Digital Screening Frequency Modulation and AM Technique Comparison

In today's complex, fully automated prepress workflow, the hot topics of discussion focus on issues such as PDF, JDF, XML, CMS, and ICC. Screening is no longer the technology most concerned by prepress specialists. However, recent developments indicate that screening, as well as other forms of network formation, remain the top concerns for prepress workflow and output system manufacturers and distributors. The benefits of optimized screening technology are to bring more vivid prints, clearer details, no moire and process stability to the printing companies, and those companies that ignore this technology missed a very important production control tool. .

Let us first review the development of digital screening technology. It evolved slowly from the process of glass and contact screen screening used in the formation of simulated dots. From the early days of offset printing technology, printing process experts have been experimenting with dot shapes, angles, and screen lines, hoping to optimize the effectiveness of printed matter and control the process. Digital screening provides us with a possible method of physically forming network points, making the process constraints no longer a problem.

When dtp (desktop publishing) appeared, the early Postscript Rip could implement four-color and multi-color digital screening, but the quality was lower than digital screening of analog screening and dedicated high-end color systems. Experienced prepress operators may still remember the extensive and intense discussion of “screening” in the late 1980s. Later on, there was a revolutionary change in the composition of optimized networks, which provided more screening angle options than Postscript Level 1. At the same time, prepress equipment manufacturers began to pay attention to Postscript Rip and applied their rich screening knowledge to desktop publishing systems.

The real revolution is the use of FM or random screening, although it is not a new idea. In the 1980s, it had been used in digital printing processes where the dot quality was not high. Instead of using different dot sizes to simulate different continuous tone, it used the same size and relatively small dots. Between the intervals to reproduce the tone. The FM/Random Screening was just launched at the height of the war on the Internet and was heavily advertised and given too much hope. Since then, the traditional screening method has been named AM (Amplitude Modulation) screening to distinguish it from FM screening.

Compared with AM, the advantage of FM is a clearer effect, because extremely small dots can produce more hues; ensuring no moire between colors and patterns, making the printing process more stable. However, it encountered many obstacles before it was applied to a wider range of non-special purpose applications. The disadvantages encountered in the early 1990s were that it was very difficult to process in computer-to-film (ctf) environments and that there was no exposure range for printing plates; particles were generated in the gray midtone and flat net areas. Patterns; and it is difficult to compensate for chromatic aberrations on the press by using ink thickness variations. Operators who used this method in the early days seldom insisted on using it all the time.

In the past decade, a small number of prepress and printing companies have begun to use the FM screening technology in a tacit agreement to produce higher quality prints. Artistic books and high-gloss catalog products with more prone moire objects, such as textile images, speaker's pupils, and architectural designs, all benefited from FM screening technology and became a technology they were happy to use.

Since ctp technology simplifies the process, it plays an important role in the recovery of FM screening technology. In addition to film, dust and registration problems have become factors that hinder the development of FM technology. With ctp technology, film edges (thickness) or dust particles do not have to be considered too much, so smaller FM dots (usually 12 to 30 microns in offset printing and 70 microns in flexo and newspaper printing) can be directly on the plate Formed on. Another advantage of the Ctp technology is that harder dots can be formed on the thermal plate for better printing results. Particles composed of laser beams can form screened dots that are predictable in imaging and printing. Any edge of the dot formed by the unfixed layer of the residual plate will not form an irregular printing effect.

Creo Corp. strongly recommends using Turbo or SquareSpot screening technology to take a step forward in the quality of hard dot. This technique uses a higher resolution in the main scanning direction (the circumference of the exposure roller) than in the secondary scanning direction (direction above the roller) to improve the accuracy of the screen dot. Compared to conventional dot-mapping (2,400x2,400dpi) technology of the same size, screen dots composed of more particles can achieve higher resolution (9,600x2,400)
Dpi).

