REGISTER MARK DETECTION

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.