1. An optical displacement sensor comprising a source of optical radiation, an array of radiation detectors, at least one modulating element having alternating first and second modulating regions circumferentially spaced around a central axis of the element, the first and second regions having different optical characteristics and the transition between adjacent first and second regions being defined by a substantially radially extending edge, the modulating element being displaceable relative to the array of detectors so that the first and second regions are exposed to optical radiation from the source and pass by the detector array to form an image of said first and second regions of the modulating element on the array, a data processor connected to the detector array to receive therefrom respective signals dependent upon the radiation falling on the detectors, and characterised in that: the detector array comprises a two-dimensional array of detector elements which produces a two-dimensional image of the first and second regions; the processor is adapted to identify the orientation of at least two different radially extending edges of regions on the at least one modulating element from the two-dimensional image and to determine the position of the centre of the element from the determined orientation of the edges.