A method is described of optically performing a joint transform correlation starting from images displayed side-by-side on a spatial light modulator illuminated by a collimated light source. The image is focused by a lens onto a camera, the image is recorded by a frame grabber, and processed by a computer. By using a two pass process, the result is a measure of the correlation of the images. The images may be preprocessed in a phase-encoded chequerboard pattern and binarised by thresholding based on the average value of neighbouring pixels.