The hue of the ink is one of the key indicators that affect the quality of printing. Therefore, the preparation of ink has become an indispensable process before printing. The basic principle of color matching is based on the theory of color synthesis and color mixing, and the effect of the same color and different spectrum colors can be obtained by the method of color mixing. With the development of electronic computer technology, computers can store large amounts of data and have high-speed computing capabilities. With the help of chromatic theory, they can process a large amount of ink basic data and color values, and match colors through human-machine dialogue. High, introducing it into the printing field can make color management and quality inspection more modern.
1. The development and characteristics of computer color matching
1. The development of computer color matching
In industrially developed countries, coloring-related industries, such as textile printing and dyeing, dyes, pigments, paint manufacturing, plastic coloring processing and inks, generally use computer color matching systems as a powerful tool for product development, production, quality control and sales. The rate is high. For example, foreign optical instrument companies have developed InkFormulation 4.0, a Grinder Macbeth ink formulation software in recent years, which can prepare precise formulas for offset printing, flexographic printing, screen printing, and rotogravure printing industries. The fast formula and multi-channel calculation features make it possible to quickly formulate low-cost formulas. The software window has a friendly interface to ensure accurate formulas and the success rate of one-time preparation.
In the past 10 years, China has successively introduced various types of color matching systems, but not many have achieved significant benefits. The software developed abroad is based on the characteristics of the European and American processing industry, and the color material quality is relatively stable; while the domestic processing industry has its own characteristics, the stability of the color material quality is relatively poor, plus the current color varieties are constantly updated A large number of new substrates are emerging, and the existing color matching systems at home and abroad lack flexible adjustment capabilities. Therefore, the practical application of color matching systems has encountered difficulties.
Shenyang Chemical Research Institute started to study color matching system in 1984. Among them, thinking color matching Chinese software is the earliest Chinese color matching software in China. With this software system, the total price of matching with domestic machines is 1/3 of the whole system. If it is matched with imported machines, the total price is 1/2 of imported. The system has been used in more than 70 manufacturers, covering many industries related to coloring, such as dyes, printing and dyeing, woolen spinning, knitting, paint, ink, rubber, wallpaper, etc. In addition, Xi'an University of Technology has developed a density computer color matching system that uses a color density meter to connect with a computer, which is convenient and extensive for popularization and application. Judging from the current development trend, computer color matching has become an important part of ink color matching in the future.
2. Features of computer color matching
(1) It can reduce color matching time, reduce costs, and improve color matching efficiency.
(2) The correction formula can be calculated in a short time.
(3) Store all the ink colors that have been used in the past in the database and call them out immediately when needed.
(4) Easy operation.
(5) The color correction formula and the calculation of color difference are displayed or printed out by the computer digitally, and the final color matching result is also stored in the memory in digital form.
(6) Other functional systems can be connected. For example: a weighing system can be connected to minimize the weighing error; reproducibility is improved. If the process is continuous, a print quality monitoring system can be set on the printed product. When any abnormality occurs, it will be immediately Downtime reduces unnecessary waste.
Second, the computer color matching principle and system
1. Kubelka-Munk theory and its limitations
The KM theory was proposed as early as 1931, but it was not successfully used in the textile printing and dyeing industry until 1958. The application of the theory in the printing industry began in the 1970s.
Computer color matching systems developed by the United States, Japan and other countries still basically use this theory.
Through a series of derivations of KM theory, the simplest form of the function suitable for color matching calculation and its derivative form are given:
K / S = (1-r) 2 / 2r
r = K / S + 1-[(K / S + 1) 2-1] 1/2
Where r represents the reflectance at the wavelength; K is the absorption coefficient, which represents the absorption rate of light in the micro-element thickness dielectric layer after the diffuse illumination light is incident in an infinitely thick planar medium; S is the scattering coefficient, which represents the micro-element thickness The scattering rate of light.
So far, the basic principles of computer color matching (CCM) still follow KM theory. For example, spectral visual matching method, computer reflection spectroscopy color matching, computer color matching approximation algorithm, etc. are all based on KM theory. But in the practical application of KM theory, there are often differences between the theoretical calculation and the specific practice. The reason can be summarized as two factors.
â‘ The K-M theory itself is derived under certain assumptions.
First, if the thickness of the color layer is x, and the light falls on any micro-layer dx, the reflection caused by the interface is not considered. The result must be that the color layer applying this theory is immersed in a medium with the same refractive index. Algorithms that simplify the problem and ignore the different refractive indices on the interface may cause errors.
Second, dx is any micro-layer within the thickness x of the color layer. The absorption coefficient and scattering coefficient obtained in this way are considered to be the same and uniform throughout the color layer when used, but this assumption is difficult to apply to extinction or semi-extinction Oily material.
Third, the colorant particles in the color layer are chaotically arranged, so that the light in the color layer becomes a form of diffuse diffusion. The particles are completely submerged in the diffusion effect, creating two channels up and down. But in practical applications, when the particles are present in the thin film of oily film, most of which are arranged in the horizontal direction, it will cause the destruction of the luminous flux assumption of the two channels.
Fourth, on the thin color layer, the light has entered the inside of the color layer before scattering. At dark tones, a lot of light has been absorbed before scattering, so these beams entering the color layer are not in a diffuse state, resulting in experimental results. Big difference.
