Benefits of Concentric Screening in Offset Lithography


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Mark R. Samworth


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Concentric Screening(tm) is a halftone screening technology in which the large solid halftone dot is divided into thin "concentric" rings. The thin rings limit the ink film thickness on the offset plate. The result is a halftone dot which prints with less variation on press, exhibits higher color saturation, enables printers to go to much higher screen rulings, and consumes 15% to 30% less ink than normal AM screening. While stochastic screening has shown many benefits on press in recent years, it suffers from one major problem: a grainy appearance. It is a fact of the human visual system that a uniform pattern appears smoother to the eye than a random pattern. A stochastic screen is a small spot that is randomly distributed. The benefits of stochastic screening are related to the small spot size, not the random distribution. By dividing the large AM halftone dot into thin concentric rings, the Concentric Screen has similar on press benefits as the stochastic screen. Because the pattern in uniformly arranged, the screens appear smoother than stochastic screens.

The benefits of Concentric Screens can be confirmed quantitatively by running two simple tests - both consisting of an AM vignette from 0% to 100% printed side by side with a Concentric vignette. By measuring the LCH values throughout the vignettes on the same sheet and plotting the relationship between Chroma and Lightness of each screen, it can be determined which screen produce higher chroma and, by deduction, thinner ink films. By printing the same sheet at different ink levels (e.g. low density and high density) the degree of stability on press can be measured. These results of these tests, described in this paper, show that Concentric Screens are superior to AM screens.

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