Application Research of Multispectral Color Copy Technology
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
There are many theories for the evaluation of color reproduction. Among them, the famous British color expert Hunter divides the color reproduction into six types through comprehensive analysis of various factors: spectral color reproduction, chroma color reproduction, correct color reproduction, equivalent color reproduction, corresponding color reproduction, and Like color reproduction.
The traditional color copying method successfully achieves both chromatic color reproduction and correct color reproduction. However, due to its essential characteristics based on metamerism, unconditional reproduction of color, that is, spectral color reproduction, cannot be realized. Multi-spectral color reproduction technology uses a multi-primary color imaging method to increase the color gamut by increasing the freedom of color reproduction. The 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 certainly become the direction of the future development of the printing industry.
First, the defects of traditional four-color printing
The traditional printing method uses CMYK four primary colors to achieve color reproduction. Due to its limitations, four-color inks cannot cover all 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 the printing copy, as long as the printed color is the same as the human eye of the original color, even if the spectral composition of the two is different, it can be recognized as the correct color reproduction. This method greatly reduces the difficulty of color copying, and in most cases can achieve the correct reproduction of colors, thus forming the basis of the traditional printing method. However, when the illumination source and the observer change greatly, the copying 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 color that can be reproduced by four-color printing is only about half of the color spectrum of the visible spectrum, and the image reproduction effect with bright color and dynamic range is often unsatisfactory. Under the current state of the art, even if people perform accurate color management and gamut matching during copying, the problem of metamerism and gamut too small can not be fundamentally solved.
Conventional printing methods focus on matching the density of the original with the printed color density. However, this type of matching pays too much attention to the discussion of ink usage and ignores the problem of matching color chromaticity and brightness. In fact, due to the limitations of the color gamut of four-color printing, it tends to be in a paradoxical situation when regulating color components and neutral ash components. If you want to increase the amount of color components to increase saturation, it is inevitable. The neutral ash component is increased to reduce the brightness. In other words, four-color printing tends to over-emphasize the reproduction of saturation, and a method of sacrificing brightness has to be employed.
Due to the above-mentioned defects of traditional copying, the copying effect is often unsatisfactory, mainly because the image is heavy in color, the stereoscopic effect is poor, the layer loss is more serious, and there are different degrees of color distortion.
Second, the superiority of multi-spectral color reproduction technology
Multi-spectral color reproduction technology enables color reproduction by acquiring, analyzing, and processing multi-spectral data. This technique uses spectral matching as a color reproduction standard, and achieves an enlargement of the reproduction color gamut by increasing the degree of freedom of color superposition. Due to the uniqueness of the spectral reflection curve, the reproduction effect can be kept stable regardless of the change of the light source and the observation conditions. In addition, the sampling of multiple bands of spectral reflectance can record the color characteristics as much 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 displays, printers, and scanners is greatly different from that of human eye vision. Due to its own limitations, the traditional four-color printing mode cannot solve the problem of color deviation fundamentally even with the help of the color management system. The multi-spectral replication technology greatly improves the integrity of data acquisition by increasing the number of sampling channels, thereby achieving high-quality color reproduction. In view of the above advantages, this technology has been successfully used in the fields of copying and saving of valuable art works and online shopping. At the same time, the technology also laid a solid foundation for future high-fidelity printing and cross-media publishing.
Third, multi-spectral replication technology process
Multispectral color reproduction techniques describe color information in spectral data by describing the spectral reflectance or transmittance of the color. The specific process flow (Figure 1) can be divided into the following steps:
1. Data acquisition
Multi-spectral image data of an original or a thing is acquired using a multi-spectral camera with a multi-color filter. Typically, acquisition systems consist of multispectral light sources, color filters, and multispectral cameras. Compared with the traditional three-color based image acquisition method, this system has the following advantages:
The light source has a short starting process, a wide spectrum and high radiation efficiency; the color filter has strong transmission and is not affected 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 to achieve high-precision spectral reconstruction. The matrix representation of the mathematical model for multispectral data acquisition is now described as follows:
Let the spectral power distribution of the multi-spectral 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-ranking operation of the matrix. In a multispectral camera, the spectral transmission characteristics of m color filters can be represented by a matrix F.
The spectral sensitivity of the detector is represented by the matrix D.
Combining the above matrix, the colorimetric integral calculation formula can be obtained, and the color value of the collected color is t=(DF)TSr. Subsequently, the tristimulus value XYZ of the color and CIELAB can be obtained by corresponding linear and nonlinear transformations. Color values such as coordinates.
