Color difference control method
The color of a color print is an important part of evaluating the print quality. In production, two methods are used to evaluate the color.
One is that people judge by visual means. However, there are some limitations in the visual inspection method. For example, it is difficult to correlate the quality with the numerical value of the visual concept. Physical, mental, and emotional factors may affect the ability to observe colors. The human eye has poor record of color. , only show accuracy when performing direct color comparisons. Therefore, the changing factors observed by the human eye are too many, and there is no standard in grasping the size of the color difference.
The second is the use of color measuring instruments to detect the color difference between the process color and the target color. If the color difference of the color is within the specified tolerance range, it is considered as qualified. This method is more scientific and objective.
1, density control method
The color difference of the printed product is caused by the impure absorption spectrum of the ink, which makes the color effect poor. If you can know the absorption of color light samples, it is easier to judge the color error. From the definition of optical density, it can be known that the size of the color density indicates the strength of the absorbed light. That is, the higher the density, the stronger the absorption of light. The smaller the density, the weaker the ability to absorb light.
Density control is an important form of color control. The density meter uses three color filters to measure the density values â€‹â€‹of the inks of ä¸«, M and C respectively. The instrument is placed on a position of a standard color sample and a print, and the density change of each ink is measured, and the Î”D is used. Indicated. The calculation method is:
â–³D=(D print--D mark)
Where: D is the density value D of a single color on the printed product, and the density value of a single color on the standard color sample.
If â–³D is within the allowable range, the color difference between the color of the printed product and the standard color sample is relatively small, and the color quality of the printed product will not be affected; if the allowable range is exceeded, the color of the printed product will be described. The color difference from the standard color sample is relatively large, and the color quality of the printed product is unacceptable. Since the density directly reflects the amount of light reflected (or transmitted) from the print or film, the depth of the color and the change in the thickness of the ink layer can be directly judged from this value.
2. Color control method
The spectrophotometer is used to measure the spectral reflectance of the color block, and the tristimulus value is calculated or the color block is measured directly by the colorimeter to obtain the tristimulus value. After converting the color detection formula, L*, a*, b are obtained. * and åŽ¶E, as the color standard, and then adjust the printing color according to this standard until the color difference meets the requirement.
The color difference between the color reproduction and the color sample can be known by the color difference between the standard color and the sample color.
â–³L*>0 is white, â–³L*<0 is black;
â–³a*>0 is red, â–³a*<0 is green;
â–³b*>0 Yellowish, â–³b*<0 Blue.
By analyzing the color difference information, it is possible to provide an accurate basis for determining the adjustment direction of the spot color ink formulation, so as to reduce the color difference and make the color of the color reproduction more similar to the color sample provided by the user.
According to "GB7705-87", â–³E should be controlled within the allowable range during the printing process.
The color difference is 0.5-1.5NBS, which means there is a small color difference. There is a slight difference;
The color difference is 1.5-3. ONBS, there are significant differences;
The color difference is 3.0-6. ONBS said that there is a significant difference in the color difference in 6.0-1. ONBS, there is a particularly noticeable difference
In gravure printing, strict control of ink quality and ink layer thickness must be enforced. Chromatic aberration can be monitored by density or chromaticity, and density control is suitable for monitoring the printing process. However, for multi-color printing, the density measurement results and people's visual density measurement language cannot clearly and effectively exchange information with customers, and such information exchange is now becoming more and more important. Therefore, the specification of the product must be explained in a way that the customer can understand. Chromaticity control has become the focus of printers' research. The reason is that it shows obvious advantages in testing:
1. The color method is more accurate than the density method. The color calculation of the color method is more scientific and more in line with the actual visual characteristics. Therefore, theoretically, the calculated value obtained is higher in accuracy.
2. The chromaticity control method is sufficient to objectively match the copied color swatch with the sample color swatches, regardless of the change of lighting conditions and subjective perception of color by the person.
3. The colorimetric control method is industrially applicable to any color matching process without any limitation.
4. Chromaticity control is an excellent way to ensure color quality in printing.
In summary, the color chromaticity control method is based on the spectral energy distribution of the light source and the surface reflection performance of the object, which is consistent with the observation and observation of human eyes, and therefore is an accurate color control method. It overcomes the shortcomings of the density control method, can accurately measure and describe the color of the original; it is based on the theory of color balance and can accurately control the color reproduction of the manuscript; its various color methods can The brightness is independently adjusted, and the color measurement method can directly detect the image and obtain various color difference information. Therefore, the chromaticity control method is an ideal control method and has a wide application prospect and development space.
(Author/Ma Yanhong Hu Gengsheng Zhuzhou Institute of Technology)
"Printing Technology" Packaging and Decoration Printing 2005, 7
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