This paper is on focused application of chosen thermal non-contact and contact methods for quality diagnostics of the transformer windings. The first part of the paper examines basic principles and application of non-contact temperature measurement on power transformers by thermovision. The second part is focused on analysis of thermal processes in distribution transformer. In the finally, it is examined experimental measurement on laboratory transformer with task to detect mechanical strength of transformer winding considering possible thermal effect of short-circuit currents. By means of the experimental measurements and following analysis it was showed the practical thermovision and contact thermal measurement by optical detectors for diagnostics of power oil transformers in field mechanical strength of winding material.