With the rapid development of computer-to-plate technology and digital printing, the application of digital proofing is becoming more and more extensive. It is based on digital publishing and printing system (CIP3/CIP4), and in accordance with the standards and specifications of publishing and printing production in the process of publishing and printing. Handle the page graphic information, without any form of simulation, directly output the sample through the color inkjet printer, simulate the final printing effect, to check the image page quality, provide reference samples for the printing process, and provide the basis for the customer to sign the sample. .
First, the necessity of digital proofing for color management
Compared with traditional proofing, digital proofing has the advantages of good image reproducibility, low requirements for personnel quality, fast output, and low system cost, and is increasingly favored by people in the industry. However, there are still some problems compared to the actual printing process.
Spot color proofing. Although many digital proofing systems support spot colors, spot colors that are outside the printer's color gamut are actually represented by CMYK four colors, which are similar to CMYK used in printing to match spot colors, which are not exactly the same as the spot colors used for printing. .
Dot structure. In addition to a few digital proofing software, such as EFI Colorproof, GMG Colorproof and BlackMagic support color inkjet dot proofing, the basic use of FM dot or no dot dye sublimation technology, and printing usually uses amplitude modulation dots for color reproduction, which will lead to screening Potential ghosting faults are difficult to detect during the process, and the print operator must check the print quality using the tone matching control method during the printing process.
Color matching. Since digital proofing generally uses glossy paper or semi-high gloss paper, matte paper, and is whiter than printing paper, the spectral characteristics of the ink used are not consistent with the spectral characteristics of the printing ink, resulting in different color ranges for digital proofing and printing. The former's color gamut is generally larger than the latter's color gamut, so their color matching needs to be further tested. At the same time, the ink used in domestic printing factories is the Japanese ink standard, while most of the printing machinery is in Europe, and the difference in climate between the north and the south of China is large, which causes the digital proofing and the actual printing to not cooperate well.
Due to these problems, it is necessary to color manage the digital proofing system in order to match the color of the digital proof with the printing. The color matching system calls the proofing device and the printer profile under the specific paper and ink combination to match the color between the two, so that the sample output from the proofing device can simulate the final printing effect. [next]
Second, the implementation of color management in EFI Colorproof XF 4.0 digital proofing solution
The following is an example of the EFI Colorproof XF 4.0 digital proofing solution to illustrate the implementation of digital proofing color management.
1. Establish a standard color environment
The stable color environment makes it possible to observe the original color, proof proof and print color. Therefore, the standardization of the color environment is the primary condition for implementing color management, which includes two aspects of the standardization of the observation environment and the standardization of the ambient light source. For the standardization of the observation environment, the chroma of the ambient color and the background color is as small as possible, and both are required to be neutral gray, that is, the neutral gray of the Munsell brightness value. Otherwise, the ambient color or the background color is disorderly, which affects the observer's judgment of the color; for the standardization of the ambient light source, the standard light source is required to have a higher color temperature and a higher color rendering index. At present, the globally recognized source for observing reflective samples is D50 or D65, which have red, green and blue energy equal properties in the visible spectrum, making it easy for observers to perceive subtle differences in color. In addition, their color rendering index is above 90, which is convenient for accurate color judgment.
2. Printer basic linearization
The printer's basic linearization is to determine the linear relationship between the input gradation and the output density, to solve the difference in the color rendering characteristics, ink volume and density, ink volume and chromaticity of different inks on the paper, so that the input and output values Maintaining a basic linear feature (input value = output value), obtaining the largest possible color density with as little ink as possible, and adjusting the proofing system to a production-compliant basis provides the basis for creating a printer paper profile. The principle is to correct the deviation of the printed color by checking and adjusting the density of the various levels of the printer, and the density can be measured by a densitometer or a spectrodensitometer.
EFI Colorproof 4.0 comes with a basic linear correction tool as a control tool for output quality stability. After entering the LinTool module, select "Printer Basic Linearization", set the connected online measuring device (such as EFI X-Rite Eye-One), ink type, paper type, resolution, color mode, etc. in the "Settings" interface, and then The system will step through the linearization process using the default settings of the linearization process.
1 ink limit per channel.
Click "Print" and the ink limit chart will be printed automatically. Wait for the ink to dry, then click on “Measure†and measure the printed chart to display the color value. If necessary, click Advanced to enter the Adjust Ink Limits screen. When setting the ink limit, it is better to have a smooth chromatic trajectory and no abnormal distortion, as close as possible to the printing gamut range and the hue hexagon apex. The measured data is separately plotted, and then the optimum value of the single-channel ink amount is found according to the curve change.
