Laser-induced surface deformation is a widely used technique for studying thin film coatings. In this technique the deformation is typically detected by using the well-known optical beam deflection technique. In this paper we report a different technique in studying the deformation by using optical diffraction effect. In this detection scheme the laser-induced thermal bump behaves as a curved reflection mirror which can either focus or defocus a second probe laser beam through optical diffraction, depending on the specific geometry used. This surface thermal lensing effect is demonstrated to be a sensitive and easy-to-handle method for thin film characterization. The potentials of this technique and its advantages over the conventional method for thin film absorption measurement and defect characterization are discussed.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.