Touching halogen bulb causes premature failure. Urban legend or not?

Factual Questions
41

In this thread about “dippy birds,” there’s a link to this explanation of how they work. The explanation contains this caution: “If you handle your bird, clean the glass with alcohol or Windex or Dawn or something; the oil from your hands has a high specific heat, which damps the transfer of heat, and a low thermal conductivity, which attenuates the transfer of heat.”

That suggests to me that finger oil retards heat emission through through the area of glass it’s on, which would cause that area to get really hot. I have seen ordinary (non-halogen) automotive light bulbs deformed from heat. If they’re contained in a small space that retains their heat well, they can bulge out, which indicates that the glass actually softened. Halogen bulbs run hotter than these ordinary bulbs, so it’s quite plausible that an oily spot could overheat to the point of melting or otherwise weakening the glass.

42

There have been a lot of good theories here but I think people are making it too complicated. It’s a fact that oil absorbs more heat than glass (as do a lot of substances, but oil is the most common contaminant). If you get oil on a bulb, then less of the energy passes through as designed, a lot of it gets absorbed by the contaminant and really heats up. With most types of glass, if you heat them unevenly, they experience some degree of cracking. Depending on the kind of glass, this could result in micro-cracks invisible to the unaided eye, or an instant explosion of shattering glass. Quartz is pretty sturdy, so the likely course of events is an accumulation of microscopic cracks weakening the bulb to its eventual failure.

The above is going to happen in any bulb. But for halogen bulbs in particular, there’s also the issue that the tungsten redeposition happens at the hottest parts of the bulb. This is supposed to be the filiament. But if you have a hotspot on the envelope, you could end up with tungsten getting redeposited on the bulb. Thus, the filament is “boiling off” tungsten but it isn’t getting redeposited as quickly as it should, which could lead to more rapid failure.

As to the manufacturer’s instructions, the only question is whether they actually tested the theory or not. If contaminating the bulb is easy to avoid and could theoretically cause problems, it makes sense to warn people off of it even if you don’t know precisely how big of a problem this is.

43

This is true but, ironically, I think you’re making it too complicated, too. In my description of the etched crucible above there is heat and fingertip residue, but no tungsten filament plating. I think it’s simply stuff from your fingertip – salts, likely, although it might be oil – recating chemically with the glass to etch it. No “heat retention” necessary, and no tungsten deposition required.

44

Also, it doesn’t take a huge effort to not touch the glass, so there’s far more to gain than lose.

I always watch out for lazy/rednecky installers who think they’re above reading instructions and frown on “book learning,” as if engineers are merely rolling dice when they design products.

45

I have a desk lamp that’s had the same halogen bulb for 12-13 years. Really, it’s true.

46

That bulb is as old as this thread!

47

I know that this is an old thread, but this post stood out to me–and not in a good way.

This is NOT how the scientific method works. This proposed experiment doesn’t even begin to account for all possible variables. For all anybody knows, the bulb to burn out first could have any number of non-obvious flaws from the manufacturing process. Or it could have been handled incorrectly, and thus damaged, during shipping.

This experiment is a good start, but only a start. Repeat the experiment a few dozen times, and IF every one turns out the same way, THEN you can start having confidence in the results.

48

I worked with stage lighting, about 50 years ago(!). Lamps then were just high intensity incandescent (dimmable). Not touching with your bare hands was definitely true. We always used a damp alcohol rag to replace them. (And you didn’t forget more than once, partly because a burned-out lamp was often very hot, and you might have less than a minute during a scene shift to replace it.)

But we sometimes saw a fingerprint on prematurely burned-out lamps. Sometimes clear enough that the stage manager threatened he would look for a fingerprint match in the lighting crew to find the culprit. Those stage lamps got very hot, just as current halogens do.

49

I’ve got halogen ‘ordinary house bulbs’ in some of my ‘ordinary house sockets’. They don’t get hotter than other incandescent globes, and don’t come with no-touch warnings!

So I guess “current” halogens covers a spread of devices. I don’t know how they manage to stay hot inside without heating the envelope or socket.

