All good points, jraef.
In the past [think theatre and stage lighting days] I've played around with dimmers of the straight-up ohmic resistive type, the autotransformer [variac?] type, and those of the SCR / triac family, and one thing I've consistently noticed is that regardless of the type, if a dimmer is very slowly and smoothly brought up from zero toward full power output, there comes a point at which the bulb suddenly 'lights,' meaning that even in low-illumination situations I've almost never seen the bulb come from blackness to incipient glow to finally visible light; instead it suddenly is lit, and rather brightly at that.
My guess is that during the run-up the filament is getting hotter and hotter, and as its resistance varies directly with temperature there comes a point when the resistance rapidly spikes up, at which point the voltage developed across the filament rises rapidly, which reduces the current draw across the source, which as a consequence causes its output voltage to rise still further, which which causes the filament to glow even more brightly, which...you get the picture.
Once the bulb would light, it was possible to dim it back down very slowly until it went completely out, but was still giving off infra red.
Fun and games.
Assumptions? I'm ignorant of any...
Calculations? I never had any inclination whatever to try.
But if you want to measure the resistive value of the bulb with a very low-voltage ohmmeter at however many Kelvin degrees room temperature is, then calculate what resistance the bulb must actually have to consume sixty watts of electricity at full voltage and use your material resistivity coefficient tables to calculate how many degrees Kelvin the bulb has to be at to have that resistance...have at 'er; I'm bored already.