A simple form of accelerometer using interference fringes of a Fabry‐Perot system is described, in which accelerations as small as 10−5msec−2(10−6g) can be measured. The variations in the light flux transmitted by the vibration‐sensitive interferometer due to relative plate displacements as small as 0.25 nm (10−8in.) are photodetected and finally displayed as a waveform on an oscilloscope. A frequency analyzer in the detection system enables any chosen harmonic component to be studied and measured. The unit can be calibrated readily in terms of the wavelength of light during measurement for any given frequency. The unit has been used to measure low vibration levels in laboratory rooms where the amplitude and frequency range of the main disturbances usually were in the ranges 0–0.05 &mgr; (0–2 &mgr;in.) and 0–50 Hz, respectively.