The halogens in the old “overhead projector” used in our classrooms in the early '70s would last only minutes if they were handled on installation.

50

I don’t know what the old style flash bulbs were made of, but if you touched them with your bare hands, there would sometimes be a bubble in the ‘glass’ where you touched them and occasionally actually pop. Not really explode since the glass so thin. I’d always get scolded if touched the bare bulb before use and people would usually wipe them before putting them into the holder.

51

Yeah, halogen covers a spread of bulbs.

The “don’t touch” warning goes with high intensity ones. E.g., the ones used in overhead projectors. They have a special quartz glass.

(And an overhead projector was device used in schools in Olden Days to allow students to take a nap in the dark during a class.)

52

From Wikipedis:

“The bulb must be made of fused silica (quartz) or a high-melting-point glass (such as aluminosilicate glass). Since quartz is very strong, the gas pressure can be higher,[11] which reduces the rate of evaporation of the filament, permitting it to run a higher temperature (and so luminous efficacy) for the same average life. The tungsten released in hotter regions does not generally redeposit where it came from, so the hotter parts of the filament eventually thin out and fail.”

"Any surface contamination, notably the oil from human fingertips, can damage the quartz envelope when it is heated. Contaminants will create a hot spot on the bulb surface when the lamp is turned on. This extreme, localized heat causes the quartz to change from its vitreous form into a weaker, crystalline form that leaks gas. This weakening may also cause the bulb to form a bubble, weakening it and leading to its explosion.[25]

The small glass envelope may be enclosed in a much larger outer glass bulb, which provides several advantages if small size is not required:[2]

the outer jacket will be at a much lower, safer, temperature, protecting objects or people that might touch it

the hot-running inner envelope is protected from contamination, and the bulb may be handled without damaging it

surroundings are protected from possible shattering of the inner capsule

the jacket may filter out UV radiation

when a halogen bulb is used to replace a normal incandescent in a fitting, the larger jacket makes it mechanically similar to the bulb replaced"

Source: https://en.wikipedia.org/wiki/Halogen_lamp

I used to have a couple of home video halogen lamps and the packaging of the bulb not only warned against touching them with you bare hands and to wipe them with alcohol if you did, but also not to move the lamps before the bulbs were completely cool as the filament was extremely fragile when heated. The same applies to regular incandescent bulbs. If you shake a hot bulb, you can easily break the heated filament into multiple pieces resulting in that tinkle tinkle sound when you shake it.

53

I don’t know about your halogen bulbs, but the ones I have installed have two glass envelopes, an inner one simply enclosing the filament, and the outer one about the size of an old style incandescent bulb. They are rated at 53watts, but give as much light as a 75watt incandescent. So, they won’t need the ‘no touch’ warnings and they probably only get as hot as the higher rated bulbs they replaced.

54

Yup, seen those myself. (sorry, zombie thread etc, etc)

55

20 years a theatre tech and I saw plenty of prematurely aged tungsten halogen lamps with a clear fingerprint etched in to the glass surface. It’s not a myth. Quartz “glass” is pretty sturdy stuff otherwise. In linear lamps, a common “old age” failure mode was for one part of the filament to grow thin, sag and stretch until it touched the quartz envelope, which would then slowly bubble out to accommodate it. Biggest bubble I’ve seen in a still working lamp hung down 6mm and was about 15mm long.

56

I’ve seen Xenon lamps in fluorometers with fingerprints clearly etched into the quartz surface. If the light isn’t getting through there, and it’s not, it is heating the bulb up instead.

57

Not to divert attention unduly, but The term “urban legend” has been used by folklorists since the 1960s and was catapulted into the mainstream by Jan Harold Brunvand with a series of books containing the term in the title. I think the first one was published in 1980-1.

58

I posted here 14 years ago! I have no memory of this thread

59

Sounds like a good SCP hook.

60

… And in my junior year, that class was scheduled in a spare room in the administration building, which was kept 5 degrees warmer than any other building in the school …

(That teacher was working at the limits of his ability, and touching the bulb was only one of the mistakes he made).