An improved opto-digital joint transform correlator reducing the influence of defocus on image motion measurement

Joint transform correlator (JTC) is a highly efficient way to measure image motion and a hybrid opto-digital JTC (HODJTC) has been proposed by us in [CHIN. OPT. LETT., Vol. 8, No. 8]. Being different from the traditional JTC, only one optical Fourier transform is needed and the optically generated joint power spectrum (JPS) is used to compute the image motion in a digital way. Although a high measurement precision can be obtained through HODJTC, the defocus will counteract the final result. In this paper, the influence of defocus is analyzed and an improved HODJTC, whose sensitiveness to defocus is reduced, is proposed. By introducing randomly generated defocus, a series of cross-correlation peak images is obtained and a subsequent spatial averaging procedure is applied to these images to generate the final cross-peak image which is used to compute the defocus invariant motion value