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Print density measurement and point area calculation

there are two commonly used point area calculation formulas: murray2davies formula and yule2nielson formula

murray2davies formula is relatively simple and most widely used. General densitometers use this formula to convert the point area. Experiments show that the calculation accuracy of mur2ray2davies formula can be improved if the value of n is set appropriately. Mathematically, if a limiting condition can be determined, such as the accurate point area corresponding to a certain order of density modulation, the value of N can be obtained. However, because the printing process is very complex, it is impossible to obtain the accurate point area of a certain step density, and the determination of n value is actually very difficult. Although many people have done more detailed research on this, the size of n value has not been unified so far. According to the information, the n value range of coated paper is 112 ~ 210, and the n value range of non coated paper is 118 ~ 510 Because the value of N acts on the index, its change has a great impact on the calculation results. If it is improperly selected, the calculation accuracy will be reduced

in order to further explain the problem of density and point area calculation, it is explained by comparing the actual measured and calculated data. Table 1 lists the density measurement values and corresponding point calculation values of the primary color ink printing ladder. The ladder is obtained by offset proofing with 128G coated paper, and the measuring instrument is x2rite528 spectrodensitometer. The relative density values dt-dw and ds-dw can be directly measured by the instrument; The calculated point value is the result of formula (7); The measured point value is directly measured by the instrument. It can be seen from the data listed in Table 1 that the calculated point value is consistent with the measured point value, and the difference is within the measurement error range, which indicates that the instrument also uses equation (7) to calculate the point area

it should be pointed out that the point values corresponding to n=1.5 (relative) and n=1.8 (relative) in the table are calculated by yule2nielson formula and relative density value (the range of n value obtained for coated paper printing is generally 115 ~ 118). The calculation data shows that the value of N has a great influence on the calculation results, especially in the highlight area. From the highlight to the middle note area, the point value decreases, and the calculation result is obviously unreasonable, but the error increases. From the general trend, with the increase of n value, the calculated value of point area decreases. This has a great impact on the highlight area, but not on the dark area. For further verification, the comparison calculation is carried out again using yule2nielson formula and absolute density value. The results are shown in the data listed in table 1n=1.5 (absolute) and n=1.8 (absolute). These data seem to be more reasonable than the results calculated with relative density values. The value of N here is arbitrarily set, and the calculation results are not necessarily ideal, but it is possible to find a more appropriate value of n to make the calculation results more accurate

from the above data, it can be seen that the selection of correction coefficient n has a great impact on the calculation results. In this experiment, whether absolute density value or relative density value is used to calculate, the result is not very ideal. Therefore, it is suggested that if the value of N cannot be determined accurately, it is better not to use the formula of yule2nielson

use chromaticity value for point area calculation

with the development of printing technology, more and more index theories that should be paid special attention to when selecting tension machine for soft packaging material detection are analyzed according to chromaticity, especially in prepress and printing color control. Color calculation is basically carried out in CIEXYZ standard chromaticity color space and ciel3a3b3 uniform color space, such as color management system. As the price of colorimeter decreases, many enterprises strengthen the testing means to improve the quality of printing products, and purchase colorimeter one after another. This will become the orientation of future testing methods. Therefore, it is of great significance to explore the method of calculating the area of printing points with chromaticity measurements

according to the principle of color addition, it is easy to deduce the relationship between excellence value and point area

it is not difficult to see that the point area calculated by chromaticity value is close to the directly measured value, which shows that the method of calculating point area by chromaticity value is feasible

discussion

it is very difficult to accurately measure or calculate the area of points, because of the lack of accurate measurement references. Because the accuracy of the absolute measurement value of the point area is not high in actual production, the relative value can be used to control the digital display all-round experimental machine, microcomputer screen display hydraulic machine all-round experimental machine, microcomputer controlled hydraulic servo electromechanical all-round experimental machine, etc., so it is usually approximately replaced. X2rite528 spectrodensitometer used in this experiment is a relatively high-grade instrument without any damage and parameter error. It can not only measure density and point area, but also measure chromaticity, color difference, overprint, printing contrast and other indicators, as well as detect ink hue error and gray scale. When measuring the area of points, you can choose either murray2davies formula (n=1) or yule2nielson formula (n 1) to calculate, and the value of N can be set by the user. The user cannot set the value of n when measuring the area of a point with a general type of densimeter. The instrument is calculated according to murray2davies formula. X2rite528 spectrodensitometer is calculated by relative density value when using murray2davies formula and yule2nielson formula. As mentioned earlier, it is more reasonable to take the absolute density value when using yule2nielso formula for calculation. If we take the relative density value to calculate and choose the correction coefficient n value at the same time, it seems to be somewhat difficult to understand in theory. Because the density value of the paper itself has been subtracted from the relative density value, and the n value is set to compensate for the light penetration effect of the paper, which is equivalent to a double correction. On the other hand, it is difficult to determine the accuracy of n value in practical application, and the previous calculation and actual measurement results show that if the n value is improperly selected, the accuracy of calculation will decline. Therefore, using n value to correct the error is not the best method, which should be carefully selected in practical application

according to the color principle of printed matter, this paper proposes a formula for calculating the point area based on the chromaticity value. In fact, the chromaticity calculation method and density calculation method are based on the same principle, which is based on the principle of light absorption and reflection of printed matter color, and the measurement conditions are also the same, except for the different ways of processing spectral data. Density measurement adopts T, e, a and other types of color filters, while chromaticity measurement adopts the unified CIE1931 standard observer function. Colorimetry is based on human vision, and the theory is relatively perfect. Using this method to calculate the point area should be a more reasonable method. Neugebauer equation is based on the principle of color addition, which is the most basic rule of colorimetry. Therefore, the physical meaning of calculating the point area with this method is clearer, and there is no error in theory. Comparing the data in Table 1, it can be found that the results of direct measurement, density method calculation and chromaticity method calculation are very similar. Due to the lack of accurate reference standards, it is difficult to prove which method is better. Next, it can be verified more accurately with the help of scanning amplification method. However, from the principle analysis and calculation results, if the results calculated by murray2davies formula can meet the accuracy requirements, the method of calculating point area with chromaticity value proposed in this paper can also fully meet the requirements. In practical application, if the printing process is controlled by measuring the dot area, then using the colorimetric method to calculate the dot area can play a role

another advantage of calculating the dot area by chromaticity method is that it can calculate the dot area after overprint, which can be obtained by solving the two-color and three-color Neugebauer Equations. The calculation of dot area by density method is based on the complementary standard property color density, and the complementary color density can only be obtained from the measurement of monochrome samples, so it is inconvenient to calculate the dot area of multicolor overprint

from the calculation results, it can be seen that the function of the correction coefficient n value is mainly to reduce the point value of the highlight to the middle note area, but has little effect on the dark tone area. Comparing the results of direct measurement, density method calculation and colorimetric method calculation, except for the high-profile area of yellow ink, the point value calculated by colorimetric method is slightly smaller than the measured value, which can be said to partially contain the function of compensating the n value

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