Research on Application of Multispectral Color Copy Technology (Part 2)

2. Prediction of the original colorant and selection of the best ink color

After the multispectral data acquisition is completed, data analysis needs to be performed. Through the prediction of the spectral distribution of the collected color, the best ink selection of the color reproduction is determined, and the effect of the same-color different-spectrum phenomenon on the color matching is maximally eliminated.

In order to achieve the best match between the copy color and the original color, it must be ensured that the spectral distribution curve of the color copy is approximated to the spectral distribution of the original color as much as possible. In actual operation, the principal component analysis method is usually used to analyze and process the spectral data, and then the actual possible optimal color material selection is predicted through the limited rotation transformation. Finally, the best ink selection scheme is finalized by comparing the predicted combination of colorants with the ink combination in the database.

3. The establishment of ink overprint model and spectral prediction

There are many theories about the establishment of the halftone model in color reproduction. In general, people use the Kubelka-Munk theory to calculate the primary color reflectance of the modified Negeber model by Yur-Nelson. Among them, the modified Negeber model (YNSN model) is the most commonly used reflectivity prediction model, which explains the correspondence between the reflectance of halftone printing color spectrum and the dot area ratio at each wavelength. The optical dot expansion problem is taken into account and its specific formula is:

λ=1...8

Among them, Rprint, λ represents the reflectivity of the printed color, n is the Yule Nielsen factor. Rp,λ is the spectral reflectance of the negboer p-type primary color, and αp is the dot area ratio of the primary color.

4. Color separation and printing based on spectral data

The color separation technology based on multi-spectral data is the core of the multi-spectral color reproduction technology, and is usually implemented by inverse transformation of the YNSN model. When using the YNSN equation to obtain the spectral value of the ink dot, an appropriate non-linearization optimization iteration method should be used to determine the color separation settings for each primary color ink.

The color look-up table of this separation technology is based on the same principle as the four colors. The difference is that the hue interval of the color space needs to be rationally divided so that the ink color matching input color can maintain the minimal degree of homochromatic spectrum, and the matching is improved. Accuracy. Since the separation effect can maximize the approximation of the original, this technique is often used for high-fidelity printing.

After color separation is completed, multi-color printers or printers can be used for multi-primary printing. Compared to traditional printing methods, multi-spectral replication technology has a larger gamut space and can reproduce more vivid real colors. In addition, the sense of layering of its prints is more realistic and the effect of visual change is closer to the original spectrum.

Fourth, the current status of multi-spectral color reproduction technology and related institutions

1. Research content of multispectral replication technology

According to different stages of color processing, multi-spectral color reproduction can be roughly divided into three directions: data acquisition, data processing, and color output. Each direction can be subdivided into several sub-directions:

Data acquisition: device characterization methods, filter design, multispectral camera tuning, and data recording.

Data processing: color space conversion, color gamut matching, spectral data encoding and decoding, etc.

Data output: establishment of a comparison table, study of color separation algorithms, ink selection, etc.

2. Related institutions

At present, many international organizations, laboratories, and research institutes around the world are working on multispectral color reproduction technology. Well-known universities include the Munsell Color Science Laboratory at Rochester Institute of Technology, North Carolina State University, University of Leeds, UK, and Chiba University in Japan. In addition, IS&T of the Society for Imaging Science and Technology, SPIE of the International Society of Optical Engineers, and the IEEE of the Institute of Electrical and Electronics Engineers of the United States have also made significant contributions to this study.

At home, universities such as Wuhan University, Beijing Institute of Technology and Jiangnan University have also conducted research in different directions on this topic.

3. Problems at the current stage of research

Although the advantages of multi-spectral color reproduction technology in spectrum matching are undoubted, the perfection and popularization of this technology still require the joint efforts of the printing industry and researchers compared to the mature traditional printing technology. At this stage, the high cost of this technology and the complicated operation techniques make most people discourage it.

In addition, this technology requires very high precision in data sampling and processing. According to the relevant research done by the Institute of Multimedia Information Technology of the Italian National Research Institute, the effect of multi-spectral color reproduction is achieved when spectral data cannot meet the required accuracy. Very unsatisfactory. That is to say, although spectral matching is the most advanced color matching, there is no doubt that the degree of spectral matching is not directly related to the difference in color difference and the difference in human vision. Obviously, although its advantages are obvious in principle, its actual implementation is actually more difficult.

V. Conclusion

Traditional four-color printing has the fundamental defect of this metamerism, and its color matching can only be maintained under certain conditions. For this reason, more and more people are focusing on multispectral color reproduction techniques that can realize unconditional color reproduction. Although this technology is still at the initial stage of development, its advantages of accurate color reproduction will certainly become the focus of future industry research and lay a solid foundation for high-fidelity printing.