Sensor for sensing absolute angular position of a rotatable body

An angular position sensor comprising at least one body at least partially surrounded by a housing, the body rotatable about an axis of rotation fixed with respect to the housing, the body having a grating element attached thereto or integral therewith, the grating element comprising a surface of revolution about the axis of rotation, the surface comprising regions of high and low EMR reflectivity, the sensor also comprising at least one EMR source and at least one array of EMR sensitive detectors, the source irradiating the surface and the array receiving incident EMR reflected from the surface, the source and the array fixed with respect to the housing, a pattern thereby produced by incident EMR on the array at any instant of time resulting from the alternating regions of low and high reflectivity on the surface of the grating element, regardless of the angular position of the body, the output from the at least one array resulting from the pattern on the at least one array at said any instant of time is processed by a processor to derive the absolute angular position of the regions with respect to the housing, and hence provide a measure of the absolute angular position of the rotatable body with respect to the housing.