Göran Ström, Sandra Madstedt

Abstract
Print mottle is one of the most severe print quality defects in offset lithography. There are several types of this phenomenon, with back-trap mottle and water interference mottle being the two most important ones.
The dot shape from halftone areas of full scale offset prints that showed water interference print mottle was characterized by image analysis. Measured quantities were areas within the dot area without ink (referred to as white spots), areas outside the dot area that carried ink (black spots) and the increase in dot perimeter (PI) relative to the theoretical value. Good correlations were found between print mottle and PI as well as with the sum of white and black spots. Poor correlation was found when only white spot area or black spot area was considered. A point-wise variation in dot shape, characterized by the coefficient of variation showed no correlation to print mottle.

The content of white spots (but not black spots) decreased with printing impression. Calendering reduced the content of white spots but increased that of black spots. A more open coating structure generated by the use of less latex binder resulted in lower content of black spots and lower content of white spots when the impression was low. Dot non-uniformity expressed as white and black spots is discussed in terms of forced and capillary absorption of fountain solution by the coating, local variations in pressure and excess water acting as weak boundary layer and lubrication layer.

Keywords: Print quality, Print mottle, Offset printing, Image analysis
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Raisa Halonen, Tuomas Leisti, Pirkko Oittinen

Abstract
The growth of on-demand and home printing has created new needs for paper producers to specify the quality potential of paper using metrics which are relevant to end-users. This study is a part of research which aims at developing a quality model and index for the visual quality of digitally printed photographs. In the study, the subjective quality of printed photographs is predicted using regression analysis with instrumental measurements from prints and computational quality attribute values separately as predictors. As the quality attributes are computed without a reference, both positive (contrast, colourfulness) and negative (blur, noise) attributes can be used for the prediction of subjective print quality. This creates a preferential model as opposed to the typical distortion models of image quality. To meet the objective, the sample set used in the study includes ink-jet and electrophotographic prints made on a wide range of papers. The results of the study suggest that the subjective quality of printed photographs can be predicted to a reasonable extent using quality attributes computed from digitized prints or instrumental measurements from objective test fields. The models given by the regression analysis clearly depended on the analyzed image content. This was seen in the set of included variables as well as the goodness-of-fit of the models.

Keywords: Electrophotography, Ink-jet, Print quality, Paper grade, Image quality model
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