There are many theories for evaluating color reproduction. Among them, the famous British color expert Hunter, through a comprehensive analysis of various factors, divided color reproduction into six types: spectral color reproduction, chromatic color reproduction, correct color reproduction, equivalent color reproduction, corresponding color reproduction, and Like color reproduction.
Although the traditional color reproduction method has successfully achieved both chromatic color reproduction and correct color reproduction, it cannot achieve unconditional color reproduction, that is, spectral color reproduction, due to its inherent characteristics based on metamerism. The multi-spectral color reproduction technology uses a multi-primary color imaging method, which increases the color gamut by increasing the freedom of color reproduction. Its spectral matching reproduction method eliminates the problem of metamerism, thereby achieving unconditional matching of colors. This technology is of great significance to the development of high-fidelity printing technology, and will surely become the future development direction of the printing industry.
1. Defects in traditional four-color printing
The traditional printing method uses CMYK four primary colors to achieve color reproduction. Due to its limitations, the four-color ink cannot cover all the spectral information of the original color. In fact, traditional four-color printing is based on the principle of metamerism to achieve color reproduction. The principle of metamerism means that in printing and copying, as long as the printed color is the same as the original color, even if the spectral composition of the two is different, it can still be recognized as the correct color copy. This method greatly reduces the difficulty of color reproduction, in most cases can achieve the correct reproduction of color, so it forms the basis of the traditional printing method. However, when the illumination light source and the observer change greatly, its copy effect tends to show a large deviation. This problem is also an important source of quality disputes in the traditional printing industry. In addition, the colors that can be reproduced by four-color printing are only about half of the color gamut of the visible spectrum, and it is often difficult to satisfy the image reproduction effects of vivid colors and large dynamic ranges. At the current state of the art, even if people perform accurate color management and color gamut matching when copying, they still cannot fundamentally solve the problems of metamerism and small color gamut.
The traditional printing method focuses on matching the density of the original with the printing color density. However, this kind of matching method pays too much attention to the discussion of the amount of ink and ignores the matching problem of the chromaticity and brightness of the copied color. In fact, due to the limitation of the color gamut of four-color printing, it often falls into a contradictory situation when regulating the color component and neutral gray component-if the amount of color component ink is increased to increase saturation, it is inevitable The neutral gray component is increased and the brightness is reduced. In other words, four-color printing tends to overemphasize the reproduction of saturation, and has to sacrifice brightness.
Due to the above-mentioned defects of traditional copying, the copying effect is often unsatisfactory. The main manifestations are heavy image color, poor three-dimensional feeling, severe layer loss, and various degrees of color distortion. [next]
Second, the advantages of multi-spectral color reproduction technology
The multi-spectral color reproduction technology realizes the color reproduction by acquiring, analyzing and processing multi-spectral data. This technology uses spectral matching as the color reproduction standard, and increases the color gamut by increasing the degree of freedom of color superposition. Due to the uniqueness of the spectral reflection curve, no matter how the light source and observation conditions change, its reproduction effect can still be kept stable. In addition, the sampling of multiple bands of spectral reflectance can record the color characteristics as detailed as possible, effectively solving the problem of low data accuracy in the traditional mode.
In the field of color reproduction, the principle of color reproduction of digital devices such as monitors, printers, and scanners is very different from the formation mechanism of human vision. Due to its own limitations, the traditional four-color printing mode cannot solve the problem of color deviation even with the help of the color management system. The multi-spectral replication technology greatly improves the integrity of data collection by increasing the number of sampling channels, thereby achieving high-quality color replication. In view of the above advantages, this technology has been successfully used in the field of copy and preservation of valuable art works and online shopping. At the same time, the technology has also laid a solid foundation for future high-fidelity printing and cross-media publishing.
Three, multi-spectral replication technology process
Multi-spectral color reproduction technology describes color information with spectral data through the description of color spectral reflectance or transmittance. The specific process flow can be divided into the following steps:
1. Data Acquisition
Use multi-spectral cameras with multi-color filters to obtain multi-spectral image data of originals or things. Generally, the acquisition system consists of a multi-spectral light source, color filters, and a multi-spectral camera. Compared with the traditional three-color image acquisition method, this system has the following advantages:
The light source has a short start-up process, a wide spectrum, and high radiation efficiency; the color filter selects strong transmission and is not interfered by background light; it can collect high-resolution data, multiple data support modes, and high imaging contrast.
After acquiring the spectral data, it needs to be analyzed and processed to achieve high-precision spectral reconstruction. The matrix representation method of the mathematical model for multispectral data acquisition is now introduced as follows:
Let the spectral power distribution of the multispectral light source be S,
The spectral emissivity of the object is r, r = [r1, r2, ... rn] T, where n represents the number of sampling wavelengths, and T represents the rank-shifting operation of the matrix. In a multispectral camera, the spectral transmission characteristics of m color filters can be expressed by the matrix F,
The spectral sensitivity of the detector is represented by the matrix D,
Based on the above matrix, it can be obtained from the calculation formula of chromatic integration. The color value of the collected color is t = (DF) TSr. Then, through corresponding linear and nonlinear transformation, the tristimulus values ​​XYZ and CIELAB of the color can be obtained Color values ​​such as coordinates.
