Abstract: Apparatus for scanning a light-beam across a sheet mounted on a flat bed, the apparatus including a bridge that extends across the bed from one side to the other and is supported at each end on carriage guided along a respective one of said sides of the bed, a scanning head that is su ported on the bridge, drive devices at each end of the bridge to drive a respective carriage, location devices at each end of the bridge to monitor the position of each carriage along a respective one of said sides, and control device to control operation of each drive devices in accordance with information of the position of the respective carriage driven by the drive devices, determined by the respective location devices, so that both ends of the bridge are moved simultaneously in timed relation to one another.