SIS-Stamp: An integrated inspection system for sheet prints in stamp printing application

Postal stamps with print defects are valuable collectables for philatelists. However, stamp printers strive to emit exclusively defect-free stamps. In order to achieve high quality of the production process, each sheet is inspected by especially trained staff. This kind of inspection is very flexible but it is tedious and leads to unstable and irreproducible results. Based on the experience in quality inspection, the ARC Seibersdorf Research Team set the goal to design an automated sheet inspection system (SIS), in which the front and rear side of the sheet are quality-inspected in a fully automatic mode using image processing methods. The print inspection covers each single stamp in the sheet and includes, among others, misalignments of the individual print phases and the perforation, detecting defects like smears, splashes and missing parts of the print. The features to be inspected, as well as the defect sensitivity, can be defined by a special setup program. The patented mechanical sheet transportation system separates the sheets, transports them to the inspection stations, and finally, stacks them in two trays for good and defective sheets. The prototype of the SIS-Stamp is installed in the Austrian State Printing Office (Österreichische Staatsdruckerei, ÖSD) for final inspection of postal stamps and vouchers.