Right - and unfortunately it is somewhat dependent on external conditions
which may not be present in any lab test because the thermal regime dictates
the resistance at any time.
One thing that can be done is to vary the voltage and measure the current
to get a V-I curve. This still doesn't get you there as then one needs to
consider external resistance and inductance to try to get a non-linear
differential equation to give what you want. Even that would be an
approximation.
A second thing is to use a scope and get traces of voltage and current on
energisation of a bulb and from this get the variation of resistance with
time, for that particular voltage source. Another source, another place,
another R vs time curve.
