The configuration of the nonconventional joint transform correlator provides an optical implementantion of the fractional correlation. The fractional correlation has the advantages over the conventional correlation such as a spectral distribution with no high peak and a very sharp correlation distribution. At the center of the correlation output plane, the fractional correlation gives the same value as the conventional correlation, though it is not shift-invariant but partially shift-variant. (C) 2002 Elsevier Science B.V. All rights reserved.