"Redbelly" writes:

Lots of factors are at play here. Changing the supply voltage will
change the filament temperature, and hence the filament resistance.
So cutting the voltage in half will not cut the current in half.

I made a spreadsheet which accounts for the changes in temperature and
resistance, and here's what I found:

Where did you get the formulas for these? As you say, an incandescent
lamp filament is far from a fixed resistor, but how does resistance and
temperature change with voltage?

Dave