Okay. I used to work at Osram Sylvania where all halogen headlamps are made; in Hillsboro, NH for a number of years. Having worked there with the “burners” A.K.A. lamp portion of the headlights, I can maybe shed a little light: 1) the glass used in MFG of them is ~ .100 thick wall; 2) is annealed at ~ 1630 F gradually up to that and back down to room temperature - simplified version - during part of its process before becoming what you see from a tube of Corning glass; 3) contains quartz. BUT the burner should never be touched by a bare set of fingers. Why? The fingers WILL leave oils, salts, etc. behind which will shorten the life of the lamp. The HBR gas mixture inside is at 8 ATMS +/- .4 and excites the tungsten filament to an extremely high temperature around it. Any oils left behind from a fingerprint will cause a local hot spot outside, and due to the transference laws of thermodynamics LOL, inside. The life of the burner is heavily dependent on the filament maintaining a temperature (insert material science nerd speak here) hence why a very hard, brittle material like tungsten does a fine job - too hot = shorter life of the filament…in a nutshell.
See, the very thin glass of the single glass halogen lamp is going to break when the fingerprint collects more heat , OR the fingerprint etches the glass…
So the glass is affected by differences in expansion , due to different termperatures, and by the etching weakening it…
Note that if the halogen gas bulb is enclosed with a larger cover, the larger cover is at a lower temperature… That is, the halogen lamps in incandescent standard sizes/sockets are actually a smaller halogen lamp inside a larger case… the high temperature is at the small bulb inside…
Welcome to the SDMB, and thank you for shedding light on the situation. I found your post illuminating.
Got any more bright ideas?
Normal bulbs use a glass which has sodium added to it, as does regular glass used for windows, bottles etc. The sodium ions lower the melting point of the glass and also makes the melt less viscous and easy to work. Unfortunately this also increases its thermal expansion and vulnerable to breakage due to thermal shock.
Halogen bulbs run much hotter than regular bulbs so normal glass will not work. Consequently they use a much purer silica glass, often referred to as quartz glass, because of its superior thermal properties. If you allow your sweat to contact the glass some of the sodium ions from the salt will leach into the glass so that you have regions in the structure with a higher thermal expansion coefficient than the rest, leading to cracking and failure.
An optical physicist colleague once told me about the sodium ion migration thing, though I did misinterpret his statement and for a while I believed that the sodium ions migrated all the way through the envelope and interfered with the halogen cycle. Local sodium contamination of the glass sounds much more plausible though.
This is one of those little snippets of information where my google-fu fails me, and I have found it very difficult to find a reputable cite for the effects of sodium ions on halogen bulbs, so it’s nice to have my colleague’s information corroborated; cheers ticker.
That’s what I’ve heard too.
I was told the same thing, in high school & college theatre.
I think the “don’t touch” applies to any bulb that normally runs very hot. That includes theatre lighting equipment, but halogens are probably the only normal household lamps where this would apply.
After eight years, has anyone figured out if this applies to halogen bulbs?
The jury is still out. Someone may come by and shed light on the matter, if he’s fairly bright.
Yes- halogen bulbs, in the sense of the little tubular jobsthat were all the rage in mid-1990s light fixtures.
Not so much in the sense of the halogen bulbs that look like a classic light bulb. Those are basically one of the little tubular jobs inside of a light-bulb shaped outer shell, so getting fingerprints on the outer shell doesn’t do any harm. (you can actually see how it’s laid out in the link)
Bit of a whoosh, I’m afraid, sir.
I have found the reverse interaction to be the more critical to remember when touching the bulb, ie fuck the integrity of the bulb, it’s the integrity of my finger as it singes and blisters touching a halogen gbulb that was just on.
Also the Doper who chimes in to tell us it’s all [current President’s or Stupid Republicans’] fault, and another rebuke from moderator for political jabs while changing light bulbs.
Agree with this.
I think not touching with bare fingers applies to all light bulbs that normally operate at a high temperature. I recall stickers warning not to touch the lamps in film projectors and also in theatrical lighting fixtures and follow spots. And in lighthouses also.
The gas in a light bulb does flow in circular paths due to convection. These convection currents carry heat away from the filament, and thus decrease the efficiency of the bulb. Double helix filaments, a.k.a. “coiled coil” filaments, were invented to help reduce the amount of heat transferred from the filament to the gas, thereby increasing its efficiency.
One to link to an XKCD strip.
Have we heard from anyone who makes their own light bulbs cheaper and better at home?