2 linearization.
Click Print and the linearization chart will be printed automatically, waiting for the ink to dry, and then the printed chart will be measured to show the color value.
3 total ink limit.
Click “Printâ€, the total ink limit chart is automatically printed, wait for the ink to dry, measure the L value, and decrement according to the L value until the corresponding color block starts to determine a reasonable total ink limit value. The amount of ink changed from the original 400 to 380.
4 quality control.
Click "Print" and the quality control chart will be printed automatically. From the visual analysis, it is required that the printed color strips reach gray balance, no curl, dirty ink, etc., and a smooth transition from light ink to normal ink. Wait for the ink to dry, click Measure, and measure the printed chart to display the color value.
At this point, the basic linearization is completed and a linearization report is obtained. The data set formed by the basic linearization is stored in the EPL format in the "...\EFI\EFI Media Profiles\My Profiles\..." folder. [next]
3. Create a profile
1 Establish a printing press ICC file.
After strict control in each process, the ECI2002 linearization chart is used as the original for color separation, plate making, and printing. 300 prints are obtained, and one sheet is taken every 50 sheets. Then, the chromaticity value of each color block is measured using ProfileMaker. The average of the results of several measurements gives an ICC file that reflects the color reproduction properties of the press, and finally saves the file in the "...\Server\Profiles\Reference" folder.
2 Create a paper profile Select “Create Profile†in EFI Color Manager to print out the ECI2002 linearization chart with the parameters of paper, ink, basic linearity and total ink volume fixed, then use X-Rite Eye- One measurement chart gets its chromaticity value, then you can “create it immediately†the ICC profile, then name the file “XT4.0_SP88.icc†and save it with the basic linearization file “SP9800_720×720_250609_143301.epl†In the same folder.
3 Package basic linearization files and paper profiles.
After creating the paper profile, use the Profiler Connector tool under LinTool to add the printer's basic linearization file as a patch to the paper profile, making it a whole and saving it to the appropriate folder instead of the previous paper profile.
4. Create a workflow
To create a workflow, follow these steps:
1 Click New Workflow in the toolbar. The Use Template New Workflow window opens. Highlight the template "Default Workflow" and click "Load", the system will automatically create a new workflow called "Default workflow 1", then enter the Property inspector and type the name of the workflow.
2 Make the necessary settings for the newly created workflow. After entering the "Print Media" tab and selecting the correct print combination, you can load the corresponding calibration set (as an EPL basic linearization file added to the selected paper type as a patch) from the drop-down list box "Calibration Set". The associated paper profile "XT4.0_SP88.icc" will be automatically selected, as shown in Figure 10. Then in the "Color Management" tab, check "Color Management" and call the printer overview file in "...\Server\Profiles\Reference" (this experiment is called EFI Colorproof because the printing machine is difficult to standardize.) The XF comes with the press overview file "JapanColor2001Coated.icc"). In general, the description files for both “Source†and “Analog†are the same, but if the RGB color is used in the job document, you need to select the source overview along with the EFI Colorproof XF or with the input device at the Source. File, and only call the printer profile at "simulation". Then choose a coloring intent from the Coloring Intent drop-down list box, depending on whether you are simulating paper white. If there is an optimization profile, it is called in "L*a*b* optimization" (the establishment process of the optimization profile "Japan_SP88_1BitTif.3cc" here will be further explained later).
3 Connect the user, workflow, and printer through a thin black line and turn the switch green to bring the workflow online. [next]
5. Printout
Select the newly set workflow in Job Explorer, click "Import Job" to load the job or use drag and drop to place the file directly in the job list area, then select the job in the job list, click "Print" or select " File\Print can output a print job.
6. Optimization profile
After printing out the output file, once it is found that the print quality can be further improved, the optimization of the profile is performed.
1 Open the LineTool module and select “Optimization Profile†to define the profile settings for optimization: select the ECI2002 color scale, select the workflow from the drop-down list box, select the newly created EPL file and the reference profile, and the paper White is set to Absolute Colorimetric, then select Create a new L*a*b* Calibration Profile in the optimization method.
