Author: Zheng Pei Yang Zhigang Zheng Yuanlin

Screening is a very important process in printing. Its basic function is to transform the processed continuous tone image into a halftone image for printing. The quality of screen technology directly affects the quality and grade of printed products. Amplitude Modulation Screening (AM), as the most commonly used screening method now, has a fixed position and angle of the dots, and obtains gradation changes and color reproduction through the size changes of the dots. Its advantages are good reproduction of midtones and smooth and natural transition of layers; the disadvantage is that it is limited by printing conditions, the number of screens is generally limited to less than 200 lpi, the image is not delicate enough, and the reproduction of highlights and darks is not ideal when using high screens. FM screening (or random screening, English is FM) The dot size is fixed, but the position is randomly changed, there is no grid angle and network line, through the change of the number of dots per unit area to reproduce the gradation and color, between the dots To distinguish its precision. The dot diameter is 40 microns, 21 microns (equivalent to 2% of the 175 lpi AM network), 14 microns (equivalent to 1% of the 200 lpi AM network), 10 microns (equivalent to 1% of the 300 lpi AM network), etc. . The traditional FM screening method can be used to print very fine prints, which can eliminate the moiré of AM screening, can copy dark tones and bright tones, and can use lower resolution output, but The printing dots are greatly expanded, and the highlights are prone to particles. The printing pressure and ink volume control requirements are very high. A small change in printing conditions will affect the tone reproduction of the entire image. Although the FM network has been popularized for many years, due to its harsh printing conditions, there are not many practical applications. Both FM and AM screening have certain limitations. With the widespread adoption of CTP, on the premise of not increasing production costs and reducing production efficiency, the quality requirements of products are getting higher and higher, so companies have successively researched and launched a variety of new screening technologies.

Although the new screening methods introduced by different companies are different, in general, the method of combining AM screening and FM screening is generally adopted, that is, the "hybrid" screening of AM and FM Method (of course not simply mixed together). In addition, some companies insist on adopting an improved FM screening method. This article will introduce some new screening technologies of each company.

1 Screen company's "Supplier" screen

The "Spekta" screening technology is a new hybrid screening method developed by the Japanese screen company in 2001, which can avoid problems such as moire and wire breakage. Visida Screening can significantly improve the printing quality without affecting the production efficiency and without changing the conventional printing conditions. If the thermal CTP version is used for output, the normal 2400dpi Visual Basic screen can achieve the same quality as the high-line screen. Vivid Screening can make full use of the advantages of amplitude modulation and frequency modulation. The difference between it and the two basic screening methods is shown in Figure 1.

Traditional Screening

FM screening

AsBida Plus

Figure 1 Comparison between Bida Screening and FM AM Screening

Depending on the different color densities of each image, Vivida Screening adopts dots similar to FM or AM. In the highlights of 1-10% and the dark-tones of 90-99%, Sebida adopts the dots with the same size of the FM network, and reproduces the gradation by changing the number of dots. The distribution of dots is optimized according to the principle of not overlapping and forming large intervals, so that the particle effect can be effectively controlled. In the part of 10 to 90%, the size of the dots changes like amplitude modulation screening, and the distribution of the dots changes randomly like FM screening, so that there will be no collision.

For the highlight part, the minimum dot can be as small as 10.5 microns at an output accuracy of 2400dpi, but it is difficult to reproduce in actual printing. Visualstar Screening solves this problem by merging small outlets. The method is to combine 2-3 small dots into a larger dot (21 or 32 microns) to improve the printability and stability of the highlight area.

The screen accuracy can be achieved equivalent to 300 lpi under the condition of ordinary 2400dpi-175 lpi with the screen of ordinary Bida, and it does not need the very strict process control as the high screen line printing. Like the FM screen, the screen distribution of the Bida screen is random, and there is no screen angle, which can completely avoid the moiré, and can also avoid the "net cover" on the fabric and wood speaker caused by the screen angle. , Can eliminate the rose spots that are easy to appear in dark gray areas or black areas. The screening method can also make the reproduction of the middle tone part more vivid and natural.

