As the market continues to improve the quality requirements of color box products, color management has also received more and more attention. However, there are various color problems in current production. High-quality originals cannot be copied well. The quality of unsuitable originals cannot be improved after copying, which makes the original attractiveness of product packaging unable to be fully expressed, and the purpose of promotion and promotion cannot be achieved. At the same time, it increases scrap and cost, and reduces the competitiveness of enterprises. The reason is that, in addition to the problems of equipment and personnel technical level, the key lies in the failure of enterprises to carry out effective color management.
The necessity of color management
Color reproduction is the primary task of printing, and color is the core of the life and quality control of printing. However, for a long time, the colors of the original manuscript, screen display, proofing, printing and other links often did not reach the same, thus causing unnecessary disputes.
There are many variables in the production and copying process of color boxes. The pictures and texts need to go through multiple stages of scanning, processing, color separation, printing and so on. It is not easy to get the originals accurately restored. At each stage, the color information will be displayed according to the color rendering characteristics of the currently used equipment, so different scanning and display devices will have different performances on the same manuscript: In addition, the RGB color display screen and C, M, Y, K or C, M, Y, K, LC, LM color printing inks have different color expression capabilities: coupled with different application software conversion data is also lack of consistency, the use of different paper and ink to obtain printed products Different. Different types of equipment, even the same type of equipment of different models, have different expressions of colors.
Therefore, it is necessary to control the color reproduction on various devices and media in the production system by adopting color management technology to ensure that the final copy can better reproduce the original color.
The concept of color management
Color management is the correct interpretation and processing of color information, that is, managing people's perception of color. The essence is the process of converting the color data of an image from one color space to another color space under the premise of minimum color distortion, to ensure that the appearance of the color from input display to output matches as closely as possible, and finally make the original and product colors consistent .
When implementing color management, the colors in a device's color space are combined with the device's characteristic file to the intermediary color space, and then combined with the target device's characteristic file to the target device color space. Since the color space independent of the device is used as an intermediary, the consistency of colors is ensured. According to this implementation process, three elements of color management are obtained:
The device-independent color space serves as an intermediary for color conversion; for each device, there must be a standard specification to describe its color characteristics and parameters; there must be an accurate conversion algorithm to realize the conversion of colors between different spaces.
In order to standardize the standards of color management, the International Color Consortium (ICC) has formulated the ICC specifications to make every element of color management clear: the color space as an intermediary is CIEXYZ or CIELab; the file format describing the color characteristics of the device It is the format specified by the ICC; the color matching algorithm is completed by the manufacturer according to the ICC specifications.
Color management workflow
The establishment of a device's color profile (Profile) is the core of color management. The description file contains the representative color characteristics of each device, such as chromaticity characteristic curve, output color gamut characteristic curve, etc. The color management system uses these color characteristics Match and convert the color space of each device.
There are multiple workflows for color image copying based on ICC, and when the image is printed as an independent file or as an object in the page file, the specific implementation methods of color management are also different. According to the different application time of the output device characteristic file, it can be divided into early binding workflow (Early-bindingworkflow) and delayed binding workflow (Late-bindingworkflow). The former converts the color information of all images from the source color space to the color space of the final output device as early as possible; the latter defers the conversion of the color information of the image to the color space of the final output device as much as possible.
Principles of color management
For color management, a series of prescribed operations must be followed to achieve the desired effect. Color management is divided into three steps in principle, referred to as 3C, namely calibration (Calibrationy), characterization (Characterization) and conversion (Conversion).
Calibration is to ensure the stability, reliability and sustainability of the color information transmission process. The input, display and output devices must be calibrated to ensure that they are in a standard working state. The calibration of each device should follow the 3C principle.
Characterization is to record the corrected characteristics of all equipment. In order for each device to achieve accurate color space conversion and matching, it must be characterized. For the input device, display, and output device, make their color gamut characterization curves, and on this basis, compare their device-independent color spaces to make their color description files. These description files are the bridges from the device color space to the standard device-independent color space (PCS). On the basis of calibrating the system equipment, the equipment description file is used, and the standard equipment-independent color space is used as the medium to realize the conversion between the color spaces of each equipment. Because the color gamut of the output device is narrower than the color gamut of the original, scanner, and display, the color gamut needs to be compressed by the method in the ICC protocol when color conversion is performed. The following describes some specific operations implemented by 3C.
