Xenon Bulb Tech Pages

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After the electrodes are allowed to cool, they must next be assembled into a seal assembly, You see, if the bulb wasn't sealed, that expensive xenon would escape and our shiny, expensive electrodes would turn blue and green. There are three basic seals employed by the various bulb manufacturers in the world; the graded glass seal, the molybdenum cup seal, and the molybdenurn ribbon seal. Each manufacturer will tell you that their seal method is the best, but the truth is that they're all good if done properly.

A graded glass seal is performed directly onto the electrode shaft, and as the name implies, requires various grades of glass. You can't attach quartz directly to a tungsten or molybdenum shaft due to the drastic difference in their expansion rates while under heat. Therefore, a glass bead is first formed around the electrode shaft and then a glass of higher temperature is placed on top of it and another on top of that until you build it up to quartz. Once the seal is built up to quartz, it may be directly sealed to the quartz side arm of the envelope assembly using a torch.

The molybdenum cup seal is also sometimes referred to as a "housekeeper seal" and as the name implies, utilizes a cup or thimble-looking part called a moly cup. Quartz will seal to very thin molybdenum and the moly cup's edge is chemically or electrochemically etched down to a thickness of .0015 of an inch. Using a torch, the quartz is compression scaled to the moly cup's edge while it is turning in a lathe under a vacuum. The vacuum is required to prevent oxidation of the molybdenum and it makes the molten quartz collapse around the cup's edge providing a quartz-to-metal seal. The electrode shaft is then brazed to the moly cup, completing the seal.

The molybdenum ribbon seal has been around a long time, This method is somewhat similar to the molly cup seal, but instead of utilizing a cup, ribbon or foil is used. This

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ribbon is also etched down on the edges and is usually spotwelded or brazed to the electrode shaft, The complete assembly is then sealed to a quartz tube or side arm in a lathe while under vacuum.

All quartz work utilizing heat on the bulb is performed while the bulb and electrodes are under vacuum. Once the parts are put inside the envelope assembly, the bulb is attached to a vacuum station by means of the fill tube and the complete assembly has a vacuum drawn on it. The bulb assembly is then placed on another lathe and the side arms are shrunk down on the electrode shafts. This is done to insure some form of mechanical integrity during handling and shipment of the bulb.

The electrode spacing must next be set. You see, the amount of gas pressure and the distance between the electrodes determines the voltage that the bulb operates at during operation. The spacing is set on a lathe on graded glass and moly cup seal lamps, and is usually set by hand by an are spacer on ribbon seal lamps. Most manufacturers usually hold the are spacing tolerance on a theatre xenon bulb to plus or minus .010 of an inch during manufacturing.

Now that we've subjected this xenon bulb to all this heat, let's subject it to a little more. You see, quartz, being a member of the silica family, is very susceptible to strain patterns, especially after all that heat we've subjected it to. If we don't want our bulb to explode right after we've turned it on, we've got to get ride of those strain patterns. This is accomplished by heating the bulb assembly to 2100 degrees Fahrenheit and then gradually bringing it back down to approximately 1200 degrees Fahrenheit on a timed temperature slope. Most manufacturers also pump a vacuum on the bulb during this operation to remove those unwanted contaminants.

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Finally, now we get to the fun part. The bulb is a xenon bulb and it's only right that we put some xenon gas in it. Using sophisticated measuring equipment, the bulb is again subjected to a vacuum of approximately 10 to the minus B. This means one part per million of contaminates for us common people. Then the vacuum system is shut off and using precision measuring equipment, the bulb is pressurized to approximately 70 p.s.i. absolute pressure, Here again, for us common folk, that's approximately four times the pressure of the air we breathe.

One thing has always bothered me. Nobody ever asks how you get the bulb off of the fill station! You see, if quartz collapses when heated under a vacuum, it only stands to reason that it expands when heated under pressure. If you placed a torch on the fill tube after filling a xenon bulb, it would blow out and that expensive xenon would escape. Somehow that positive xenon pressure has to he reduced to that of a vacuum, so that the quartz will collapse like it did when we made the cup seal. This is accomplished by placing a cup of liquid nitrogen under the pressurized xenon bulb. The liquid nitrogen freezes the xenon in the bulb forming a "little pile of snow" in the bottom of the bulb. Therefore, we've taken a pressurized gas and formed an unpressurized solid and in turn, a vacuum, because the only gas in the bulb was xenon. A torch is now used to remove the bulb from the fill station by heating the fill tube close to the envelope. Once the bulb is removed from the liquid nitrogen, the solid xenon heats up and returns to a pressurized gas, Alas, the birth of that expensive xenon bulb!

Wait a minute -- we re not done yet! It seems that all lamphouse manufacturers design their systems to utilize different bulbs. This is done to insure your bulb business, because the lamphouse is the razor and the bulb is the blade,

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which requires periodic replacement, A 2000 watt bulb is a 2000 watt bulb, right? Yes, to an extent. But how that bulb is connected to a lamphouse varies from manufacturer to manufacturer. The final step in making the bulb is the end fitting or end ferrule configuration. The installation of these fittings is quite critical as it affects the overall focus of the bulb in the lamphouse. This operation, typically called basing or mounting, is accomplished on precision mechanical machinery utilizing optical comparitors and microscopes. Most manufacturers maintain plus or minus .010 of an inch on end fitting installation to guarantee repeatability in configuration.

Well, what's left? How about a little testing and quality control? I know it's hard for the guy who puts the bulb in the lamphouse and it doesn't work to believe, but I really believe that all manufacturers do test their bulbs. Most manufacturers even run them for a considerable amount of time to eliminate premature failures in the field. The bulb is tested in a lamphouse or burn-in fixture to ascertain that it meets the industry standards for voltage and amperage. Some manufacturers even spot-check their bulbs to determine lumen or light output, Last, but not least, enters the quality control inspector who checks the operational, mechanical, and optical quality of the bulb. Once his eagle eye has analyzed the product and the documentation associated with it, the bulb finally goes to packaging for encasement in its "tomb" until you unpack it to install in your lamphouse on a Friday night with people screaming,"WE WANT A SHOW!"

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