The invention disclosed includes a method for determining the magnitude and phase calibration of dynamic motion sensors (SUTs), such as accelerometers, velocimeters, and dynamically responding displacement transducers at low vibration frequencies, using an optical encoder as the reference. To calibrate the sensitivity, the dynamic motion sensor is mounted to a shaker mechanism which imparts harmonic linear motion in the axis of the sensitivity of the sensor. In the case of an accelerometer, the mechanism commonly includes another accelerometer as a reference sensor. However, the output of conventional reference accelerometers becomes inadequate at low frequencies. The subject of this patent is an optical encoder to measure the displacement, at frequencies at which the optical measurement is more accurate than that of an accelerometer or other dynamic motion sensor.