Prediction of the perceived quality of streak distortions in offset-printing with a psychophysically motivated multi-channel model

Abstract

The evaluation of printing machines poses the problem of how distortions like streaks caused by the machine can be detected and assessed automatically. Although luminance variations in prints can be measured quite precisely, the measured functions bear little relevance for the lightness of streaks and other distortions of prints as perceived by human observers. First, the measurements sometimes indicate changes of luminance in regions which are perceived as homogeneous by humans. Second, the measured strength of a distortion correlates often weakly with its perceived strength, which is influenced by a variety of factors, like the shape of a streak’s luminance profile and the distribution of luminance variations in its spatial surround. We have used a model of human perception, based on fundamental neurophysiological and psychophysical properties of the visual system, in order to predict the perceptual strength of streaks (i.e. the distortion as perceived by a human observer) from the measured physical luminance signal. For the evaluation of the model, tests with naive and expert observers have been conducted. The results show that the model yields a good correlation () to the assessments of human observers and is thus well suited for use in an automatic evaluation system.

Keywords:

• image quality
• offset printing
• human visual system
• quality control

Acknowledgments

This work was supported by NCT Bremen, Sächsisches Institut für die Druckindustrie Leipzig and Forschungsgesellschaft Druckmaschinen e.V. Frankfurt am Main. The authors would like to thank the accompanying working group of specialists (FGD Arbeitskreis Streifenmessung) for the fruitful discussions. Parts of this work has been presented at the 18. Workshop of the German Color Group in Darmstadt, Germany in 2012.

Notes

Since the locations of the streaks are marked, they will typically receive a grade 1, even if critical testing without markings would presumably reveal that some participants are not able to see them.