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LEAKER
A leaker, as the name implies, is caused by a seal failure
which allows the xenon to escape from the bulb. This can be detected by
monitoring your system's ammeter. A leaker will have higher-than-normal
amperage and lower- than normal voltage. It is also signified by low screen
illumination and if you observe the are through the system's observation port,
you will notice a much larger flame of bluish color between the electrodes.
When looking at a removed bulb that is a leaker, you will sometimes sec
discolored electrodes and at times, depending on the severity of the leak, a
white or blue-colored envelope.
Three things can cause a seal failure: It wasn't made properly
to begin with; It was overheated in the system; or, It was mishandled during
unpacking or installation, The individual bulb manufacturer usually knows if
they've had a high incidence of leakers in a certain model of bulb. A good
theatre supply dealer can also usually tell you if they've returned more bulbs
of a certain model than another.
An overheated end seal is usually evident by the discoloration
of the lamp's end fittings. While a bulb's envelope really requires little, if
any, external cooling, that's not the case with the end seals. The maximum safe
end seal temperature recommended by most bulb manufacturers is 250 degrees
centigrade. The season for this on both molybdenum and graded glass seal lamps
is that tungsten and molybdenum oxidize at approximately 300 degrees
centigrade. When this happens, the mechanical structure of the refractory metal
is destroyed and it can no longer contain the pressure.
A discolored end fitting does not always mean that the system
has overheated the bulb. Inside the end fitting, where you can't see it, is a
braze of braided nickel wire to the
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tungsten or moly
shaft, This wire is there to provide a flex joint between the electrode shaft
and the end fitting, if the bulb manufacturer makes a high resistance or poor
brats of this wire to the shaft, the seal will also overheat and fail. Keep
good bulb records on the individual systems to see if a pattern is
established.
The last method of seal failure is caused mechanically. A lot
of times this occurs during unpacking and installation in the system. Don't put
excessive pressure on the end fittings of the bulb. The seal is only an inch or
so away and you can easily fracture it, Never install an end cable without
cupping the fitting you are putting it in, with your other hand!
LOW LIGHT OUTPUT
There are several things that can cause low light output of
the xenon bulb. As we've discussed earlier, both a leaker and a contaminated
bulb can cause low light output along with other problems. However, if a bulb
is operating within electrical specifications and is not contaminated, then the
problem lies elsewhere.
The light output of a xenon bulb falls off drastically over
the first 200 hours of operation due mostly to the cathode electrode. With a
brand new xenon bulb the origination of the are is much like the point of a
pin. There are a large number of electrons confined to a very small area, and
the lamphouse's optical system is focused on this point. With time, this
pinpoint widens to one that more closely resembles a ten-penny nail and, while
the same number of electrons are still present, they are spread over a wider
area and cannot be as efficiently optically collected by the lamphouse's
optical system. This phenomenon is very evident if the bulb
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is operated in a micro-radiance fixture which projects the
bulb's are, It is called a diffuse are and cannot be corrected with the current
state-of-the-art technology. Therefore, be certain to install a lamphouse and
bulb that will meet your illumination requirements when the bulb has 20UO
hours.
In theory a xenon bulb, when first installed, should meet your
illumination requirements when operated at 80 percent of its rated amperage, At
the end of the warrantied bulb life, the same bulb should meet your
requirements when operated at 1OO percent of its rated amperage. However,
simply raising the current won't guarantee the results. One should check the
focus of the bulb in the lamphouse at approximately halflife of the bulb and
readjust if required. It's also a good idea to periodically clean your lenses
and porthole glass. If a clean single sheet of glass causes approximately 8
percent light loss through an optical system, just think what a dirty one must
cause!
UNSTABLE ARC OR FLICKER
To analyze this problem, you must first determine what problem
you have. This is accomplished by observing the are through the lamphouse's
bulb observation port. You can usually sec an unstable are, while you can only
observe screen flicker on the screen, Screen flicker is usually caused by the
light level being too high, above ZD foot lamberts. If you see screen flicker,
try turning the power supply amperage down and see if the problem corrects
itself. Don't forget what happens if you run the bulb at too low of an amperage
setting -- you may have to defocus the bulb to maintain proper amperage
settings.
On the other hand, an unstable are is a much more complex
problem. There are several things that can cause
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this phenomenon, both within the bulb and within the
lamphouse. Let's examine the most common ones.
Cathode electrode - An improperly
processed cathode will not have sufficient surface thoria and in turn will
raise the work function of the electrode. This will cause the ball of the are
to constantly seek an area with a lower work function on the cathode surface.
On older bulbs with more rounded, or ten-penny-nail-looking cathodes, the are
will constantly pop from a diffuse are to a point source on the cathode as the
electrode heats up during operation. Another possibility is an improper vacuum
prior to filling of the lamp. Oxygen, while we need it to breathe, is extremely
detrimental to the cathode electrode. When oxygen is present within the bulb
and it is operated, the oxygen molecules combine with the thoria on the cathode
tip to form thoria-oxide, which raises the work function of the cathode.
Power supply - A power supply with a
high ripple percentage can also cause an unstable are. This is caused because a
theatre xenon bulb is a point source device and requires very pure direct
current. You'll never get flicker on a perfect xenon bulb if you operate it on
batteries. However, this is impractical and very expensive. In layman's terms,
ripple is defined as an AC component in a DC line of a power supply. If you're
a technician, you can check for this by setting your multimeter on AC and
reading the power supply AC output. There are far more accurate ways to read
this however, and it makes sense to contact your dealer or manufacturer for
their recommendations.
Overcooling - In the earlier days of
the xenon lamphouse and bulb, we were always taught that you could never have
too much exhaust or airflow. This is far From the truth! The cooling of a xenon
bulb is to prevent seal failure only. There are 30,000 watt bulbs in solar
simulators