Abstract: This invention concerns a heterodyne interferometer for measuring the amplitude of vibration of a vibrating object. The invention is particularly useful for measuring the amplitude of vibrations of objects having reflectivities below about 0.02% or for measuring vibrations amplitudes below about 10.sup.-17 cm.The invention comprises a laser and a beam splitter for producing an object light wave and a reference light wave; at least one modulator for changing the frequencies of the object light wave and the reference light wave thereby producing a predetermined offset; means for directing the object light wave onto the vibrating object; and a photodetector for producing, at a frequency equal to the predetrmined offset, a beat signal which varies according to variations in the interference resulting from combining the reflected object light wave and the reference light wave.