Incandescent lamps failing

[winnie]

120/130 squared is about 0.85, or 15% less power. [...]
Is there more to this? Perceived brightness from lower color temperature?
George

Yes and yes.

The _vast_ majority of the output of an incandescent lamp is in the form of invisible infrared radiation.

The proportion of the output that is in the IR depends upon the temperature of the filament. The cooler the filament, the less total radiation it puts out, _and_ the greater the percentage that ends up as invisible IR.

When the lamp is operated at 120/130 of its nominal full voltage, then the filament resistance goes down about 4%, and the power consumption goes down about 12%.

This means that the _total_ output will go down about 12%.

But the output is also shifting more toward the IR, so the _visible_ output goes down about 24%.

The term 'color temperature' is a way of describing the spectral distribution of a light source. A 'perfect black body radiator' at a particular temperature will emit light of a particular spectral distribution. Color temperature is the thermodynamic temperature of a black body who's spectrum best approximates the real source being described. A 'black body radiator' is an idealized hot surface, giving off light with no reflection or color of its own. A tungsten filament is actually a pretty good approximation of a black body, so the color temperature of an incandescent lamp is pretty well the temperature of the filament itself.

Please note that these are only approximate numbers.

-Jon

 

[infinity]

Trevor, even if the resistance didn't drop as the filament cooled, I don't understand your 24%. Power is voltage squared divided by resistance ... 120/130 squared is about 0.85, or 15% less power. BUT, the resistance will be lower (I don't know how much), so it will probably be between 10% and 15% dimmer.

Is there more to this? Perceived brightness from lower color temperature?

George

Remember that a visible light output change is not as simple as a proportional reduction in voltage. If you took a 100 watt 120 volt lamp and put 60 volts on it you would not get 1/2 the amount of light. Compared to applying 120 volts, at 60 volts you would get hardly any light at all. The same would hold true when applying 120 volts to a 130 volt lamp, you would get about a 24% reduction in light output. Watts consumed and light output are not directly proportional. Winnie gave an explanation of this a few posts back.