This is one of those things that “Everyone knows”- that if you touch a halogen bulb with your bare hands, it won’t work. I’ve asked people who’ve told me this to elaborate and the answer is usually “Because. That’s why.”, or something to that effect.
However, I know that this crowd is a lot smarter than the people I come into contact with (OK, for the most part 
), So can someone please explain?
WAG ALERT!!!
I always thought it was because you left oil or dust from your fingers on the bulb.
Maybe it has something to do with…well, you know how you can put a pan of water on to boil and it will boil faster if there’s something in it that serves as a place for the bubbles to start…I figured it had something to do with that. Somehow the oil or dust caused a weakness at that spot on the bulb when it got hot, resulting in breakage.
Then again, I’m pretty much an idiot, so I’m sure I’m wrong.
Yeah, but you still have a much better explanation than anyone I’ve met so far.
People seem to be under the impression that if you touch it it will (POOF!) not work.
Hmmm.
I don’t know if consumer halogen bulbs get this hot, but in my stage and movie lighting days, we never touched new lamps with our fingers because the oil will boil and rupture the glass, causing the lamp to explode. (I’ve seen this happen, it makes a hell of a mess.)
Wow. Thanks!
There is another more fundamental process at work as well.
The oils left in your fingerprint also contain body salts and a host of other chemicals. There is a substantial amount of energy departing a halogen filament that is in the infrared region of the spectrum.
I would venture that most halogen bulbs have a quartz envelope. Quartz is virtually transparent to infrared light. If you cause any part of the quartz to become opaque, (i.e., leave a fingerprint on it) it is no longer able to transmit the infrared component of the optical spectrum and that energy is absorbed by the “glass” of the bulb.
The concentration of heat promotes a process known as “vitrification”. This drives a cascading reaction in the envelope of the bulb that causes a non-uniform or localized build-up of heat in one area of the lamp’s globe. The thermal stress involved can eventually shatter the bulb.
This same exact mechanism manifests in large “bell jar” type epitaxial reactors used in the semiconductor industry.
http://www.howstuffworks.com/question151.htm :
“A halogen lamp also uses a tungsten filament, but it is encased inside a much smaller quartz envelope. Because the envelope is so close to the filament it would melt if it were made from glass. The gas inside the envelope is also different - it consists of a gas from the halogen group. These gases have a very interesting property - they combine with tungsten vapor! If the temperature is high enough, the halogen gas will combine with tungsten atoms as they evaporate and redeposit them on the filament. This recycling process lets the filament last a lot longer. In addition, it is now possible to run the filament hotter, meaning you get more light per unit of energy. You still get a lot of heat, though… And because the quartz envelope is so close to the filament it is EXTREMELY hot compared to a normal light bulb.”
I always thought you don’t touch them cause they are really really hot!
I’ve seen the oil effect on a halogen lamp (BTW - the correct term for the fixture is ‘lamp’, not ‘bulb’, damned if I can remember why off the top of my head), and i’ve seen glass ‘blisters’ that were wider than the main body of the lamp, with the filament pulled into the blister. Oddly enough, these tyhings will continue to burn, but you get an inferior , smudgy light out of it (stage instruments are VERY dependent on lamps being in the correct position).