XM (Cross Modulated) Screening Technology

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Rosa Fowler
5 years ago
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1 XM (Cross Modulated) Screening Technology Increasing Print Quality in a Computer-to-Plate (CtP) Workflow Introduction Eliminating film in the plate-making process has done more than shorten the workflow. Removing the intermediary image transfer has eliminated prepress registration errors and shortened press makeready. These on-press improvements provided immediate benefits for the printer. Now that CtP technology has been widely implemented, customers are demanding higher print quality as well. CtP enables the use of higher screen rulings a practical method to deliver the quality that customers now demand. However, conventional AM (amplitude modulated) screening technology, which uses a grid to arrange halftone dots, is limited in its ability to capture all the benefits of a finer screen ruling. Fine AM screening can lose much of the information in images rich with highlight and shadow detail, since CtP can now deliver more detail than the press can hold. FM (frequency modulated) screening, which randomly places microdots, addresses this problem, by keeping the dots no smaller than the press can hold. However, FM screening comes with its own set of technical imperfections such as the appearance of grain in flat tints. In addition, fine FM techniques can be difficult to implement on press, thus slowing down the process. Hybrid screening technology represents an attempt to compensate for the shortcomings inherent in AM and FM screening. It applies AM algorithms to midtones, and FM to highlights and shadows. The problem with hybrid methodology occurs at the changeover from AM to FM. In most hybrid techniques, the intersection between AM and FM is discernible with the naked eye, and the complicated screening algorithms can slow the pre-press process. The challenge, therefore, has been to develop a new screening technology that combines the desirable behaviours of AM and FM screening without the drawbacks of a conventional hybrid solution. This paper will demonstrate how XM screening technology accomplishes this task, allowing the print provider to obtain unhampered high-screen frequencies with no loss of detail loss and with no extra effort on press. XM screening enables its users to realise the true benefits and potential of a computer-to-plate workflow.

2 Computer-to-Plate, the Enabling Technology Computer-to-plate technology has brought with it a number of benefits. By eliminating the film intermediary and imaging digital data directly to plate, registration is exact. Make ready is reduced by as much as 50%. The ability to more precisely control the halftone dot has opened the door to new methods of achieving higher quality. The production benefits afforded by CtP have also driven printing companies to seek a new competitive edge. In the attempt to achieve higher quality, they are printing at high screen resolutions of 175 and 200 lpi (lines per inch). Others have revisited FM screening technology which had been cast aside because of the difficulty holding the smallest microdots from the film to plate transfer. Yet, printing with conventional screens at high line rulings or controlling a 10-micron FM dot still requires a number of workarounds, leaving the user s desire for higher quality with ease of use unrequited. To understand the characteristics needed in a new screening technology, it is important to look at the benefits and advantages of various screening methods. AM Screening Technology AM screening, or amplitude-modulated screening, is still the most widely used halftone screening method. The AM process places a fixed number of dots on an orthogonal grid. The grid is measured in lines per inch (lpi). The size or amplitude of the dot modulates according to the tonal values of the image. Darker tones generate large dots and brighter highlight areas have smaller dots. This black & white photo illustrates the typical representations of tonal values in the shadows, mid-tones, and highlights as rendered with AM screening technology. For duotone or four-colour process printing, the line screens are placed at angles, so that one colour does not over print another. In four-colour process, the screen angles create a rosette dot pattern. At high screen rulings, AM screening renders flawlessly in the midtones. However, it loses dots, ergo image detail, in both the highlights and shadow areas as the dots (or open areas) become too small to hold on press. 2 of 7

When applying a fine AM screen to the left image, the bright spots in the supporting colour (cyan: centre top, black: centre bottom) of the skin tone will clip.

Although the cyan plate and its resultant clipping has been highlighted the clipping on the black plate is more pronounced, and results in a posterised effect on the printed sheet. CtP Quality vs.