As a screen-making technology, the screen-to-screen technology is directly applied to the thermal plate-making machine of the Perak Publishing God series of screens, and is supported by the screen RIP and workflow, thereby achieving high-quality output of CTP direct platemaking.

In summary, the positions of the Bida outlets are random. In this respect, it is like an FM network. In the mid-tone part, although the size of the dots is variable, there is no fixed center distance and angle of the amplitude modulation screen. By increasing the minimum idea, the problem that the FM network of the highlight department is difficult to print is solved, and through the complex dot positioning and combination technology, the level mutation phenomenon is minimized.

2 Agfa Sublima Jinghua outlet screening technology

Sublima Jinghua network screening technology combines the advantages of amplitude modulation (AM) and frequency modulation (FM), and is called the over-frequency network (XM). It is a brand new screening technology launched by Agfa after the crystal outlets (the Chinese name of Jinghua outlets is "crystal sublimation").

On the premise of having certain printing conditions and not increasing costs, Sublima achieved high screen count printing. The print image is very delicate, and the net flowers and dots are difficult to identify with the naked eye at 340 lpi.

Sublima uses AM dots to express the middle part (8-92%), and FM dots to express bright tones (0-8%) and dark tones (92-100%). Moiré and rose spots have been basically eliminated.

Sublima uses Agfa's patented XM algorithm. When AM transitions to FM, the random point of FM continues the angle of AM point, which eliminates the transition trace from AM to FM. See Figure 2.

Figure 2 The random point of FM continues the angle of AM point

Sublima adopts the concept of "minimum printable dots" (more than 175 lpi 2% AM dots). When transitioning from midtone to bright and dark tones, the minimum dots always meet the technical requirements of ordinary printing, so as to truly achieve 1 on the printed matter % -99% of the outlets are restored. At the same time, the calculation of network outlets adopts the "step-by-step accurate calculation method", which significantly improves the calculation efficiency.

When outputting Sublima dots of various network lines (210 lpi, 240 lpi, 280 lpi and 340 lpi), CTP equipment can use 2400dpi output accuracy, and produce high-screen printing plates without reducing output efficiency.

In fact, the overclocking technology is to use FM dots in the bright and dark areas and AM dots in the middle, and to achieve a smooth transition at the dot conversion. For example, when reproducing the dots in the bright tone area, the amplitude dots will gradually decrease to the smallest size that can be copied, and then fade out and be replaced by FM dots. In the same way, the dots in the dark area are also transitioned from amplitude modulation dots to FM dots, and no traces are left at the junction. The dots in the bright and dark areas have the same size and the density is different, but they are not real FM dots. The dots under the control of the FM mode are used, which are linearly arranged at a certain angle and become a new kind of dot arrangement.

At the Shanghai All-India Exhibition in 2003, Agfa used the Sublima dot + Xcalibur45 CTP machine to make the printed proofs not only very delicate, but also the layered colors.

Figure 3 The 340 lpi Sublima dots have the same size at 8%, 2%, and 1%, and the hierarchy is expressed by reducing the density of the dot distribution; the same method applies to 92% to 99% of the dots. 340 lpi Sublima minimum dots are greater than 175 lpi 2% AM dots, if the printer can copy 175 lpi 2% AM dots, 340 lpi Sublima 1% -99% levels can be copied

When printing in Figure 4, the 240 lpi AM highlight part of the dot is lost because the dot is too small to be copied by the printer; and the 240 lpi Sublima dot can completely copy the highlight part (also can completely copy the dark part) because the dot Always maintain printable dot size

3 Screening technology of Esko-Graphics

1) Offset printing dots

Monetscreens and highline screens

First- and second-order monet screens. Monet's first- and second-order "frequency modulation" screening is currently mainly used for offset printing screening. You can get images without net flowers, moiré patterns and unusual sharpness. In order to perfectly match the different needs of each type of printed matter, Monet dots of different sizes can be mixed in the same printed matter, so this dot is no longer a simple FM dot.

