Analysis of reflection density calculation and application in printing
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
First, Overview
There are many uses for reflection density in printing, mainly for printing quality control, and sometimes for the analysis of printing inks and substrates, which is an important parameter in the industry. The reflection density is called differently for various specific purposes, and its physical meaning is also different. In the last century, the Americans summed up the Marley-Davis formula for calculating the reflection density of dots, and gradually gained the attention of printing scholars all over the world. Soon, yur-Nielsen revised this formula. At the same time, densitometers for measurement have also been developed and widely used. ISO introduced an international standard for reflection density measurement in 2000. Two years later, China released the national standard on its basis and it is still in use today. In order to better realize the data quality of printing quality control and promote the development of printing materials, this paper systematically analyzes the calculation and application of printing sample dot density and field density, so that it can play a better role in the industry.
Second, the theoretical basis
1. Reflection density
The reflection density in printing mainly depends on the measurement position, measurement method, and application purpose, and mainly includes the density of the solid, the dot density, the three-color density, and the equivalent neutral gray density.
1.1 definition. The reflection density is a kind of optical density in physics. It is a characteristic measure of the absorption of light by an object, that is, the ratio of the amount of incident light to the amount of reflected light, expressed by the decimal logarithm of the inverse of the reflectivity, as shown in equation (1).
D=lg(1/R) (1)
Wherein R represents reflectance; D represents reflection density. Obviously, as the reflectance R increases, D decreases.
1.2 measurement and calculation. The measurement principle of reflection density is as shown in (1), the measuring instrument is the density meter specified in ISO 14981, and the measurement methods of various density values are provided in GB/T18722-2002. The density value measured using the densitometer is the actual measured value, and the density value calculated using the mathematical formula is the theoretical calculated value. The former is the actual reflection of the latter, the latter is the theoretical basis of the former, and the two complement each other. The Marri-Davis formula is as in (2) and the Yule-Nielsen formula is (3).
D=-lg{1-a[1-antilg(-d)]}(2)
D=-nlg{1-a[1-antilg(-d/n)]} (3)
Where a is the percentage of the area occupied by a dot, D and d are the dot density and solid density of the dot area a, lg is the logarithm, and anti lg is the antilog. It can be seen from equations (2) and (3) that the latter introduces a parameter n and n values compared to the former for correcting the influence of diffuse reflection and number of screen lines due to light penetration of paper and ink film. .
2. Application of reflection density
By measuring the reflection density, the basic property values of the ink, the dot gain value, the printing relative contrast, the overprinting ratio, and the equivalent neutral gray density (END) can be obtained. By analyzing these parameters, the entire printing process as well as printed materials can be evaluated and controlled.
2.1 Parameters related to reflection density.
Color intensity: also the solid density, the primary density of an ink. It reflects the color range and depth of the ink. The color intensity was measured directly using a densitometer.
Hue error: Hue is the combined result of the absorption and reflection of the ink on the visible spectrum. The hue error indicates the degree of purity of the hue. The hue error is calculated from the data measured by the three color filter densitometer, as shown in equation (4).
Among them, H, M, and L respectively represent the highest, middle, and lowest density values measured by the densitometer under the three-color filter. The lower the hue error value, the more pure the hue.
Grayscale: Reflects ink color saturation. The lower the gray value of the ink, the higher its saturation, and the calculation method is as shown in equation (5).
Color efficiency: The comparison between the ink and the ideal three primary color inks for the combined results of absorption and reflection of the visible spectrum, reflecting the degree of correct representation of the color, which is calculated as equation (6).
(5), (6) In the two formulas, the meanings of H, M, L and (4) are the same.
Other control parameters can also be obtained by calculating or measuring the reflection density, as shown in Table 1.
Table 1 Other parameters related to reflection density
2.2 Application of reflection density and related parameters.
In the objective evaluation stage of the quality of printed products, the printing characteristic curve and the color hexagonal figure can be drawn by obtaining the reflection density of the four-color scales and color scales on the sample sheets, and then the values of the comprehensive quality evaluation can be obtained according to them and the above parameters, and then passed. Comprehensive evaluation and visual inspection will provide a comprehensive evaluation of the quality of printed products. By separately measuring the reflection density of the sample and the specific calculation and analysis of the above parameters, it is possible to control the pre-press screening, printing and equipment correction, adjust the amount of ink in printing, printing pressure and color sequence, and finally ensure the faithfulness of color reproduction. stable. In addition, these parameters are also used to evaluate the printability of inks and paper.
