Abstract: A system for maintaining lateral alignment of web employs a rotating mirror which reflects a light beam from a source of coherent light along a track across the web path. A mirror driven by a synchronous motor is located at the focal point of a parabolic reflector which converts the circular sweep of light from the mirror to rectilinear light that scans along the track and is incident on the track from the same direction all along the length of the track. A retro-reflector on the track reflects the light back to a detector. Thus the system generates an electrical pulse as the light beam crosses the web edge and this pulse is compared to the phase angle of the power driving the synchronous motor. This phase relationship is then used to develop an error signal to return the web to its reference location.