..........
>
> Almost certainly, because the Q of the filament is so low that the
> reactance probably doesn't count below 1 MHz or so. There are tubes
> specifically made for power measurement which consist of two identical
> straight filaments mounted in an evacuated envelope. The power to be
> measured is fed to one filament and the current in the other adjusted
> from a separate source to give equal brightness (the eye is very
> sensitive to small brightness differences), at which point the two
> powers can be considered equal. I've used a similar method to measure
> powers at frequencies as high as 1200 MHz and the accuracy, compared to
> a VHF wattmeter, seemed to be within 10% or better.
And the inaccuracies are probably evenly divided between the lightbulb and
the wattmeter, or due to things like mismatch.