Abstract: At least one dimension of the transverse cross-section of an object is measured in a predetermined direction by scanning the cross-section at constant speed with a laser beam which is moved in the plane of the cross-section but remains perpendicular to this direction. Successive pulses are generated representing the times, opto-electronically detected, at which the downstream side beam disappears and reappears. An electronic window is generated from said pulses during each of the time intervals of interception of the beam by the object. Clock pulses are counted during each of the windows and each of the counts obtained is compared with a predetermined digital threshold to validate the respective measurement.