[BillA]

all done and said is predicated on assumptions, they are unavoidable
but it is instructive to be aware of such, and their potential for error when ignored

wrt heat transfer, 'ideal' contact areas are assumed
and even more significantly from an experimental viewpoint:
1) the 'source' is assumed to be perfect - which is never true wrt flatness
1A) while it is merely constant if only one source is used
1B) AND if no wear or degradation is occurring - which is known not to be true
2) the DUT is assumed to be representative of all such items - which is 'true' only if statistically validated
3) AND the source/sink thermal coupling is 'correct' - again, 'true' only if statistically validated

the above does not mean that testing cannot yield conclusions,
just that extreme prudence is needed in the interpretation of test results (a sample of one is always questionable);
all of the above listed 'variables' are interrelated and indeed interactive,
change one slightly and the apparent results are quite different

for illustration an enhanced bp image is attached made with an optical flat in a reflex viewer using monochromatic light from a filtered mercury bulb,
each 'fringe' (light/dark band) represents a height (difference) of ~0.000273mm (0.000010749in.).
- the example shown has a huge valley running across it, the square outline in the center is 10x10mm