After several years of development and application, digital proofing technology has been rapidly developed. Now, digital proofing has occupied a high proportion in the proofing of many printing companies, and has replaced traditional proofing in the fields of commercial printing and book printing, and has become the mainstream proofing technology.
First, the source of digital proofing
Compared with traditional proofing, digital proofing has several major advantages: First, the cost is low, equipment investment and manpower investment are reduced; second, high efficiency, not only reduces the time of proofing, but also improves the entire prepress department Work efficiency; The third is to adapt to the CTP work process; Fourth, the stability is good; Fifth, the technical factors of the staff are less dependent, for daily proofing operations, do not need to have high technical skills.
Despite all of the above advantages, in practice, there are different opinions in the industry regarding the color of digital proofing. Some people think that the color of the digital draft is very accurate and stable, and it is easy to follow when printing; However, people think that the digital manuscript is not as good as it was when it was printed. Which of these two completely different opinions is correct? What is the reason? It is necessary to have a deeper understanding of digital proofing.
Digital proofing is a kind of "analogue" technology. Whether using inkjet printers or laser printers, some color control techniques are used to let ink-jet and laser samples simulate the colors of prints. The accuracy of color simulation, that is, the accuracy of proofing. In other words, a number proofing proofs, if it is easy to print, then the proofing is successful, otherwise, even if the color is beautiful, it is also unqualified proofs. Because the color of the printer and the printing itself is not the same, it is necessary to perform the color value conversion and then print. The job of the digital proofing software is to complete the conversion work.
Second, the basic production of digital proofing parameters
Digital proofing is based on color measurements. People use their eyes to observe colors and determine the proximity of colors. Proofing software uses the measured values ​​to determine whether the colors are similar or not. Most of the proofing software uses Lab values. Color measurement is a key factor for digital proofing. The production of digital proofing parameters is based on measurement data. Having some understanding of color measurement and Lab color space can help to make better proofing parameters.
Digital proofing software requires interpolation during color numerical conversion. Whether it is making printer parameters or making printer parameters, it is impossible to print out all the colors and measure them in reality. We can only print or print a certain number of Color table, such as ECI 2002 color table (1485 color blocks) or IT 8.7/3 color table (928 color blocks). The digital proofing software finds the color rule based on the measured values ​​of these color patches and generates color parameters. Similarly, the color parameter file is only a limited node and does not contain all the colors. In the actual proofing process, for the specific color in the proofing file, it is necessary to interpolate its color data according to the rules embodied in the data of these nodes. For example, the ICC profile used for digital proofing based on ICC represents the color rule of a printer or a printer. Usually, digital proofing uses the color look-up table (CLUT) method to perform interpolation operations.
First, the source of digital proofing
Compared with traditional proofing, digital proofing has several major advantages: First, the cost is low, equipment investment and manpower investment are reduced; second, high efficiency, not only reduces the time of proofing, but also improves the entire prepress department Work efficiency; The third is to adapt to the CTP work process; Fourth, the stability is good; Fifth, the technical factors of the staff are less dependent, for daily proofing operations, do not need to have high technical skills.
Despite all of the above advantages, in practice, there are different opinions in the industry regarding the color of digital proofing. Some people think that the color of the digital draft is very accurate and stable, and it is easy to follow when printing; However, people think that the digital manuscript is not as good as it was when it was printed. Which of these two completely different opinions is correct? What is the reason? It is necessary to have a deeper understanding of digital proofing.
Digital proofing is a kind of "analogue" technology. Whether using inkjet printers or laser printers, some color control techniques are used to let ink-jet and laser samples simulate the colors of prints. The accuracy of color simulation, that is, the accuracy of proofing. In other words, a number proofing proofs, if it is easy to print, then the proofing is successful, otherwise, even if the color is beautiful, it is also unqualified proofs. Because the color of the printer and the printing itself is not the same, it is necessary to perform the color value conversion and then print. The job of the digital proofing software is to complete the conversion work.
Second, the basic production of digital proofing parameters
Digital proofing is based on color measurements. People use their eyes to observe colors and determine the proximity of colors. Proofing software uses the measured values ​​to determine whether the colors are similar or not. Most of the proofing software uses Lab values. Color measurement is a key factor for digital proofing. The production of digital proofing parameters is based on measurement data. Having some understanding of color measurement and Lab color space can help to make better proofing parameters.
Digital proofing software requires interpolation during color numerical conversion. Whether it is making printer parameters or making printer parameters, it is impossible to print out all the colors and measure them in reality. We can only print or print a certain number of Color table, such as ECI 2002 color table (1485 color blocks) or IT 8.7/3 color table (928 color blocks). The digital proofing software finds the color law based on the measured values ​​of these patches and generates the color parameters. Similarly, the color parameter file is only a limited node and does not contain all the colors. In the actual proofing process, for the specific color in the proofing file, it is necessary to interpolate its color data according to the rules embodied in the data of these nodes. For example, the ICC profile used for digital proofing based on ICC represents the color rule of a printer or a printer. Usually, digital proofing uses the color look-up table (CLUT) method to perform interpolation operations.
