How is the color reproduction capability of Landa devices?

Landa digital printing equipment uses nano ink technology, which has the advantage of ultra-small pigment particle size, only tens of nanometers, compared to the particle size of traditional inks of about 500nm. These nanoscale pigment particles can better penetrate and adhere to the surface of different substrates, forming an image thickness of only 500nm. This thickness is less than half that of traditional offset ink images. At this time, the ink only adheres to the surface of the substrate and does not penetrate inside, and the color saturation and clarity of the printed picture are excellent. Landa digital printing equipment can achieve 4~8 color printing by inkjet printing at 600dpi or 1200dpi resolution, of which the sheetfed equipment supports up to 7 colors (CMYK+OGB) and the rotary equipment supports up to 8 colors (CMYK+OGB+white). According to official data, the 4-color CMYK configuration can cover 84% of the Pantone color gamut, while the 7-color CMYK+OGB configuration can cover up to 96% of the Pantone color gamut.

This paper relies on the Landa sheetfed digital printing equipment of Shenzhen Jiuxing Printing and Packaging Group Co., Ltd. to test and analyze its color reproduction ability on white cardboard with a quantitative capacity of 300g/m2. Firstly, the equipment is linearized to measure the saturation and gradation uniformity of its monochrome, and then the ICC file of the equipment is analyzed to evaluate its color gamut performance and spot color coverage performance.

Investigation of core algorithm of color reproduction of 7-color digital printing system

01

Types and principles of linearization algorithms

Linearization of digital printing equipment is a key technology to ensure a linear relationship between device input and output signals. The 7-color channel linearization has significant technical complexity compared with the traditional CMYK 4-color. The first is the increase in the number of channels, from 4 to 7 means that the size of the lookup table increases exponentially. Common linearization algorithms include the following 4 types:

(1) Polynomial fitting algorithm is the most basic linearization method, which realizes linearization by fitting polynomial curves of input and output data. The advantages of this algorithm are simple calculations and fewer parameters, but the disadvantage is that it has limited modeling ability for complex nonlinear relationships.

(2) The lookup table (LUT) algorithm is the most commonly used linearization method in digital printing. 1D LUTs are the simplest form that only processes a single channel of the image, defining an output value for each input value (0 to 100). The essence of 1D LUT is a lookup table in one-dimensional space, and each input value is "repositioned" by the LUT to obtain a new output value, presenting a one-to-one corresponding relationship. A typical ICC printer profile configures a 1D lookup table (1D LUT) based on the number of color channels on the device, and then uses a 3D lookup table (3D LUT) to complete color gamut mapping and color conversion.

(3) The local linear regression algorithm performs well in color management, especially in the small and medium-sized sample scenarios estimated by digital printing lookup tables, and its performance is better than neural networks, polynomial regression and spline functions. The core idea of the algorithm is to use the local linear regression set of neighborhood points for each grid point to fit the linear hyperplane by the weighted least squares criterion, and estimate each output color component separately.

(4) Deep learning algorithms represent the latest development direction of linearization technology. Modern technology has been able to realize the linearization model of printed color channels based on deep learning networks, and with the online feedforward multi-dimensional nonlinear color density compensation method, it can achieve wide color gamut, high linearity, and continuous and stable digital printing output.

02

Multi-channel color management algorithms

Multi-channel color management for 7-color devices requires special algorithm support. In the traditional 4-color CMYK system, color management mainly focuses on the balance of four colors: blue, magenta, yellow, and black, while the 7-color system needs to consider the interaction of 7 colors at the same time. In a 7-color system, each color may interact with the other 6 colors, and this multi-dimensional color relationship requires more complex mathematical models to describe. In the traditional CMYK system, black is mainly used for grayscale balance and ink saving, while in the 7-color system, the addition of orange, green, and blue makes color mixing more complex. Commonly used color separation algorithms include the following two types:

(1) Composite Neugebauer models are important tools for processing multi-color printing. This model is a generalized version of the Neugebauer model that subdivides the entire XYZ color space into several volume partitions, predicts the color component weights within a given partition, and serves as a function to determine the XYZ values of the three basic colors for that partition. This method can effectively handle complex color relationships in a 7-color system.

