Many biological structure present periodic patterns. At the microscopic scale, skin surface, epithelial cell cultures or muscle cell striations (sarcomeres) are typical examples. Dynamic sarcomere length measurements are in no way easy to realize, due to the low contrast of transmission images, even with phase contrast techniques, the small amplitude of contractions, and the high deformation speed. The device presented here is based on a CCD matrix coupled to a PC computer via a specially designed interface. An image of the periodic pattern is projected onto the detector and accumulated along the lines ('binning' technique). After real time frequency domain transformation, the fundamental spacial frequency and the peak bandwidth are computed, stored into the PC memory and displayed on the screen at a speed up to 1000 values per second. Such high rates require a combination of hardware analog storage, flash A/D conversion and FFT algorithm optimization. As an example of results, the sarcomere length (approximately 2 mu) of a given cell can be measured every millisecond, with a standard deviation better than plus or minus 20 nm, even under low contrast and high random noise conditions.