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What are short arc
lamps?
A short arc lamp is generally a spherical or slightly oblong quartz bulb with two
electrodes penetrating far into the bulb so that the tips of the electrodes are only a few
millimeters apart. An arc is formed between these electrodes. The bulb is filled with
xenon, mercury vapor, or a mixture of these at high pressure. The wattage can be anywhere
from a couple hundred watts to a few kilowatts. With the small arc size and this amount of
power, the arc is extremely intense.
Short arc lamps are usually used in movie theater projectors, searchlights, some stage
spotlights ("followspots"), and some scientific equipment requiring an extremely
intense light source. The high cost, short lifetime, limited efficiency, and major safety
hazards of short arc lamps make them impractical for general purpose lighting.
Hazards of and
safety precautions for short arc lamps
Short arc lamps have numerous hazards. The arc is about as intense as a welding arc or a
carbon arc. One should not look directly at the arc. If your eyes are already adapted to
bright light, you may get away with looking at the arc for up to a few seconds with no
permanent eye damage - but doing this is NOT a good idea. You won't hurt your eyes looking
at the arc through #12 welding glass - but there are other hazards. Filters made for
looking directly at the sun will also make the arc safe to look at directly. Most other
dark transparent materials will not protect your eyes since they let through enough
infrared to risk cooking spots on your retina.
The arc emits almost every kind of ultraviolet in the book, including large amounts of
UV-A, UV-B, and some UV-C. These different ultraviolet bands are bad for different parts
of your eyes. Number 12 welding glass and sun-viewing filters will protect your eyes.
However, the UVC, UVB, and the shortest UVA wavelengths can sunburn your skin. Serious
sunburn and increased risk of skin cancer may result from significant exposure to this
radiation. These wavelengths are blocked by ordinary glass, but for other reasons below it
is HIGHLY RECOMMENDED to only operate short arc lamps in fixtures designed and made for
them by qualified personnel - typically engineers and technicians who work for the fixture
manufacturers.
The shortest ultraviolet wavelengths emitted by short arc lamps can generate ozone, and
require that they be operated in an adequately ventilated area. If you can smell the
ozone, it can noticeably irritate your lungs within hours.
Short arc lamps operate at red-hot temperatures of up to 900 degrees C (approx. 1650
degrees F) and contain very high pressure usually in excess of 10 atmospheres and often
near or over 20 atmospheres. They can explode. Rarely, but who knows how rarely, they
explode when nothing can be detected as wrong. If operated in any way other than as
specified or past their intended life expectancy, the risk of explosion increases.
Explosions can be dangerous since they can result in red-hot pieces of glasslike quartz
being shot out in all directions, possibly with considerable force. These lamps should
only be operated in fixtures made for these lamps and designed to contain a lamp
explosion. Some of these lamps contain mercury, so you might not want one to explode in
your home even if there was no risk of fire or red-hot glass shrapnel.
Some short arc lamps require forced air cooling. This is another reason to operate them
only in fixtures designed for the particular lamp being operated. Some short arc lamps
have pressure well above atmospheric pressure even when they are cold. You don't want to
drop one and have it break.
Short arc lamps have quartz bulbs operated under stress at high temperature. The usual
halogen lamp rules for bulb cleanliness apply. Carbon deposits can absorb light and make
hot spots. Traces of any sort of ash, salts, metal oxides, or alkalis can leach into hot
quartz and cause strains which can weaken the quartz. You should not touch the bulb. If
you touch the bulb, you can clean it with alcohol so that no skin oil traces are left on
the bulb. It may also be a good idea to rinse the bulb with distilled water. Do not
operate a short arc lamp that has been scratched or chipped.
Short arc lamps should be operated only at the wattage and with the type of current for
which they were designed. Overpowering them is obviously bad. Underpowering an arc lamp
can also be bad, since a slightly-too-cool electrode does not emit electrons easily, and
the voltage drop in the cathode region of the arc increases and causes positive ions to
hit the cathode harder, which "sputters" off cathode material. This will
discolor the bulb and may cause the bulb to overheat or have an abnormal temperature
gradient somewhere and could make the bulb explode. Discolored bulbs as well as abused
electrodes can really overheat if operated at full power.
