New Screening Techniques for CTP
By Debora Toth October 30,
By WhatTheyThink Staff
Published: October 29, 2006
By Debora Toth October 30, 2006 -- There’s no doubt in any printer’s mind that computer-to-plate systems have raised quality levels significantly. Eliminating film and imaging directly to plate gives pressmen cleaner, more precise dots to use and frees the press from many of the limitations of the four-color process. It also eliminates most dot distortions and can more accurately compensate for dot-gain on press. Due to these advantages, printers are now able to more easily print with higher screen rulings. In turn, more manufacturers are introducing a variety of stochastic screening products to be used with computer-to-plate systems. Traditional halftone screening, often called AM Screening, consists of dots placed in a regular grid, which vary in size to produce lighter or darker re-gions. AM screening is very predictable and behaves much better on press than stochastic screening. The limiting factor has been the ability of printers to maintain dots at the low and high ends of the grayscale range; as linescreen goes up, it becomes more difficult to hold highlight dots and keep shadow screens open. Frequency modulated (FM) screening or stochastic screening uses a different approach than AM, holding dot size constant and varying the distance between dots. Because the dots are smaller and closer together over most of the tonal range, FM screening results in more apparent detail in the midtones and also removes a major source of moiré patterns. The images look like continuous-tone photographic quality. But, because FM screening uses very small dots, the problems associated with highlight dots in AM screening are present over most of the tonal range in FM screening. Before computer-to-plate, it was hard to hold the microdot from film to plate. Computer-to-plate has completely eliminated that problem. Many of the new screening options are a mixture or hybrid of both AM and FM. Hybrid screening uses a traditional AM halftone pattern over most of the tonal range, and an FM pattern in the highlights and shadows. By using hybrid screening, offset printers can increase the linescreens they use over the AM values, without increasing the difficulty on press. Here is a brief description of the variety of screening products on the market. Agfa Agfa offers a screening technology that it says has captured the benefits of both AM and FM screening. :Sublima is a patented technology combining both screening techniques into a single solution. :Sublima is the first implementation of XM technology, which is patented by Agfa. Two Agfa technologies were used in the development of :Sublima, :ABS (Agfa Balanced Screening) and :CristalRaster FM screening technologies. Here’s how :Sublima works. In the midtones, :Sublima uses Agfa Balanced Screening (:ABS) for clear, accurate reproduction. In difficult highlights and shadows, :Sublima uses FM technology to reproduce the subtle tones and every shadow detail. But the software doesn’t simply switch from one screen to another. It uses patented technology to determine the precise changepoint between AM and FM screening and smoothly transition from one to the other so there is no visible crossover normally associtated with hybrid screening. Although the FM areas use smaller dots controlled in FM mode, they are still aligned using the screen angles established by :ABS. The end result is an entirely new species of modulation. :Sublima generates line-screen equivalents of 210, 240, 280 and 340 lpi. :Sublima software takes into account all possible variables. It calculates the smallest dot size that a plate can hold on specific presses so it handles like AM screening, allowing the pressmen to make adjustments on press—a major advantage over FM screening. Built-in calibration curves automatically compensate for differences in dot gain. It can hold a 1-99 percent tonal range throughout long press runs. Among the technology’s greatest advantages is its performance in prepress and on press. At the highest screen ruling—340 lpi—it RIPs at only 2400dpi resolution. The same 340 lpi screen behaves on press no differently than a 150-lpi screen—including ink-water balance and plate performance. Artwork Systems Artwork Systems Group NV, a world leader in professional prepress software, offers Concentric Screening technology. Concentric Screening, says Artwork, is a revolutionary halftone dot technology that divides the conventional round dot into thin concentric rings. These rings limit ink film thickness on the offset plate, thereby providing greater stability on press and increased color saturation. Concentric Screening is the first halftone dot that uses a different internal shape. It's the world's first ‘non-solid' halftone dot. Artwork claims that printers are able to increase—even double—screen rulings without experiencing the traditional problems of