â‘¡The printing industry must consider the interaction between light and pigment particles and the physical properties of the ink when describing the ink superposition effect. In practical applications, it should be said that the KM theory contains two double constants, namely an absorption coefficient K and a scattering coefficient S. The light scattering ability of the ink is negligible compared with the scattering ability of the substrate. Therefore, the color rendering principle of the ink is mainly The ink selectively absorbs light, and the ink's ability to absorb incident light is affected by the ink layer thickness and ink concentration. The KM theory is based on the premise of opaque media, and the inks used in printing are transparent or semi-transparent. Therefore, the KM theory is very inadequate.
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2. Use tristimulus values ​​for computer ink color matching
(1) Tristimulus value color matching
At present, the mathematical model used by computer color matching systems at home and abroad is based on the K / S function. In view of the limitations of K / S and the characteristics of the printing industry, this paper proposes a method of color matching using tristimulus values. This method does not use K / S values, reflectance and other color-measuring indicators, but only uses tristimulus values ​​as color-measuring indicators.
On the basis of K / S theory, tristimulus value matching can also be carried out, but the database of K / S value and concentration needs to be established in sections to study the relationship between tristimulus value and concentration, that is, between the tristimulus value and the percentage of dots Relationship. In printing, the method of converting the tristimulus value and the percentage of dots is mainly useful for the conversion of Newburger equation, conversion by matrix change method and conversion using a lookup table. In this paper, the chromatography is used to establish a lookup table for conversion.
(2) Color matching principle of three stimuli
According to the CIE standard colorimetric system, any color in nature can be represented by the spectral tristimulus values ​​X, Y, and Z. At present, most advanced color measuring instruments use this colorimetric system, that is, the color of any object can be expressed by the tristimulus values ​​X10, Y10, and Z10. The principle of computer color matching mainly uses the principle of metamerism, that is, if the tristimulus values ​​X10, Y10, and Z10 of two color samples are respectively equal, the two are the same color.
The look-up table established by chromatography describes the relationship between the tristimulus value and the percentage of dots of each color ink. Suppose a color sample is overprinted by three inks a, b, and c. The dot percentages of these three inks are l, m, and n, respectively, then the ratio of inks a, b, and c is l: m: n, white ink. Account for (1-l) + (1-m) + (1-n). This color matching system uses CIE standard light source D65 and 10 ° field of view data for calculation, and uses the CIELAB color difference formula: ΔEab = [(ΔL) 2+ (Δa) 2+ (Δb) 2] 1/2 to calculate the standard color The color difference between the sample and the color sample.
We can see that using tristimulus values ​​for computer color matching can make the color of the color sample to be matched under a specific light source represented by data. There is a corresponding relationship between the tristimulus value of the color sample and the ink ratio, and the color difference can be used. Verify that the formula meets the requirements.
(3) Tristimulus value color matching method
Enter the tristimulus value of each color block of the chromatogram and the dot percentage of each ink into the computer to establish a basic database. During color matching, the tristimulus value of the target color sample is input into the system, and the system calculates the mixed ink and its proportion, and outputs the formula prediction result. After the ink sample of the color matching result is dry, the tristimulus value is measured, and the computer calculates the excellent difference according to the color difference formula, and makes further correction instructions, and can quickly prepare a higher quality metamerism color.
Regarding the quality requirements of color reproduction, according to the clear requirements of the national standards for the color difference ΔE * ab of the same batch of color decoration prints, this paper chooses ΔE * ab≤3.
The color spectrum includes most of the common colors. For the colors in the color spectrum, you can directly find the ratio of the ink, instead of the colors in the color spectrum, you can first find the color with the smallest color difference in the color spectrum, and then use linear interpolation Solve.
3. Computer color matching system
(1) Function of color matching system
Computer color matching system is a modern equipment integrating color measuring instrument, computer and color matching software system. The basic function of computer color matching is to store the color data of the color matching ink in the computer in advance, and then calculate the mixing ratio of the sample color matched with these inks to achieve the purpose of the predetermined formula.
(2) Composition of color matching system
â‘ Hardware part of computer color matching system
Computer: use Windows operating system, hard disk storage space of at least 20MB; spectrodensitometer; chromatography.
â‘¡Computer color matching software system
Software main menu: Display the program directories in the color matching system software, so that the operator has a general understanding of the color matching software, and enable the operator to select and call the programs displayed in the directory according to his own purpose.
Basic data files: Use Microsoft Access to create database files, including three parts: two-color overprint, three-color overprint and spot color overprint.
This file includes basic data file establishment, management, data processing and recipe storage procedures.
â‘¢ Formula calculation and correction
Call this program to calculate the color difference between the color sample and the standard sample, select the formula based on the color difference, and correct the formula.
The color matching system software has a strong human-machine dialogue function. The operator can obtain the required ink formula by entering the corresponding parameters and data according to the prompt on the computer screen.
Color matching is a complex technical project involving light color theory, ink, paper, process, etc., using tristimulus color matching by chromatography, which overcomes the limitations of KM theory and is suitable for the characteristics of the printing industry, reducing the burden of color matching personnel , Improve the product's color quality, color matching speed, precision, and increase economic benefits. Although there are still many areas to be improved, such as the color difference calculated by tristimulus value matching under different light sources, the accuracy of color matching has a great relationship with the accuracy of chromatography. However, with the continuous updating of computers, more sophisticated instruments, the continuous emergence of various mathematical methods, and the gradual standardization and dataization of materials, computer color matching will inevitably show incomparable superiority.
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