Some short arc lamps are designed for AC, and typically have two identical main
electrodes. Others are designed for DC and DC lamps usually have two visibly dissimilar
main electrodes. Operating an AC lamp on DC will overheat and/or excessively age at least
one electrode. Operating a DC lamp on AC will overheat and/or excessively age at least one
electrode, as will operating a DC lamp with reversed polarity. Some DC lamps require
well-filtered DC for full life expectancy and for proper function of aging electrodes.
Aging electrodes have limits in the peak rate at which they can emit electrons without
overheating or sputtering, and peak current has to be minimized. Excessive peak current
may accelerate aging of electrodes that are in good shape.
Short arc lamps may have to be operated in a specific position so that convection currents
- internal and external - don't cause vulnerable parts of the lamp to overheat. This is
another reason to use them only as directed in proper fixtures designed specifically for
the lamp being used. Follow all directions that come with the lamp and all directions that
come with the fixture.
With all this trouble and the high cost and short life expectancy of short arc lamps, it
is no wonder why they are only used where there is no substitute for a small, very intense
arc. Short arc lamps are not good toys for casual experimenters.
Please be VERY careful with any high pressure xenon lamps, especially the short-arc type.
Even when cold, they still represent a very significant explosion risk, with cold fill
pressures of 10-25 atmospheres. You drop one of these on the floor......... think of a
hand-grenade! All xenon lamps are now shipped in multi-layer protective packaging, with
many warnings about the use of FULL face and body protection BEFORE even opening the box!
As to the operation of these lamps, don't try to make your own gear, it's just too
expensive, and the ignition voltages required sometimes exceed 75kV. UV is a definite
problem, and also the explosion risk, so operation of this lamp will require a protective
enclosure.
Short arc lamp types
Short arc lamps can be mercury vapor, xenon, or mercury-xenon.
Mercury vapor short arc lamps contain a low pressure gas fill, typically argon, plus
mercury. These lamps start easily without multi-kilovolt starting pulses but require a
warmup period and have poor color rendering. The spectrum consists mainly of mercury lines
but also has some continuous spectrum. The color rendering is better than that of a
general-purpose unphosphored mercury lamp, but not good enough for color film projectors
since the spectral output is low on red.
Xenon short-arc lamps do not require a warmup period and have excellent color rendering.
The color is an icy pure to very slightly bluish white, with a color temperature usually
in the mid 5,000's Kelvin. This is very slightly more blue than noontime tropical sunlight
with clean air. The visible spectrum is almost entirely continuous spectrum, with faint
visible xenon lines. Near-infrared xenon lines are more significant and detract somewhat
from the efficiency at which visible light can theoretically be produced.
Xenon short arc lamps have the drawback of requiring very high voltage starting pulses
around 30,000 volts. The actual voltage requirement varies with what model lamp is used.
Mercury-xenon short arc lamps are a compromise between mercury and xenon in both
advantages and disadvantages. These lamps contain xenon at a pressure around 1 atmosphere
(when cold). When started, there is significant light from the xenon arc. Warmup may take
around a minute, at which time full light output is achieved. The spectrum has both
significant continuous spectrum from xenon and moderately strong mercury lines. The color
and spectrum are somewhat like that of a "daylight" fluorescent lamp, except the
yellow mercury line is stronger with high mercury pressure and this makes the short arc
lamp not quite as blue as the "daylight" fluorescent lamp.
Short arc lamp operation
A ballast is required to limit the current flowing through the lamp and also to provide
any necessary high voltage starting pulses.
Some short arc lamps have only the two main electrodes. Others have a third electrode for
applying the starting pulse.
In most short arc lamp fixtures, current limiting is achieved with an inductor. Some newer
fixtures have electronic ballasts. Since the voltage across the arc is typically low
compared to the line voltage, an inductor ballast will allow nearly constant current to
flow through the lamp even if the voltage across the arc changes due to aging or warmup.
Some electronic ballasts provide nearly constant power, and the lamp does not overheat as
badly if the arc voltage rises due to aging electrodes.
If the lamp has a starting electrode, the high voltage pulse may be obtained from simple,
inexpensive circuitry. If a lamp with only two electrodes requires a high voltage pulse,
then series triggering or pseusoseries triggering is required.
In series triggering, the secondary of a transformer that generates the high voltage pulse
is in series with the lamp. This transformer may be the current limiting choke, with the
secondary being the main winding and the primary being an auxiliary winding used only for
high voltage pulse generation. With xenon lamps, the necessity of making a ballast main
winding withstand 30,000 volt pulses can increase the cost of producing such ballasts.
