Interferometer

An interferometer with adjustable optical path length difference comprises a light source 4, an interferometer arm adjustable in length eg by movement of prism 3, and a photoelectric receiver 17 to detect the interference signals. An incremental sensor is coupled to a device 18 for adjusting the arm length and produces a reference signal which, in dependence upon the speed of the change of the optical path length difference, changes its frequency in the same manner as that of the interference signal. An interference pattern is projected by the interferometer onto the back of an eye 12 via polarising beam splitter 10 and quarter wave plate 11 for measuring the distance between different layers. Reflected light passes through the plate 11, splitter 10 and further splitter 15 to the receiver 17 and an observer 16. The incremental sensor may comprise fixed and movable optical gratings producing two signals 90{ out of phase.