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.
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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
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