(2) The multi-channel color space conversion algorithm needs to consider the mapping relationship between different color spaces. When converting from a device color space (CMYKOBG) to a standard color space (such as CIE Lab), you need to establish precise conversion functions. Studies have shown that it is an effective technical scheme to establish the relationship between device space and CIE XYZ space through three-dimensional relationship, and to achieve color separation by using three-linear interpolation between the values of the lookup table and table columns.

Experimental preparation and testing

01

Testing equipment and equipment

(1) Test equipment: Landa digital printing equipment, 7-color nano ink (CMYK+OGB);

(2) Test paper: 300g/m2 Asia Pacific Symbo Yinbo white cardboard;

(3) Measuring instrument: X-rite i1io spectrophotometer;

(4) Test software: EFI Fiery Color Profiler Suite (CPS);

(5) Environmental conditions: temperature 25±2°C, humidity 55%±5%.

02

Testing process and steps

(1) Step 1: Print the linearization chart. Preheat the Landa digital printing machine for more than 30 minutes and output linearized charts using the EFI Fiery Color Profiler Suite (CPS). The Landa digital printing system is equipped with a linearized color table of 4 to 7 colors. This paper takes 7 colors as an example, each channel of the 7-color color table has 54 colors, a total of 378 color blocks, and the dot area coverage rate is 0~100%.

(2) Step 2, measure the linearization chart. Wait for the linearization chart to dry and complete the data measurement of the 7 color channels using CPS+i1io.

(3) Step 3, draw the gradation curve. Seven channel gradation curves are plotted according to the measurement data and theoretical data. The difference between the measured data and the target data is analyzed, the appropriate linearization algorithm is selected, and the linearization curve is calculated.

(4) Step 4: Print the ICC file to make a chart. Call the linearization curve in step 3 and print the diagram to make the ICC file, such as iT8.

(5) Step 5, calculate and generate the ICC file. After the iT8 chart is dry, the iT8 is measured using CPS+i1io, the data is saved, and the appropriate color separation algorithm is selected to generate an ICC file. This ICC file is the largest color gamut file for current equipment and paper combined.

Data collection and analysis

01

Device linearization analysis

The measured values of the linearized data chart are shown in Figs. 1 and 2. Fig. 1 shows the relationship between the area of each color dot and the corresponding color CIE Lab brightness value L*, the dots in the figure are the sampling points of each channel, the curve is the fitting of the 2nd spline curve, the fitting of the 2nd spline curve cannot express the mapping relationship between the dot area rate and the brightness, and a more complex mapping function is needed to describe the correspondence between the area of the dots and the visual brightness level.

Figure 1 Relationship between dot area and luminance value

Figure 2 shows the hue variation and maximum saturation for six color channels. In the figure, the purple and magenta channels bend noticeably as saturation increases, indicating that the hue uniformity of these two sets of colors is not very good. Of course, hue uniformity is also related to the uniformity of the CIE Lab color space. For the yellow and orange channels, inhomogeneity in chroma is also quite evident. For example, in the yellow channel, the spacing between points is uniform below a b* value of 50, but becomes larger above 50; the orange channel is similar to the yellow channel, and around 40, overlapping points appear, resulting in outliers. Therefore, issues such as hue bending and chroma inhomogeneity will increase the complexity of developing linearization and color separation algorithms.

Figure 2 Color Saturation and Hue Performance of Each Channel

By combining Figure 1 and Figure 2, the optimal saturated color of the device can be determined. Table 1 shows the correspondence between the maximum chroma of the 300g/m2 white card used in this study and the chroma of ISO 12647-2 Type 8 substrates.

