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