Recently, Zenobi, Hahn and Zare introduced thin-film resistance sensors for the surface temperature measurement of dielectric materials heated by pulsed laser radiation. They concluded that absorption or reflection by their thin-film sensor is in any case negligible, thus causing no systematic errors. We show theoretically and experimentally that the thin-film sensors can be very effective absorbers for the infrared radiation used by Zenobi et al. The absorptance of the thin-film sensor is negligible only if its square resistance is greater than 10 k-OMEGA.