High-line screen technology (highlinescreens)

The characteristic of high line count dots is that it can get very high screen line count under the general output resolution. Esko's high wire count screen technology is a new AM screen technology that combines the characteristics of AM and FM screen. Its advantage is that it can show the details of the image, and it has stability and reproducibility. Using high line count dots can achieve 423 Ipi output at 2400dpi accuracy.

2) Flexo printing dots

Samba outlets (sambaflexscreens)

The Samba outlet won the FIexoTech Award as early as 1999. The Samba outlet combines mid-tone AM screening technology and FM screening technology in highlight and dark tone. This ideal combination of different dots is suitable for flexographic printing, corrugated paper printing and screen printing. Samba outlets can provide a smooth transition between AM and FM screening, so you can't see changes at the intersection of outlets.

groovy outlets

The latest direction of flexo screens focuses on improving color contrast and density of dark areas, which is a major focus of flexo printing. Especially when printing PE materials, density and contrast problems will occur. Groovy can produce higher density maps and can use less ink while achieving the same printing effect. Printing tests prove that it will produce better color contrast.

Ec-centric dots for corrugated paper printing

Although the prepress of corrugated paper printing is about the same as that of flexo printing, it also has some unique features, including extremely low screen lines, challenging printing materials and printing registration issues. Eccentric dots are especially used to eliminate Rosette patterns that are easily seen in four-color images with low screen lines. Ec-centric dots can reduce dot expansion and make the tones smoother.

4 Creo TV Fangjia FM Screening

In the latest issue of Japan's "Printing Information", a powerful print of a photographic work was published, and the details are reproduced very delicately. The printed product is printed with Staccoto FM screen of 10 microns, Heidelberg Speedmaster CD102 printing machine, the paper is Turin coated paper, printed by Japan Jinming Printing Company.

Staccoto FM

Screening combines the advantages of random screening and traditional screening, eliminating the problems of halftone rose spots, screened moire, insufficient gray scale, and skipped tones. Shifangjia FM screening also improves the stability of colors and halftones on the printing press. It can still guarantee the quality of printing when the printing volume is extremely large, supplemented by the accuracy and consistency of SQUARE spotTM square spot thermal imaging technology It is precisely because of the square dot technology that random screening really moves towards the practical stage of printing). It can produce stable and smooth colors. In the traditional primary FM screening, defects such as particles often appear in areas with few colors, and secondary screening can avoid these problems. Staccoto screening uses first-order FM screening in highlights and dark areas, and in the midtones where problems are prone, use dots of different sizes to avoid problems with flat screen areas. Its second-order FM screening uses a secondary screening method. Shifangjia's secondary screening is clustered in a regular form, which can effectively reduce low-frequency noise and instability, and avoid visible particle patterns brought by other FM screening.

Shifangjia FM screen can be printed with a very thin ink film, and the printing material is wide. The dark tone part can also get a good performance without sacrificing solid density, contrast and color gamut. The thinner ink film means that the drying speed is faster than amplitude modulation and screening, which is especially beneficial for double-sided printing.

One-time FM screening only uses a random algorithm. When the middle tones of multiple color plates are superimposed on each other, it will produce water wave-like stripes, which is what we call low-order harmonics. In response to this phenomenon, we will reorganize the FM outlets that have been completely broken up first, and then break them up again to obtain the required secondary FM outlets.

Generally, the dot expansion of FM screening is relatively large, because the dot expansion only occurs at the edge. The secondary frequency modulation screen connects adjacent small dots when reorganizing the dots, greatly reducing the total circumference of the dots, making the dot expansion easy to control, as shown in Figure 5.

