Abstract: A method and apparatus for predicting the position of a register mark within an image on an elongate web relative to a reference position of the mark when the image is at a reference position is described. The apparatus includes a detector for obtaining successive samples of the image; a comparator for comparing the samples with a reference threshold so as to generate a binary sequence; and a comparator for comparing the binary sequence with a reference sequence. The apparatus is operable to carry out a series of comparisons between the two sequences with the sequences offset by successive one bit intervals and at each offset position, the number of coincidences between binary values is summed by counters. The resultant sums are then compared and the highest is taken to indicate the degree of offset between the two sequences and is used to predict that the register mark has the same offset.

Abstract: A method of and apparatus for detecting a register mark on an elongate web (3) is described. The web (3) carries a number of copies of an image (4) spaced apart along the web (3) and a region of the path along which the web (3) moves is irradiated with infra-red radiation by a source (5). The region is viewed with a series of detectors (1D-1F) as the web (3) passes through the irradiated region and the output levels of the detectors (1D-1F) are monitored for changes in their output levels. An edge extending transverse to the direction of movement (2) of the web (3) is then selected within the image (4) when the monitored changes in the output levels of all the detectors (1D-1F) satisfy predetermined criteria. The edge is then used for subsequent register control.