The performance of phase-only optical correlators is usually reduced i
f the filter-plane phase differs from that prescribed for the classica
l matched filter. Current spatial light modulators, which frequently p
roduce less than 2 pi phase modulation, and interface circuits, which
quantize or incorrectly amplify signals placed on the spatial light mo
dulator, both can produce systematic phase errors. We examine these ef
fects using a model of correlation-peak. amplitude as a function of ph
ase error. The correlation peak is reasonably approximated as the prod
uct of an average of unity-amplitude error phasors multiplied by the a
verage amplitude across the filter plane. The trends predicted by this
new model compare favorably with computer simulations that use gray-s
cale images.