A new method is presented to measure in‐plane vibration amplitudes of the order of some 10 nm. The measurement system utilizes the speckle effect and consists of only a few mechanical components. The evaluation is done by a microcomputer. The whole system is insensitive to environmental noise. The method has been applied to investigate the amplitude distribution of plano‐convex AT‐cut quartz crystals as they are commonly used in thin‐film thickness monitors.