a quality - control device includes : an illumination system for illuminating one peripheral face of each folding box , an image capture device for forming a multiple image of the respective peripheral faces of a bundle of folding boxes , an image processing system for : i ) detecting within the multiple image each peripheral end of the slots of each box that separate its adjacent flaps , ii ) generating a data set indicative of the geometry of each peripheral end , and iii ) analyzing the data set to determine the width of each peripheral end .