Cover
EVERYTHING YOU
WANTED TO KNOW
ABOUT XENON BULBS ....
(But Were Afraid to Ask)
By: RAY BOEGNER
Page One
WHAT!! BULB COSTS $700.00 ? BY: Ray F. BOEGNER -
STRONG INTERNATIONAL
I'm sure that most theatre managers and owners at one time or
another have questioned why a xenon bulb costs so darn much money, It's a
question that I think we have all asked, but probably never received a good
answer to. It's hard to believe that you have a choice between buying a decent
used car and a light bulb for the same amount of money. But there is a major
difference -- the used car won't put enough light through your projector to
even see the credits!
First of all, let's start pronouncing it right. The word is
"xenon", pronounced zee-non, not x-enon or z-on, Xenon is a rare gas derived
during the production of liquid oxygen. Is it expensive? Yes, compared to
oxygen and some other common gases such as argon and helium, The reason for the
expense of the gas is the small amount of it that is contained in the
atmosphere and the purity of the gas which bulb manufacturers require, Most
manufacturers buy xenon certified to be 99.9995% pure in order to minimize
sources of possible contamination in the bulb.
Why do they use xenon? The reason is simple; xenon at elevated
temperatures, such as the are of a bulb, most closely matches the desired color
temperature of daylight (6000 degrees Kelvin). The result of this is blue sky
in an outdoor scene on your theatre screen.
If it's a light bulb, where's the filament? There isn't one to
be exact. A xenon bulb such as the one you use in your lamphouse is a compact
arc discharge lamp that containstwo electrodes, an anode and a cathode. It's
kind of like
Page Two
encapsulating your
positive and negative carbon in a quartz envelope, but with one major
exception. You don't have to change the electrodes, because they are
nonconsumable. That's another reason we use xenon; it's an inert or noble gas
with a very low reaction rate.
The big electrode in the bulb is called an anode and is made
of pure tungsten. This is the electrode that glows so brightly when you view it
through the bulb observation window of your lamphouse, But don't be deceived by
its glow, because it provides no light at all through the lamphouse's optical
system. The reason that it is so big and glows so brightly is that it is being
bombarded by electrons from the cathode or smaller electrode, That also helps
to explain why it is made of tungsten, The surface temperature at the face of
the anode (closest to the are) is over 2000 degrees Centigrade and tungsten,
being a refractory metal, has a liquidous temperature of almost 3000 degrees
Centigrade. If a manufacturer were to use stainless steel or aluminum for an
anode, it would simply melt away in less than a minute.
The smaller, pointed electrode is called a cathode, This
electrode is also made of tungsten but with a slight twist, The tungsten is
doped with thorium which lowers the work function of the electrode and provides
more free electrons into the are. The thoriated tungsten also facilitates
easier starting of the bulb just as thoriated welding electrodes start easier
than non-thoriated ones. The cathode, while you can't see it with the naked
eye, provides the light for the optical system of your lamphouse. The focus
point, or F1, of the lamphouse reflector focuses on approximately
1/3 of the are closest to the cathode electrode. The F2,
or second focal point of the lamphouse reflector, is just past the aperture in
the projection lens.
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Three
Tungsten, itself, isn't found in your everyday rock, and as
with xenon, bulb manufacturers insist on extremely pure tungsten to minimize
possible bulb contamination. Pure tungsten, such as the anode material, is
extremely difficult to machine and requires expensive ceramic tool bits and
lots of time in machining. The cathode electrode requires the same tool bits
but is much easier to machine because it is thoriated tungsten. In most
circumstances, the electrode heads are press-fit or brazed onto an electrode
shaft to reduce material costs. The shaft material on the anode electrode must
still be tungsten due to the elevated temperatures which the anode operates,
However, the cathode electrode shaft is often molybdenum, which, while it is
still a refractory metal, it is much less expensive.
The envelope surrounding the electrodes and containing the
xenon gas is made of quartz, As with everything else in the bulb, temperature
once again enters the picture. The temperature of the quartz envelope is
between 600 and 700 degrees Centigrade during bulb operation, If one were to
use regular glass, you would have a molten mess within a few minutes of
operation.
What's this ozone stuff? Well, it's really quite simple. Ozone
is actually O3 which is an extra oxygen molecule. A xenon bulb is
capable of producing O3 if the quartz envelope allows light
transmission in the 180 nanometer level. Early xenon bulbs allowed this
phenomenon to occur. In certain circumstances, based upon the application,
ozone generation is desirable. But the projection booth is not one of those
applications. Henceforth, the development of "ozone-free" quartz, Ozone-free
quartz is quartz that is doped with titanium in the ingot stage of
manufacturing. This doping acts as a shield and blocks light transmission below
220 nonometers, The resultant is no ozone generation during bulb operation.
Now that we've discussed the key components of the xenon bulb,
we have to put them all together to make it work.
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First we have to make the envelope, This is done on a large
lathe using extremely hot flames of either hydrogen or propane and oxygen.
While a piece of quartz tube is inserted and turning in the lathe, it is heated
to a near molten point, and a glass blower blows into the inside of the tube to
form the desired envelope. They all end up looking the same because the glass
blower uses a carbon paddle as a guide or template when blowing. You can always
tell the glass blower in a xenon bulb plant, as he's the one with the
year-round suntan or burn, Glass blowers in a xenon bulb plant make pretty good
money and rightfully so. The molten or working temperature of quartz is around
1400 degrees Centigrade, and the glass blower sits only a foot or so away while
making the envelope.
After the envelope is made, it is usually cleaned in a bath of
hydroflouric acid which is real potent stuff and can actually dissolve quartz
if it is left in an undiluted mixture long enough. Now you have to put the fill
tube on the envelope, This is the dimple looking thing that you see on the
anode side of your bulb's envelope. It's put on that side to keep it out of the
path of the light, The hole for the fill tube is either blown or drilled in the
envelope by the glass blower and then a smaller piece of tubing is fused to the
envelope using a smaller hand torch.
The electrodes go through quite an ordeal themselves after
machining. First they are ultrasonically cleaned and degreased to remove any
machine lubricants. Then the anode electrodes are heated in a vacuum chamber to
around 2400 degrees centigrade and stay at that temperature for several hours.
The reason for this is to pump out those contaminants that would otherwise
outgas or evolve during bulb operation. The cathode electrode is also heated in
a vacuum chamber both for cleaning and for thoria activation. The thoria
activation is accomplished by elevating the electrode temperature for a short
period of time. This causes the thorium to migrate, or move, to the cathode
surface.