A warm up current is sometimes maintained through incandescent lamps by
theatrical dimmers to reduce the inrush current and to smaller rise time
to bring the lamp to full (it's already part way there when it's "off"). I
think that when any current runs through the filament, you get light, it's
just a matter of how much (and what color, perhaps more infrared than
visible). We've set up dimmers where the lights looked like they were out,
but the customer calls back and says the lights are "ghosting." The light
becomes visible in the dark as your eyes adjust to the dark.

Long ago, a vacuum tube similar to an incandescent lamp was used as a
constant current source. These were "ballast tubes."

Radios in the 1930s used an incandescent lamp as a volume expander. The
lamp was across the audio at some point in the audio chain. Loud
programming would increase the lamp resistance, allowing the audio to get
louder. Quiet programming would decrease the lamp resistance, making the
audio quieter.

HP used incandescent lamps in their first audio generators to stabalize
the feedback gain. Too high an output level (headed towards clipping)
would increase the lamp resistance, decreasing the oscillator loop gain.

I've used incandescent lamps as current limiters in an electric vehicle
battery charger.  A near constant voltage source is applied to the battery
string through an incandescent lamp. As the battery voltage increases
during charging, the lamp resistance decreases, keeping the current from
falling as much as it would without the lamp.

Useful devices!

Harold


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