A system (1) is used to excite an object (2) at a vibration frequency, in order to induce stationary or travelling waves having the vibration frequency on the surface of the object (2). An optical interferometer is configured to use optical interference to determine vibration amplitude and phase data of the stationary or travelling wave at each of a plurality of points on the surface, relative to a reference phase. A processing system (4) is used to apply a spatial derivative filter to the vibration phase data, and the resulting spatial-derivative-of-phase data is processed to determine a property of the object (2), and is further processed to generate graphical-representation data for outputting on a display device (10).