Printing Quality Control--Color Control (5)

As already mentioned, the printability of the image is determined by the contrast of the image. In order to describe this characteristic, we classify the image according to the magnitude of contrast, set the image with very low contrast as 0 level, the image with extremely high contrast as 3 levels, and most of the colors representing the color mixing are set as 2, 1 The image contrast is less or there may be a relatively large sensitive color surface, such as flesh. The steps represented by the color balance hexagonal inner and outer rings are also related to the contrast level of the image, and a small color balance change like 1% is very significant for a 0-level image. Therefore, when printing a zero-level image, the allowable dot gain should be within the innermost hexagon. For a level 1 image, the acceptable variation should fall within the next hexagon, and so on. The importance of this method lies in the fact that for a given image to be copied, it provides an objective way of determining the quality of the print and improves the method of transferring image-copy information. Previously, the image's color balance was visually determined based on the printed gray balance block. However, in order to evaluate the correctness of print adjustment and color separation adjustment, the correctness of color balance needs to be mastered. The color balance hexagon can be displayed according to the dot coverage change. Accurate color offsets can simplify the adjustments that need to be made.

Another innovation of the expert system is to simplify the measurement technique by using a densitometer with three decimal places to the end to get all the required information from the midtone and dark tone gray balance blocks, with the traditional monochrome dot and field blocks. Comparing methods for determining color balance, image levels, and total contrast, readings that need to be measured are reduced by 2/3. The middle and dark tone grey weights are part of the control panel of the printing expert system. Other elements include: each color has 25%, 50%, 75% dot and solid blocks, overprint blocks and other necessary elements.

The printing expert system can use the adjustment map to display the relevant measurement values ​​of the image on the screen, and Fig. 5-12 is an example showing the adjustment map of the four ink areas in the horizontal direction of the paper. Each area consists of a color-balanced hexagon and a hierarchy. The window and the total contrast tolerance window are composed. The right block diagram and table are for the purpose of improving the printing effect. The values ​​recommended by the printing expert.

For example, in the left-hand color-balanced hexagon, the position of the gray squares indicates that the midtones have a color shift in the yellow-red direction, with a total offset of 2%. This can be read from the color-balanced hexagon or the characters from the lower side. The amount of change. A character indicates that the color appears to have a dot gain increase of 1%. The color balance of the dark tone is indicated by the white square. It can be seen that the color of the dark tone in the blue-violet direction has changed, but it is slight.

Above the color hexagon is the image level tolerance window. The unit is expressed as a percentage of the dot (the percentage is shown on the left side of the figure). The level of the whole image is expressed as the average value of the yellow, magenta, and blue dot increase. In the figure, a single spring is used to indicate that each single-color dot gain is indicated by a small colored rectangle symbol, and the upper and lower limits of the hierarchy window are determined according to the standards established by each company. In the illustrated example, the dot gain of the image is obviously greater than The range is high.

Near the top of the adjustment graph is the Total Contrast Tolerance window, which is the density of solids measured on the paper using the densitometer. The upper and lower limits of the window are determined by the specific process conditions. Small solid rectangles represent the solid density of each ink. In this example, the solid density of each ink did not exceed the tolerance window. Although the density of the yellow ink was already at the lower limit, it did not exceed the tolerance range.

There is no black ink reading in this example, if any, black ink dot gain and solid density can be displayed on the left side of the tolerance window. The four adjustment maps placed side by side show the complete profile of the sheet, which is very important because the color balance, the image level and the total contrast of the various areas of the image displayed on the screen during the preparation phase and during printing are Important.

In addition to the graphical display, the printing expert system also provides the operator with necessary operational recommendations. For example: This example shows various deviations in the adjustment map. In order to correct these deviations, the operator can make choices based on the expert advice displayed (Table 5-2): Reduce yellow ink, magenta ink, and even blue ink. The printing expert calculates the optimal adjustments so as to maintain the optimal color balance of the image while adjusting the gradation and total contrast towards the center of their respective tolerance window.

The bottom block diagram in Figure 5-12 shows how the dot gain should increase or decrease and the relative increase or decrease. In this example, all three colors are recommended to reduce dot gain because the dot gain of the image is high. In order to improve the color balance, yellow ink should be reduced the most, in the figure with double bars, magenta is the second, and blue is the least. Sometimes the adjustment of the display of the tone according to the adjustment may not be appropriate, and the printing expert system also allows the operator to adjust according to the tone.

Table 5-2 Expert opinions

Blue ink: Even if the dot gains are appropriate, the density in the field is low. Please check* - Is the ink low in viscosity? - Dot distortion or ghosting? Is it too thick?
Magenta ink: Even if the dot gains are appropriate, the density in the field is low. Please check* - Is the ink low in viscosity? - Dot distortion or ghosting? Is it too thick?
Huang Mo: Even if the dot gains are appropriate, the density in the field is low. Please check*—is the ink low in viscosity? - Dot distortion or ghosting? Is it too thick?
Black ink: Even if the dot gains are appropriate, the density in the field is low. Please check*—is the ink viscosity low? - Dot distortion or ghosting? Is it too thick?
The recommended data provided by this adjustment map can be used for reliable color control of several colors at the same time.

The error caused by the simple visual attempt to control the printing state.

In the case where the color balance and the image level are optimized only by adjusting the ink transfer, the data displayed on the upper right of the adjustment chart represents the deviation of the solid density values ​​of each ink area. When the deviation is greater than 0.15, it is represented in red to emphasize the printing condition. Exceed technical requirements and make necessary adjustments. In this example, all values ​​are displayed in red because the image's gradation is too high. If you simply adjust by reducing the ink, it is likely that the total contrast is too low or the field density is too low. Large deviations like this indicate Basic changes must be made, such as changing the blanket or adjusting the viscosity of the ink.

The printing expert system can provide expert advice. In order to correct a certain problem, what items should be checked, and according to the actual printing results, simply pressing a key can provide suggestions for further research.

The printing expert system can use statistical analysis to express print quality stability information. Tolerance hexagons show the color balance measurement results for each sample measurement in gray squares (Figure 5-13), and the average color deviation in the entire printing process is shown in pink squares (pink blocks are light gray in the illustration). Irregular polygons in the graph represent the 95% confidence limit for color change. To help understand the meaning of this irregular polygon, it is also possible to display a regular hexagon of the same area. The total color variation shown here is between 2% and 3% (mesh coverage), which means that although the entire printing process is red-yellow, this range of variation is difficult to replicate for a level 1 image. Is allowed.

The printing expert system can also record the ink density in the field, the dot gain and its average value and standard deviation in time sequence, which can be used in statistical process control analysis.
If necessary, 25%, 50%, and 75% of dot gains can also be displayed and analyzed.

Source: Certified Color