This paper develops a more accurate method of measurement for thermal diffusivity using photothermal displacement spectroscopy. In previous studies, thermal diffusivities for materials with a finite thickness have been determined by the angle of deformation and the phase difference, with regard to the relative positions of the pump and probe beams. In this study, a complete theoretical treatment and experimental measurement, through the photothermal displacement technique, have been conducted for semi-infinite solid materials. To verify the method developed, the measurement has been carried out for an alloy of pure copper and nickel with both finite and semi-infinite thicknesses. In the results of the study, the measured values for semi-infinite materials differed from those reported in the literature by at most 1% of the standard deviation, while the corresponding differences for finite materials were at most 3% of the standard deviation. Consequently, the model for semi-infinite solids is proposed as a highly accurate method for measuring thermal diffusivity. (C) 2009 Elsevier B.V. All rights reserved.