|---|
| Helium | Whitish orange; under some conditions may be grayish, bluish, or green-bluish | Used by artists for special purpose lighting. |
|
| Neon | Red-orange | Intensive light. Used frequently in neon signs and neon bulbs. |
|
| Argon | Violetish pale lavender blue | Often used together with mercury vapor. |
|
| Krypton | Grayish dim off-white. May be greenish. At high peak currents bright blue-white. | Used by artists for special purpose lighting. |
|
| Xenon | Grayish or bluish-gray dim white, at high peak currents very bright green-bluish | Used in xenon flash lamps, xenon HID headlamps, and xenon arc lamps, and by artists for special purpose lighting. |
|
| Nitrogen | Similar to argon, duller, more pinkish; at high peak currents bright bluish-white, whiter than argon | ||
| Oxygen | Violet-lavender, dimmer than argon | ||
| Hydrogen | Lavender at low currents, pinkish magenta over 10 mA | ||
| Water vapor | Similar to hydrogen, dimmer | ||
| Carbon dioxide | Slight bluish-white, in lower currents brighter than xenon | ||
| Mercury vapor | Light blue, intense ultraviolet | In combination with phosphors used to generate many colors of light. Widely used in mercury-vapor lamps and Hydrargyrum Medium-Arc Iodide lamps. Often used together with argon. | |
| Sodium vapor (Low pressure) | Bright yellow | Widely used in sodium vapor lamps. |
|
Examples
- Neon signs may use either direct ilumination or (to obtain certain colors), indirect phosphor excitation.
- low-pressure sodium lamps of street lighting. They are orange and the color rendering index is below 100.
- Xenon flash lamp. This lamp is commonly found in film and digital cameras, even in single-use cameras. This lamps have produced interesting illumination effects in theatre and dancing. More robust versions of this lamps can produce short intense flashes repeatedly, allowing the stoboscopic examination of repetitive motion (useful in certain balancing applications). These were at one time popular, "freezing" the motion of the actors or dancers. This type of lamp was also used to demonstrate persistence of vision, where an entire room would be iluminated by multiple lamps behind difusing wall panels. In this otherwise darkened room a periodic flash would cause every detail of the occupants to be imaged on the observer's retina, completely frozen in motion.
- High intensity gas discharge lamps. They are now employed as automobile headlamps, and although favorable to the driver their intense blue light can be uncomfortable to other drivers. This can be somewhat ameliorated by highly accurate beam shaping to reduce stray illumination.
External links
- [Museum of historic discharge lamps]
- http://www.thebakken.org/artifacts/Hauksbee.htm
See also
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