For the prompt detection of incorrect thickness on aluminium strip, patented laser triangulation sensors are employed as a modern alternative solution to isotope measurement systems. To save costs the places on the strip which are outside the permissible thickness tolerance are not provided with a foodstuffs-compatible coating and so the strip can be recycled without problem in production. At the point of measurement the sensors are mounted opposite one another, above and below the passing aluminium strip. Irrespective of the exact height position of the strip, the exact material thickness is obtained by a simple distance signal coupling, F=Ref. - (A + B), of both sensors.