Perceptual assessment of simulated print noise with random and periodic structure

Lightness variation in a supposedly uniform printed surface is referred to as ''print mottle.'' This phenomenon is one of the most detrimental to overall print quality. A fundamental problem in the evaluation of human sensitivity to a print quality variable such as mottle is the difficulty in controlling external variables, such as variations in color or in average print density, that arise in the printing process. These variables can influence the rating of quality. To analyze the impact of a systematic mottle pattern compared to that of the common case of a random pattern, a digital simulation technique was used to create gray test samples with various amounts of stochastic and systematic noise of different characters. The samples were printed using a high quality inkjet printer and evaluated by a panel of judges. Two different evaluation methods were used. Observers rated dissimilarity and preference in a pairwise comparison task, and also by positioning samples in the horizontal and vertical directions of a digitizing tablet. The results show that individuals rate the samples in a very consistent way and that systematic noise is perceived to be more annoying than random noise of a similar physical magnitude. Furthermore, the consistency between the two different evaluation methods is very good, which suggests that two-dimensional scaling on a digitizing tablet is a viable method for grouping samples in a plane. The results also show that digital simulation of print artifacts is a powerful tool for creating samples with controlled disturbances.