We are presenting a method to quantify the similarity of local mottling patterns in consecutively printed copies in sheet-fed offset printing. It employs image registration to extract precisely defined image regions in a series of printed copies. The similarity of these image regions is calculated using a point wise correlation method. By analysing the similarity in the mottling patterns in printing runs over several thousand copies we demonstrate that there are mottling patterns that are time stable and in fixed positions of the print. We termed this repeated occurrence of mottling patterns over hundreds of printed sheets a 'memory effect' in sheet-fed offset printing. Between 20% and 50% of the print mottle in two separate print trials consisted of such location and time stable patterns. It can be speculated that this time stable mottle is related to printing press parameters (e.g. rubber blanket, printing form....), whereas the remaining, non-stationary mottle is related to local variations of the printed paper or stochastic variations in the printing process. print mottle; image analysis; pattern; offset printing
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