Input correction: including brightness, contrast, black and white field correction. Taking scanner calibration as an example, when the scanner is initialized and zeroed, the same image data should be obtained for the same original no matter when it is scanned. [next]
The first is to adjust the relevant parameters of the scanner to the allowable range, such as the light source should not exceed the corresponding life, the reference white clean and colorless. Next is to generate the device characteristic file of the scanner, which is the core of color management. The specific customization process is to scan a standard color mark composed of 264 color blocks representing the entire CIELab color space, generate an RGB file, and then use ScanOpen to read the scanned RGB file and the original standard RGB file. Compare The color difference between the two files generates an ICC curve, which is the device characteristic file of the scanner. Finally, in the actual scanning, the ICC file that has been generated is called to complete the standard conversion of the color space.
The color space conversion of the scanner has gone from the Lab color space of the original to the RGB color space of the scanner to the Lab color space, and finally the process of generating the RGB file of the original by calling the JCC file of the scanner. It should be noted in the color management of the scanner that the parameters of the device will change over time. Therefore, ICC files must be customized and replaced frequently to ensure the latest working status of the scanner.
Display calibration: The display is a device with RGB color space. Use the display calibration system to set its brightness, contrast, color temperature, and the gamma value of the entire display system. Make the display characteristics of the monitor conform to the ideal parameter values set in its own device description file, so that the graphics card accurately reflects the colors of the CMYK color space on the display according to the color data of the image data. Professional color management schemes should be used in image processing before printing. That is, use the hardware measurement method to generate the device characteristic file of the display. Such as Heidelberg's ViewOpen and the supporting suction cup screen colorimeter and X-Rite's PulseColorElite system. In order to ensure the accuracy of the display, the environment around the display should also be improved. If given
The display is equipped with a hood to reduce the interference of stray light around: install a standard light source; change the background color of the monitor desktop to neutral gray, etc. The color monitor should be installed in a room with gray walls and ceiling. After the above-mentioned transformation, the display can meet the requirements of professional soft proofing and achieve true WYSIWYG.
Output calibration: It is the final step of the calibration process, including the calibration of printers, imagesetters, printing presses and proofers, according to the device description file provided by the device manufacturer, its characteristics are corrected, so that the device is in accordance with the standard characteristics of the factory Output. When doing printing and proofing correction, the printing materials such as paper and ink used in this equipment must meet the standards. Correct the gray balance and black plate parameters of the output device. This is the correction of the printing material parameters such as paper and ink. Taking digital proofing as an example, the introduction is as follows.
Digital proofing is the use of color management technology to realize the color of lithographic offset printing on the inkjet printer. As the last color output link before printing, the accuracy of the color is directly related to the customer's recognition of the printed product. First, we must calibrate and determine the equipment parameters used in the normal operation of the inkjet printer, the performance of the ink head, and the variety of inks and substrates that are normally used to ensure that the materials used in the formation of the device characteristic file are the materials used in normal work. Secondly, use the calibrated inkjet printer to print the standard color code combination (commonly used IT8.87928 color block combination). Then, the standard color standard is combined to make a color plate, and printed on a calibrated printing machine to find a representative printing sample. Finally, the spectrophotometer is used to measure the printing sample and printing sample of the standard color code combination, and the data is input into PrintOpen to generate the device characteristic file of the printing machine. In use, the color management of digital proofing is realized by calling the digital proofing ICC curve file.
Because the paper and ink used for proofing have a great influence on the color, the device characteristic file of digital proofing should be re-customized when replacing new paper and ink. When re-customizing, it is only necessary to re-measure the newly-generated standard color code combination of the print sample and the previously input print sample data to compare, without having to print again.
Implementation of color management in color box enterprises
System composition
A complete color management system should include input, display, and output device characteristic files, characteristic file creation software, and standard inspection objects, color management models, characteristic file connection spaces, and application software interfaces required when creating characteristic files. At present, there are a variety of feature file making software on the market, such as ColorFIow and ProfileEditor software. Standard inspection objects include those required when acquiring scanner, display characteristic files, and output device characteristic files.
It can be said that to do color management, a standard environmental light source is required; high-quality color characteristic file generation software; advanced color management system: standard measuring instruments and test tools; standard color target: standard printing management; standard Remote digital proofing color management; standard scan reduction color management: standard digital camera color management; standard display calibration and screen soft proofing.
System Implementation
During the implementation of color management, carton companies should focus on the following aspects:
Standardization management of printing process: Carefully do the standardization, standardization and data production management of each process in the whole printing process, which is the premise and foundation of applying ICC color management. Enterprises must strictly formulate quality standards for fine products, intact standards for raw materials and equipment such as paper and ink, standards for measuring tools and environmental light sources, and develop standardized management documents, etc., and require everyone to be strict
Follow the standard. The ICC files and parameters of each device should be standardized, and quantified within the standards and specifications, summarizing and summarizing the data that can guarantee the quality.
