OPTICAL DISPLACEMENT SENSOR

An optical displacement sensor (34) comprises a reflective surface (36) and two or more diffraction gratings (44, 46) which, together with the reflective surface, each define a respective interferometric arrangement. The reflective surface (36) is moveable relative to the diffraction gratings (44, 46) or vice versa. Light from a light source (52) propagates via the interferometric arrangements to produce an interference pattern at a respective set of photo detectors (54, 56). Each interference pattern depends on the separation between the reflective surface (36) and the respective grating (44, 46). A collimating optical arrangement (48) at least partially collimates the light between the light source (52) and the diffraction gratings (44, 46). The optical displacement sensor is configured such that each diffraction grating (44, 46) comprises a set of parallel grating lines extending in a different respective grating line direction and/or such that a respective separate light beam (62, 64) is provided to each grating (44, 46) using a beam-separating arrangement (48) or plural light source elements.