Detektieren von Wasserzeichen

A system is disclosed for detecting watermarks or other marks made along a web of paper at regular intervals. The watermarks are detected by a video camera (40) activated by a frame grabbing pulse (136) in a control loop. The video for each frame grab pulse signal is digitized and processed to detect the watermark and to measure its position (124) within the frame. The error between the measured position and a frame reference is used to adjust (128) the value of a parameter (NEWCOUNT) representing the distance or pitch between successive watermarks. This value is in turn used to determine the moment of the next or a succeeding frame grab pulse. All timing is done in terms of measured length of web run by measuring pulses from an encoder (32). The control loop acts to track variations in the watermark pitch. The system may include circuitry (140) for detecting and measuring every Mth mark while at the same time generating an interpolated pulse (24) for each watermark usable in the control of a machine for cutting sheets of stationery paper from the web. Procedures are also described for automatically acquiring initial detection of watermarks on running the web up to speed. A fast infra-red strobe apparatus cooperable with the camera (40) is also described as a source (42) for illuminating the web.