Printing Quality Control--Objective Evaluation (3)

The color gamut of the test ink may have the following methods, each with different display characteristics.

(1) CIE Standard Chromaticity Representation: The tristimulus values ​​X, Y, and Z of the three-color inks can be measured with a spectrophotometer or a colorimeter, and the chromaticity coordinates X, Y can be converted to the ink with the relative brightness Y. Colors are punctuated in the CIE Chromaticity Diagram and can be accurately displayed in the display range, as shown in the YRMBCG polygon range in Figure 6-17.

The color properties of the ink varieties are different, and their color gamut is also different. Comparing with the color gamut of the color artwork pigment (yrambcg polygon in the figure), it can be predicted that there is a gap in the color reproduction of the original document.

When performing spectrophotometric measurements, many factors (eg, reference whiteness, spectrophotometer illumination, paper surface properties, etc.) will have an impact on the measurement results. At the same time, each punctuation location in the CIE chromaticity diagram is closely related to the thickness of the ink layer, and the spacing between the points and the points also does not correspond to the visual chromatic aberration. The brightness of the ink represented by the same punctuation can be different, and it is easy to misunderstand. The data can be color specifications, but the ink color and color differences are still inadequate. If you use the revised CIE1976 uniform color space for coloring, you can achieve satisfactory results in line with the actual visual color.

This color-coding method requires the use of a spectrophotometer or a colorimeter. Instruments and tests are relatively complex. Calculations and calibrations are also not simple. They are not widely used in actual production.

(2) Circular color gamut diagram: The circular color gamut diagram is shown in Figure 6-18. The three primary colors of yellow, magenta, cyan, and secondary colors are red, green, and blue-violet, and they are marked on the circumference according to six equal divisions. Equal amounts of two primary colors can be mixed into three secondary solid colors. The colors on the circumference are saturated colors with 100% color purity. The more the center of the circle is, the lower the color purity is along the radius. The center of the circle is black or gray, and the color purity is zero.

Using a color reflection densitometer, the solid ink density of the solid ink of the three primary colors and the density of the ink between the two primary inks were measured to obtain the maximum density H, intermediate density M, and minimum density L of the respective three-color light. Then, Calculated using the formula:

Color shift=M-L/H-L×100%
Gray ratio = L/H × 100%

The hue deviations and grayscales of the six solid colors Y, M, C, R, G, and B calculated will be measured and plotted in Figure 6-18. Each hue will be determined in terms of its direction of deviation and percentage of color cast. On the central angle, and then from the circumference along the radius to the center of the circle according to the color and percentage, you can determine the location of the six solid color points, connect the points, which constitute a standard printing area of ​​the printing ink hexagonal.

Then measure the actual print color on the ground and the two-color solid imprinting color scales, calculate their respective hue deviation rate and hair color rate, also punctuation and connect the color area on the circular color gamut map. Compared with the ink of this kind and the standard color rendering field, if the position of the primary color point is changed, it is explained that the printing operation has an unfavorable influence on the ink color. If the position of the color point changes, the overprint effect has changed. Similarly, the three primary inks of different varieties or different combinations have different color regions.

The chromaticity characteristics of the color film pigment and the density meter filter are the same. The circumference of the circle gamut can represent the standard gamut of the color film. Then, on the circular gamut chart, the specific ink color area Blank areas outside the hexagons to the circumference indicate the color of the original that cannot be recovered by the ink. These colors, which cannot be faithfully restored, are mainly to change and reproduce the color that may be close to it in terms of color and lightness, and indicate the inevitable degree of error. Therefore, a more realistic evaluation of color reproduction can also be made. If it is to approximate the main color on the original, the ink can also be selected based on the color gamut.

(3) Primary color hexagonal color gamut diagram. CATF hexagonal color gamut map does not require formula calculation, can directly use the three-color optical density of the ink to describe, can be simple and quick and preliminary evaluation of the print monochrome and overlay color quality, make timely quality and operational control.

In the colored regular hexagon, the six typical hue of primary yellow, magenta, cyan and inter-red, green, and blue violet are marked on each vertex in turn, and the two opposite colors in the center correspond to complement each other. From the center to the vertex, there is a scale line with a density increase of 0.02. The center density is zero. The higher the ink color density, the farther it is from the center, as shown in Figure 6-19.

When the primary color ink and the two-color and three-color overprint color points are plotted, their respective three-color optical densities can be directly used. For example, the optical densities of R, G, and B in yellow ink are 0.03, 0.05, and 1.03, respectively, and the gray density value 0.03 that is commonly absorbed by the three colors can be subtracted from the other two color densities. Its color cast density is 0.02, and its saturation saturation remains at 1.00. Then, along the direction of the hue axis (ie, the opposite direction of the color light), the position of the color point can be marked with a density of 0.02 and a density of the color purity at a density of 0.02. The other primary inks and stacked inks can be similarly measured and punctuated. The ink hue and chroma hexagons shown in Figure 6-19 can be connected.

Hexagonal color gamut maps can be used to quickly assess print quality and sample proofs, as well as differences in the quality of sampling and standard prints during the printing process. When the positions of the inks of the two original inks are not the same, whether they increase or decrease, it means that the ink supply, the ink balance, and the pressure have changed and need to be adjusted. If the position of the three-colored ink dot is not changed and the position of the color dot is shifted, it indicates that the ink has been badly printed.

In the hexagonal color gamut diagram, the ink brilliance value is subtracted and the ink color brightness cannot be expressed, and only the hue and purity of the primary color ink anti-overprinting ink color can be evaluated.

(4) Primary color triangle gamut diagram. GATF subtractive triangle gamut map, the ideal three primary colors yellow, magenta, blue standard in the three vertices of the equilateral triangle, the ideal three color red, green, blue and purple are located in the middle point of the three primary colors on the opposite side. Corresponding complementarity, as shown in Figure 6-20. Its coordinates are also delineated by ink hue deviation rate and gradation rate, and more description of color content can be expressed on this graph.

Using a color reflection densitometer to measure the three-color optical density of the actual printing tri-color ink standard in the field, using the color shift formula and the gray formula to calculate the percentage of color shift and the percentage of gray for each ink, in the same way as in the previous circular color gamut diagram. In the method, the three-color ink color points are marked in the three-character domain map of the primary color of FIG. 6-20, and they are connected into a triangle, such as ΔYMC. In the same way, the two-primary inks were printed and printed on the ground to print out the color deviation ratio and the gray-scale ratio of the secondary color. Three points R, G, and B were also marked in the triangle of the primary color.

According to the characteristics of three primary colors inks and superposition between two colors, several printing situations can be judged:

1 A straight line connecting two primary color ink dots represents the ideal color produced by the overprinting.
2 If the color points of the overprinting of the two primary inks are on both the line of the two primary ink dots and the vertical line of the side of the primary color triangle, the overprinting of the two color inks is very satisfactory.
3 If the overprinting color point is on the connecting line of the primary color ink dot, but not on the perpendicular line on the side of the primary color triangle, it indicates that there is a color shift in the overprinting color. Which color is biased indicates that the color ink is overprinted or the ink is poor in transparency, or that another color ink is insufficient for overprinting.
4 If the overprinting color point is outside the connection line of the two primary colors, it indicates that the overlapped glossiness has increased. If it is located within the line, it indicates that the inter-color glossiness of the overprint due to ink penetration or the like decreases.