On the two or three things of digital printing color management
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In this era of individuality, customization and personalization have become the development trend of all walks of life. As product diversification intensifies, the demand for packaging flexibility becomes more and more obvious, and the awkward traditional printing can't adapt to this fast pace in terms of timeliness and cost. Digital printing, known as "short, flat, fast", is favored, even today's digital printing is not enough to counter traditional printing in terms of print quality and printing speed.
International printing standards have detailed target regulations and guidelines for traditional printing. We also have accumulated rich experience in color management of traditional printing. What should we do for color control of digital printing? What should we pay attention to? Down with you, you will walk into the color management of digital printing. Traditionally, color management requires three steps. These three steps are called "3C", namely Calibration, Deviceisation, and Conversion.
Preparation: Determining color goals
Before managing the printing color, you must first determine the color target. The traditional printing color target is commonly used by Fogra, GRACoL, JapanColor, etc. The realization of these color targets depends on the support of the ink. Selecting a set of primary color matching inks is the prerequisite for the realization of color management goals. condition. Unlike the inks used in traditional printing, digitally printed toners and inks are almost in line with digital printing equipment models, and the degree of freedom of consumables is not high. Therefore, it is necessary to test the digitally printed toner and ink first, and fully understand its color characteristics to determine the color target that can be achieved. It should be noted that Fogra, GRACoL, JapanColor, etc. are all developed for traditional printing. Digital printing does not need to blindly pursue these color targets, but should be determined according to actual conditions. Yutong's digital printing is mainly used for color proofing and short-single production, proofing to match traditional printing colors as the target, while short-single production is more focused on color consistency between different batches, that is, the digital printing equipment is stable under its own state. Color is consistent as a goal.
Step 1: Equipment calibration
Equipment calibration content generally includes equipment parameter adjustment, ink solid density adjustment, plate compensation curve adjustment and so on. The continuity of color management needs to be realized in an environment with stable equipment status. Therefore, the calibration equipment must first adjust the parameters of the equipment to ensure the continuous stability of the equipment status. Adjusting the density of the ground and the compensation curve of the printing plate is a process of correcting the color performance of the device from the aspects of color gamut and tone level. The adjustment of the density of the solid printing by the conventional printing needs to be controlled within a certain range according to the printing suitability of the ink. Similarly, the range of adjustment of the solid density by the digital printing should not be too large. Although most of the printing equipment has ink volume adjustment, the range of adjustment is limited. Under normal circumstances, the default ink density/ink volume of the device is a relatively stable state after passing the factory test. It is not recommended to adjust the ink volume in a wide range. Otherwise, the dynamic balance of the ink transfer process will be destroyed, resulting in a more unstable printing state. Conventional printing usually uses a method such as TVI (print dot enlargement curve) to correct the print tone level. These correction methods can also be used for digital printing. It is worth noting that due to the different printing principles, the dot enlargement of digital printing is not as inevitable as traditional printing, but is close to zero expansion, so the amplitude of the printing calibration curve is different from that of traditional printing.
In the HP Indigo digital printing device using electronic ink, the CMYK four primary colors are close to the GRACoL color target, and the RGB secondary colors are far apart, as shown in Figure 1. The HP Indigo digital press uses the LUT curve to control the print tone. The curve can be edited directly through the device operating system. It can also be calculated and imported by the software such as Curve3. The printing curve of some digital presses is directly calculated by the device operating system. No editing or importing functions are available.
Step 2: Device characterization
Device characterization is the process of printing a calibrated device and printing the status with a data logging device. It is necessary to use ECI (standard color table printing proofs) and other color table for printing and scanning, and then calculate and generate an ICC characteristic file, which records the color characteristics of the printing device in the form of data. There are many factors affecting the stability of digital printing equipment. It is necessary to ensure that the sample of the scanning measurement can represent the characteristic color of the equipment.
Step 3: Color space conversion
In traditional printing, we usually use the prepress process system or third-party software to convert the color value of the printed documents. In the prepress process system, different processing methods, conversion intentions, digital printing can be set for the objects such as graphics, images, and text. The device operating system does not provide similar functionality, just a simple setup from "source space" to "target space." If you use the functions in the device operating system to perform color conversion directly, the printed proofs will appear dirty in the original color and change the mesh point in the field. To avoid this kind of primary color from getting dirty, you can color-process the file before printing, and properly convert it to ensure the original color and other elements. However, digital printing is generally short-term, and the number of printed documents is naturally not small. If each printed document needs to be separately processed in advance, it will not only take time and effort, but also increase the probability of error.
Yutong is more willing to use the digital printing device's own operating system to automatically and uniformly process the color of printed documents. For the printing effect of primary colors, the DeviceLink method is adopted (usually used for color conversion from one output device to another). . Through the third-party color management software, the target color ICC profile and the digital printer ICC profile are loaded, and the primary color, secondary color, and color tone are retained according to actual needs, and then the DeviceLink ICC profile is generated and applied. In the digital printing device operating system.
Different levels of color retention settings bring different printing effects to primary colors, primary color tones, secondary colors, and secondary color tones.
The DeviceLink property file is divided into three different settings: “Retain primary color”, “Keep primary color + secondary color” and “Do not retain color”. The corresponding print data is shown in Figure 3. It can be seen that the object with the color retention avoids the dirty visual effect, and the color performance also continues the original characteristics of the toner and the ink, so the color matching process may not achieve better results; if no color retention is done Using noise to make up for the difference between ink color and target color, it will perform well in color matching of primary color CMYK or secondary color RGB. The matching degree of overall color with Fogra39L is shown in Figure 4, which meets the digital proofing standard. ISO12647-7 requirements. Therefore, when making color retention settings, you need to pay special attention to the actual situation to make trade-offs.
Color management is to better control our printing colors using predictable and controllable methods. Compared with traditional printing, there is no essential difference in the color management of digital printing in terms of methods and processes, but the process needs to be combined with the characteristics of digital printing. Targeted processing. With the increasing use of digital printing, color accuracy, stability and other requirements will be higher and higher, color management capabilities will inevitably become an important factor in the development of digital printing in the industry.