1. In an apparatus for controlling positioning of a workpiece relative to a welding head while the welding head produces a welding arc by directing light in a predetermined pattern at the workpiece and sensing the light image from the pattern reflected from the workpiece to obtain information about the workpiece, the combination comprising: an image-sensing means positioned to avoid sensing of light directly from the welding arc, means for directing the light at the workpiece in at least one pulse having a peak intensity substantially greater than intensity of the welding arc, and means for blocking the light image from the sensing means except during a pulse from said light-directing means.