mottle, dot gain, and variability typically associated with high screen rulings. Concentric Screening has all the benefits of second generation FM combined with the printability, ease of use, and smoothness of conventional AM screening. Concentric Screening is available with the Nexus and Odystar workflows from Artwork Systems. Esko-Graphics In September 2006, Esko introduced PerfectHighlights, a new and unique screening technology for flexo printing. PerfectHighlights provides new tools to apply Esko's, as well as converters' and label printers', flexo printing know-how to the pre-production and platemaking process. Printers can create complete screening sets optimized for specific printing environments using specific inks and substrates on specific presses, thereby increasing the value delivered to the packaging buyers and creating a stronger competitive differentiation. PerfectHighlights incorporates a number of methods to print 1-2% highlight dots. The result can be combined with the full range of Esko screening technologies optimized for midtone and shadow reproduction. In addition, Esko offers a wide set of screening technologies including SambaFlex, and Groovy Screens, two hybrid screening technologies using traditional AM screening throughout most of an image and either FM screening in the highlights (SambaFlex) or a line pattern (or grooves) into the shadow areas to improve color saturation and solid rendering. Esko also offers a high frequency AM screen called HighLine, used to produce very high screen rulings at lower output resolutions than was traditionally required for platesetters. For example, a 423 lpi screen can be created at 2400 dpi resolution. Fujifilm Fujifilm offers two proprietary screening technologies: Co-Res AM Screening (for common resolution) and Taffeta FM Screening, which was introduced at Drupa 2004. Co-Res Screening is a revolutionary AM high definition screening software developed by Fujifilm that allows customers to print high-screen rulings while using standard output resolutions. As a result, users get the increased productivity of high-line screening and are able to achieve superior reproduction of details in highlight areas. Fujifilm has also developed Taffeta screening, a second-order FM software that allows computer-to-plate customers to print projects with enhanced detail and stunning quality. Taffeta incorporates Fujifilm’s Image Intelligence, a specially-designed, highly sophisticated digital imaging technology that helps printers improve noise issues associated with FM, provide more detailed color reproduction, print with more than four colors and eliminate moiré. The new Taffeta FM Screening also employs a graininess optimization algorithm, based on simulation using visual characteristics, as well as a screen pattern optimization algorithm for both minimizing graininess and maximizing printability. Fuji says that Taffeta provides: - Complete elimination of moiré and rosettes - Improved primary and secondary color saturation - Better reproduction of image detail - Improved texture and printability - A reduction in unevenness and graininess Heidelberg Heidelberg introduced its new Prinect Hybrid Screening method to the US market in April 2006. This new screening method combines the benefits of amplitude-modulated (AM) and frequency-modulated (FM) screening technologies. It enables higher screen resolutions and finer reproduction of detail in print, all of which enhances the overall print image quality. Prinect Hybrid Screening is available for the Suprasetter family of thermal imagesetters, the Prosetter series with violet technology, and the Topsetter. Depending on the CtP platesetter technology and plate type used, screen resolutions of up to 400 lines per inch (160 lines per centimeter) are possible. Prinect Hybrid Screening defines the smallest dot for highlight areas and the smallest dot for shadows and does not allow the dot size to fall below these values. Unlike stochastic FM technology, the dot follows the angle of the AM screen. This produces a smooth transition between tonal values. In contrast to other hybrid methods, Prinect Hybrid Screening uses sophisti- cated dot distribution at the relevant angle to enhance detail and eliminate moiré. Prinect Hybrid Screening uses Heidelberg’s own Irrational Screening (IS) technology in the tonal range reproduced with AM technology. This eliminates undesirable moiré and delivers smooth results for difficult skin tones. For black/white reproductions, the black separation has been set to a better angle. The new screening method does not necessitate any major adjustments to the process flow. The user can increase the resolution step by step as required, taking the usual screen rulings as his starting point. Heidelberg has also enhanced its Satin Screening FM screen technology and integrated it into the Prinect workflow as Prinect Stochastic Screening. Prinect Stochastic Screening