ICC-based digital proofing color adjustment technique
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The core of EFI Digital Proofing Color Management is the creation of a basic linearization and paper profile ICC. This method relies too much on original ink and proofing paper, and the color cast when using non-original ink and paper proofing cannot be adjusted. This paper uses Profilemaker to adjust the paper profile ICC to achieve color cast adjustment.
First, the factors affecting the printing color characteristics
1. Colorant
The colorant has a very significant effect on the color reproduction of the device. The color characteristics of the colorant determine the range of colors that the device can replicate. The colorant used in the ink jet printer is a dye or pigment type inkjet ink. Its performance is very different from that of printing inks, and its reproduced color gamut is greater than the color gamut of the printing press. And most of the digital proofing equipment adds a light C, M or K ink to the original CMYK four-color ink, which in turn expands the color gamut of the printer. Therefore, the proof after replacing the ink is easy to cause color cast after the basic linearization due to the change of the color gamut.
2. Black and white points
Black and white dots are two other important factors that affect the color characteristics of printing devices. The printer black dot determines its dynamic density range, which is the range of maximum and minimum brightness colors that the device can copy. The white point is the reference for the eyes to observe other colors. The white point of the printer is the result of the combination of the illumination source and the color of the paper itself, so the whiteness of the paper must first be assessed before printing the proof. The black dot of the printer is achieved by overprinting CMYK four-color ink.
3. Tonal replication characteristics
The color and density of primary white and black dots can only represent the extreme state of the device, the most saturated color, the brightest white and the darkest black. In order to fully describe the color characteristics of the device, it is also necessary to understand its tone reproduction characteristics. The tone refers to the range of densities that the image contains from the brightest to the darkest parts and the density levels that can be distinguished. The tone reproduction characteristics of the printer are very complicated and are affected by a combination of paper and ink. Therefore, it is necessary to collect a large number of representative tone values from the highlight to the dark portion when evaluating it. The process of making the printer profile is actually the process of collecting the information of a large number of patches with the above color characteristics, and the output patch file is designed according to the above characteristics.
Because the ink and paper used in the digital proofing equipment have great differences from the traditional printed ink paper, the color reproduction has a big difference. The traditional proofing mainly controls the color by controlling the density of the solid and the dot value. Digital proofing uses a color-matched color control method to achieve the same color gamut as the print gamut, ie, to create an ICC Profile file. The digital proofing ICC Profile can be created by special color management software. Gretag ProfileMaker can also be used by the color management module in the proofing software, such as the Colormanager module in EFlColorproof.
In the process of digital proofing, the basic linearization is first made. The purpose of the printer's basic linearization is to calibrate the printer and compress the color gamut of the printer to the color gamut of the press. The basic linearization correction is to determine the linear relationship between the input gradation and the output density, to solve the difference in the color rendering characteristics, the amount of ink and the density, the amount of ink and the chromaticity of different inks on the printing medium, so that the input and output Maintaining a basic linear feature, the proofing system is adjusted to a production-standard basis, ie the printer prints the color correctly after basic linearization correction. However, such problems are often encountered in actual production. Even after re-linearization after ink replacement, there is still a serious color cast problem. Based on in-depth research, the author summarizes an effective color matching technique. The experimental process is as follows.
Second, the experimental part
1. Experimental purpose
Taking digital proofing as a serious reddish example, Profilemaker adjusts the paper profile ICC to achieve color cast adjustment.
2. Experimental equipment and software
EpsonPro 4880 digital proofing machine, Eyeone Pro spectrophotometer, Gretag ProfileMaker 5.0, EFI 3.0 digital proofing software, Xiamen free printing ink, coated paper.
3. Experimental process
Step 1: Create a paper profile to generate a print ICC
Open "ColorManager" in EFI 3.0, click the "Create Print Media Profile" button, pop up the interface as shown in Figure 1, select the measurement device and generate the reference profile.
Print the measurement to generate a paper profile ICC and save it to the appropriate location.
Step 2: Regenerate ICC in ProfileMaker. Since EFI and ProfileMaker have different color connection spaces, editing cannot be called directly. In the Measurement Data list, select the Import ICC Profile option, as shown in Figure 2.
Click "start" to generate a new ICC and save it to the appropriate location.
Step 3: ICC adjustment in ProfileEditor
Open the ProfileEditor and load the source, simulation, and target ICCs. Click the button labeled in the figure to start editing this ICC.
Click the “Gradation” button, as shown in Figure 3, because the dark portion of the proof is obviously reddish, so the cyan, yellow and black channels are closed to adjust the magenta channel curve, as shown in Figure 4.
Select a few key points to adjust, the dark adjustment area of the proof paper is obviously reddish. After repeated experiments, it is found that the following adjustments are needed: input 100 output 95, input 70 output 65, input 85 output 78. The adjusted curve is shown in Figure 5.
Click "OK" to return to the ProfileEditor main panel. Select the "file-save as-ICC profile..." command to save the ICC after the series. In the Save Profile window that pops up, select Manual Mode and B2A conversion mode. After the selection is complete, click to save the edited property file. The file name is P5-WHDX-090329-E.
4. Experimental results and analysis
Using the adjusted ICC print color chart, it was found that the color cast was corrected. It can be seen from the comparison of ICC color gamut before and after adjustment that the color is no longer reddish (blue is the printing color gamut, red is the pre-adjusting color gamut, white is the adjusted color gamut), as shown in Figure 7. This method can effectively improve the color cast, but it also has shortcomings. Firstly, the adjustment of ICC has high requirements for editors; secondly, it is necessary to repeatedly experiment to determine the adjustment value, and the operation is not simple; the process involves multiple softwares, and the cost is high.
Third, summary
The successful implementation of digital proofing is closely related to the rational implementation of color management. Basic linearization and adjustment of the ICC curve are effective ways to implement digital proofing color management. Due to the influence of various variables in the proofing and printing process, the actual printing effect is always different from the digital proofing effect. Therefore, we need to continuously optimize the printing and digital proofing process to control the difference within an acceptable range. Inside.