Pulse light is applied through a piano concave lens (14) to a thin metal film (15), which is evaporated on the concave side of the lens. The lens, which is formed of a high thermally insulating material such as glass, for example, and the film are disposed within a liquid (11) such as water, for example. The pulsed light is absorbed by the metal film, which converts the thermal energy produced by the pulsed light into elastic waves in the form of acoustic waves and simultaneously focuses the acoustic waves on an ojbect (20) within the water at a selected focal plane. The metal film is as thin as possible, consistent with complete absorption of the light energy by the metal film, so as to utilize the high thermal expansion coefficient of the water.