The instructions with a replacmeent quartz halogen bulb (for the car or for a floodlight or whatever) always say not to touch the glass with your bare hands.
What is the reason for this?
Some websites say that the finger grease is burned off. Others say that the grease causes minute cracks to develop in the quartz glass. Are either of these correct?
And does the effect of finger grease on a bulb actually make a noticeable difference in real life?
The message from Svend Gr contains these words:
I've seen bulbs with a fingerprint neatly done in black on them where someone's handled them before installation. Whether this has shortened their life I could not say, but I can certainly imagine that the black spots would give local heating and anyway - they reduce the light output slightly.
The grease tends to cause a hot spot, which I guess may cause differential expsnsion and glass failure. However, I've changed halogen bulbs where the hot spot has actually caused a bulge in the glass!
It's not that hard to avoid touching the bulb, so just take care.
I assume that the presence of the grease causes that part of the glass to heat up (and hence expand) at a different rate to the surrounding glass which will causes a stress which makes the glass break.
Presumably there is some scientific evidence to support the suggestion that it does. No consumer is ever likely to have enough controlled experience to determine this in real usage.
tim
Film type sparks ignore these instuctions - and QI bubbles are very common. If it does shorten the life, it's probably not that significant. Will depend what's on your hands - although I'd have thought the remains of a location catering bacon butty would be a severe test.
However, for us lot here it's not really much of a problem to use the supplied sleeve.
I was told that the grease boils befor burning and this can shatter the glass.
I always replace halogen bulbs by holding them with a piece of kitchen paper then wipe them over with some meths to remove any fingerprints. It seems to do the trick!
You are also often told not to move equipment containing halogen bulbs, unless the bulb is not illuminated, in case the shock and vibration damages it. So, what is the most common use of halogen bulbs? Yup, in motor vehicles, where they get one hell of a shaking and a rattling!
I suppose automotive lamps will be designed slightly differently to ones intended for static use, but I do sometimes wonder how much of these precautions are OTT.
Rick
Some bulbs are designed to such a low price that they bulge anyway. I had a batch of 500W bulbs that I was using indoors, when I needed good light. They were the ones that came in security lights. The filliment was inadequately supported, and sagged a bit. When the filliment got close to the bulb, it made the bulb glow dull red (when switched off). If the bulb was not turned 180 degrees, at this time, then the bulb swelled out from this point, till it burst.
Yes, I've seen a good one where the fingerprint was clearly visible, and the side had bulged out. It really looked like a large finger had pushed it out from the inside.
The bulge is caused by the salts and organic products in your finger print being absorbed into the high temperature quartz, which causes the quartz to soften. Some of these lamps operate at reasonable internal pressures (unless they explicitly say no extra glass shield is required), so if the quartz softens, it is likely to bulge out.
I have heard a few times now that a lot of the smaller quartz lamps like those used in cars are actually not quartz any more, but I can't personally verify that.
Rarely, dirt and grease from handling will create a hot spot that causes the quartz capsule to fail. People who work with large numbers of quartz lamps (staging, film, etc.) see this regularly. Some Metal Halide lamps cost hundreds, or even thousands, of dollars. They, usually, are pretty careful with those. (Most such lamps come with an alcohol infused wiper (like a little wet nap from Swiss Chalet) that is used to clean the lamp after installation.
le moo
"Richard Sterry" wrote | You are also often told not to move equipment containing halogen | bulbs, unless the bulb is not illuminated, in case the shock and | vibration damages it. So, what is the most common use of halogen | bulbs? Yup, in motor vehicles, where they get one hell of a | shaking and a rattling! | I suppose automotive lamps will be designed slightly differently | to ones intended for static use, ...
Yes, and 12V or 24V bulbs have a filament 10 or 20 times thicker than a 240V one to start with.
It used to be standard practice to use transformers to step mains down to LV for task lighting on machinery for that reason.
Owain
Salt in your sweat acts as a flux to catalyze the crystallization of amorphous fused silica to cristobalite at operating temp. When the bulb is turned off fused silica, crystobalite, and quartz have different coeffcients of thermal expansion and densities, and surface microcracking occurs. When the bulb is again turned on and the cracks expand in tension, BOOM.
-- Uncle Al
Does AC or DC supply make any difference on the LV bulbs?
Adam
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