Heidelberg applied this technique to their ctf imaging machine a few years ago. Creo combines this precise network of dots, the company's Staccato screening technology and consulting services to enable FM screening technology to recover again. It advocates controlling on-machine profits, including reducing the amount of ink used, obtaining higher color stability, and making ink easier to reach balance. Better printing process stability has gradually shifted the offset from the newspaper printing of 65 screen lines per inch to 100-110 lines per inch (40-44 lines per cm). In commercial offset printing, the number of screens is increased from 133 lpi (54 l/cm) to 175 lpi (70 l/cm), and very high quality printing requirements can reach 200 lpi (80 l/cm).

A similar approach to FM as an alternative to the AM screening technique is to use very fine screens for screening. This method has become widespread in Japan and is commonly used by many professional printing companies in the Western Hemisphere. This method usually uses a line number of 250-350 lpi or even higher to optimize the printing effect and achieve a clearer, more elaborate effect through the high-density ink. Breakthroughs in the number of ultra-high-gauge lines have encountered the same problems as FM, such as harsh process conditions and the need for high-resolution scanning to generate maximum profits.

In many locations in Europe and the United States, 300 dpi scans and modest false light masks are often the default settings, while the scan standards in Japan are set at 350 dpi and slightly higher levels of unclear masks to cater for higher screen cables The number of requirements. Associated with the number of super-high screen lines is a higher image resolution, which means that more gray levels can be generated.

For the 4,000-dpi line, the imaging process is significantly slower than the 2,400 or 2,540 dpi more commonly used by the Western world in AM and FM screening. Fuji Corporation proposed a Co-Res screening method using a traditional screening angle, which is comparable to the use of relatively high-resolution image effects. From this, a 1,219 dpi image can reach 175 lpi. Or a 300 lpi effect can be obtained from a platemaker that does not have this effect at a lower image resolution. To date, the technology is still limited to use in Fuji's Corporate Rip and platesetters. The most immediate advantage is a leap in the overall production volume, because there is less data to be processed, and it is not necessary to change the operating habits of the printing press because the screen angle and dot are exactly the same as the original.

The disadvantage of the hybrid screening method is that it requires interactive masks on high-end workstations, such as Screen's TaigaPress. Therefore, the labor intensity is high, and it is only suitable for high-value printing projects. For packaging applications, printing methods with more than four colors are generally used. For the coarser screen printing lines of flexo printing, Barco and Artwork Systems have developed corresponding hybrid screening methods to not only avoid the occurrence of moiré patterns. You can also produce more detail in lighter and darker areas for more clear, more saturated printing. This hybrid screening method can provide FM details and continuous tone reproduction of bright and dark areas without affecting most flat tone and midtone grainy graphics because these are implemented using AM screening. .

In the area of ​​newspaper printing, Agfa has just completed testing of the hybrid screening technology Sublima. This technology has been tested in Norway, using FM CristalRaster outlets in some regions and AM ABS screening in the middle portion. Screen Corporation has recently introduced Spekta, a new hybrid screening technology to achieve the best quality of its thermal Ctp imagers. Spekta Screening Technology combines the advantages of FM and AM Screening in dot formation, color mixing, moiré, and ease of operation compensation on the press.

Like other hybrid screening methods, Spekta uses FM screening in 0-10% and 90-99% of replication areas, and uses AM round-network screening in the 10-90% range. Since all the dots are randomly distributed within the total dot area, there is no limitation of the screening angle between different colors, and therefore no problem of moire occurs when colors are mixed. Spekta provides an optical effect comparable to 300+lpi compared to other high-mesh screens. There is no productivity limitation, but 2,400 should be used in a standard image setting area such as 175 lpi (70 l/cm). Dpi screening.

Under normal circumstances, adding text to the image will give better results, such as wine labels and maps. Using Screen's Spekta Screening Technology eliminates noticeable rose spots and moire, along with Screen Taiga and Trueflow, provides users with a unique PlateRite thermal imaging machine, which is a versatile, most suitable screening solution. New technologies related to thermal ctp imaging technology can now produce clearer, more replicable dot quality. Based on this, prepress customers are constantly looking for the best screening mode to get the maximum productivity in order to achieve the best reproduction effect in the easiest way. In the process of printing and copying, outlets are undoubtedly the most important issue. The organic combination of AM and FM technologies will produce better results.