Light Bulb Aging
Most recent answer: 10/22/2007
Q:
After long periods of use a grey spot develops on the inside of the bulb. Why is it so?
- Frank (age 20)
canada
- Frank (age 20)
canada
A:
Frank- I’m pretty sure that the cause is evaporation of the tungsten
from the hot filament. Some of the tungsten atoms fly off the hot
filament and recondense on a nearby part of the cooler bulb, forming a
coating which I believe is the grey stuff you see. When too much
tungsten has evaporated, the filament gets thin and breaks.
There’s an interesting way to reverse this process. Tungsten forms a gaseous compound with certain halogens (e.g. bromine) at room temperature. At high temperature, that compound breaks down. So when a molecule of it happens to hit the hot filament, it decomposes and redeposits the tungsten on the filament. That allows the bulb to run with a very hot filament for a long time. That’s how halogen bulbs are made. The bulb has to be kept small, so the tungsten has a good chance of getting back to the filament. In a small bulb, the walls need to be made of material that doesn’t absorb much light and can withstand a lot of heat. Hence we have quartz halogen bulbs, rather than glass halogen bulbs.
Mike W.
There’s an interesting way to reverse this process. Tungsten forms a gaseous compound with certain halogens (e.g. bromine) at room temperature. At high temperature, that compound breaks down. So when a molecule of it happens to hit the hot filament, it decomposes and redeposits the tungsten on the filament. That allows the bulb to run with a very hot filament for a long time. That’s how halogen bulbs are made. The bulb has to be kept small, so the tungsten has a good chance of getting back to the filament. In a small bulb, the walls need to be made of material that doesn’t absorb much light and can withstand a lot of heat. Hence we have quartz halogen bulbs, rather than glass halogen bulbs.
Mike W.
(published on 10/22/2007)