Color monitoring: printing digital color management is more dense and perfect

1. The color reproduction requirements of the printing industry are increasing day by day

Printing is a graphic communication industry. The image and graphics parts are now in color. Even newspapers have a large length of color. Therefore, image reproducibility and color reproducibility play an important role in quality. Coupled with the packaging of many products, the color of the image and the background dots constitute the color, which is the basis for many customers to distinguish between authenticity and authenticity. Today, the division of labor in the printing and peripheral industries is becoming increasingly dense, regardless of the hardware and software driven by electronic digital technology, it can be more accurately measured, simulated, and controlled. Therefore, the users of printed materials have stricter requirements on the difference in color reproduction of printed products, and some of them are more demanding, not only beyond the ability of visual recognition, but also use instruments to measure dark parts and dark parts. The error value of the eye is insensitive, and the measurement requirements beyond the necessity are made, and the degree of the required scope of the visual product is lost.

Second, the color management from closed to open

During the 1970s and 1980s, an electronic scanning color separation machine was installed, and technicians would request the output of color screens for printing, proofing, and printing to establish a closed color reproduction gamut and color balance system for the printing plant. If there are different printing papers, It also divides bright curve copper plate, matte copper plate and non-coating three curves for color reproduction management. The author asked him what if the plate-making factory responded to many different printing plants? He could not respond to them one by one, only to achieve the color management system until the proofer made the proofs.

Today, we no longer rely on the R, G, and B color filter density values ​​for color management, because it cannot perform the calculation of color difference values, and most of them use Lab three-dimensional color gamut space, L color brightness value and ab and- The position of the hue plane of ab, when it is the color value data, the two color data regardless of RGB to CMYK, RGB to different RGB performance, CMYK to RGB, CMYK to different CMYK output performance change value, can use Lab this color space To calculate the conversion. Moreover, the two color gaps can also be used for the color difference display of the plane or three-dimensional gap with the value of △ E, so the open prepress and printing equipment using this color gamut space has become the mainstream of color management. In different brands of equipment, Different hardware, software color materials and media combinations are used to display and adjust the color values. Today's ISO 12647 system also uses Lab data values ​​as the main body of color specifications and operations, making color reproduction more open and consistent.

3. Image input, shielding display and various output management

The traditional image input center has been equipped with professional roller or platform CCD scanner with good IT8 7-1 7-2 for transmission and reflection color correction to capture image files. But today it has become a general amateur CCD camera, and even the image files taken on mobile phones are reproduced under different hardware CCDs and software transcoding and color mechanisms. In addition to the changes in the ambient color temperature of lighting, many prepress jobs today have no funds And time, do manual adjustment of common sense color and version tone. Therefore, if many prints are not taken by the studio today, there are color managers, many of which are image pictures with a large difference between version tone and color. When printing without a plate, the situation is worse and worse, so the image trimming software of AI artificial intelligence is necessary.

Regarding the color reproduction of the screen, it is more important now to promote soft proofing of long-distance screens and soft proofing of paperless printing. Some advanced screens have automatic color correction equipment and programs, or color management software, and use measuring instruments for screen color output. The revision is also very effective. However, different screens have different color gamut performances, such as the color rendering of CRT using cathode ray tubes (the ones with large backs), the use of electron rays to stimulate fluorescent toners, and the thin LCD TFT screen, which uses a backlight to emit white light. And then use the filter to produce the color, which is quite different, and the color rendering of the 53LCD backlight board module itself is also inconsistent. Basically, the color rendering of the screen is wider than that of the printed matter, but it does not mean consistency and long-term stability. Even after the power is turned on, the color will be stable for 10 to 20 minutes. In addition, the human eye judges that the color has a very perceptual change in the optic nerve and visual compensation. For example, if you go from a dark place to a bright place for a while, you will feel darker when you return to the dark place. of.

As for output, printing output has been the main goal for a long time. Color reproduction has plate-making process, plate conditions, printing press conditions, ink conditions, paper media conditions, lithography and more ink-water balance conditions, temperature and humidity conditions and printing. Pressure and other conditions, synthesizing the print version and color reproduction conditions, and the biggest pillar of color management is the stability and consistency of these conditions as the cornerstone, otherwise there are many changes in its own conditions, then all information-based color management is built on the beach Pavilion. The use of IT8 7-4 is the standard for color management of future output prints. The original IT8 7-3 color management swatch tool has 928 color gaps and tight color intervals. The plate-making method is used to enable the IT8 7-4 swatch tool to output color samples under stable printing conditions, and then use the measuring instrument to input the IT8 7-4 color scale data of the printing machine as a reference. Then use IT8 7-4 file to submit digital proofing such as inkjet proofing machine to output digital IT8 7-4 color samples, and then input the data by the measuring instrument. The former is used as a benchmark to simulate the digital color data of inkjet samples , Calculate more than 1500 color scales, the trimmed color â–³ E value of each color scale, form the ICC Profile characteristic correction file, if you can make it a few times and the printing machine IT8 7-4 color scale is very similar, the human eye can not see the gap â–³ E is 0.8 Digital proofing within the gap of ~ 1.5, because some pre-press departments can make contract proofing in accordance with digital proofing, for printing test sample comparison printing, on different media, paper surface, IT8 7-4 color reproduction management data should be revised Another set of new standards. When a period of time changes or a production condition or material condition changes, the ICC Profile must also be re-calibrated to avoid losing color reproduction standards.

4. Conclusion

Today, many printing and digital printing equipment are equipped with monitoring instruments for color reproduction measurement control, and many of them can be automatically fed back to adjust the color reproduction. Some sheet-fed offset printing machines are equipped with offline color marks and printing. Streak measurement and monitoring, as well as adjusting the feedback system, contribute to the stability of long-term color reproduction. However, for the overall color reproduction system, the hardware and software selection must be done in advance, and the standard color reproduction data must be established in order to have the possibility of subsequent instrument automatic control production. Today's instrument sensitivity and computer high-speed calculation capabilities can handle 16,000 sheets per hour in A1 size, 0.5mm per pixel printing online quality monitoring, but smaller area pixels or more sensitive gap resolution can be expected to continue to develop If it is often detected that the defective rate of the finished product is more than 30%, then the subsequent cost of labor to do the quality inspection is overwhelming. Whether it is necessary to find a balance in the pursuit of accuracy and practicality, is it feasible color and quality control system. Today, the use of precision color measurement instruments for digital color management and the use of color measurement instruments also affect the accuracy of color calibration. Therefore, the calibration and adjustment of the instruments according to regulations are also important pillars to ensure good color management.

The author Chen Zhengxiong was engaged in the zinc relief process in 1958, and was good at color imaging. In 1976, he and his brother founded "Printer" magazine, which is the CEO of Taipei International Printing Machinery Exhibition and publisher of "Printer" magazine.

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