I've come upon this interesting paragraph on piclist.com (don't I already
know what is written there? Well, no, in fact now that there is about 3GB of
text on the site, I find more and more often that I have no clue were stuff
came from. Rather fun actually!) and I was curious about the unanswered part
of it: How does one know how much voltage to put through the lamp to heat
the filament without producing light?

"One feature of incandescent bulbs that you can exploit is that when off, an
incandescent filament is very low resistance- much lower than when it is
glowing. This allows lamps to be used to protect against short circuits.
=A0Also, you can trickle a small (fractions of a milliamp) current through =
the
bulb- if there is current flow, the bulb is good, none, the bulb is burnt
out. This method can only tell you the health of the bulb when the bulb is
off. Make the current small enough not to light the bulb. The reduced
resistance of a cold bulb results in larger current draw when it is first
powered; called "in-rush current". The filament color temperature (Kelvin)
at which the lamp is designed to operate determines the inrush current. A
300'K lamp will have an in rush current about the same as its operating
current, but a 2,500'K lamp will draw about 12 times the current when cold.
Most lamps will heat to normal current ratings in approximately 30-100
milliseconds. The in rush current can be reduced by using a "keep alive"
voltage to pre-warm the filament while not producing light."

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

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