Non Periodic (Stochastic) Screening, Offset Negative and Positive Tone Reproduction, Part 2: Tracking the Dot/Spot Gain in Non Periodic Printing

Three different manufacturers' non periodic screening systems were used in this study. Two would be considered First Order FM the other Second Order FM. User generated linearized negative and positive image film vignettes were used to characterize the output [film, plate, press] tone reproduction and tint gain of these systems. This paper attempts to follow the change of these attributes as the image passes through the repro train. In non corrected Non Periodic Negative acting printed images, the rate of change in tone values initially increases at a rapid lineal rate up to 40% tint area. At that point the rate of change becomes exponential up to input levels on average of 20%. At levels higher than this the rate becomes asymptotic to 100% tint area. Such a response exhibits, on average, tint levels of near 60% output for an input value of 20%, and 90% for input values of 40%. In contrast Periodic Negative acting images exhibit a less rapid incline in tint response with 20% input about equal to 40% output. The Periodic sample also exhibits an extended inflection point. Therefore, an 80% input tint results in a 85% output image. Additionally, the Periodic tone curve does not become asymptotic to 100% tint area. Non Periodic Negative acting Tint Gain Curves are skewed. Reading tint gain at a 50% input tint level may be misleading. A major portion of this gain was found to occur at the platemaking step. Non Periodic Positive tint gain is considerable less than that of periodic tints. Non periodic Positive images are more susceptible to sharpening than periodic images. Tint sharpening occurs when Positive films are contact printed onto Positive acting plates. Platemaking exposure induced sharpening is an effective additional means of achieving a closer tint match from Positive non periodic to periodic images. Some Positive non periodic images may not need more than film to plate induced sharpening to match periodic images. Therefore, the tone reproduction scale (hence the tint gain) of Positive non periodic images can be made to be a close match to Positive periodic images.