AVOIDING THE PHOTON-COUNTING HOLE IN SPECKLE IMAGING BY MEANS OF CROSS-CORRELATION TECHNIQUES

Real photon-counting detectors are unable to distinguish overlapping p hoton events. This defect limits considerably the performance of speck le-imaging methods. In particular, this defect results in the so-calle d photon-counting hole in the center of the average image autocorrelat ion. Cross-correlation techniques are investigated, and it is demonstr ated that they avoid the photon-counting defect for the purpose of spe ckle imaging with the bispectrum and the Knox-Thompson methods. Furthe rmore, a method that requires the recording of only two sets of simult aneous images is proposed for obtaining a quasi-unbiased estimator of the The validity of these methods is checked against simulated data an d under various flux levels. Signal-to-noise-ratio considerations are addressed. (C) 1997 Optical Society of America.