Two of my colleagues have been told to never touch light bulbs while fitting them into the socket, something I’d not heard before today.
One’s been told that it’s something to do with the grease left behind on the glass effecting the flow of gas inside the bulb. I can appreciate marks might cause a minor variation in the temperature of the glass, but not the gasses inside.
We’re talking about 60w reflector bulbs with Tungsten filaments.
Can anyone offer a scientific explanation for why finger marks would effect a bulb like this?
And am I right in thinking the bulbs are filled with Nitrogen? Or do they use Noble gasses?
In some cases, grease residues from your fingers can cause the glass to heat unevenly and melt or crack - usually, this advice applies to bright halogen lamps, but there may be other types for which it is standard procedure to avoid touching the envelope.
I suppose it’s possible that uneven heating of the glass could set up some kind of convection inside, but if that’s the actual mode of failure for these cases, it’s news to me (not that I’m an expert in the field anyway).
Incandescent lamps have a doubly-coiled wire, usually of tungsten, inside a sealed glass bulb. The bulb can have a fairly hard vacuum or can have a weak vacuum with some particular gasses inside. If the glass envelope operates at lower temperatures of maybe 100 C or so, tungsten that evaporates from the filament builds up on the glass, leaving the filament weaker and the glass partly blackened. So, they came up with tungsten halogen bulbs, in which the envelope is designed to get very hot, hundreds of degrees. The backfill gas is a halogen or mixture of them (fluorine, chlorine, bromine, iodine, etc). This hot envelope and the halogen gas work together to get the tungsten to move back from the glass to the filament.
I think the idea about fingerprints is to avoid leaving grease on the glass, so that that spot does not get very hot and make the envelope break, but it might also change the circulation of the gas inside if it changes the temperature profile of the outside.
I change a lot of lightbulbs as part of my job testing new aircraft. The only lightbulbs that require extra care are the halogen bulbs, we clean them with isopropyl alcohol and wear white cotton gloves when handling them. All other bulbs are handled with bare fingers.
As noted, this is for halogen bulbs. The reason is that halogen bulbs are run at much higher temperatures than normal incandescent bulbs – they have to, in order for the reaction that re-deposits the tungsten on the filament to work. At these higher temperatures, it’s more likely for residue from your fingers to etch away at the glass envelope and damage it.
I’ve never seen this effect with light bulbs, but I HAVE seen my fingerprints neatly etched into the sides of quartz crucibles (that I thought had been cleaned) that I used for growing alkali halide crystals. These were taken to much higher temperatures – around 700-800 degrees C. And, even so, they didn’t compromise the integrity of the crucible. I have to assume that the bulb manufacturers are just being extra cautious.
To remedy having contaminated a halogen bulb, you can clean it with a cotton ball with isopropyl alcohol when it’s cool to the touch.
I have seen brown discoloration on a halogen bulb, in the pattern of a fingerprint at which point it’s not going to clean up. I seen a broken halogen bulb I think failed from something on the surface.
I have seen finger prints on incandescent bulbs too. I doubt there is much risk of the glass on one of those breaking.
I would also assume the crucibles are made of much thicker glass than that typically found in lightbulbs. Which, if the ones I’ve managed to break over the years is any indication, is very thin–less than a mm.
Crucibles are made of all sorts of materials, including, for some specialized purposes, graphite and platinum (I’ve used the latter – it’s good for making glass, which is incedibly corrosive when it’s molten). We had some quartz crucibles that I used for melting salt and growing crystals.
My advice: Don’t do it. Alumina is much better for salt crystal growth. And your fingerprints don’t get etched into it.
QED – the halogen bulbs I’ve used (and i’ve changed a LOT of them) have pretty thick envelopes. It’s hard for me to see them getting fatally etched by just fingerprint detritus. But it DOES look ugly.
My colleague has now said he was talking about Halogen bulbs, and not the ones we had a pile of on the counter. The ones I had in my hand when he started talking about this.
When I did high school theater we we told not to touch the bulbs because the oils would cause the bulbs to fail. There were a few bulbs we kept around because they had a spot on the side which had softened and the pressure from the hot interior had caused the bulb to expand there. It looked like the bulb was preparing to spawn another bulb asexually.
This is not only an issue with halogen bulbs. I am responsible for several fluorescence microscopes, and they have mercury vapor lamps that you’re not supposed to touch either. It is not an issue with normal incandescents or fluorescents, as they do not get hot enough for a little oil to matter.
One to answer the question
One to point out the hidden assumptions
One to answer the question given the new assumptions
One to cite Unca Cece’s answer in the column
One to say the first four are all idiots and get the thread Pitted.
I’ve seen bulbs fail because of finger oil. I service the television’s in a couple of sports bars. Two of the sets are older DLP rear projectors and use UHT bulbs. The most recent re-lamping of both units was not done by me, but by the bar owner and one of his employees. The one done by the employee failed prematurely, and I had to replace it. There were fingerprints on the glass, as expected.
Aside from the fingerprints etched into the jacket, was there any other indication of why the bulb failed?
I treat halogen bulbs carefully, and use things to held them in place while I install them, but I still don’t understand why finger oils would cause failure, rather than mere cosmetic blemishes. I’ve never seen a “budding” bulb such as gazpacho describes.
You forgot the Doper between the third and fourth position who asked for a cite; the Doper(s) who came in to say that while Celecil’s answer may have been correct when the column was written in 1992, it had been rendered obsolete by (cite, cite, cite, cite and cite); the ones who then argue that this is proof that Cecil does not and indeed never did exist; a plaintive question by the OP, who is a Guest and doesn’t associate Cecil Adams with the SDMB; a rebuke by a Moderator telling everyone to keep a civil tongue; and a SECOND Pit thread about the existence of Cecil.
Two identical sets, used identical amounts of time. The fingerprint was on the glass on the face of the bulb, and anything that impedes the light and heat in it’s path out of the bulb is a Very Bad Thing.
