Hybrid digital/optical system for pattern recognition

An initial Fourier transform on input data is performed by digital electronic means 2 using a specialised fast Fourier transform chip, the resulting spectral phase data being digitally added 4 to pre-stored template data 3 at high speed and loaded onto a phase modulating spatial light modulator 5. A subsequent optical Fourier transform 9 is used to generate a two dimensional correlation between the input scene and template. A non-linear thresholding spatial light modulator 10 is used in the correlation plane to detect a strong correlation peak at a speed commensurate with the template search rate. An area photodiode 11 is employed to convert the optical correlation peak to an electrical signal independent of its location in the output plane. This hybrid correlator arrangement exploits the asymmetry in the processing time requirements between the initial Fourier transform performed on the input image, which needs only to be performed once per input cycle, and that of the stored reference image. This may require several iterations to achieve recognition and so must be performed between 10 and 1000 times faster than the initial Fourier transform to maintain the required system response time.