Abstract: A method of detecting the presence and position of labelled material in a sample in which the labelled material either gives off light or can be stimulated to do so. The sample is imaged onto a CCD array which is scanned following each exposure. A cooled CCD camera or an image intensified CCD camera may be used. Measurements are performed on the data signals so obtained, to identify clusters of data values from adjacent regions of the CCD array caused by light incident on those regions. The measured signal values are compared with at least one threshold so as to distinguish clusters resulting from light emitting regions of labelled material from the remainder of the sample and the centroid of each light produced cluster of data values is computed with reference to the camera array, and a signal value corresponding to the centroid coordinates is stored in a memory together with the centroid coordinates of any other light produced clusters identified during the same interrogation.