FORMATION OF PAPER AND MOTTLING OF SOLID PRINTS

The evenness of optical density is one of the major requirements of so lid prints in order to avoid mottling. This evenness is affected by th e uniformity of the two-dimensional ink distribution on the paper surf ace characterized by a stochastic grey level distribution. To obtain a high correlation with the visual perception the parameters structure and contrast of this distribution must be taken into account. Whereas the contrast corresponds to the first order statistic the parameter st ructure can be described by algorithms based on second order statistic s such as co-occurrence matrix or two-dimensional autocorrelation func tion. These parameters can be measured by means of an image analyser. There exist relationships between the parameter specific perimeter (po wer spectrum), correlation length (autocorrelation function) and corre lation (co-occurrence matrix). Correlation COR is a measure describing correlation length which also reflects the average feature size e.g. the average area of ink distribution or the average flee size of unpri nted paper (measured in transmitted light). The contrast is the overal l variation of the grey tone image of printed or unprinted paper which can be simply quantified by its variation coefficient. A combination of both parameters leads to the mottling index or formation index. Mot tling index and formation index have been evaluated with respect to wo odfree and wood-containing offset papers. According to our findings th e effect of formation on the unevenness of solid prints is not signifi cant. But there exists a high correlation between the structure of an unprinted paper (quantified by parameters of the co-occurrence matrix such as COR-value) and the structure of the ink distribution on paper.