Incandescent Bulb "Wear Out"

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Long ago, and far away, in my vacuum tube class as an EE, we discussed tungsten filaments. As time goes on, three things occur, if I remember correctly:
1. Some tungsten evaporates on to the globe, reducing the light output.
2. The diameter of the filament decreases, eventually leading to "run away," and the filament burns out.
3. Also, as time goes on, the filament becomes more fragile as it evaporates.
You get "hot spots," which speed the failure process. Adding argon or nitrogen help on the evaporation issue. Halogen lamps work even better.
There is also the "bathtub" curve issue. Many DOA, or "infant death," then quite a period of useful life, then they start to fail. Curve looks like an old-fashioned bath tub in cross section.
73 /paul W3FIS
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Nicely said. You learned well.
Your #3 is what usually leads to lamp failure as I understand it from the lamp engineers. The filament becomes thinner due to tungsten evaporation and it also becomes brittle and more sensitive to shock and vibration. At some point, even a slight movement or vibration of the fixture is enough to make the filament fail. Often, of course, the filament fails at turn-on when it heats quickly and may be subject to movement (like in a portable lamp) at the same time.
The "bathtub curve" applies to solid state devices, but I've not seen such a curve for inancescent bulbs. Early failures due happen, however.
Tomsic
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I broke one filament when it was on its way from the package to the fixture. These things are *fragile*. The ones with more supports are much better.
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put a dimmer on any incadescent bulb, run it around 95% of full voltage. Bulb life will be forever, they wouldnt burn out, but the insides of the glass will blacken and light output get so poor you will finally replace the bulb....
I saw this in a BIG chandlier at a hotels meeting room, for service training. The place was very dark, so much so people brought in extra lights just to see what we were working on.
Just a few of the hundred or so bulbs were brite, staff said those had burned out and been replaced
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Yes, certain bulbs will blacken much faster than others. Vacuum bulbs, which are usually the small low-wattage types, get black very fast. Gas-filled bulbs blacken more slowly because the pressure of the gas slows down the escape of tungsten from the filament. The Lighting Handbook says that a gas-filled bulb will have an average light loss of 15% and end up with a light output 20% below initial. Halogen bulbs recycle the filament particles that burn off, so their average loss of light is about 5% and only 6% at end-of-life.
Wall-box electronic dimmers - even when set at full output - don't operate the bulb at full voltage. There is a few volts drop just because of the circuit, but I've heard dimmer manufacturers say in meetings that some dimmers are designed with a 10% drop to lengthen bulb life. I've never checked that, however. Does anyone have voltage data that they might share?
Tomsic
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bob haller wrote:

That sounds correct to me. When we built our home some 27 years ago the four 25 watt "vanity" bulbs across the top of our main bathroom's mirror/medicine cabinet were a bit too bright for my taste.
I put a variable dimmer in a box inside the top of the cabinet with its knob shaft sticking up through the top of the cabinet where it's out of sight to anyone less than 7 feet tall.
I cranked the dimmer down a bit until the brightness was comfortable, and to this date all four of the original bulbs are still working fine.
And, just last nite the 150 watt incandescent bulb in the end table lamp I read by burned out on turn on and after I replaced it there was noticeably more light. It would have been fun to anticipate that and have a light meter handy to check the difference, but I haven't had one of those in about 50 years now, ever since cameras had them "built in" in one form or another.
Jeff
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In the Good Old Daze (1975) I had a spec booklet from GE that had exctlty that info for their incandecent and other lamps. It also had a diagram showing the surface temperatures in different positions and locations.
But try finding that today....
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