A comparison of the perception of juxtaposed tones in newspaper images when screened with conventional and random dot screens

The halftone process has been the conventional means of reproducing photographic artwork on newsprint. The use of relatively coarse screen rulings, dictated by the paper characteristics, produces an image with a noticeable dot pattern. Furthermore, the printing process is susceptible to extreme dot gain variation, which affects tone reproduction and the pictorial communication of the image. There were good reasons to suspect that a random dot screen could give more consistent tone rendition than a conventional halftone screen. A comparative study of the consistency of perception of conventional and randomly screened images when printed on newsprint has not yet been reported. The present study is an attempt to determine which screen can reproduce more consistently perceptible images when extreme dot gain occurs . To test this comparison, an original black and white photographic image, composed of geometric shapes of varying densities superimposed on backgrounds of varying densities , was reproduced from a computer generated slide. This test target, designed to encourage objective answers from observers, was photographed through 65-and 85-line conventional halftone screens, and 75-and 150- line mezzotint (random dot) screens. The production of film, plate, and the press run were the same for the four screened images. The pressman purposely varied the ink film thickness to simulate dot gain. Selected samples of minimum, average, and maximum dot gain were presented to observers, who were asked to identify geometric shapes on 12 printed images. The recorded responses were averaged, and the collected data was represented in graphs. In this manner it was possible to determine which screens can reproduce more consistent tonal distinctions. The results of the experiment proved that the random dot screens are not capable of reproducing more consistently perceptible images. The 65-line conventional halftone screen produces more consistent perceptual responses in the highlights, while the 85-line conventional halftone screen produces more consistent responses in the midtones