In addition to the above methods, you can also use the principal component analysis method (PCA) to select the best color filter design and achieve more accurate spectral reconstruction. This method is often used for the mutual conversion of integral density and analytical density in photography technology, and also for the establishment of high-precision device characteristic files for scanners.
2. Prediction of original color and selection of best ink color
After the acquisition of multispectral data is completed, data analysis is required. By predicting the spectral distribution of the collected colors, the best ink color choice for color reproduction is determined, and the effect of metamerism on color matching is eliminated to the greatest extent. In order to achieve the best match between the copy color and the original color, it must be ensured that the copy color spectral distribution curve closely approximates the original color spectral distribution. In actual operation, the principal component analysis method is usually used to analyze and process the spectral data, and then the limited rotation transformation is used to predict the actual best possible color material selection. Finally, by comparing the predicted color combination with the ink combination in the database, the best ink color selection scheme is finally determined.
3. Establishment of ink overprinting model and spectral prediction
There are many theories about the establishment of halftone models in color reproduction. Generally, people use the Kubelka-Munk theory to calculate the primary color reflectance of the modified Niebel model of Yule-Nelson. Among them, the Niebel model (YNSN model for short) revised by Yule-Nelson is the most commonly used model for predicting reflectivity. This model clarifies the correspondence between the spectral reflectance of halftone printing color and the dot area ratio at each wavelength. Taking into account the expansion of the optical network, the specific formula is:
λ = 1 ... 8 (4)
Among them, Rprint, λ represents the reflectivity of the printed color, and n is the Yule Nielsen factor. Rp, λ is the spectral reflectance of the p-type primary colors of Negerber, and αp is the dot area ratio of the primary colors.
4. Color separation and printing based on spectral data
The color separation technology based on multi-spectral data is the core of multi-spectral color reproduction technology, which is usually implemented by the inverse transformation of the YNSN model. When using the YNSN equation to obtain the spectral values ​​of ink dots, an appropriate nonlinear optimization iteration method should be used to determine the color separation settings of each primary color ink. The color look-up table of this color 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 reasonably divided, so that the ink color matching input color can maintain the minimum metamerism and improve matching Accuracy. Because the color separation effect can approximate the original to the maximum extent, this technology is often used in high-fidelity printing.
After the color separation is completed, multi-primary colors can be printed using a multi-color printer or printer. Compared with the traditional printing method, the multi-spectral copy technology has a larger color gamut space and can reproduce more vivid and true colors. In addition, the level of the printed product is more realistic, and the visual change effect is closer to the original spectrum.
4. Research status and related institutions of multi-spectral color reproduction technology
1. Research content of multispectral replication technology
According to different stages of color processing, multi-spectral color replication can be roughly divided into three directions: data acquisition, data processing, and color output, and each direction can be subdivided into several sub-directions:
Data acquisition: device characterization methods, color filter design, multi-spectral camera tuning and data recording, etc.
Data processing: color space conversion, color gamut matching, encoding and decoding of spectral data, etc.
Data output: establishment of a comparison table, research on color separation algorithms, ink color selection, etc.
2. Related agencies
At present, many international organizations, laboratories and research institutions in the world are devoted to the research of multi-spectral color reproduction technology. Some of the more well-known are the Munsell Color Science Laboratory at Rochester Institute of Technology, North Carolina State University, Leeds University, and Chiba University in Japan. In addition, organizations such as IS & T of the Imaging Science and Technology Society, SPIE of the International Society of Optical Engineering, and IEEE of the International Institute of Electrical and Electronics Engineers have also made great contributions to this research.
In China, universities such as Wuhan University, Beijing Institute of Technology and Jiangnan University have also made research on this topic in different directions.
3. Problems in the current research
Although the advantage of multi-spectral color reproduction technology in spectral matching is undoubted, compared with the mature traditional printing technology, the improvement and popularization of this technology still require the joint efforts of the printing industry and researchers. At this stage, the high cost of this technology and the complicated operation technology make most people prohibitive. In addition, this technology has very high requirements on the accuracy of data sampling and processing. According to the relevant research done by the Multimedia Information Technology Institute of the Italian National Academy of Sciences, the effect of multi-spectral color replication in the case that the spectral data cannot reach the required accuracy Very unsatisfactory. That is to say, although the conclusion that spectral matching is the highest level of color matching is unquestionable, the increase in spectral matching is not directly related to the reduction in color difference and human visual difference. It can be seen that although its advantages are obvious in principle, its specific implementation is actually more difficult.
V. Conclusion
The traditional four-color printing has the fundamental defect of this metamerism, and its color matching can only be maintained under specific conditions. For this reason, more and more people are focusing on multi-spectral color reproduction technology that can realize unconditional color reproduction. Although this technology is still in the initial development stage, its advantages in accurate color reproduction will make it a hotspot for future industry research and lay a solid foundation for the realization of high-fidelity printing.
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