Your question about Accuracy when performing a Qualitative scan is answered by considering that you are using the same instrument to measure relative temperature differences within the same image background(in most cases). Two schools of provided comparison of like data are commonly used by todays thermographer. The first compares a suspected fault to a background(like the cabinet) temperature within the same image and the second compares the suspected fault to an assumed similarly loaded component(like connections on C phase with the suspect fault on similar connections on A phase). In both cases you assume your instrument to be in cal and as such, linear in error. Your temperature difference between the two compared points should have only instrument and cal errors between them(assuming like components have a like E, no reflection and similar transmittance errors). This temperature delta is reflective of temperature difference under similar conditions and, with the same instrument, are fairly accurate representations of that difference temperature. Remember that in qualitative thermography you are providing a difference(delta) between two points(Qualitative) and not a temperature based on a reference point(like the freezing point of distilled water or absolute zero), A Quantitative figure.