A Comparison of Different Print Mottle Evaluation Models

Print mottle, i.e. unwanted reflectance variation patterns, is perhaps one of the factors most detrimental to general print quality, and it is therefore important that it is evaluated in a proper and understandable manner. Several models have, over the years, been developed to evaluate mottle instrumentally. Today, there is also an ISO Standard for evaluating print mottle. The theoretical foundation of this standard is not however entirely reassuring. This paper attempts to examine a number of different print mottle evaluation models, including ISO 13660, conceptually and to compare the extents to which they correlate with visual print mottle assessment. Results suggest that three aspects of stochastic monochrome print mottle must be considered in any attempt to evaluate print mottle instrumentally: the amplitude of the variation, the coarseness of the variation, and the mean reflectance level of the print. The way in which this is carried out is however somewhat less crucial. We question whether an ISO standard for print mottle evaluation should indeed be based on a specific model such as the one described in ISO 13660. A standard based on a rigorous visual assessment of artificially created mottle would perhaps serve a better purpose.