So in between the exhortations to chew xylitol gum and the complaints about the dead trees and cobwebs left hither and yon by the landlord, my dentist grouses about the short life of the high-intensity bulbs that light the field of view of the bugeyed supermagnifiers she wears over her glasses.
I may not know much about how to keep my teeth from rotting (or at least not much that I translate into action) but I do know a thing or two about halogen lamps. You can google "halogen lamp cycle" if you want, but none of the initial references are much better than the version I'm going to give you:
Halogen lights contain several atmospheres of a mild halogen gas (typically iodine) instead of the usual vacuum or inert-gas fill in an incandescent lamp. When a filament gets really hot and starts glowing, tungsten vapor boils off the filament and condenses on the outer glass envelope, which is bad because
But when there's halogen gas around, as long as the temperature of the inside surface of the bulb is above about 250 C, the gas molecules react with the tungsten deposits and etch them away, leaving the bulb clean and a bunch of tungsten iodide (or bromide) gas floating around the inside. Better yet, when the tungsten iodide comes close enough to the very-high-temperature filament (2700 C and up) it breaks up into tungsten and iodine, and the tungsten deposits back onto the filament. So halogen bulbs have filaments that glow hotter (and hotter means brighter, because emission goes as something like the 4th power of temperature) and last longer than ordinary bulbs.
So why were my dentist's bulbs burning out so often? (Well, partly because they're not made in an outsourced factory somewhere where quality control isn't what it once might have been.) When she's not staring intently at a brightly magnified view of a patient's mouth, like any frugal yankee she turns the lamp down. Which is exactly the wrong thing to do with a halogen lamp: if the temperature of the inside of the bulb goes below about 250 C, the halogen cycle stops. No etching, no cleaning, no miraculous redeposition. The dimmed lamp burns out much faster than the one burning brightly.
It turns out that if you run the lamp at full power for a while after every dim period, you can clean off the tungsten deposits on the inside of the bulb. Unless the lamp has been running dim for too long, in which case either the filament will blow, or the thickly deposited tungsten will absorb enough heat to soften the quartz envelope, and the bulb will explode. (Sometimes if you catch it just right, it will bulge out into some kind of weird glassblowing shape, and you can turn the lamp off and have a mutant bulb to play with.)
So the next time I see my dentist I'll find out if her lamps are lasting longer. Oh, and the other thing: you can't run them at full brightness when you're going certain kinds of fillings, because the composite in the filling is cured by ultraviolet light, and the halogen bulbs put out enough UV to set it up rock hard before it's even properly slathered into place. Go figure.
Posted by wallich at August 28, 2005 09:43 PM