We introduce a new filter for an optical correlator that produces the
spatial derivative of the optical correlation between an input object
and a reference object. The derivative signal has a dark spot at the c
enter of the correlation output. The width of this dark spot is narrow
er than the width of the conventional correlator bright spot and provi
des more precise location of the input object as well as a higher sign
al-to-noise ratio. Experimental results are presented for the case in
which the new filter is written onto a programmable magneto-optic spat
ial light modulator. (C) 1995 Optical Society of America