Abstract: A method for observing a moving web having printed thereon a regularly repeated pattern and control features along the web, comprising mounting two photoelectric cells a fixed distance apart in the direction of movement of the web, the photoelectric cells being adapted simultaneously to scan identical data in different locations on said web, the cells being connected in an electrical circuit which is balanced when they observe identical data and which produces an output when they observe non-identical data due to one cell but not the other observing a control feature.