Third, the case analysis
1. Reflection density calculation
Table 2 is the reference value of n in the Yule-Nielsen formula introduced by the United States. Obviously, the value of n increases as the number of screen lines increases, and decreases as the gloss or smoothness of the surface of the paper increases, but is generally between 1 and 5. When n = 1, the Yule-Nielsen formula is equal to the Marri-Davis formula.
Table 2 Reference values of n in the Yule-Nielsen formula (US B.E.Tory)
The relationship between the dot density and the dot percentage calculated by taking different values in the case where the n value in the formula (1) is 1.40 in the solid density is shown in Fig. 1. As can be seen from the figure, the dot density calculated by different values of n has a certain difference. The larger the value of n is, the closer the curve is to the straight line, which means that the paper with better surface performance can obtain a uniform tone printing effect under the higher number of screen lines.
Figure 1 Curve of reflection density and dot area ratio (d=1.40)
Figure 2 Comparison between calculated and measured values of n=1 and n=1.6N
Figure 3 shows the position of the ink in the color circle diagram and its color gamut comparison
Figure 2 is a comparison curve of the reflection density calculated by different n values and the actual measured reflection density when printing coated paper with 150 lines. The dotted line in the figure indicates the case where the calculated value is equal to the measured value. Obviously, when considering the surface properties of the paper and the number of screen lines, the calculated density obtained by taking n=1.6 is closer to the measured density, and the two have better consistency.
2. Reflection density application
The results shown in Table 3 were obtained by measuring the reflection densities on the color marks of the two groups A and B, and then calculating according to the formulas (4), (5), and (6).
Table 3 Calculation results of basic properties of two sets of three-color inks
It can be seen from Table 3 that the yellow inks of Groups A and B have the highest color efficiency, and the color efficiency of Group A is higher than that of Group B as a whole; for gray scale, Group A is lower than Group B. . According to Table 3, the GATF color circle diagram of Fig. 3 is further drawn. The closed hexagon surrounded by the solid line and the broken line respectively reflects the color gamut range of the inks of the A and B groups.
It is obvious that the two hexagonal corners of the yellow area are closest to Y0 on the circumference, which means that the two sets of inks perform well and do not differ greatly from each other in the reproduction of the yellow color; in magenta and overprinted In the blue region, the performance of the ink of group A was significantly better than that of group B; from the overall point of view, the color gamut of group A was larger than that of group B. If the A group is an ideal three primary color ink, then the hue and gradation of the B group ink are obtained from this figure, and the B group ink can be evaluated. If the A and B groups are printed on different papers of the same kind of ink, it can be obtained that the paper used in the A group has better printability for color reproduction than the B group. If the A group is the proofing result and the B group is the printing result, then it can be known that the matching between the proofing proof and the printed proof has the above difference, and the printing adjustment and the proofing system calibration can be instructed. In addition, this chart can also indicate the blending color range of a set of three primary inks, and the color effect of overprinting.
3. Section
The results obtained by taking different n values in the calculation of reflection density are different. The calculation of the reflection density of the surface properties of the paper and the number of screen lines can be in good agreement with the measured values. After calculating the basic properties of the two groups of inks A and B, the GATF color circle diagram is drawn. The analysis shows that the color of Group A ink is larger than that of Group B, and the analysis results can be applied to pre-press equipment calibration, printing process adjustment, and evaluation of printed materials.
Fourth, the conclusion
Based on the significance of reflection density in the field of printing, this paper makes a comprehensive summary of the related concepts, measurement and calculation of reflection density, and the scope of application. As discussed in the Example Analysis section, the value of the parameter n in the reflection density calculation can correct the calculated value and achieve consistency with the measurement result, providing accurate data for the next application. The value of the reflection density obtained from the sample can be used to calculate the basic properties of the ink, and the GATF color circle diagram is drawn. According to the comparison of various starting points, different evaluation results are obtained. In this paper, there is no specific analysis of the influence factors of reflection density in the calculation and printing process, and the specific application of other parameters related to reflection density is not fully developed. These are worthy of further discussion.