James Newton, Host wrote:

> How does one know how much voltage to put through the lamp to heat
> the filament without producing light?

I think you need to specify that better. As I understand it, the filament
sends out radiation the moment it gets heated, no matter how little. That
radiation is in the infrared range, and the hotter it gets, the higher the
spectrum it sends out. So there's probably not really a point where it
starts producing light, it just becomes more and more.

You're getting probably closer by watching power, rather than current or
voltage. 

> So again, how do you know what the filament resistance / heat / current will
> be given a steady voltage?

Generated heat + emitted light = electric power. To know how much heat and
how much light, you'd need to know the temperature of the filament. This is
probably non-trivial, given that you don't know that much about the
filament. If you know the material, you maybe can estimate the temperature
from the increase in resistance from the cold resistance. The temperature
then gives you an estimate of the emitted spectrum, and that gives you the
percentage of energy emitted as light. This relationship probably can be
found described in some manufacturer literature, I'd hope.

As easier methods there are thumb-rule percentage of the rated power, or a
thumb-rule resistance increase from the cold resistance. The thumb rules
maybe created using the above...

Gerhard

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