2009 Volume 2
Oxfordpp 1293-1323Probabilistic analysis of small-scale print defects with aligned 2D measurementsAbstractPDF
We present an analysis of the pointwise relationship between the reﬂectance of print and the surface topography of the paper before printing. We have measured the surface topography and reﬂectance of paper before and after printing in a sheet-fed pilot offset printing press. The 2D measurement maps have been aligned to obtain local print reﬂectance and surface topography values for every spatial position on the samples. In contrast to the various deterministic modeling approaches, which imply an a priori deﬁned underlying mathematical model, we apply probabilistic analysis. Therefore we ﬁrst estimate joint probability density functions (pdfs) of local topography and print reﬂectance using Gaussian Mixture Models (GMMs). From these pdfs we select paper regions with unusual properties, i.e. regions from the tails of the pdfs. These anomaly maps are analyzed for interrelations between the print reﬂectance and surface topography, its gradient and local variance. The degree of interrelation is characterized by the mutual information (MI), a measure to quantify statistical dependence without making assumptions about the linear or nonlinear nature of the regression dependence. The signiﬁcance of the MI values is conﬁrmed by simulation based statistical hypothesis testing. The objective is to offer answers to the question: How does the observation of an exceptional topography point on the paper surface change our information about whether the print quality attainable at that point will be exceptional or not? The results suggest that topography in combination with its local variance have the most prominent interrelation to small scale print anomalies. Furthermore it is shown that regions with abnormal topography have at least ten-fold higher probability to exhibit exceptionally high print reﬂectance, compared to randomly selected regions.
Linting is the removal of material from the surface of uncoated grades of paper during offset printing. Excessive linting reduces image quality and can reduce press productivity. In this paper, web-fed and sheet-fed linting trials have been used to investigate the effect of important press and paper variables on linting. Two of the most important printing parameters affecting linting were the take-off angle from the nip and the printing tone. From analysis of the effects of take-off angle and printing tone, two forces were identiﬁed as being especially important to linting: a ﬁlm ﬂow force in the nip and a tack force from the splitting of the ink ﬁlm. A simple model was presented that could qualitatively explain why printing press speed, printing pressure and ink tack all had smaller effects on linting than would be expected from consideration of tack force alone. Laboratory printing tack tests and other measurements of paper properties were compared with lint measured in the sheet-fed trials. The tack force measured in laboratory printing was found to be lower for improved newsprint compared to newsprint, while the lint in both sheet-fed and web-fed trials was higher for the improved newsprint. Differences in the ﬁlm ﬂow in the nip were suggested to be responsible for both effects. The improved newsprint was also found to have a lower surface strength, as measured by delamination.