On 08/12/2015 09:32 PM, Marwan Daar wrote:
Why not? Where else does the energy go? The filament is held up by the ends, so there is very little conduction. The bulb is evacuated, though a tungsten-halogen lamp may have some convective and chemical means of energy transport, the actual energy losses should be tiny as a fraction of the whole. And that may be balanced by the warm glass envelope radiating some heat back at the filament when the bulb reaches a steady state.
Do you accurately know the surface area of the filament? The
emissivity is only useful if you know the watts per square meter
leaving the surface from the temperature and the emitting area. In
the bulb we have a certain amount of emission which is caused by
the input electrical power; but we would get just the same from
another bulb with half the emissivity and double the surface area.
Either way, it seems that you can't assume that all the input electrical power is converted into radiant flux.