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 closely mimics 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. Xenon headlamps in automobiles actually use metal-halide lamps where a
xenon arc is only used during start-up. Types
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 Each consists of a fused quartz or other
heat resistant glass arc tube, with a tungsten metal electrode 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.
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, these lamps saw wide use in movie projection, where they replaced
the older, more complicated carbon arc lamps. The white continuous light
generated by the xenon arc is spectrally similar to daylight, but the lamp has 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 power ratings ranging from 900 watts up to 12 kW. Omnimax
projection systems use single xenon lamps with ratings as high as 15 kW.
Lamp construction All modern xenon short-arc lamps use a
fused quartz envelope with thoriated 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, 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 not 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 deionized to make it electrically
non-conductive, 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 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, 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 discarded. 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 contains only xenon gas; and
xenon-mercury, which contains xenon gas and a small amount of mercury metal.
= Pure xenon= 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 with a color temperature of
about 6200K and color rendering index close to 100. However, 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. Light intensity ranges from 20,000 to 500,000
cd/cm2. An example is "XBO lamp", which is an OSRAM trade name for pure xenon
short-arc lamp. For some applications such as endoscopy
and dental technology light guide systems are included.
= Xenon-mercury= 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 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 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,
although in modern times they are being displaced by single mode laser diodes
and white light supercontinuum lasers which can produce a truly diffraction
limited spot. 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
substantial ultraviolet radiation. 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 readily passes UV
radiation unless it is specially doped. 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 shortwave UV blocking
coating on the envelope and are sold as "ozone free" lamps for solar simulators
applications. The company WACOM has also a long history of xenon lamp production.
Some lamps have envelopes made out of ultra-pure synthetic fused silica, which
roughly doubles the cost, but which allows them to emit useful light into
the 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 output power ratings with either
UV-transmitting or blocking windows. The reflector options are parabolic or
elliptical. They are used in a wide variety of applications, such as video
projectors, fiber optic illuminators, endoscope and headlamp lighting, dental
lighting, and search lights. Power supply requirements
Xenon short-arc lamps have a negative temperature coefficient like other gas
discharge lamps. They are operated at low-voltage, high-current, DC and
started with a high voltage pulse of 20 to 50kV. As an example, a 450 W lamp
operates normally at 18 V and 25 A once started. 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 proper power
supply. Automotive headlamps
In 1991 "xenon headlamps" were introduced for vehicles. These 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 and scandium are vapourised by the heat of the xenon arc.
The lamp envelope is small and the arc spans only a few millimetres. An outer
hard glass tube blocks the escape of ultraviolet radiation that would tend to
damage plastic headlamp components. The first xenon headlamps contained mercury;
newer types do not. 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.
See also List of light sources
References External links