3 When applying a fine AM screen to the left image, the bright spots in the supporting colour (cyan: centre top, black: centre bottom) of the skin tone will clip. This will make break lines (fake contours) show up (right image). Although the cyan plate and its resultant clipping has been highlighted the clipping on the black plate is more pronounced, and results in a posterised effect on the printed sheet. CtP Quality vs. Press Capability Fall-off or plugging is not solely the result of the screening technology. It also has to do with the limitations of the press and pressroom conditions. In most printing environments, the imaging quality of the computer-to-plate system exceeds the capabilities of the press. A very fine line captured on the plate cannot be transferred easily to the sheet. In the effort to retain the line, the shadows will plug; or to keep the shadows open, the fine line will be sacrificed. Regardless of technology, visible light or thermal, CtP often delivers details on plate that cannot be held on press. In this example, a 10.6 micron line is imaged nicely on plate, but in order to hold the fine line on press, shadow detail is lost. FM Screening Technology Frequency Modulated (FM) screening, also known as stochastic screening, overcomes many of the limitations of AM screening. FM screening modulates the number or frequency of dots rather than the size. FM uses tiny microdots from 10 to 21 microns as small as the platesetter and press can hold. Instead of arranging dots in a grid, FM clusters the microdots depending on the density or tonal value of the image. Although they appear to be random or stochastic, these dots are carefully calculated and placed, using a tiling method, to reduce image artefacts. The clustering effect makes FM capable of rendering the finest detail. Because there are no screen rulings, the image prints more like a photograph than a halftone. Typical FM rendering creates clusters of same-sized dots. Problems with FM screening occur in the midtones, where it becomes difficult to control how the dots cluster. When the dots connect or overlap, noise or mottle can occur. This is especially noticeable in flat tints. 3 of 7

Unlike AM dots, by their nature, FM microdots on press resist added ink densities. As a result, it is difficult to make colour or tonal adjustments.

Because the dots are never distributed homogenously over the tile and the tiles are repeated, an undesirable patterning can result comparable to patterns on wall paper.

Hybrid screening technologies have attempted to combine the benefits of both and eliminate the drawbacks.

4 Unlike AM dots, by their nature, FM microdots on press resist added ink densities. As a result, it is difficult to make colour or tonal adjustments. Attempts to reduce clustering result in worm effects, producing an unpleasant artefact. In addition, the FM tile can become visible. Because the dots are never distributed homogenously over the tile and the tiles are repeated, an undesirable patterning can result comparable to patterns on wall paper. Hybrid Screening Technology Clearly there are advantages and disadvantages to AM and FM screening. Hybrid screening technologies have attempted to combine the benefits of both and eliminate the drawbacks. The ultimate goal is to take advantage of the high-quality and high-productivity capabilities of CtP and provide a screen technology capable of rendering flawless high screen rulings with minimal effort in prepress or on press. Hybrid Solution: First Approach The first approach divides images in parts, using stochastic screens in the detailed areas to capture nuances and AM for rendering flat tints without noise. Unfortunately this solution takes more time to calculate, thereby interrupting productivity. In addition, the intersection between the AM and FM screen becomes visible, creating a distracting artefact in the rendered image. First Approach Hybrid Solution: Second Approach Here AM screens are applied to the midtones and a dithered implementation of FM to the highlights and shadow areas. With this approach, the AM screen provides soft rendering and because of its high line ruling it is still showing the finest details. The FM screen ensures that the dots will not become smaller than what can become reproduced on the platesetter and press. To prevent noise, the number of dots is reduced. But here again, the intersection between the FM and AM algorithms becomes clearly visible. Second Approach Hybrid Solution: Third Approach A third approach distributes AM dots using the FM method. This delivers good detail at modest frequencies. However, it does not overcome the inherent limitations of FM screening and its grainy appearance in midtones and flat tints. Third Approach 4 of 7

