An optical scanning device for scanning a document to produce image signals related to the density of the image on successively scanned document areas includes a platen for supporting a document and a surface of substantially uniform reflectivity extending along the edge of the platen. A bar of light is directed such that it strikes the document and the surface of substantially uniform reflectivity and moves across the document in synchronization with optical scanning of the document. Light reflected from the surface of substantially uniform reflectivity is directed to a plurality of reference transducers which sense the position of the bar of light. A servo control loop, responsive to the reference transducers, positions a reflector during the scanning operation such that the area on the document being scanned is properly illuminated.