In addition to the above methods, principal component analysis (PCA) can also be used to select the optimal color filter design and achieve more accurate spectral reconstruction. This method is often used in the mutual conversion of the integrated density and the analytical density in photographic technology, and is also commonly used in the establishment of high-precision device property files for scanners.
2. Prediction of original color material and optimal ink color selection
After the multi-spectral data acquisition is completed, it needs to be analyzed. By predicting the spectral distribution of the collected color, the optimal ink color selection for color reproduction is determined, and the effect of metamerism on color matching is eliminated to the utmost extent. In order to achieve the best match between the copied color and the original color, it is necessary to ensure that the copied color spectral distribution curve maximizes the spectral distribution of the original color. In practice, the spectral data is usually analyzed by principal component analysis, and then the optimal possible colorant selection is predicted by the constrained rotation transformation. Finally, the best ink color selection scheme is finally determined by comparing the predicted colorant combination with the comparison of the ink combinations in the database.
3. Establishment of ink overprint model and spectral prediction
There are many theories about the establishment of halftone models in color reproduction. In general, Kubelka-Munk theory is used to calculate the primary color reflectivity of the Niel-Nielsen revised Niebel model. Among them, the Niel-Nielsen revised Nygger model (referred to as the YNSN model) is the most commonly used reflectivity prediction model, which 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 optical network points, the specific formula is:
λ=1...8 (4)
Where Rprint, λ represents the reflectivity of the printed color, and n is the Jur Nielsen factor. Rp, λ is the spectral reflectance of the p-primary color of the negbral, and αp is the dot area ratio of the primary color.
4. Color separation and printing based on spectral data
The color separation technique based on multi-spectral data is the core of multi-spectral color reproduction technology, and is usually implemented by inverse transformation of the YNSN model. When using the YNSN equation to determine the spectral value of the ink dot, an appropriate nonlinear optimization iterative method should be used to determine the color separation setting of each primary 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 degree of metamerism and improve matching. Precision. This technique is often used for high fidelity printing because the color separation effect can maximize the approximation of the original.
After the color separation is completed, multi-color printing can be performed using a multi-color printer or printer. Compared to traditional printing methods, multi-spectral replication technology has a larger color gamut and can replicate more vivid colors. In addition, the print layering is more realistic, and the visual change effect is closer to the original spectrum.
Fourth, the research status of multi-spectral color reproduction technology and related institutions
1. Research content of multi-spectral replication technology
According to the different stages of color processing, multi-spectral color reproduction can be roughly divided into three directions of 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 logging.
Data processing: color space conversion, color gamut matching, encoding and decoding of spectral data.
Data output: the establishment of the comparison table, the study of the color separation algorithm, the choice of ink color, and so on.
2. Relevant institutions
At present, many international organizations, laboratories and research institutions in the world are working on multi-spectral color reproduction technology. More famous are the Munsell Color Science Laboratory of Rochester Institute of Technology, the University of North Carolina, the University of Leeds, and the University of Chiba, Japan. In addition, the Institute of Imaging Science and Technology, IS&T, the International Society of Optical Engineering, SPIE, and the Institute of International Institute of Electrical and Electronics Engineers, IEEE, have also contributed greatly to this research.
In China, universities such as Wuhan University, Beijing Institute of Technology and Jiangnan University have also conducted research on this subject in different directions.
3. Problems at the current stage of research
Although the advantages of multispectral color reproduction technology in spectral matching are unquestionable, compared with mature traditional printing technology, the perfection and popularization of this technology still requires the joint efforts of the printing industry and researchers. At this stage, the high cost and complicated operation technology of this technology have made most people discouraged. In addition, the accuracy of data sampling and processing is very high. According to relevant research conducted by the Institute of Multimedia Information Technology of the Italian National Research Institute, the effect of multi-spectral color reproduction is obtained when the spectral data cannot meet the required accuracy. Very unsatisfactory. That is to say, although the conclusion that spectral matching is the most advanced color matching is undoubted, the increase in spectral matching is not directly related to the difference in chromatic aberration and human visual difference. It can be seen that although its advantages are obvious in principle, its implementation is actually quite difficult.
V. Conclusion
Traditional four-color printing has the fundamental flaw of this metamerism, and its color matching can only be maintained under certain conditions. To this end, more and more people are focusing on multi-spectral color reproduction techniques that enable unconditional color reproduction. Although the technology is still in its infancy, the advantages of accurate color reproduction will make it a hot topic in the future, and lay a solid foundation for the realization of high-fidelity printing.