Xenon arc lamp

A xenon arc lamp is an artificial light source. Powered by electricity, it uses 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.

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 they 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 efficiency in terms of lumens of visible light output per watt of input power. 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.

Lamp construction

All modern xenon short-arc lamps use 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 (25 atmospheres for an IMAX bulb) 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, large lamps are water-cooled. In those used in IMAX projectors, the electrode bodies are made from solid Invar and tipped with thoriated tungsten. An O-ring seals off the tube, so that the naked electrodes do contact 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 media dissolve into the water.

In order to achieve maximum efficiency, the xenon gas inside short-arc lamps is maintained at an extremely high pressure (up to 300 Atmospheres), which poses safety concerns. If a lamp is dropped, or ruptures while in service, pieces of the lamp envelope can be thrown at high velocity. To mitigate this, large xenon short-arc lamps are normally shipped in protective shields, which will contain the envelope fragments should breakage occur. Normally, 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. As lamps age, the risk of failure increases, so bulbs being replaced are at the greatest risk of explosion. Because of the safety concerns, lamp manufacturers recommend the use of eye protection when handling xenon short-arc lamps. Because of the danger, some lamps, especially those used in IMAX projectors, require the use of full-body protective clothing.

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 passing through the plasma cloud strike the anode, causing it to heat. 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 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 in 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 size of the arc makes it possible to precisely focus the light from the lamp. 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 employed in searchlights where narrow beams of light are 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 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 roughly 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.

Ceramic Xenon lamps

Xenon short-arc lamps also are manufactured with a ceramic body and an integral reflector. They are available in many wattages with either UV transmitting or blocking windows. The reflector options are parabolic (for collimated light) or elliptical (for focused light). They are used in a wide varitey of applications such as video projectors, fiber optic illuminators, and search lights. [ [http://optoelectronics.perkinelmer.com/content/Manuals/GDE_cermax.pdf Cermax Guide ] ]

Power supply requirements

Xenon short-arc lamps are low-voltage, high-current, DC 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. As an example, a 450 W lamp operates normally at 18 V and 25 A.

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 the lumen output of the lamp significantly, the xenon gas being used only to provide instant light upon the ignition of the lamp.

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.

References

External links

* [http://www.youtube.com/watch?v=SVpD8SWzKFM Exploding Projector Bulb] , filmed accident illustrating the hazards of handling high-pressure short-arc lamps
* [http://optoelectronics.perkinelmer.com/content/Manuals/GDE_cermax.pdf] , ILC/Perkin Elmer Cermax reference guide

ee also

*List of light sources


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