Re: [Phys-l] 50 year incandescent bulb

["Larry Woolf" <larry.woolf@ga.com>]

At 10:21 AM -0700 on 5/1/07, Jason Alferness wrote
> But then, I'm still a little confused why they can't design a 50 year incandescent bulb...

Chuck Britton replied:
My (limited) understanding is that a 50 incandescent bulb could be
designed - but it would not give off as much light as you would
expect.
I notice that (some?) 'long life' incandescent bulbs are 'rated' at
130 volts rather than the more common 120 volts.
I.e. the filament is cooler when it is used at 120 volts and is
putting out less than the rated wattage (wattage rated at 130 volts
=> longer life due to 120 volt use)
============================================
 
Bulb lifetimes are limited by evaporation of the tungsten filament.  At lower filament temperatures, the vaporization rate of the tungsten filament is reduced, but so is the light output. So the filament temperature is a compromise between light output and lifetime.
 
Larry Woolf
General Atomics