The main purpose of digital proofing is to provide reference proofs for printing and provide proof basis for customers. A set of digital proofing system includes a printer, a set of digital proofing software (integrated color management function) and a measuring instrument.
In this process, achieving color management is the core and key to digital proofing.
Color management for digital proofing can also be summarized in four steps:
â‘ Printer basic linearization; â‘¡Make digital proofing paper Profile; â‘¢Package linearization file and Profile file; â‘£Match printer Profile and printing machine Profile.
This article takes the famous digital proofing software Best Screenproof developed by a German software vendor as an example to introduce the principles and methods of digital proofing color management.
1. Matters needing attention in implementing printer color management
Summarized as follows:
â‘ The color target should be printed in its original size.
â‘¡ The paper size is at least A3.
â‘¢ If the measuring instrument is DTP41T, the paper size is at least A1.
â‘£The measuring instrument should be calibrated when it is used. If it is BestEye, the light source is about 2mm away from the paper when measuring.
⑤ The resolution calculated by RIP should be less than or equal to the resolution of the printout.
â‘¥The color matching method is selected according to actual needs. Best has developed the "Perceptual-absolute" matching method, which is suitable for photography, especially the conversion and matching of large color gamut to small color gamut can get very good results.
⑦If re-linearization is to be performed, the selected proofing parameters such as paper, ink, printer, measuring instrument and proofing software must be consistent with the basic linearization.
⑧ Use high-quality ink and paper as much as possible to ensure that the color gamut of the printer is larger than the color gamut of the printing press. This is the basis for implementing color management.
2. Basic linearization of the printer
1. Why should the printer be linearized
Basic linearization determines the physical characteristics of paper and ink, and optimizes the total amount of inkjet and single-channel ink of the printer, that is, the smallest possible ink volume is used to obtain the largest possible color density. Creation provides the foundation and optimization features, thus establishing the foundation of color management.
In fact, most inkjet printers are not in a linear output state. Generally, the maximum color density value is obtained when the total ink volume is about 50%. After that, even if the ink volume increases, the color density will not change significantly, as shown in the figure One shows.
This leads to the fact that the test color block used to make the profile cannot distinguish the chromaticity values ​​of higher areas, so it is essential to determine the maximum total ink volume and single-channel ink volume by basic linearization.
For example, when an inkjet printer prints a yellow patch, the maximum color density value can be obtained where the ink output is about 50%; even if the printer continues to eject ink in an area over 50%, the color density value is not Will increase again, but will make the output color slightly reddish. The same happens when the printer prints magenta and cyan blocks.
This color block is included in all color targets that create the Profile. If the target is printed and measured with a spectrophotometer, the color density value changes very significantly in the area where the inkjet volume is 0% to 50%, exceeding 50 % Area, the color density value changes very little, and the measurement results are very similar. These areas are not meaningful for making a profile.
If the printer is linearized in advance, the range of small output density changes will be ignored, and the chromaticity values ​​represented by the remaining inkjet range will be evenly distributed. That is to say, the range of the printed yellow patch seems to be The average distribution between% -100% is expressed as a linear change in density, that is, the printer input density value is approximately equal to the output density value. The profile obtained by a linearized printer is more ideal than that obtained by no linearization.
The result of the basic linearity is saved in the file format of the extension bpl. When performing proofing output in Best Screenrproof, the relevant basic linear file must be required to limit the ink output of the printer.
2. How to perform basic linearization
Basic linearization only needs to measure the color density value, so it can be measured by any densitometer or spectrophotometer. Best Screenrproof supports online measurement, such as DTP41, Gretag Macbetheye-one, BestEye, etc., and provides corresponding color targets for different measurement devices. Users can open Color Manager Charts under the installation folder to find them.
The test steps are as follows:
â‘ Import all the color targets to be printed out into the best.color Queue. Assuming that the measuring instrument is eye-one, you can import Total Ink Limit_BestEye, Ink Limit Channel_BestEye, Lin6c_BestEye, and Quality Control_BestEye, which are used to automatically print and measure the maximum total ink Volume, single-channel ink volume, linearization and quality control;
â‘¡Enter the basic linearization interface and set the parameters such as measuring instrument, resolution, paper and ink type, as shown in Figure 2;
â‘¢ Print and measure Total Ink Limit_BestEye, the total ink volume measurement result will be automatically generated and activated;
â‘£Print and measure InkLimitChannel_BestEye and Lin6c_BestEye, get single channel ink volume measurement value, and automatically activate, the total ink volume and single channel ink volume can be applied to the subsequent color target printing;
⑤ Print and measure Quality Control_BestEye, check the linearization result, require that the separation line between each color block and color block is clearly visible, and there is no abnormal phenomenon such as serious grading, ink bar and penetration, then it prompts the successful creation of the basic linear化 File.
3. Make Printer Paper Profile
Based on the created linearization file and the measured ink volume, the printout is used to make a color profile for a paper profile, such as IT8.7 / 3, which is measured and created in Printopen or other ICC software. As shown in Figure 3, it is the wizard interface for making Printopen Profile.
Fourth, packaging printer linearization files and paper Profile Best Screenproof provides a Profile keeper plug-in, which is specifically used for packaging printer linearization files and paper profiles. The so-called packaging is to associate the two and make them closely related.
5. Object-oriented color management function
Use Best Screenproof software to match the profile characteristics of printing and digital proofing, use the color gamut of digital proofing to simulate the color gamut of printing, and finally make the colors of printing and digital proofing look consistent. After we have obtained a profile file that reflects the characteristics of the printing process (Best Screenproof called the reference profile) and a profile file that reflects the characteristics of the color printer and printing paper (Best Screenproof called the paper profile), put these two Profile files in In the Best Screenproof software, under a certain matching method (Renderintent), after compiling and calculating, you can print out a sample very close to the printed matter.
Best has object-oriented color management functions. In the reference profile options, you can select the reference ICC files of CMYK, RGB, and LAB color spaces for conversion and matching of different color spaces. You can also find the check files under the Best Screenproof 4upC \ ICC Profiles \ Checker Profiles directory Profile of color space, such as RGB Checker Profile, if you select it, the RGB image in the file will be output as a simple green block when you print it out, users can easily find the problem; at the same time, in the lower part of this panel There are two options. According to the above method, you can check whether each file has an embedded profile, or use the embedded profile instead of the reference profile set by Best.
After the color space conversion, a new CMM data set is generated, which records the mapping relationship between the data in the above two Profile files in the intermediate color space and is the matched data file.
After the linearization of the printer and the profile matching of the printer and the printing machine are completed, the output file can be arbitrarily printed in the established queue without other color settings. This output file is based on color management of digital proofing. Therefore, in turn, it can be used as a basis for printing and a reference for sample tracking.
(Author: Department of Printing and Packaging, Wuhan University)
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