Table 1 Comparison of Color and Chroma Between Landa Digital Printing System and ISO 12647-2 Type 8 Substrates

Table 1 data indicate that, except for magenta, whose chroma is lower than that of ISO 12647-2 CD1 paper, the chroma of the primary colors of the Landa digital printing system can fully cover the chroma of the eight types of paper defined by ISO. Therefore, it can be inferred that the Landa digital printing system, through further linear adjustments, can perfectly match the offset printing standards of ISO 12647-2, and of course can also meet the requirements for certifications such as G7 and C9.

02

Device Gamut Analysis

After linearization, the generated ICC file reflects the current color characteristics of the digital printing system. As shown in Figure 3, it compares the gamut of the Landa digital printing system with that of Adobe RGB (1998). The gamut of the Landa digital printing system and Adobe RGB (1998) do not simply have a containment relationship. In the medium-lightness blue-to-green region and low-lightness red-to-blue region, the Landa digital printing system gamut encompasses the Adobe RGB (1998) gamut. In contrast, in the high-lightness green-to-yellow region and red-to-yellow region, it is encompassed by Adobe RGB (1998).

Figure 3 Comparison of Landa Digital Printing System and Adobe RGB (1998) Color Gamut

This situation indicates that when using the experimental white card paper with the Landa digital printing system for high-fidelity printing processes, the reproduction capability for saturated yellows, oranges, and greens is slightly weaker. This may improve if paper with higher whiteness is used.

Figure 4 shows a comparison between the color gamut of the experimental Landa digital printing system and the GRACoL2006_Coated color gamut. From the comparison, it can be seen that the color gamut of the Landa digital printing system basically encompasses the GRACoL2006_Coated gamut. In particular, the blue-to-green area and the red-to-blue area of medium lightness are completely within the GRACoL2006_Coated gamut; however, in the very high-lightness green-to-yellow area, the GRACoL2006_Coated gamut is slightly larger. This indicates that combining the experimental white card paper with the Landa digital printing system can reproduce the colors of ISO 12647-2 offset printing, and using paper with slightly higher whiteness can achieve better color reproduction in high-lightness areas.

Figure 4 Comparison of Landa digital printing system with GRACoL2006_Coated color gamut

Figures 5 and 6 use ORIS X Gamut's spot color simulation function to calculate the proportion of Pantone spot colors that the Landa digital printing system can reproduce under two color difference tolerances: ≤3 and ≤5. As shown in Figure 5, when the tolerance is ≤3, 94.9% of the 2,390 Pantone color patches can be matched; Figure 6 shows that when the tolerance is ≤5, 98.6% of the 2,390 Pantone color patches can be matched. This experiment confirms the accuracy of Landa's official claim that the 7-color CMYK OGB configuration can cover up to 96% of the Pantone color gamut.

Figure 5 Landa Digital Printing System Coverage of Pantone Color Gamut (Color Difference Tolerance ≤3)

Figure 6 Coverage of the Pantone Color Gamut by the Landa Digital Printing System (Color Difference Tolerance ≤5)

In summary, this experiment tested the color reproduction capability of the Landa digital printing system using 300g/m² white cardstock, which is commonly used in the company's products. Analysis of key data captured during the process revealed that: the CMYK primary color capability of the Landa digital printing system can match ISO 12647-2 CD1 paper and fully cover the other seven types of paper; compared to the Adobe RGB color gamut, the Landa digital printing system's 7-color gamut is relatively smaller in high-luminance areas and slightly larger in medium-luminance areas. For high-fidelity printing using Adobe RGB as the primary color space, it is recommended to use paper with higher whiteness. The 7-color gamut of the Landa digital printing system basically encompasses the GRACoL2006_Coated color gamut, can fully match the ISO 12647-2 color standard, and when the color difference is ≤3, it can match over 94% of the Pantone color gamut.