In the past, the color matching of spot color inks was mainly done manually, and the manual color matching was highly dependent on the experience of the color matching personnel. The control of the color matching process was mostly based on personal feeling and experience, and lacked standardization. The color matching process was difficult to control, and the ink material consumption was large and time-consuming It is also long, the accuracy of the spot color ink prepared is poor, and the color matching efficiency is also low. In order to obtain the same color matching effect in different environments and different operators, and to improve the efficiency of color matching, people have introduced a computer color matching system. The computer color matching system is a modern automatic color matching facility integrating measuring instruments, computers, and color matching software systems. The basic principle of the computer color matching system is to express the color sample to be matched in the form of color data, and then determine the color of the color sample according to the color theory The required ratio of each base ink. That is, the reflectance spectrum data of the color sample to be matched (or the chromaticity value of the color sample to be matched) is measured by a color matching measuring instrument, and it is input into the computer, and the ink color database and related color matching software stored in the computer are used to treat The relevant color measurement data of the matching ink color sample is processed, and the computer software is calculated, iteratively corrected, and spot colors are deployed, and the ink formula that meets the requirements of the standard color sample is screened and output, and the computer's automatic color matching is completed. Nowadays, the color matching system has been widely used in many printing companies. At present, there are more color matching software used in CHINA, such as DataMacth and X-RiteInkFormulation of DataColor Company. The basic working process of computer color matching is shown in Figure 1:
The first step is to establish the basic database of color matching. base of different ratios
The reflectance or L, a, b chromaticity value of the color is input into the system, and the basic database of color matching is established. The color matching database is the core of the computer color matching system. All data used in color matching calculations come from this database, so the accuracy of the database will directly affect the accuracy of ink color matching.
Step 2, input the relevant color data of the specified color sample into the color matching system
Establish standard values for standards. The color sample can be the actual color sample, it can be the tristimulus value X, Y, Z of the color sample, or it can be the spectroscopic curve of the color sample, among which, the most common one is the tristimulus value X, Y, Z of the actual color.
The third step is to run the color matching software. According to the color difference threshold, color matching cost, ink characteristics, spectral curve matching index and other limited conditions, the color matching system will give a variety of optional color schemes.
Step 4, according to the actual situation, select a good color matching formula for proofing.
Step 5: Measure the chromaticity value of the proofing color with a color measuring instrument, input the standard value of the color matching system and the standard sample color for analysis and comparison, and calculate the color difference between the two.
The sixth step is to iteratively correct the initial formula according to the color difference value until the most satisfactory ink formula is obtained.
Computer color matching uses a lot of basic knowledge of color science, and requires a high level of color knowledge for operators. Operators should have a certain understanding of the relevant knowledge involved, so that they can use the color matching software well and get satisfactory color matching results. In actual color matching practice, operators should pay attention to the following issues:
1. Understand the working principle of color matching software
The basic algorithm principle of computer color matching is mainly divided into two methods: reflection spectrum matching and tristimulus value matching.
Reflectance spectrum matching method: also known as unconditional color matching, the purpose is that the spectral reflectance of the color matching result is exactly the same as that of the sample to achieve spectral matching. For ink, it is the reflection spectrum that ultimately determines its color, so the reflection spectrum matching is a good color matching, and the color matching can be maintained for any light source and observer. Although the spectral matching method has many advantages mentioned above, this method requires that the target ink material and attachment medium are exactly the same as the colorant and attachment medium of the sample, the conditions are relatively harsh, and the success rate of prediction is very low . Although the spectral color matching method is a very good color matching method, it is difficult to implement. Unless there are very special requirements, this method is generally not used in actual production.
Three-stimulus value matching method: The principle of the three-stimulus value matching method is that as long as two colors with equal three-stimulus values give people the same color perception, color matching can be achieved. The premise of this color matching method to achieve isochromaticity is that the illuminant, observer and color measuring instrument that affect the tristimulus value are all the same, otherwise color difference will be caused. Tristimulus color matching method is a widely used and practical color matching method. The computer color matching based on the tristimulus value is to express the color of the color sample to be matched under a specific lighting source in the form of chromaticity data, to find the quantitative relationship between the tristimulus value and the quality of the basic ink, and to use the color difference to determine whether the formula is qualified or not. to control.
From the user interface of the color matching software, it is difficult to see clearly what for color matching calculation. But as users we should be clear about the difference between these two algorithms.
There are two types of color reproduction, one is same-color and same-spectrum reproduction, that is, the spectral reflectance of two colors is required to be exactly the same at each wavelength point; the other is metamerism, that is, the spectral reflectance of two colors is not required to be completely consistent , only the tristimulus values X, Y, and Z of the two colors are required to have the same size. It is very difficult to achieve the same color and spectrum reproduction. It requires that the color components are exactly the same, and the coloring medium also has the same material, gloss, diffuse reflection and even texture. However, the metamerism replication can ignore the material and ink composition requirements, as long as the color values are consistent. In the principle of computer color matching, the reflection spectrum matching method belongs to the same color and spectrum replication, and the tristimulus value matching method belongs to the metamerism replication. We can better understand the above reasons from the calculation formula of the tristimulus value.
Many articles say that the three-stimulus value matching method may have , and this is indeed the case in theory. But what I can tell you is that this difference can be ignored. The above formula is the CIE1931 standard chromaticity observer spectral tristimulus value. Observers of standard chromaticity are actually people who represent the vision of ordinary people. Their ability to perceive color is that the color perception ability of the body will be different from it, but the difference will not be very large. They can also pass the color difference when matching colors. The setting of the value threshold reduces this effect.
3. The choice of basic ink should cover the maximum color gamut as much as possible
According to the intermediate color law of Grassmann's color mixing law, the new color produced by the two colors involved in the mixture is between the two colors, and on the chromaticity diagram, the new color is located on the straight line of the two colors involved in the mixture. In order to match as many colors as possible, the larger the base ink coverage, the better, because the new colors that can be produced by color matching can only be within the color gamut formed by the base ink. It can be seen from Figure 2 that if yellow, magenta, and cyan are used as the basic ink, the color gamut is the range surrounded by the dotted line; The color gamut of the basic ink is the range surrounded by the solid line. It can be seen that the color gamut between the two is very different, and the number of colors that can be matched by the six-color basic ink will be much more. It should also be clear that if the color swatch to be matched is out of gamut, an exact match is simply not possible, only an approximate match. Therefore, in order to judge whether the color matching sample can be realized, the coordinates of the color matching sample can be measured first, and marked on the chromaticity diagram. If it is outside the basic color gamut, it is impossible to achieve accurate color matching.
For this reason, in order to match as many colors as possible, the choice of basic ink is particularly important. Generally, when choosing the basic ink, you should not choose at will. You should choose colors similar to the primary colors yellow, magenta, and cyan, and inks similar to the secondary colors red, green, and blue. The more base inks, the more colors can be matched.
