Digital proofing scheme based on Best and Printopen

Linearization and ICC Profile Production

EFI Colorproof software centered on Best technology is a Postscript RIP with LogoSync ICC color management software package. It fully supports ICC standards and forms the basis of proofing. It automates the simulation of printing color effects. It puts all Colors are automatically calibrated with two ICC files and six color adjustment curves are used to achieve optimal color matching.

Digital proofer linearization

When the digital proofer is installed, linear adjustment is very important before entering the normal working state. The color management function can be used for printing only after the linearization is properly performed. Maintaining consistency is an important part of the ICC workflow, so a good digital proofing system must include tools to maintain print stability. Basic linear and linear correction tools within EFI Colorproof can be used as a control tool for output quality stability. The basic linear correction is to determine the linear relationship between the input gray and output density, to solve the difference in the color rendering characteristics, ink volume and density, ink volume and chroma performance of different inks on the print media, so that the input and output values Maintain the basic linear characteristics and adjust the proofing system to the production standard.

The linearization of the printer is based on the principle of correcting the deviation of the printed color by checking and adjusting the density of the printer at various levels, and the density can be measured with a densitometer or a spectrodensitometer. Different measurement tools correspond to different linear patterns. Figure 3 is a color chart for printer linearization adjustment, which is a common pattern used by EFI Colorproof for printer linearization.

When EFI Colorproof performs basic linearization, a linear pattern file is printed without performing color management so that the printed proof can detect the density deviation of the printer.

The software supports measurement devices that have online data transmission capabilities, such as the X-Rite DTP41 automated scanning spectrophotometer. After such measurements, the measured density data can be transmitted directly to the EFI Colorproof software. The color table used for printer linearization adjustments is stored in the "Linearization Charts" folder in the EFI Colorproof installer. There are many files in this folder. Files used for printer linearization start with "Lin_" followed by Followed by the name of the measuring device used in the printer's linearization process. For example, "Lin_DTP41.tif" indicates that the measuring equipment used is X-Rite's DTP41 automatic scanning spectrophotometer. Of course, if there is no online measuring device, Best has a file "Lin_Manually.tif" under this folder, which can also be used for linear adjustment of the printer (as shown in Figure 4).

Whether it is manual measurement input or online measurement, when using the linear assistant, the whiteness of the paper used for output proofs and the solid density of the four primary colors of C, M, Y, and K (that is, the amount of ink in each channel) should be measured. The amount of ink is adjusted.

The above operations only determine the density range that the digital proofing system can represent. After determining the density range of the printed paper (between 0 and 100%), the density value of each color patch in the linearized test strip should also be measured using a color density meter, and then the corresponding gray levels of each ink are obtained. The density value of the printout, the density value obtained by the measurement is input into the linear assistant menu, and the system automatically generates a corresponding print relation curve. According to the actual printing requirements, the relationship between density and ink supply is adjusted so that the input gray scale maintains a linear relationship with the print output (as shown in Figure 5).
After adjustment, EFI Colorproof software automatically generates an adjustment curve with the suffix “.mcc”, which is automatically activated once it is generated. After adjustment, recalculate and print out the same linear color proofs, and use the linear assistant to measure the density again to check whether the final print density is consistent with the density of the basic linearization. If there is still a deviation, you need to Adjust until the print density is consistent with the density at the time the printer was initially linearized and the linearization of the printer is complete.

Another way to linearize printers is in EFI Colorproof software, called Visual Linearization. However, because this method is based on the absence of measurement tools under the conditions of the operator to determine whether the printed color cast color, it is only suitable for experienced operators.

After calibrating the digital proofer, it is possible to output a digital proof color scale and measure the color value of each color block in preparation for the generation of a digital proofer ICC Profile.

Create Digital Proofer's ICC Profile

1. Ink: Inkjet printers must use original color ink jet printing ink, otherwise it will affect the reproduction color gamut of digital proofing. The model used in this experiment is the Epson Stylus Pro 10000CF, six-color ink printing, so it can guarantee the output color gamut range is greater than the print color gamut.
2. Paper: The color gamut is related to the selected printing paper in addition to ink. Therefore, in addition to selecting high or semi-high gloss paper and matte paper as required, the most important point is that the color gamut printed by the color printer is larger than the color gamut reproduced by the printer using the CMYK four-color ink. It is possible to use a color inkjet printer to simulate traditional proofing, which is the basis of digital proofing. This experiment uses Epson special photo paper, because it has a larger color space than the print color space.
3. After the printer is linearized, the proofing color patch is output without using color management.
4. Measure 210 color patches in the same way.
5. Generate color gamut charts with colorimetric data.
6. Through the calculation, generate a digital proofing profile file, the file name is "Profile_digital CMYK UCR-370 K-95 GCR-0", this file will be used for the color gamut conversion.

Color gamut conversion

EFI Colorproof software is used to match the ICC Profile of the printing and digital proofing, so that the color gamut of the digital proofing simulates the color gamut of the printing, and finally the colors of the printing and digital proofing look the same. In the past, we have obtained an ICC Profile that reflects the characteristics of the printing process (referred to as the Reference Profile by EFI Colorproof) and an ICC profile that reflects the characteristics of the color printer and the printed paper (EFI Colorproof calls it the Paper Profile). The profile files are placed in the EFI Colorproof software. Under a certain matching intent (Relative Intensity method is selected in this experiment), after compilation and calculation, proofs that are very close to the printed matter can be printed out.

After the color space conversion, a new CMM data set is generated, which records the mapping relationship between the data in the above two ICC profile files in the intermediate color space and is a matched data file. In theory, digital proofing has been used to simulate printing colors. However, only the first color space conversion is unable to accurately convert the color. According to the actual production conditions, the CMM data set needs to be further modified and adjusted.

Chromatic aberration formula 1: ΔELab=[(△L)2+(Δa)2+(Δb)2]1/2, this formula is close to 75% of human eye observation.

Chromatic aberration formula 2: ΔEcmc=[(ΔL*/lsL)2+(△Cab*/cSc)2+(ΔHab*/SH)2]1/2, this formula is close to the human eye observation is 95 %.

On the national standard, ΔELab ≤ 6NBS or △Ecmc ≤ 4 is used as the standard for color reproduction. However, in reality, the requirements of production companies are more stringent: for general products, ΔELab ≤ 5NBS and more than 95% of color patches are achieved. Requirements, general requirements for packaging products △ELab ≤ 3NBS; for neutral gray, memory color requirements △ELab is better than or close to 2NBS (△ Ecmc ≤ 1), the average color difference of all color blocks △ELab ≤ 3.5 (△ Ecmc ≤ 2 ).

It is generally difficult to achieve the above requirements after the first color space conversion. Therefore, professional color management software and tools are still required to further correct the ICC document so that its color reaches the index, that is, the color error is determined by multiple matching. E minimum. There are usually two ways to implement the software: 1 In the Printopen software, you can modify the dot gain rate, gray balance data, UCR and GCR parameters, total ink overprint, etc. of the ICC file; 2 It can also be done manually at EFI Colorproof. Calibrate the density curve of each basic color to adjust the color deviation. After the color correction is performed, it needs to be recalculated and matched, and the standard proofs are printed again, and the color of the printed colors is judged by eyes. If there is a color cast, four basic colors of cyan, magenta, yellow, and the like can be independently used by the linear assistant tool. Black adjusts separately (as shown in Figure 6).
The adjusted curve can be saved as a ".vcc" file. After the file is archived, the file is automatically activated, and the standard sample sheet is recalculated and printed under the curve. This is repeated until it is satisfied.
(End of the article)

Reprinted from: Digital Printing

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