Apparatus for sorting sheets according to their patterns

Apparatus for sorting banknotes in accordance with the printed patterns on their surfaces comprises: an illumination source (S) for illuminating the banknote; a scanning device (1), for example an array of photodetectors, for collecting light from a plurality of illuminated pixels of the pattern; a moving device for moving the banknote relative to the scanning device; and an analogue-to-digital converter (6) responsive to signals from the scanning device representative of the intensity of light from each pixel, to produce intensity signals in digital form; a digital memory (7) for storing the digital signals; digital correlator device (8, 10) for correlating the stored signals with a previously-stored set of signals representative of a set of standard patterns, the correlation being performed pixel by pixel; and a sorting device, responsive to the result of the correlation to divert the banknote in accordance with the most likely match.