Offset printing image color control technology(one)
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The eternal theme of printing is the transmission and reproduction of colors. How to control the quality of offset printing images that are commonly used in color printing has been improved by color control techniques, which is a challenge faced by domestic enterprises. On the basis of summarizing the most advanced international offset printing image color control technology, this paper puts forward a new viewpoint for the development trend and characteristics of China's printing industry, and points out the research direction for future color control technology.
It is widely believed that it is an inevitable and unstable printing method due to the printing principle of offset printing. The "oil" in the ink and the "water" in the ink tank are not always as good as one would expect, so for printer manufacturers and consumer goods suppliers, it will be necessary to continue to control the entire process. process. For printers and print buyers, this means that only a compromise can be achieved in a fairly long calibration process, that is, to achieve "commercially acceptable quality" rather than printing that can be foreseen in advance. quality.
But this situation has changed. The current application of digital technology in the print and publishing world is the key to their success. Digital technology makes color quality control and consistency control no longer a very technical thing. Such as CTP reduces the erratic uncertainty in the plate making process. The now exposed version can be determined to have a completely consistent plate when re-plated after 3 months.
The current printing presses already have good control performance. On the press, the auxiliary plate-loading device will reduce the problems caused by the unevenness of the liner, and the temperature-controlled roller eliminates other uncertainties. Several common color control methods are as follows.
1 Color control method based on ICC file
Take the American Reader's Digest as an example. Their magazines are available in 48 editions and 19 languages, and are distributed in more than 60 countries around the world. There are 20 editions in Europe. In addition to magazines, they also distribute other products, including books, such as music, video and other special products. As their distribution and delivery methods expanded, the company expanded its range of printed products, including electronic media and traditional print media.
Image printing is the task of all printing sites that print readers' digests, and it involves a lot of repetitive work and expenses. Different printing places make image quality control of different versions almost impossible, and color management is especially a problem. Differences in color quality and appearance can occur from inconsistencies to severe color deviations. To this end, they adopted a set of Colour Systems to avoid similar problems. This is a website dedicated to managing all European version files of Reader's Digest. Includes library files, product files, images, graphics, and more. The company has now established a new workflow to ensure consistency between different media and printers. In different parts of Europe, operators can access this website with special URLs and passwords to ensure print consistency.
Colour Systems processes the RGB image input by the drum scanner. Each scan uses a scan-corrected ICC file (also known as an ICC device connection file) to convert the color of the image into the CMYK color space of the SRD (Selected Reader's Digest). When needed, the scanner performs frequent maintenance and inspections based on established control procedures and documentation. This ensures that both the original image input and the final output image color and ink characteristics are up to standard, ensuring color reproduction and matching on the press.
2 FM screening technology combined with chromaticity color control
These changes have also aroused people's interest in FM screening and other screening technologies. For example, Agfa's Sublima studies the size of the smallest dot that a printer can consistently replicate. If the dot size is 60 μm, it can represent 6% of the 150 lpi screen lines or 8% of the 250 lpi screen lines. In the usual case, this means that the press is limited to a lower resolution. With Sublima, you can achieve higher higher-profile parts without having to try to print less than the minimum dot size of the press, but by reducing the number of dots in less than 6% of the area. This effect is immediate, the press can print more pleasantly than the expected network cable, the high-profile part is brighter and the middle-level level is more abundant.
Creo is one step ahead in advancing the use of FM screening, but it not only emphasizes the impact of very small dots on print quality, it also promotes the application of oil and water distribution balance due to the consistency of its dot size. . This result is better, the press can be controlled more consistently, the amount of ink is reduced, and the printing process can be better controlled. This is critical for long-run web offset printing, which is why 40% of magazines in the US use FM screening. As Polestar began to use Creo's Stacatto FM screening in its web offset press, FM screening was also used in UK magazine printing.
Between 1970 and 1980, register control on web presses was a very high-tech technology, and now the color closed-loop adjustment capability, or adaptive, is added. The color closed-loop control system from QTI and GMI, once set, ensures that the same color is maintained throughout the printing process. As the ultimate owner of QTI, QuadGraphics is trying to ensure that no human intervention is required on the press. In contrast, GMI takes a big step forward in using chrominance values to achieve quantitative printing.
3 spectrophotometer color control method
GMI (Graphics Microsystems Inc.) added a "Press Analysis System" to its ColorQuick color closed-loop tool. This system records a large amount of data obtained by spectrophotometers and is stored in a standard SQL database. The analysis tool can track the portion of the press that is printing a print, or it can track the press for one week, one month, or any particular time period or during any time period for any customer. Health status.
This system can generate dot enlargement and density over any period of time, and by calculation can monitor the difference between the originally set target color and the color measured on the press. When applied in reverse, replicas can predict the performance of the press and can be taken into account when making plates. In other words, by setting a target color, the printer can read the colors and copy them out.
Printing only needs to monitor this data to ensure that the press is functioning properly. It is also possible to provide publishers with these data indicating the operating status of the press. Since the GMI uses a spectrophotometer, it can measure the value of the color and then know how to adjust it by evaluating the neutral gray produced by the three-color overprint. Since neutral ash is the most sensitive color, any deviation from the set standard is immediately detected and can be corrected before affecting the overall balance of the print.
Even if the printer is operating at the correct density, the color may not be correct, which is why the color control is now switched to color control on the press. This way the printer will reach the gray balance on the monitor when the correct density value is reached. US publishers have stated that their goal is to omit all hard copy proofs, which is the last largest market for digital processes. They look forward to valuable advice on digital printing.
Because of the vast size, sending hard copy proofs by courier is a time-consuming and expensive thing. If a printer wants to match the set chromaticity value, then a visual assessment of the proof is not necessary. The main purpose of this is to reduce waste. In general, more than 1,000 sheets are wasted when printing.
What appeals to printers is the introduction of a multi-point control press method, which is what RR Donnelley is studying. All operations are performed under the same standard and will produce the same print. Similarly, the place to attract consumer companies is to give them a wealth of information about how their products are printed on the press.
However, the installation of such a spectrophotometer on a sheetfed press is still very rare. Even with such control systems installed on top-of-the-line offset presses, they are not widely used in markets where labels and cartons are critical to color fidelity requirements. The printer has not yet believed that it is a reasonable cost to install each device, and now almost no printer purchaser requires such a crowded analysis system.
However, the problem of improving color consistency on the press still exists. The ICC file has its place, but it turns out to be difficult to apply. In order to create an ICC file, the printer needs to print a complete set of test samples of different color blocks and measure them one by one. Through a series of long readings, a very accurate image averaging is obtained to form the ICC file for that day. In fact, for most printers, this work is too cumbersome to be practical.