We can use an ICC Profile Inspector tool to open an ICC profile to understand the process of interpolation. Represents a process from CMYK to Lab. Ch0, Ch1, Ch2, and Ch3 from the red line represent the four channels of CMYK. It can be seen that the value of each channel is only 0 to 16 total 17 nodes, and the actual CMYK The color value of each channel 0 to 255 is compressed to 0 to 16. The value of the blue box represents the Lab's value for this node (here 16 bits and 8 bits, respectively). If you pull the slider on each channel of CMYK, the Lab value will change accordingly, as shown. However, if the CMYK value of the desired color is not exactly this node, it is necessary to take the interpolation value to calculate the Lab value according to the numerical rules of these nodes. The represented color model can help to understand the working principle of the color table, and find the values ​​of all colors by the color rules represented by the limited nodes. Because CMYK four-dimensional data cannot be visually represented by a stereogram, the RGB space is used in this paper. The working principle of the two is the same.
Representing a color table from Lab to CMYK, you can see that the process is similar to the previous one.
Understand the working mechanism of the color table, we can see that the accuracy of the search value depends heavily on the color law reflected by the node, which is what we should pay attention to when making the digital proofing parameters. We know that a proofing parameter, such as a printed ICC profile, is generated by the ICC profile generation software (Profilemaker) based on the measurement file of the printing color table. The software itself affects the regularity of this color. This is us. Hard to change. What other factors will affect the quality of this rule? Or how can we improve the regularity of color?
Third, the accuracy of digital proofing evaluation
Printing control
Printing itself is one of the key factors. We often hear this saying: The printed ICC profile represents the color law under a “state†of the printing press. This shows that under different control, the printer can be in different states, such as different density control, pressure control and color sequence. The digital samples printed by the proofing software according to the specific “state†proofing parameters need to be controlled by the printer to that particular “state†to be able to match the colors of digital samples. This is understandable to everyone.
Not only that, but different "states" also cause differences in the regularity of color. The author found in the work that for sheetfed offset printing, if no strict color control is used when printing the color chart, or the control index is not correct, the sample is proofed according to the ECI 2002 color table production parameters, and the proof using EFI Colorproof 3.0 is used. Software, without any color adjustment, the average color difference ΔE of 1485 color patches is 2.5 or even higher; while the index of ISO 12647-2 is complied with when printing a color chart, the average color difference ΔE can be 1.5 or less. Although the software's color correction function was used later, the average color difference ΔE was adjusted to 1.0 or less. However, after a period of time, the former will cause some colors to be difficult to follow, while the latter will be much easier to print.
This shows that the reasonable printing control has a great influence on the proofing. In many cases, the color of proofing is not satisfactory. It is difficult to do everything with a variety of methods. This color is good, and others are not good. In the end, it is still necessary to reprint the color chart and do a good job of printing control. The parameters can get more accurate digital proofs. Therefore, when digital proofs are difficult to print, it should be checked whether the printing is controlled properly.
2. Color gamut
For digital proofing, color gamut is also a factor that should be considered. The following is the author's two digital proofing tests. The software uses EFI Colorproof XF 3.0 and uses the same printing parameters. For the first time, Epson Stylus Pro 7800 printer and Easicolor 517 digital proofing paper were used. The test results were as shown. The second time the laser printer and coated paper were used, and the test results were as shown. Obviously, the results are indeed very different. The first result is much better than the second one. Whether it is the average color difference, the maximum color difference, or the simulation of paper white, there are big differences. The main reason can also be seen from the fact that the first proofing system can cover 97.78% of the printing color gamut, while the second time only covers 90.10%. Therefore, the second optimization of the proofing cannot be improved by the second time, and the chromatic aberration becomes larger. Excessive color gamut is the main reason for this problem. Any color proofing system is powerless beyond the color of the color gamut. Therefore, for a successful proofing system, there is a need for a color gamut, which cannot be too different from printing. When the linearity of the printer is made, the color gamut of the printing system can be appropriately increased by changing the setting of the ink amount control, but more importantly, the selection of the printing paper can be assisted by the color gamut comparison function in some software tools. The size of the gamut, choose the print paper that suits your needs.
3. Vision
For assessing the accuracy of digital proofing, many people nowadays are accustomed to using digital methods, that is, to examine the average color difference and the maximum color difference, which is a great improvement compared with the basic visual evaluation method several years ago. However, we also need to pay attention to the fact that the evaluation of the color difference also has its limitations. After all, the proof draft is ultimately viewed by the human eye. Whether it is a sample or a print, it is in line with the human visual requirements. The author has also seen the measurement of color difference is very small, but due to excessive correction, the level of serious loss of proofs, so completely in accordance with the digital way of assessment can not fully reflect the accuracy of the proofs, should use digital assessment methods combined with the human eye. In the international digital proofing competition, the content of visual assessment has also been added. Because of the existence of metamerism in color, when observing proofs and prints, the use of standard light sources for color science should be followed, so that correct conclusions can be obtained.
Although digital proofing technology currently has some deficiencies, such as color science can not be said to be very perfect, although the Lab color space has been homogenized, but it is not completely uniform, some color measurement of the color difference and the human eye feel there are still some The difference is that the color is not the only feature that the human eye pays attention to. The difference in the glossiness and surface roughness of the proofs and the printed matter can also cause visual differences; many spot colors cannot meet the requirements in digital proofing. All these have caused the application of digital proofing in the field of packaging and printing to be less popular than commercial printing. But now there are also many new technologies, such as the increasing number of high-fidelity inkjet printers, which are suitable for directly printing UV printing technology for printed materials. It is believed that with the continuous improvement of technology, the application of digital proofing will become wider and wider.
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