1. A method of operating a pyroelectric array comprising a two dimensional array of detector elements onto which the image of a scene is focussed by optical means, whereby events within the scene cause temperature changes at the detector elements to produce electrical signals, wherein during normal operation the image is stationary with respect to the scene and only events within the scene cause the elements to produce electrical signals and wherein the image of the scene is displaced with respect to the array to generate a set of detector element signals corresponding to a reference image indicating the position of static objects in the scene.