Optical sensors for acoustic waves and photoThey are easy to scan and they can have acoustic effects are used because no mechanical contact needs to be made to the solid material being examined. very good definition. The major disadvantage is that they are extremely insensitive compared to piezoelectric detectors. In photacoustic applications, they are employed to detect modulation of t he surface temperature, surface displacement, and surface slope of a solid, as well as the temperature variation of transparent solids, liquids, and gases. A number of well-established optical detectors, their applications, and their limitations are described. Particular a ttention is given to a new type of Wollaston prism detector, used in combination with a semiconductor laser, which is very simple and exhibits very high sensitivity.