An interferometric position sensor for sensing the position of an object, the position sensor comprising: a) a light source arranged to emit light; b) an optical element having a partially-reflective surface arranged to split the light between first and second optical paths , which first and second paths are configured such that the split light is recombined so as to form an optical interference pattern dependent on the difference between the optical path lengths of the first and second optical paths; and c) a detector array arranged to measure the intensity of at least a part of the optical interference pattern, wherein the detector array comprises first and second photodetectors (A, B) spaced apart at a distance approximately equal to an integer number of fringe widths plus a half fringe width of the optical interference pattern or an integer number of fringe widths minus a half fringe width; wherein the optical element further comprises a reflective surface from which light propagating in the second optical path is reflected; wherein at least one of the first and second optical path lengths is dependent on the position of the object such that changes in the optical interference pattern can be related to changes in the position of the object; wherein the sensor further comprises a digital counter in communication with one of the photodetectors, the digital counter being arranged to count the number of fringes passing said one of the photodetectors, and wherein the digital counter is triggerable at an initial position of the object such that corrections to the initial position can be determined in dependence on the output of the digital counter; and wherein the sensor is configured to determine the velocity and acceleration of the object. a) a light source arranged to emit light; b) an optical element having a partially-reflective surface arranged to split the light between first and second optical paths , which first and second paths are configured such that the split light is recombined so as to form an optical interference pattern dependent on the difference between the optical path lengths of the first and second optical paths; and c) a detector array arranged to measure the intensity of at least a part of the optical interference pattern, wherein the detector array comprises first and second photodetectors (A, B) spaced apart at a distance approximately equal to an integer number of fringe widths plus a half fringe width of the optical interference pattern or an integer number of fringe widths minus a half fringe width; wherein the optical element further comprises a reflective surface from which light propagating in the second optical path is reflected; wherein at least one of the first and second optical path lengths is dependent on the position of the object such that changes in the optical interference pattern can be related to changes in the position of the object; wherein the sensor further comprises a digital counter in communication with one of the photodetectors, the digital counter being arranged to count the number of fringes passing said one of the photodetectors, and wherein the digital counter is triggerable at an initial position of the object such that corrections to the initial position can be determined in dependence on the output of the digital counter; and wherein the sensor is configured to determine the velocity and acceleration of the object. <EOS>