A theoretical and experimental analysis of the finite laser beam size effects in thermal wave interferometry has been reported. The limits of the heat transfer one-dimensional approximation, that is usually employed in this measuring method, have been discussed. This study has been performed by considering the actual three-dimensional temperature distribution induced by gaussian laser heating. Measurements on yttria partially stabilized zirconia coatings plasma-sprayed onto two different metallic substrates were carried out for different values of the heating beam dimension. ©1997 American Institute of Physics.