Control color reproduction before printing

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Prepress control color reproduction

there are many changing factors in prepress workflow. Therefore, the workflow process must be controlled to ensure the acceptance of printed products. With the improvement of electronic publishing, color evaluation has gradually approached the creative stage, and color management system and color proofs have become indispensable prepress workflow tools. Color management systems are often used to produce predictable color matching between one device and another. Color proofs (digital or analog, prepress proofs or contract proofs) are key tools for monitoring every aspect of the image reproduction process. Using the sample, users, designers, color separations and printers can judge the reproduction effect of live parts based on it. In the early stage of scanning and color separation, samples are the guiding means for processing originals using additional tools and adjustment processes, and they are also an integral part of living parts. In the process of moving parts development, control tools (ladders, indicators and color control strips) have become necessary accessories to confirm the stability and accuracy of color. Finally, the function of the sample is the contract that the customer requires the final print to accurately simulate. The most useful place is that the samples must be made according to the characteristics of the printing process, such as color, density, dot magnification and image resolution. The correctly made samples are used by the printing operator to guide the control of printing (ink intensity, color balance and printing quality), so as to match the printed samples with the samples provided. Once the color is recognized, what customers expect is that the characteristic quantities such as hue, color density, color balance and dot enlargement will remain stable throughout the printing process

color management

how many times have you or your users complained about the mismatch between the printed sample and the contract sample, or the different display effect between the printed copy and the screen? With the change of our industry from analog to digital, the problem of color reproduction has become very critical. We need to use color management to ensure better, faster and cheaper creation of color images, because the process of creating images is difficult to sort and cannot always achieve this effect. The ratchet wrench changes the main parameters of the experimental machine: results. In addition, many images are now reused, or reused in various media (for example, page images displayed on computer screens or computers on CD ROMs), and it is often hoped that these images will match the printing effect

* gamut

the most basic problem of color matching comes from how color images are copied. No printing equipment or color display screen can copy the color range that can be felt by all human eyes. The unique color range that each device can produce or display is called the color space of the device. Some devices have a large color space, while others have a very limited color space. The color spaces of different systems can overlap, but they do not match

imagine the following example, if we want to print and copy the color of pink sweater, and hope it can be displayed by standing on the monitor at the same time. The actual color may exist in the color gamut of the transmission film, but may fall outside the offset reproduction color gamut, and may also be outside the screen color gamut, not to mention the color gamut of the proofing equipment used to predict the color in the process. In this case, the image of this pink sweater must be converted according to the output device (printer or computer screen) before output to best match the original image. The digital color management system has been further improved to achieve the purpose of color conversion in the process of production replication

color management system

color management is how to control and describe the image colors we see on the computer screen, captured by the scanner, on the color sample and printed by the printer. From image creation or color capture to final image output, color conversion is performed in a systematic manner. In the conversion process from one device to another (whether from computer to printing machine, or from sample to printing machine), the color management system tries to maintain and optimize the fidelity of color

color management system is generally divided into three parts: calibration, characterization (device profile file), conversion (gamut correspondence). Calibration is the method of adjusting each device (display, scanner, printer, etc.) to the defined standard state to ensure that it meets or is accurate to the manufacturer's specifications. Because the equipment is changing at any time, which will affect the way of color display and generation, the calibration process is very important. Calibration, or characterization process, is the description of the color gamut of each output device, which plays a role in identifying pigments and operating modes. The profile files of these devices are often used in the conversion process from one device to another. The conversion function draws the color of one device to another device, and it must change, so that the two devices can display or produce similar colors. Creating a user's profile is important not only for every device, but also for every kind of paper used

* spectral data

the most accurate way to describe color is spectral data, which describes what color is, not just what its appearance is or how it is copied. To obtain spectral data, a spectrophotometer is usually used to measure a color sample (for more details, see the "color measurement tools" section)

nowadays, desktop spectrophotometers that many enterprises can afford are adopted, and more and more color tools that can work with spectrum appear on the market. After the design is completed, the spectrophotometer can accurately define the color of any object; Samples, prints, color blocks, ink samples, etc. Spectral information is used to create profile files for color devices. As the production workflow continues to shift to full digitalization, the monitoring of color at each stage will gradually rely on spectrum based information

* color management standards

the printing industry is developing color management standards, which provide a series of standards to be followed in the whole production process. According to the standard, hardware and software investors can establish cross platform device profile files that identify the characteristics of color devices. Now there are two standards committees working on the topic of color management: the image technology committee and the international color Union (ICC)

the image technology committee 8 of American National Standards Association (ANSI) was recognized in 1987 and engaged in the development of digital data exchange standards. The standards developed by the committee are it8.7/1 and it8.7/2, which are used in the color management software package to define transmission and reflection color targets for input scanner calibration. ICC, although not certified by any standard color organization, has developed experimental data for obtaining various data, which is suitable for the profile file format widely accepted by the image technology industry, and has reached the stage of device independent color management. ICC was founded by eight industry investors in 1993. It created a device profile format standard that represents the characteristics of color devices to serve the cross platform environment. And the color is defined by using CIELab color space. Before that, each investor had its own independent system, which was incompatible with other systems

* ICC profile file

icc profile file provides the color management system with the necessary information to convert the local color data of a device (like the color range and color gamut that a device can generate or obtain) into the device independent color space. ICC. 1: The specification divides color devices into three categories: input, display and output devices. For each device, there is an algorithm mode that performs the color conversion process. These mathematical models provide a certain range of color quality according to the different requirements of memory needs, execution process and image quality

based on chromaticity data from spectral data, ICC profile makes it possible to describe and translate the capabilities of color equipment produced by different manufacturers in a standard and portable format. The device profile is created by the physical measurement of a selected group of color patches with a spectrophotometer. These color patches are created or captured into electronic files (in the case of input devices) or output forms (in the case of output devices). Then, calculate a mathematical description related to calculating the device related chromaticity value (such as RGB or CMYK) of the device into the device independent CIELab color space value. The process of making ICC profile correctly is to accurately convert all RGB or CMYK values into CIELAB values

The goal of ICC is to establish a color management module that can communicate and process images in a standardized way, and allow the color management process to be carried out across platforms and operating systems. Successful color management requires 1) device independent color space (ICC agrees to use CIELab color space to define colors); 2) Profile files that accurately describe the color characteristics of digital color devices; 3) Good color gamut conversion technology. The ICC specification is still open to the manufacturer's own special interpretation process in key parts such as color gamut conversion, which means that users should avoid confusing and matching profile files produced by different manufacturers. (end)

source: innovative printing art

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