Pseudoseries triggering is easier to accomplish with DC lamps than with AC lamps. In
pseudoseries triggering of a DC lamp, a diode or several diodes are in series with the
lamp. These diodes must withstand the pulse voltage since they prevent the main part of
the ballast from shorting out the starting pulse. The starting pulse device, typically a
low power very high voltage transformer of one kind or another, is connected directly
across the lamp and is reasonably high impedance so that it is not adversely affected by
the main voltage. The starting device ionizes the lamp, and then the main current flows
through the diode(s) and the lamp.
Pseudoseries triggering has its own drawback with pure xenon lamps. A diode stack that
withstands 30,000 volts can drop 20 to 30 volts when the main lamp current is flowing,
which is almost as much voltage as the arc normally has across it. The diode stack would
consume almost as much power and make almost as much heat as the lamp does.
Once the arc is established and the lamp is warmed up, the voltage across the arc is quite
low - often around 30 volts. Very high pressures are required just to get the voltage drop
of a short arc that high.
Limited efficiency
and lifetime of short arc lamps
Short arc lamps are not as efficient as general purpose high intensity discharge lamps.
Since the voltage across the very short arc is low, a high percentage of this voltage is
electrode losses. The voltage across the arc may only be around 30 volts. The electrode
losses are typically a 10 to 15 volt drop, so nearly half the power delivered to the lamp
is wasted heating up the electrodes. Even after allowing for electrode losses, a xenon
short arc lamp is less efficient that theoretically expected for a 5500 Kelvin blackbody
radiator, since the xenon arc has strong near-infrared lines. Secondarily, some of the
power delivered to the main body of the arc is conducted and convected from it as heat
instead of being radiated.
Thermal conduction losses can be significant for lower wattage short arc lamps. This loss
is not as bad as it is in general purpose high intensity discharge lamps since the arc is
shorter, but is still in addition to the horrendous electrode loss.
Expect short arc lamps to have unimpressive luminous efficacies around 40 lumens per watt
delivered to the lamp. Since the arc voltage is low and current is high, ballast losses
may be quite significant.
Short arc lamps have a limited lifetime, typically a few hundred hours. Any evaporated
electrode material will darken much to all of the inner surface of the bulb. This is
unlike conventional discharge lamps which have the electrodes in the ends of a longer arc
tube so that only the ends and not most of the arc tube are discolored. Metal halide lamps
may also have some sort of a halogen cycle that keeps the inner surface of the arc tube
clean, but most short arc lamps do not have halogen since halogen vapors cause starting to
be even more difficult than it already is. In addition, halogens have some effect on the
hot quartz which is already stressed enough as it is. This can result in silicon deposits
on the electrodes. Silicon can be vaporized from the electrodes onto the inner surface of
the bulb, adding discoloration.
Halogen problems have been overcome to an extent that permits the manufacture of short arc
metal halide lamps, discussed in a separate section below. However, the arcs in these
lamps are not as small nor as intense as in short arc mercury, xenon, and mercury-xenon
lamps of similar wattage so these lamps are considered "medium arc length" and
not true short arc lamps.
Short arc metal halide
lamps (HTI/HMI lamps)
Some metal halide lamps have short arc construction. These include most metal halide lamps
under 100 watts, as well as the HTI and HMI lamps. However, the arc is larger in a metal
halide lamp since the metal vapors in the arc easily glow at lower temperatures than
mercury vapor and xenon do. For this reason, the arc is less intense than the short
mercury and xenon arcs.
Conventional metal halide lamps of lower wattages have been widely available only in
recent years. The arc tube construction often resembles that of a short arc lamp. In the
Philips 70 watt metal halide lamp, the arc tube is a small sphere. The HID lamps used in
some auto headlights are metal halide lamps, with some xenon to give usable light output
before they have warmed up.
HMI lamps are special high intensity metal halide lamps used in some followspots and are
sometimes used for shooting movies. Larger multikilowatt HMI lamps used in moviemaking
have a spectrum with a very large number of lines, and the visible spectral content is
close to that of noontime sunlight. HTI lamps are a particular variation of HMI lamps.
Although the arc in an HMI lamp is intense, it is not as small and concentrated as the arc
in mercury and xenon short arc lamps. In fact, "HMI" literally means
"hydrargyrum medium-arc-length iodide". When maximum light concentration is
required, "true" short arc lamps based on mercury and/or xenon are used.
Written and copyrighted by Don Klipstein.
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