Detection And Location Of Very Small Print Defects In Real Time For High-Speed Digital Printing - Art. No. 60760e

This paper addresses the variations and adaptations of processes needed for verifying print quality in real time for high-speed digital printers. The term high-speed is here defined to be more than 750 printed pages per minute. The processes used for print verification are based on very high speed image processing technology applied to scanned images of the printed pages. Print verification is done by scanning the printed copies on the obverse and reverse web sides thereby forming two streams of digitized images. Digitized images from each scanned image stream are then spatially aligned page by page, line by line and pixel by pixel with digitized images from a corresponding source streams. The accuracy of alignment needed to compensate for the poor dimensional stability of paper creates a need for considerable sophistication in image alignment. The source and aligned scanned images are compared to find pixel sequences that are different. The intent is to identify all differences, either missing-black or unexpected-black, that are larger than a 0.01 inch (0.25 mm) square. These differences may represent unacceptable defects in the printed copies.