It may be time to rethink the old, reliable halftone screen.
For more than a century, halftone screening enabled newspapers to reproduce photos. Throughout that time, the process has remained unchanged at its most fundamental level a continuous-tone image is broken into a series of dots that can be reproduced on press and fool the eye into perceiving a continuous picture.
At first, the new computer-based methods for producing halftone dots didn't change the basic process. Raster-image processors simply produced halftone screens digitally rather than photographically. For the past several years, though, many pre-press vendors have been offering an alternative--stochastic, or frequency-modulated, screens. These products have been promoted as a solution to moiré and dot-gain problems and as a method of obtaining significant improvements in color quality.
Today, both conventional and stochastic screens can be generated in the RIP. The differences between the two methods lie in the size and spacing of the dots (see diagram, below).
Conventional halftone dots are generated and imaged in a fixed, regularly spaced pattern. The size of the dot varies, and this variation determines the darkness, or tone, of that area to the viewer.
In a stochastic screen, the dots are not regularly spaced and are generally much smaller. (Twenty-one microns is typical.) The number, or frequency, of dots in a given area determines its darkness. In some methods of stochastic screening, both the frequency and the size of the dots are varied.
Conventional screens cause several problems in a newspaper environment, principally dot gain in the midtones and a tendency to produce moiré patterns. In theory, stochastic screening addresses these problems and produces much higher quality.
Bob Holt, director of industry marketing for Scitex America Corp., claims that with careful attention to quality control in pre-press preparation, the very small dots typically used in stochastic screening can significantly reduce midrange dot gain. Phil Rose, product-marketing manager at Prepress Solutions Inc., states that the smaller dot used in stochastic screening yields a better image since it has the "same effect as a going to a higher line screen."
Moiré patterns are the result of interference between either regularly spaced rows of dots in overlaying screens, or between regular patterns in the original image (such as fabric weaves) and the regular pattern of the halftone screen. Since the dots in stochastic screens are not laid down in regular patterns, interference does not occur, and moiré is eliminated.
Given its advantages, one might expect stochastic screening to be widely used in newspapers, but it's not. The reason comes down to two words--quality control.
The extremely small dots generated by most stochastic-screening RIPs place very heavy demands on processes between the workstation and the press. Exact control of negative-to-plate draw-down during the platemaking process is particularly important, since a small variation in vacuum pressure or timing can produce large change in image quality.
The demands of newspaper production haven't lent themselves to the degree of control necessary to achieve the best results from stochastic screening, and indeed, most vendors report greater sales to commercial printers than to newspapers.
Pre-press systems vendors generally agree that the future of stochastic screening in the newspaper industry rests on the wide adoption of direct-to-plate processes, which eliminate much of the optomechanical variation that prevents stochastic screening from reaching its full potential.
Until then, in the words of Jon Guerringue, business manager for marketing at Linotype-Hell Co., "newspapers will get more benefit from good, sound color management than from stochastic screening."
Steven Ostrofsky is president of Publishing Productivity Systems, Denver. E-mail, stevevelo@aol.com; phone, (303) 733-5268.
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