The influence of additional filters upon the output of an optical correlator is studied for the case of a spatial filter measuring the size of rectangles. Additional filters leading to increased sensitivity are derived by means of a series expansion. One type of additional filter, consisting of several transparent slits, yields a linear relation between output signal and the deviation from the desired size. The other additional filter, effecting a quadratic output characteristic, is a properly dimensioned bandpass. Experimental results demonstrate the enhancement of sensitivity achieved with additional filters. As this method can easily be extended to a correlator measuring the diameter of circular apertures, additional filters may be of interest in the field of automatic production control.
A. Engel, "Additional Filters Enhancing the Sensitivity of an Optical Correlator," Appl. Opt. 12, 743-748 (1973)