The acousto-optic technique presented by Wevers, Devolder, Leroy, and De Meester [Appl. Phys. Lett. 66, 1466 (1995)] was based on the information concerning the phase shift between the incident and the reflected ultrasound at critical angles. Two laser beams were used, one being diffracted by the incident ultrasound, the other being diffracted by the reflected ultrasound. Information concerning the geometrical shape of the sample as well as information concerning the thickness of a thin layer could be obtained from measurements in the near-field of the light diffraction. In this letter it is shown that using only one laser beam, traveling through the intersection region of both ultrasounds, exact thickness information of the layer can be obtained. Theoretical calculations are derived and compared with practical measurements for a 3 mu m copper layer on a stainless-steel plate. (C) 1996 American Institute of Physics.