Print non-uniformity, or mottle, is an important factor in print quality. The ultimate judge of print quality is the printer or print buyer, so print quality measurement should be representative of human perception. Most methods that are currently available to systematically quantify print mottle do not consider eye response in the calculation. Instead, the user has to select appropriate scales for the analysis by comparing with separate visual ranking experiments for each new set of prints.
We developed a method to process digital images of mottled black prints to provide a mottle index that takes into account eye response. The mottle indices obtained for a range of paper and
board grades were compared with the results of separate visual rating experiments, and there was very good agreement between them. The mottle index outperformed other parameters also used for the quantification of mottle. Based on these results, the mottle index is deemed reliable enough to decrease the need for separate visual assessments by panels. The mottle index algorithm removes the need for the operator to make subjective choices on the appropriate analysis scales for sample sets where print uniformity is the dominant quality criterion.
The proposed mottle measurement method allows systematic and objective quantification of mottle. The method can easily be implemented to analyze test prints using the analysis software we
developed, and an appropriate desktop scanner that will require calibration to relate the greyscale to reflectance values.