Xenon arc lamp
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Xenon arc lamps are an artificial light source. Powered by electricity, they use ionized xenon gas to produce a bright white light that closely mimics natural daylight.
Xenon arc lamps can be roughly divided into three categories:
- Continuous-output xenon short-arc lamps
- Continuous-output xenon long-arc lamps
- Xenon flash lamps (which are usually considered separately)
Each consists of a glass or fused quartz arc tube with tungsten metal electrodes at each end. The glass tube is first evacuated and then re-filled with xenon gas. For xenon flashtubes, a third "trigger" electrode usually surrounds the exterior of the arc tube.
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[edit] History and modern usage
Xenon short-arc lamps were invented in the 1940s in Germany and introduced in 1951 by Osram. First launched in the 2 kW size (XBO2001), these lamps saw a wide acceptance in movie projection where it advantageously replaced the older carbon arc lamps. The white, continuous light generated with this arc is of daylight quality but plagued by a rather low lumen efficiency. Today, almost all movie projectors in theaters employ these lamps with a rating ranging from 900 watts up to 12 kW. When used in Omnimax projection systems, the power can be as high as 15 kW in a single lamp.
[edit] Lamp construction
All modern xenon short-arc lamps utilize a fused quartz envelope with thorium-doped tungsten electrodes. Fused quartz is the only economically feasible material currently available that can withstand the high pressure and high temperature present in an operating lamp while still being optically clear. The thorium dopant in the electrodes greatly enhances their electron emission characteristics. Because tungsten and quartz have different coefficients of thermal expansion, the tungsten electrodes are welded to strips of pure molybdenum metal or Invar alloy, which are then melted into the quartz to form the envelope seal.
Because of the very high power levels involved, the lamps may be water-cooled. In those used in IMAX projectors, the electrode bodies are made from solid Invar and tipped with thoriated tungsten. In (continuous wave pumped) lasers the lamp is inserted into a fixed lamp jacket and the water flows between the jacket and the lamp. An O-ring seals off the tube, so that the naked electrodes do not get into contact with the water. In low power applications the electrodes are too cold for efficient electron emission and are not cooled, in high power applications an additional water cooling circuit for each electrode is necessary. To save costs, the water circuits are often not separated and the water needs to be highly deionized, which in turn lets the quartz or some laser mediums dissolve into the water. Theoretically the water can be pressurized to disburden the hot quartz, practically it only alleviates the effect of lamp breakage.
In order to achieve maximum efficiency, the xenon gas inside a short-arc lamp has to be maintained at an extremely high pressure. With large lamps this presents a serious safety concern, because if the lamp is dropped or ruptures in service, pieces of the lamp envelope can be ejected at high velocity, causing bodily injury or death. To mitigate this risk, large xenon short-arc lamps are shipped inside special protective shields (see photograph), which will contain the envelope fragments if the lamp is dropped and explodes. The shield is removed once the lamp is installed in the lamp housing. When the lamp reaches the end of its useful life, the protective shield is put back on the lamp, and the spent lamp is then removed from the equipment and disposed of. The risk of explosion increases as the lamp is used. For this reason, it is important to observe the manufacturer's recommended operating life limits.
Even with the protective shield in place, eye protection should always be worn when handling xenon short-arc lamps. For some lamps the manufacturers also recommend artery protection.
There is another type of lamp known as a ceramic Xenon lamp (Developed by Perkin-Elmer as Cermax) - this type uses a ceramic lamp body with an integrated reflector.
[edit] Light generation mechanism
Xenon short-arc lamps come in two distinct varieties: pure xenon, which contain only xenon gas; and xenon-mercury, which contain xenon gas and a small amount of mercury metal.
In a pure xenon lamp, the majority of the light is generated within a tiny, pinpoint-sized cloud of plasma situated where the electron stream leaves the face of the cathode. The light generation volume is cone-shaped, and the luminous intensity falls off exponentially moving from cathode to anode. Electrons that manage to pass through the plasma cloud collide with the anode, causing it to heat up. As a result, the anode in a xenon short-arc lamp either has to be much larger than the cathode or be water-cooled, to safely dissipate the heat. Pure xenon short-arc lamps have a "near daylight" spectrum. The light output of the lamp is relatively flat over the entire colour spectrum.
