BooKs HuB: 2013

Sunday 20 January 2013

Xenon Arc Lamp

A xenon arc lamp is a 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 that is close to natural sunlight. Xenon arc lamps are used in movie projectors in theaters, in searchlights, and for specialized uses in industry and research to simulate sunlight.

Types

Xenon arc lamps can be roughly divided into three categories:

Ø Continuous-output xenon short-arc lamps

Ø Continuous-output xenon long-arc lamps

Ø Xenon flash lamps

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. The lamp has a lifetime of around 2000 hours.

Xenon Short-Arc-Lamps

Xenon short-arc lamps use a fused quartz envelope with tungsten (and small amount of thorium for long life) 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. Because tungsten and quartz have different coefficients of thermal expansion, the tungsten electrodes are welded to strips of pure molybdenum metal, which are then melted into the quartz to form the envelope seal.

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. In order to achieve maximum efficiency, the xenon gas inside short-arc lamps is maintained at an extremely high pressure up to 30 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 speed. To mitigate this, large xenon short-arc lamps are normally shipped in protective shields

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. The output of a pure xenon short-arc lamp offers fairly continuous spectral power distribution. Some applications include light guide systems such as endoscopy and dental technology.

In xenon-mercury short-arc lamps, the majority of the light is generated in a 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 Ultraviolet output. These lamps are used primarily for Ultraviolet curing applications, sterilizing objects, and generating ozone.

The very small size of the arc makes it possible to focus the light from the lamp with moderate precision. 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. All xenon short-arc lamps generate substantial ultraviolet radiation.

The Ultraviolet 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.

Xenon short-arc lamps have a negative temperature coefficient like other gas discharge lamps. They're operated at low-voltage, high-current, DC and started with a high voltage pulse of 20 to 50kV

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.

Automotive Headlamps

Xenon automotive headlamps are actually metal-halide lamps; the xenon gas is used only to provide some light immediately upon lamp startup, as required for safety in an automotive headlamp application. Full intensity is reached 20 to 30 seconds later once the salts of sodium is vaporized by the heat of the xenon arc. The lamp envelope is small and the arc spans only a