Performing photothermal surface displacement (PTD) technique at &lgr;=10.6 &mgr;m in a vacuum chamber, the measured signal is obtained by the change in probe beam deflection at the bulge of the reflecting heated sample surface. The influence of the accuracy‐limiting quantities such as angle of incidence of the probe beam as well as its finite lateral extension, and the detector geometry and the temperature‐dependent reflection at the sample surface has been investigated by a numerically performed simulation in order to optimize the measuring procedure. The maximum resolution power is achieved for probe beam diameters small compared to the heating beam diameter. An unambiguous measurement may be carried out satisfying the following condition between the quad‐cell slit widthdslit, the probe beam diameter 2rPB, and the heating beam diameter 2rHB: 2rHB≥2rPB≫dslitkeeping the probe beam angle of incidence &agr;PBas small as possible. Moreover, it has been shown that the contribution of the temperature influenced reflection on the change of the probe beam intensity can be also neglected at &lgr;=10.6 &mgr;m.