The faults associated with departures from width tolerance esp. for sheet, show up as differences in brightness relative to the surroundings. Two conventional photoelectrically derived partial width signals are summed to give the total measured width and this is compared with a nominal value set on the equipment. The position is derived from the total width, with the two separate partial widths. This stage is not carried out when information on the centre line is available from a supplementary detector on the roller table.