5 Achieving automatic, artefact-free, high-resolution screening requires the combination of techniques. The solution needs to apply FM screening in highlights and shadows to capture fine details, and AM screening in midtones to achieve smooth gradations. It needs a smooth indiscernible transition from one screen to the other. It has to take the limitations of the printing press into consideration. And finally, it must do all of this productively. XM (cross modulated) screening technology is the first to accomplish all of these tasks. XM does, in fact, use FM screens in the highlight and shadow areas and AM in the midtones. But it uses a patented technology to smoothly transition from one screen to the other. It calculates the precise change point where the image can no longer benefit from AM screening. For example, the AM dots become smaller when rendering highlights until the minimum reproducible dot size on press is reached. From that point on, dots are withdrawn from the grid providing the desired tone. Similarly, in the shadows, the screen smoothly evolves from one screening type to the other with no visible intersection. XM (Cross Modulation) Once XM reaches the smallest reproducible dot for a specific press, it no longer makes the dots smaller. Instead, it uses a patented method to take-out dots. Even though the dots at the highlights may appear to be random, you will notice the dots continue to align along defined AM angles. The shadows and highlights may look stochastic. However, this is not true stochastic screening. Although the FM areas use smaller dots controlled in FM mode, they are aligned as a continuation of the AM screen angles established in the midtones. The result is an entirely new order of screening, aptly named XM or cross-modulated screening. Because the FM dots are placed at the established AM angles throughout the mid-tones, there are no technology intersections. Flat tints are reproduced using these same AM angles whether they reside in the highlights, mid tones or shadows. Gradations are flawless. XM Productivity and Press Performance At its highest screen ruling 340 lpi XM technology renders at only 2400 dpi. It can achieve a 180 line screen at 1270 dpi. XM methodology starts from the principle that the press is part of the imaging system. In fact, it takes platesetter, plate, chemistry, press, blankets, ink, and paper into consideration. Set up requires establishing a tone curve for each screen ruling 210, 240, 280 and 340 lpi for commercial applications. From that point on no extra effort is required in prepress. Traditional AM screening has an advantage over fine FM screening in that it is a more stable and forgiving process on press. Since XM looks and behaves like AM screening, the press operator can move, adjust and maintain colour as if using conventional screening. 5 of 7

6 Left image: 2% dot 175 lpi AM screen Right images: 1 & 2 & 8% dots 340 lpi XM screen XM requires no extra work in the pressroom even at double the line ruling. The 175 lpi 2% AM dot at the left is actually smaller than the 8, 2, and 1% XM dots at the right at 340 lpi and yet at nearly twice the frequency. The number of rows of dots in the 8% XM screen is twice that of the AM screen both in the X & Y axis. Instead of making that higher frequency dot smaller, XM gradually removes the dots (in this case at 8% at 340 lpi). This continues to give the correct tonal reading, but with fewer, not smaller dots, as XM renders the lighter tints. This same technique is used in the shadows. Therefore, if a press can hold a 1 or 2% 175 lpi AM screen now, XM enables printing at twice the line ruling, with no extra effort on press. However, AM stability decreases in highlights and shadows as the line ruling increases. Here, FM behaves opposite to AM. XM screens combine the best performance characteristics of both, enabling a wide range of stability on press. Since the smallest XM dot at 340 lpi is no smaller than a 2% AM dot at 175 lpi, it delivers the highest detail and smoothness possible with modest efforts on press. The finest details and the toughest subjects are easily rendered without typical AM image artefacts such as subject moiré. And, at such fine frequencies, process type and flat tint backgrounds can be built not in spot colours but in CMYK. The characteristic of FM screening not to respond to requested colour moves on press is countered by XM s use of AM screening. This helps the pressman maintain grey balance due to ink-train limitations, and speed and temperature fluctuations during long press runs. XM Implementation :Sublima Screening Technology :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. Agfa s goal with :Sublima has been to work within existing press conditions, and optimise the system from there. Based on different market applications, :Sublima screening is optimised for different press/paper attributes coldset newsprint, flexo and anilox rollers, as well as commercial sheetfed and heatset web. Once :Sublima determines the smallest and easiest reproducible dot on press for different applications that becomes the smallest dot that :Sublima generates and no smaller. For commercial applications, :Sublima generates screen rulings of 210, 240, 280 and 340 lpi. 6 of 7

7 Tests with :Sublima at beta sites around the world have yielded the following results: With no apparent screen, images look like photographs. Fine lines, even delicate typefaces print like solid spot colours using four-colour process. Solid and process tints reproduce evenly, with no coarseness or trace of colour mix. Flesh tones are perfectly smooth and colour accurate. Enlarging or reducing images have no effect on quality or the ability to sustain details. This also widens the variety of papers that you can use at high line rulings from high-gloss coated stocks to translucent vellums and newsprint. 7 of 7

:Sublima Advanced Screening Technology

:Sublima Advanced Screening Technology Meeting the demand for higher quality, with no extra effort on press. 2 What is :Sublima technology? You're looking at it! This entire piece has been printed with