I said that, because if I now make a measurement on my ear fixture, it will definitly look different than on another fixture.
And since we don't want to measure different silicon ears, but the headphones, we need to take the ear part out of the measurement, which i don't know and is not easy to identify in generel.
Exactly, and because the impulse response is a compound of an amplitude and it's phase (or the other way round) either the impulse response or the phase would be nice to know.
I don't think inaccurate is the correct term. A measurement is a measurement. It exists and is validated in and of itself (if it was made correctly).
What you are referring to is a consequence of the fact that this is a field of research that is still short of some big findings to become fully appreciated and useful.
For example, your rig (the one with the ear canals) can still tell you if headphones A are brighter or darker than headphones B.
What it can't tell you yet is how close it comes to what a balanced sounding pressure wave should measure like at the eardrum. Because it's missing a target curve.
A target curve to me is exactly that. The frequency response at the eardrum of a sound pressure that is considered balanced sounding. I'll try to avoid another spin-off conversation about speakers, but alas that's where the most reliable data as far as balanced sound comes from, to this day.
So the target curve to me has already, intrinsically and undistinguishably, the acoustic properties of the measurement rig embedded in itself.
The correction would be the EQ necessary (both amplitude and phase) to make a certain headphone measurement match with the target curve.
I understand the want for a measurement that is just for the headphone, 'cleaned' from the pinna and canal contribution, but that would be of little value in terms of translation to a sonic meaning to the listener. Because the behavior of a sound source in a certain setting (how headphones sound when worn on our head) can only be measured meaningfully when they are in fact placed and measured on a (mannequin) head.
Just consider the difference in the behavior of the diaphragm when it's used in free air or in a more or less enclosed space such as it's the case when you wear headphones. These differences can't be corrected for by a lump factor that represents only the contribution of the rig. The headphone behavior is rig specific and can't be measured in and of itself (which is also the reason why we need a rig that accurately represents a human head to make the most accurate measurements possible).
If you do research at a university and your department has enough funds, I'd suggest getting a B&K 5128 and try to find a target for it. This involves the use of speakers that have a balanced response. If you don't know what I'm talking about we can chat privately. I can most definitely land a hand here.
You can also use your own rig and save the $41K, provided that it's a rig that has accurate ear canals and made of material that bends similarly to a real pinna (I could help here too, but only on the first requirement, unfortunately).