(This article is an original article of Bisheng.com and it is forbidden to reprint, and reprinting will be investigated!)
I can always hear from some friends who do printing. When printing, you can often see some small colored blocks on the edge of the printing (outside of the cutting line), except for the most common ink blocks or ink plates that explain the color of the printing plate. It ’s outside the color scale, and I do n’t know what it ’s for, so what do these seemingly useless things that will eventually be cut off do?
The objective criteria for evaluating the print quality of color images are mainly tone reproduction and color reproduction of the printed matter. Because the images are different, it is very difficult to directly detect a certain quality item on the printed sheet and then control the printing quality. Therefore, select several main quality assessment indicators, design some standard images, and combine each standard image in different ways to form a printing quality control strip. The printing quality control strip mentioned here is what we mentioned as "useless".
The standard image that constitutes the printing quality control bar can be the original image pixels of the image, such as solid blocks and screen lines; it can also be standard pixels that are not in the printed image, such as stars and line blocks that check the deformation of the dots. Quality control strips are generally divided into signal strips and test strips. Signal strips are used for visual inspection and provide qualitative information. The test strip can measure density data with a density meter. At present, most printing quality control strips are composed of signal strips and test strips, which can detect the deformation of dots, and cooperate with a density meter to check the solid density, dot increase, relative contrast, and ink overprint rate of various printing ink And other major quality indicators.
GATF
GATF star.
The GATF star is shown in the figure, which is composed of 36 black wedge-shaped lines arranged in a circle at equal intervals. Due to the geometric characteristics of this arrangement, it can clearly show various imprint defects on the printed sheet, such as "ghosting" and "dot increase". When "ghosting" or "dot increase" occurs, the wedge line will behave as shown in the figure below, producing an equal amount of expansion width. Just like the dot expansion value of different sizes, the wedge tip is the smallest dot and the wedge tail is the largest dot , The same amount of expansion or contraction will be clearly displayed in the center of the circle where the wedge tip is relatively concentrated.
If the ink supply is too large, the printing liner is not suitable, or the speed difference between the cylinders occurs during the printing process, such as "ghosting", "pasting", "dot deformation" ... The dot diameter increases by 0.025mm , A diameter reduction of 0.575 mm will occur in the middle of the circle, which is about a 23-fold change (that is, a 23-fold enlargement). This magnifying effect of the star can help the printing operator find the problem in time. For several faults as shown in the figure below, the stars have obvious changes.
Usually the dots on the printed sheet are not deformed or ghosted. When the ink supply of the printing plate is appropriate, the center of the star is white. If the ink supply is too large, the center of the star is a large black circle. The larger the black circle, the greater the ink supply and the more serious the dot increase. If the ink supply of the printing plate is two small, there will be a relatively large white circle in the center. If the dots are deformed longitudinally, then the star is deformed laterally. Conversely, if the dots are deformed longitudinally, the dots are deformed horizontally. If there is a ghost image in the dots, the middle part of the star disappears, and the rest of the outline is a GATF star. Although it is a high-precision star-shaped round mark, after repeated copying, the accuracy of the original star is difficult to guarantee. The effect is not very good when using the star, and it is often used together with the GATF digital strip.
GATF digital signal bar. The GATF digital signal bar is shown in the figure. The left side is marked with numbers from 0 to 9, each number is a step. The number of dots for each level of 1 ～ 7 digits increases to 3% ～ 5%, and the number of dots for 7-9 digits increases for each step to more than 5%. The digital color gradually fades from 0 to 9. The dots on the number are equivalent to 80 lines / cm, and the substrate portion is equivalent to 25 lines / cm. The increase in fine dots is more sensitive than the coarse dots to determine the dot increase value.
When printing, take a certain number as the standard, observe the depth of the color display around the standard number to adjust the printing conditions, and control the printing quality. For example, a factory stipulates "3" as the standard digital according to the printing process conditions. In the actual printing process, when printing is normal, as shown in the figure, "3" indicates that it is not clear. The figure shows that the dots increase, and the No. 4 becomes deeper. Figure C shows that the dots have shrunk and the number 2 or even 1 has become shallower. According to the step that the number becomes darker or lighter, the dot expansion value can also be roughly estimated. When the dots on the printed matter are deformed, the density of the horizontal and vertical lines on the signal bar will be different. "SLUR" is darker than the background color, indicating that the dots are deformed longitudinally. .
Test strip.
GATF stars and GATF digital signal bars can only provide people with qualitative quality information, but cannot provide people with quantitative data. In order to assess the amount of network expansion or contraction, relative contrast, etc. A density meter is required to accurately measure the density of the ground and the dot density, so there are test strips composed of the field test block and the dot test block to control the printing quality.
For field test blocks, generally round dots are used. There are 20%, 25%, 40%, 75%, 80% and several other dot tone blocks. Connect the solid block and the selected dot tone block together, and measure the solid density and dot density with a reflection densitometer to calculate the dot expansion value and relative contrast in printing.
In addition, there are multi-color overprinted solid plots for measuring ink overprint rate and gray-balanced color scales.
Let's introduce the GATF color signal bar. The GATF color signal bar is 0.25 inches wide and 20 inches long. Every 5 inches is a unit. Each unit consists of 25 1/4 inch × 3/16 inch signals. Block composition. Each signal and test block includes the following:
Blocks 1 to 7: solid blocks with monochromatic or two-color overprint;
Block 8: Three-color overprinted solid blocks;
The ninth block: the yellow and magenta color printing blocks of 40% network cable;
The 10th block: 40%, 30% magenta and yellow gray color blocks;
Block 11: Black block of 40% network cable;
Block 12: A gray block made of 40% cyan, 30% yellow, and 20% magenta;
Block 13: Black solid block;
Blocks 14-19: solid and 40% dot color blocks for each color of cyan, yellow, and magenta;
Blocks 20-22: Overprinted color blocks of 40% dots in two colors;
Blocks 23 to 26: GATF stars.
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