Even in a high pressure lamp, there are some very strong emission lines in the near infrared, roughly in the region from 850-900 nm. This spectral region can contain about 10% of the total emitted light.
In xenon-mercury short-arc lamps, the majority of the light is generated within a tiny, pinpoint sized cloud of plasma situated at the tip of each electrode. The light generation volume is shaped like two intersecting cones, and the luminous intensity falls off exponentially moving towards the centre of the lamp. Xenon-mercury short-arc lamps have a bluish-white spectrum and extremely high UV output. These lamps are used primarily for UV curing applications, sterilizing objects, and generating ozone.
The very small optical size of the arc makes it possible to focus the light from the lamp very precisely. For this reason, xenon arc lamps of smaller sizes, down to 10 watts, are used in optics and in precision illumination for microscopes and other instruments. Larger lamps are also employed in searchlights where narrow beams of light are to be generated, or in film production lighting where daylight simulation is required.
All xenon short-arc lamps generate significant amounts of ultraviolet radiation while in operation. Xenon has strong spectral lines in the UV bands, and these readily pass through the fused quartz lamp envelope. Unlike the borosilicate glass used in standard lamps, fused quartz does not attenuate UV radiation. The UV radiation released by a short-arc lamp can cause a secondary problem of ozone generation. The UV radiation strikes oxygen molecules in the air surrounding the lamp, causing them to ionize. Some of the ionized molecules then recombine as O3, ozone. Equipment that uses short-arc lamps as the light source must be designed to contain UV radiation and prevent ozone build-up.
Many lamps have a low-UV blocking coating on the envelope and are sold as "Ozone Free" lamps. Some lamps have envelopes made out of ultra-pure synthetic fused silica (trade name "Suprasil"), which rougly doubles the cost, but which allows them to emit useful light into the so-called vacuum UV region. These lamps are normally operated in a pure Nitrogen atmosphere.
[edit] Power supply requirements
Xenon short-arc lamps are low-voltage, high-amperage, direct-current devices with a negative temperature coefficient. They require a high voltage pulse in the 50 kV range to start the lamp, and require extremely well regulated dc as the power source. They are also inherently unstable, prone to phenomena such as plasma oscillation and thermal runaway. Because of these characteristics, xenon short-arc lamps require a sophisticated power supply to achieve stable, long-life operation. The usual approach is to regulate the current flowing in the lamp rather than the applied voltage.
[edit] Technology outlook
The use of the xenon technology has spread into the consumer market with the introduction in 1991 of xenon headlamps for cars. In this lamp the glass capsule is small and the arc spans only a few millimetres. Additions of mercury and salts of sodium and scandium improve significantly the lumen output of the lamp, the xenon gas being used only to provide instant light upon the ignition of the lamp.
[edit] Xenon long-arc-lamps
These are structurally similar to short-arc lamps except that the arc-containing portion of the glass tube is greatly elongated. When mounted within an elliptical reflector, these lamps are frequently used to simulate sunlight. Typical uses include solar cell testing, solar simulation for age testing of materials, rapid thermal processing, and material inspection.
[edit] See also
| Incandescent: | Conventional - Halogen - Parabolic aluminized reflector (PAR) |  |
| Fluorescent: | Compact fluorescent (CFL) - Linear fluorescent - Induction lamp | |
| Gas discharge: | High-intensity discharge (HID) - Mercury-vapor - Metal-halide - Neon - Sodium vapor | |
| Electric arc: | Arc lamp - HMI - Xenon arc - Yablochkov candle | |
| Combustion: | Acetylene/Carbide - Candle - Gas lighting - Kerosene lamp - Limelight - Oil lamp - Safety lamp - Petromax | |
| Other types: | Sulfur lamp - Light-emitting diode (LED) - LED lamp (SSL) - Fiber optics - Plasma - El wire | |
Categories: Lamps | Xenon
