Abstract: A method for investigating a volume change of a sample under tension, with a sample being scanned by a first light beam and the light beam reflected from the sample being detected and further processed. The sample is then illuminated with at least one additional light beam of finite width, with the light beam being converted into a first voltage behind the sample, and a transverse pattern of different reflectivity is scanned by the first light beam longitudinally, and with the change in frequency, produced by stretching, in the reflected light intensity-modulated in this fashion being converted into a corresponding second voltage and the voltages are compared with one another, possibly after calibration.