Abstract A linear combination threshold function, which is defined as the linear combination of input and reference power spectra, is proposed to binarize the joint power spectrum of a joint transform correlator (JTC). The performance of binary JTC depends on the linear combination coefficients of input and reference power spectra. This relationship is investigated and an optimal linear combination threshold function is determined to get a trade-off between some of performance criteria for character recognition. The theory analysis and verification by computer simulation are described. The proposed technique is also investigated by a hybrid optoelectronic system.