Detecting the position of labelled material in an image

To detect the presence and position of labelled material in a sample 11 in which the labelled material either gives off light or can be stimulated to do so, the sample is imaged onto an image intensified CCD camera 22 which is scanned following each exposure. The measured data values from adjacent regions of the CCD array 20 are compared with at least one threshold so as to distinguish clusters of labelled material from the remainder of the sample, and the centroid of each cluster is computed with reference to the camera array. The centroid coordinates are stored in a memory together with those of any other clusters identified during the same exposure. The coordinates from each of a succession of exposures of the same sample may be accumulated for subsequent listing or display. Different detection wavelengths may be used for differently labelled materials. A tapered fibre optic array may collect the light from the sample 11.