On the evaluation of print mottle

Abstract: Print Mottle is perhaps one of the most disturbing factors influencing overall Print Quality. Mottle has traditionally been evaluated by estimating the reflectance variation in the print. Although the amplitude of the reflectance variation is probably the most important aspect of print mottle, other aspects may also influence the perceptibility of mottle. Since the human visual system is optimised to fit the conditions prevailing in its surroundings, it is also important to consider aspects such as mean reflectance factor level, spatial frequency content, structure of the mottle, and colour variations.In this thesis, a new evaluation model for the estimation of print mottle is proposed. The model is best explained as a six-step chain. First, a digital RGB image of the print is acquired with a scanner. The digital RGB image is then calibrated and transformed into the L'a'b' colour space. Next, the three colour components are transformed into the frequency domain by a Fourier transform and the power spectra are calculated. The power spectra are thereafter filtered with respect to the contrast sensitivity functions representing the human eye’s sensitivity to spatial variations in the three colour channels. To account for systematic variations in the sample, the spectra are filtered a second time with texture enhancement filters, which are based on local calculations of chi-square measures in the power spectra. The energy within the visually detectable area of the filtered power spectra is then integrated to obtain a single measure of the variation for each colour component. A single mottle estimate is obtained as the square root of the sum of the squared variation measures for the three components. To acknowledge the influence of mean lightness level on perceived print mottle in a way that agrees with the results presented in Paper I, the mottle estimate obtained is finally multiplied by the sixth root of the mean reflectance factor level.The theoretical foundations of the model are consecutively developed through the first five papers of the thesis. The first paper considers the influence of the mean reflectance level on perceived print mottle. The second and third papers describe the contrast sensitivity filter and the texture enhancement filter applied. The fourth paper compares the new model with other models for print mottle evaluation. The fifth paper extends the grey-scale version of the model into colour. The sixth paper presents the unified model that takes all the mentioned factors into account.To test the model, samples from both simulated sets of prints with various degrees of colour and/or systematic mottle and sets of real prints from various conventional presses were analysed a) visually, b) with traditional print mottle evaluation models, and c) with the new model. Results obtained using the different evaluation models were compared with visual assessments of the sets of prints. In each one of the evaluations the new model was found to be as good as or superior to the traditional print mottle evaluation models in its agreement with visual assessment. The new model is particularly promising in cases where the evaluated prints show colour and/or systematic disturbances.