enables smoother tints while retaining all the advantages of stochastic screening, i.e. elimination of moiré during compo- site printing of process colors, ink savings over conventional screening, and color stability. The new screening methods can be ordered with the new Version 6 of Prinect MetaDimension RIP technology. The autotypical screens are now grouped under the product name Prinect AM Screening. As before, they are included in the scope of delivery of Prinect MetaDimension. Kodak Kodak Staccato software is a hybrid second-generation FM screen that delivers smooth, flat screen tint builds. It is suitable for all offset applications on virtually any substrate. Staccato screening produces high-fidelity, continuous tone images that exhibit fine detail and an extended color gamut, creating a photographic experience free of visible printing artifacts, such as subject moiré and rosettes. It is available in dot sizes from 10 to 70 microns. Staccato screening, according to Kodak, eliminates gray level limitations and abrupt jumps in tone, while improving color and halftone stability. Staccato technology also reduces image degradation due to press misregistration. Leveraging a 10,000 DPI laser, Kodak Sqaurespot thermal imaging technology delivers the resolution Staccato screening requires to reproduce very fine screens in a reliable and practical manner for routine daily print production. Staccato is available in four unique patterns for four-color process printing. Staccato screening is also available with six additional extended screen patterns to support Kodak Spotless printing technology. Rampage Systems Rampage Systems offers two screening systems for its prepress workflow. Liso is a multi-phase hybrid option, and Segundo is a second-order stochastic option. A new class of hybrid screening, Liso is the Spanish word for smooth. Pronounced "lee-zo," it combines the benefits of stochastic screening (fine detail reproduction) with the benefits of conventional halftone screening (midtone control and smoother blends). As a result, says Rampage, printers can hit and hold high linescreens without changing their press conditions or procedures. Liso uses conventional halftone screening in the midtones. Then, at a specified point where dots cannot be reproduced on press, Liso shifts to a fixed dot size and removes them as necessary in a pseudo-random fashion to hold detail in extreme highlights and shadows. This eliminates abrupt artifacts as the screening changes from midtone (variable dots) to highlight or shadow (fixed dots). Liso adds a user-definable transition phase, where dot size can still vary, but the process of removing them begins and increases in frequency as shadows darken and highlights lighten. Segundo, a second-order stochastic system, uses a fixed spot size only in the highlights and shadows while allowing the midtones to vary slightly. It achieves visual density by increasing both the size and frequency of spots, enabling better tonal control and smoother, more natural reproduction. Additionally, unlike other stochastic technologies, Segundo does not resort to tiles for dot placement, which can cause repetition artifacts. Instead, Segundo uses error diffusion placement made possible by today’s faster computers. Equally important, advanced algorithms virtually eliminate the worm-like patterns inherent to diffusion screening. RIPit In September 2006, RIPit, the primary workflow and computer-to-plate provider to the majority of quick printers and franchises in North America, announced a new version of its OpenRIP Symphony that includes a greatly expanded set of PerfectBLEND AM/FM hybrid screening choices for offset printing as well as enhanced features specifically designed for flexographic printing applications. RIPit’s PerfectBLEND hybrid screening product for both computer-to-plate and film systems combines the benefits of AM screening and FM screening while allowing the user to hold linescreens up to 240 lpi. Screen At Graph Expo 2006, Screen introduced its newest hybrid screening process Spekta 2 HR. Spekta 2 achieves the fine pattern reproduction of FM screening, combined with the consistent results of AM screening. Spekta 2 delivers a level of print quality and image detail comparable to high-frequency line screens of 350 lpi or higher without changing from conventional 175 lpi conditions. Screens can be output at 2,400 dpi with no loss of productivity. Spekta 2 makes dot gain adjustments easier, says Screen, because of Screen's exclusive 12-bit screening technology. Spekta 2 eliminates moiré and is free of screen angles. It ensures the integrity of extremely fine patterns. And it brings out the true-to-life color in flesh tones. Spekta 2 improves overall color control by eliminating banding, moir and jagged lines, ensuring smooth, detailed printing reproductions. According to Screen, Spekta 2HR achieves detail comparable to line screens of 650 lpi or higher.