Originally Posted by MichaelB
Seriously, I take it any residue of any kind on the surface of the bulb shortens the lifespan?
TL ; DR : If fingerprint discoloration isn't obvious, the most likely failure is from line voltage being higher than what the bulb is designed for. A dimmer might help.
Yup. The residue from fingerprint oils or similar stuff on a quartz bulb will become very hot when the lamp is on. That will char the oils to become black. Any trace of light absorbing material on the quartz bulb leads to additional heating at that spot, which can produce failure through overheating, cracking or even blistering of the quartz. The bulb material will look 'funny' in the contaminated area, with some sort of discoloration or marking.
I've pulled bulbs that had a nice fingertip-sized bump with a black fingerprint showing.
Halogen lamps made using some specialized glass don't have this particular problem, but may still discolor from contamination.
This isn't the only way halogen lamps fail, though. The most common failures are simple filament burnout. The act of switching a lamp on rapidly heats the filament material, which places a stress on it. The initial current drawn by the cold filament is much higher than that drawn when hot. (As temperature rises, the filament resistance rises.) That in itself is not a big deal, but over time, as the lamp ages, the filament may lose metal from evaporation and develop a thin spot. Eventually a thin spot will mechanically fail from the temperature change when switched on.
The halogen lamp tries to run at a very high temperature, while prolonging the life of the filament with a nifty chemical reaction. The filament tungsten that evaporates would normally deposit on the inside of the quartz or glass envelope. The halogen reacts with the tungsten vapor at the moderate temperature near the envelope, preventing it from depositing to form that black coating seen in conventional bulbs. The tungsten halide compound breaks down at the very high temperature of the filament itself, re-depositing the tungsten metal onto the filament. The gotcha is that the tungsten doesn't necessarily re-deposit where it came from, so the filament can still develop thin spots.
The halogen concentration is picked to match the rate the tungsten evaporates at for the lamps designed voltage. At a slightly low voltage, the tungsten evaporation rate may be lower than the halogen gas pressure was picked for, and the halogen cycle won't regenerate the filament properly. This leads to early bulb failure.
At greatly reduced voltage, as with a light dimmer turned down, there isn't much evaporation from the filament, and the bulb won't blacken very much in spite of the halogen cycle not working. Bulb life is extended, although not as much as the old incandescent lamps would see. Effectively a dimmed halogen bulb is just a tiny incandescent lamp in this condition.
At a voltage above the design point, the lamp life becomes shorter due to the higher rate of evaporation of tungsten. The amount of halogen may not be sufficient to match the evaporation rate, leading to blackening of the envelope and eventual failure of the filament. Wikipedia
says the lamp life is proportional to
, and a 5% boost to voltage will cut the lamp life in half.
Lots more here, including pretty pictures:
ZEISS Microscopy Online Campus | Tungsten-Halogen Lamps