Based on the theoretical configuration optimization, surface thermal lens(STL) and photothermal detuning(PTDT) experiments with a continuous-wave modulated laser beam excitation are performed with highly reflective films coated on BK7 and fused silica substrates. The amplitudes of STL and PTDT signals as a function of the modulation frequency are experimentally measured. The sensitivities of STL and PTDT in measuring the weak absorptance of optical coatings are compared. Compared with STL technique, the PTDT technique is experimentally simple and has a higher spatial resolution for absorption mapping of optical coatings, while maintaining the high measurement sensitivity. The experimental results show that for samples with a large temperature coefficient of reflectivity, the PTDT technique can obtain high measurement sensitivity. Agreement is obtained between the experimental results and the theoretical predictions.