SCREENING ALTERNATIVES -- Screen Out the Competition
BY MARK SMITH
Technology Editor
Few things are as fundamental to the color offset printing process as screening. Add to that the experience built up with conventional screening, general resistance to change and some technical issues, and it's easy to understand the industry's caution when it comes to alternative screening solutions.
The category of enhanced screening technologies now has been expanded to include what are referred to as "hybrid" systems, along with true stochastic or frequency modulated (FM) screening. Hybrid technology claims to combine the benefits of conventional, or AM (amplitude modulated), and FM screening. One group of specialty screening solutions that won't be considered here are the new spot color replacement systems that belong in a product class by themselves.
To help printers gauge the merits of alternative systems, the Graphic Arts Technical Foundation (GATF) has conducted several independent tests. The basics are covered in the "Stochastic and Hybrid Screening Printability Study" report that GATF released in 2003.
According to the researchers, this study was designed to show the differences between currently available conventional and stochastic/hybrid screens, particularly the products from the seven suppliers that participated in the study. Measurements were collected for density, tone value increase, print contrast and gray balance, then the data were compared to show the differences between conventional and stochastic/hybrid screening solutions.
Raise Your Tone Value
The major differences seen with stochastic screens were in tone value and print contrast, notes the report. Tone value, on average, was 10 percent higher compared to conventional 150- to 200-lpi screened work. This can be attributed to the fact that there are many more dots in a stochastic image, which introduces more area for tone value increase, the researchers explain.
However, samples printed with 10 micron stochastic screens were found to exhibit the same degree, or even less, of tone value increase as 20 to 28 micron work despite having more dots. This was because a curve had been applied to the plates, the report continues.
Another factor cited as possibly influencing the amount of tone value increase associated with stochastic screening is spot shape. Spot shapes associated with stochastic screens vary by manufacturer and, in some cases, vary within a manufacturer's screening options.
Hybrid samples, on the other hand, had tone value characteristics similar to the sheets with 175- to 200-lpi conventional screens, the study found. This is said to be due to the fact that hybrid screen dot patterns vary throughout the tonal range, and in midtones are very similar to a conventional screen.
According to GATF, one of the major benefits of using hybrid screening over stochastic is the ability to image at lower resolutions (2,400 dpi instead of 3,600 dpi), while still achieving a higher resolution appearance. This enables more plates to be produced per hour, without sacrificing the benefits of stochastic screening like moiré suppression and photo-realistic images.
Due to the increase in tone values exhibited in the stochastic samples, print contrast was lowered by 6 to 16 percent, on average, the study found. However, measurements of print contrast on hybrid samples came out comparable to the conventional screen samples.
Anticipating the impact of stochastic screening on tone value and print contrast, some manufacturers attempted to compensate by taking weight out of the plates, the researchers point out. They recommend users perform a linear press run of the stochastic screened work and feed the print characteristics back to prepress in order to properly separate the images.
Among the other conclusions noted in the report were:
* Color gamuts of the stochastic samples were found to be no larger than those of the conventional screening printed samples. The reason for this is that color gamut is limited by the ink and paper selected. Midtone color gamut areas in the stochastic samples did show a greater gamut area, but this was attributed to the greater tone value increase.
* In order to adopt stochastic printing successfully, a full understanding of the production process is necessary. Plate curves or compensation amounts should be determined by print characteristics collected from a series of press runs. Most importantly, the press will need to be run day in and day out. This means that standard operating procedures will need to be established, including guidelines for target density, dot gain and gray balance values.
More GATF Research
GATF has since done another stochastic research project that compared printability and runnability of the screening on coated and uncoated paper versus 133-lpi conventional screens, reports John T. Lind, director of research. "We measured the amount of ink consumed by stochastic and conventional screens and, depending on how much weight was removed from the curves, we were able to show that stochastic printing consumes less ink—assuming a constant density for all printing conditions," he says.
On the downside, the stochastic print runs showed a faster rate of ink and paper buildup on the blankets, referred to as non-image area piling, Lind continues. As a result, GATF has planned a followup research project in conjunction with MAN Roland, paper companies, ink suppliers, fountain solution suppliers and blanket manufacturers to address these piling issues, he adds.
Take Your Pick
Printers deciding to step up to an enhanced screening solution have a growing list of products from which to choose. In practical terms, though, any given printer's options are limited by the digital front end and platesetter combination that the shop has installed. As the following sample of leading systems shows, most will at least have the choice to run hybrid or stochastic screening—or maybe both.
Agfa's newest screening solutions is Sublima XM, which it classifies as a cross-modulated system. It uses AM (Agfa Balanced Screening) in the midtones, but smoothly transitions to FM screening in highlight and shadow areas. The smaller FM dots are aligned using AM screen angles. Sublima supports 210-, 240-, 280- or 340-lpi screen rulings at 2,400 dpi with 21 micron spots. The company also continues to offer its CristalRaster stochastic screening.
Creo Staccato screening is a second-order FM solution offered in versions from 70 micron down to 10 micron, with the 10- and 20-micron systems targeted to commercial printing applications. It is designed to work in combination with Creo SQUAREspot thermal imaging technology. The company also offers Turbo screening for higher screen rulings and Maxtone hybrid screening depending on the application.
EFI introduced an AM/FM hybrid screening system, called OneFlow Hybrid Screening, as an option for its workflow solution. It is said to minimize registration problems and color shifts while improving the smoothness of screen tints.
Fujifilm Taffeta 20 screening, from Enovation Graphic Systems, is a 20 micron FM system. It employs a graininess optimization algorithm based on visual characteristics and a screen pattern optimization algorithm to both minimize graininess and maximize printability. The company also offers Co-Res screening, an enhanced AM solution that uses the same graininess algorithm, varies dot shape and employs Multi-Template Technology (with specially arranged smaller dots) to suppress moiré and to improve image quality.
Heidelberg's Satin Screening is a second-order stochastic solution that is based on a new micro structure that produces advanced dot clustering. Individual dots are said to grow together earlier to create quasi-random patterns. Diamond screening, the company's first-generation FM system, remains in its product portfolio. Heidelberg also still considers its HQS (High Quality Screening) and I.S. (Irrational Screening) technologies to be enhanced screening systems.
Spekta hybrid screening from Screen (USA) uses a fixed dot size at varying densities (FM) in the 1-10 percent and 90-99 percent tones. Randomly placed round dots that vary in size are used in the midtones. The company also offers Randot X screening, an FM solution based on Harlequin HDS technology. It is offered in 10, 15 and 20 micron versions. Both systems are said to leverage optical gain to reduce ink usage by up to 20 percent.
Rampage Systems has expanded its screening offerings to include Liso (hybrid) and Segundo (second-order FM). At a point where AM dots cannot be reproduced on-press, Liso shifts to a fixed size and removes dots as necessary to hold detail in extreme highlights and shadows. Segundo uses diffusion dithering instead of tiles to reportedly prevent repetition patterns.
Monet is a first-order FM screening solution from Esko-Graphics for commercial printing. However, the manufacturer promotes its HighLine Screens as an enhanced screening system even though it is based on AM techniques. The system is said to produce high screen rulings at lower output resolutions.
Artwork Systems has added two new systems to its Paragon screening product family for litho applications. Paragon Organic is a second-order FM screening technology that is said to result in better color stability on-press and a less grainy appearance. Paragon Radiant is a hybrid FM-AM-FM system for high screen ruling applications. The company also offers Classic and Quantum hybrid screening as options with its NexusRIP for label and packaging printing.
