High accuracy image centroid position determination with matrix sensors: An experimental comparison of methods

There is increasing utilization of optical position measurement techniques in research and industry as demands for precision increase. A number of methods have been developed for this purpose. In the present study attention was directed toward the use of matrix array sensors such as the CCD or CID sensors used in digital cameras. The primary objective of the study was to determine the accuracy with which the centroid of a laser beam or the image of a circular incoherently illuminated object (such as a mirror) could be determined. Five different methods for finding the image centroid were compared experimentally to determine which would be preferable. These are the intensity, Kortegaard, quadrant, Fourier descriptor, and cross-correlation centroid methods. The quadrant centroid method is presented here for the first time and the other methods are briefly reviewed. Experimental results show that the intensity and cross-correlation methods had the best accuracies for both the laser beam and incoherent circular images. In both cases the maximum error observed was about 1.7 μm in-the image plane using a sensor with a pixel spacing of approximately 46 μm. The effects of laser beam wander and beam wander compensation methods were also studied. It was found that beam wander due to the thermal boundary layer at the front face of a laser can be largely eliminated by attaching an open extension tube or enclosed insulated tunnel to the laser.