A laser interferometer for sub-nanometre measurements in the cochlea.

A modification of a light microscope is described here which allows measurement of nanometre movements along the optical axis of the microscope. The light path is reflected off 10-mu m-diametre small glass beads which are individually imaged with the microscope objective. The interferometer is under computer control to allow it to remain in quadrature and so maximise sensitivity. The algorithm is described. Although the techniques are applied to detection of movements of the cochlear partition, they can be used to measure sub-micron movements of any reflecting structures accessible to microscopy.