A module (28) for sensing the edges of a banknote as it passes through the module. The module consists of an array of LEDs (32) optically coupled to a light diffusing element (36) past which the banknote moves. A CCD array (40) has photoelectric conversion elements coupled to the path in which the banknote moves by a self-focusing fibre-optic lens array (38). The CCD array consists of four segments (42,44,46,48) having individual shift registers which are read out in parallel, and converted to digital signals by a number of comparators (56). The comparators compare the CCD pixel output with individual threshold levels which are stored in a digital memory (54). The pixel threshold levels are converted to an analog signal for comparison with the CCD output signals by digital to analog converters (54). The digital data streams produced are divided into series of data which are analysed to determine the presence of an optical transition in the corresponding pixel series of the CCD arrays. If a transition is identified, the location of the pixel series is transmitted to a memory (58) for use in banknote validation.