2 Print and measure the optimization target, the system will calculate the average E increment, E incremental peak, paper white and gamut percentage target values, and display it in the table, as shown in Figure 13. If the color difference is not within the normal range, click “Optimize and Print†to perform the optimization calculation and print the measurement again until the “Average E Increment†is less than 2 and the “E Incremental Peak†is less than 5 (usually three or four optimizations can be performed. Meet the requirements). Finally click "Finish" to save the optimized profile (.3cc) to the "...\EFI\EFI Media Profiles\My Profiles\..." folder.
3 After the optimization is completed, it can be called. Go to the "Color Management" tab, select the 3cc file you just generated from the "L*a*b*Optimization" drop-down menu and save it (as mentioned in the workflow settings), then you can print the output job again. . [next]
Third, the maintenance of digital proofing system
The digital proofing system uses the EFI Colorproof XF digital proofing control software + inkjet printer to ensure that the digital proofing process and color quality are well guaranteed. However, in order to ensure the stability of digital proofing color quality, it is necessary to do some daily inspection and maintenance work.
1. Printer maintenance
The maintenance of the inkjet printer is mainly to ensure the problem of cleaning the inside of the printer, so the timing cleaning of the whole machine is a compulsory course for the user. When cleaning, it is common to wipe the outside of the printer with clean water to remove deposited dust. Then use a soft brush, a smooth dry cloth or an alcohol cotton ball to clean the inside of the printer and wipe off any dust and debris inside the machine.
In addition to the cleaning work, after starting up every day, you need to print the printer self-test strip to check if the nozzle is blocked. If a wire break occurs, the printer nozzle is clogged and nozzle cleaning is required. If more than five lines of cleaning are used, the continuous lines in the pattern are missing some line segments, and a deep cleaning operation is performed until there is no wire breakage problem.
After all the print jobs are completed every day, in order to keep the nozzles in optimal working condition and avoid clogging the nozzles due to solvent evaporation, turn off the power, wash the nozzles with special cleaning solution, and then pour the cleaning solution onto the non-woven fabric. Soak it on the surface of the nozzle, and move the machine head back to the cleaning station at the left end and tightly wrap the nozzle with plastic wrap.
2. Relinearization
Digital proofing needs to ensure that the color difference is within the allowable range (the average E increment is less than 2, and the E increment peak is less than 5), so the quality inspection map needs to be printed for quality inspection every day before the normal printout operation.
First, select the control bar “ugra fogra-mediawedge v3.0a†in the control bar settings. In this way, when the printout detection map is printed, the control bar will also be printed, and the digital proofing quality can be measured from both subjective and objective aspects. . Visually check the map first and evaluate the print quality subjectively. Then use the measuring option in the ProfileMaker/MeasureTool module to complete the evaluation of the control bar. Select the standard control strip data (which prints the "ugra fogra-mediawedge v3.0a" control strip after the printer's basic linearization/optimization is completed in the previous stage, and measures it), and then uses the X-Rite Eye-One to measure the newly printed control strip. After the internal comparison, a related report is automatically generated to indicate whether the parameters such as the average color difference and the maximum color difference are in compliance with the standard. If it is, you can carry out daily production. If the color difference exceeds the standard, it needs to be re-linearized in LinTool.
Select "Re-linearize according to the measurement results", pop-up window, find the EPL file that the digital proof is using now, select the connected measurement device, print the chart, wait for the ink to dry, and then measure the printed chart to display the color value. The average E increment and the E delta peak are calculated separately and displayed in the table. Based on these values, LinTool will notify you if you want to further improve the EPL and recommend re-linearizing the printer or redefining the ink limits for each channel and then re-linearizing the printer. After the completion, save the EPL file and you can continue to print normally.
If the chromatic aberration display is still too large after re-linearization, then the basic linearization must be done again. Of course, if paper or ink is replaced, basic linearization must be performed to regenerate the EPL linearization file. Once the EPL file is regenerated, the paper profile needs to be regenerated and the profile optimized, until the measurement results meet the quality requirements, and the daily print job can be performed. [next]
Fourth, the conclusion
Digital proofing creates a mapping relationship between paper profiles and press profiles through color management, which solves the problem of color matching for proofing and printing. However, in the actual operation process, due to the existence of objective factors such as the stability of the proofing system itself, the error of the measuring instrument and the limitation of the color of the measuring chart, absolutely no chromatic aberration is still impossible. Therefore, the implementation of digital proofing color management requires correction and optimization, and try to control this color difference within the ideal range.
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