Figure 5 Schematic diagram of merged outlets

5 Fuji Photo Co-Re Screening Technology

Figure 6 Comparison of traditional AM screening, random FM screening and Shifangjia FM screening

The Co-Re screen technology has achieved a breakthrough in obtaining high screen count dots with lower output accuracy, and won the Japan Printing Association Technology Award. The previous technology obtained 175 -200lpi dots with 2400dpi output accuracy, and Fuji Photo's technology achieved the output of 300 lpi screened images with 2400dpi accuracy output or 175 lpi screened images with 1200dpi accuracy output, and the image quality did not decrease . Using this technology, the existing CTP, RIP and other software and hardware can be used, and the image quality can be greatly improved at a lower cost (purchasing network software), and the production efficiency is nearly doubled.

According to previous printing common sense, in order to reproduce the printed image, each dot needs about 200 tones to express. According to this calculation, if the output accuracy is 2400dpi, the number of screen lines is 2400 / √200 = 170lpi, which is theoretically bounded by around 175-200lpi. It is generally believed that when the number of screens is higher than the theoretical limit value, insufficient gradation will result in quality degradation.

However, judging from the characteristics of the human eye, an area larger than 175 lpi (a square dot area of ​​145 square micrometers) (a circular area of ​​320-340 micrometers) can be "feeled" by integration, so the density is different. The complex dots are mixed to express the mid-level multi-mode technical solution. A dot composed of 50-60 tone can achieve the same smooth tone effect as 200 tone dots. In this way, not only in the sample of the printed matter, but also in principle negate the common sense of the past printing dots.

Another great feature is the in-depth research in theory. In fact, when using 2400dpi output to produce 300lpi dot image, the biggest technical issue is the monochrome moiré caused by the dot frequency and the scanning distance of the output machine. Elimination of uneven frequency faults.

This technology is a simulation system made in consideration of the characteristics of the CTP material, the characteristics of the recording curve of the CTP device, especially the characteristics of the human eye, which can "record the dots on the printing plate through CTP and visually evaluate it." All operations are automatically calculated and processed on the microcomputer.

The dot is applied to Fuji's thermal imaging CTP, photopolymer CTP and dot type DDCP in an optional manner. For printing plant users who get 300lpi dot plate thermal imaging CTP at 2400dpi output, they have made great achievements in printing products such as high-quality monthly magazines and other high-quality products required by department store owners. Through the construction of a simulation system starting from material characteristics, output optical characteristics, and visual characteristics, a technology has been developed to produce products with a high network line count at a very low resolution. "Simulation of the actual situation, considering visual characteristics, automatically produces the best The "outlet" is the idea of ​​this technology and can be used in various output systems.

6 Screening technology seen at DRUPA 2004

One trend in the prepress field is new products of screen technology. Many CTP plants are developing FM and AM combined screen technology. A new generation of FM screening is introduced, achieving high quality.

1) New FM screening

Ran-dotX FM screening of Great Japan Screen / MTC. The screening method is the second-generation FM screening method (HDS screening method), which can reproduce 20 micrometers, 15 micrometers, 10 micrometers dots, no moiré, no disconnection, and achieve high quality. By controlling the expansion of optical dots, ink consumption is reduced by 25%.

Fuji Photo released its FM screening technology-TAFFETA. TAFFETA FM screening eliminates the original FM-specific visual roughness, and the company ’s previous Co-re screen technology has expanded the screening method corresponding to the type of printing.

Creo, which first introduced the staccato FM screening method, has launched Spotless4, a software that can reproduce four colors and spot colors, and can be further combined with FIVI-six or PANTONE Hexachrome, etc., and further expressed with six colors. Spotless X in spot colors.

2) Mixed screening of AM and FM

Agfa's Sublima screening (CrossModulation, mixed modulation).

The Groovy screen of Esko-Graphics (added to the ordinary AM network, adding linear patterns to the solid part, improving the density and coloring of the solid part).

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