1. Tricolor CMY curve correction
After the first printing, check the chromaticity value in the fourth column of P2P. Neutral gray may or may not do well. If it is not good, please go directly to part (2).
(1) If the gray balance has been reached
a, measure P2P
Select the prints that meet the requirements and dry them. Measure the relative density value in the fourth column of P2P. Note that at least two readings should be taken from different areas and the average value should be taken.
b. Draw the actual NPDC curve
Download free drawings on the official website of GRACoL and make an actual NPDC curve.
c. Determine the standard curve
In the NPDC fan chart, find the target chart that is closest to the actual production SID value. If not, you can use the curve board to analyze and draw from the two close curves.
d. Determine the calibration point
Check the actual curve of the institute to see where it bends the most, and then determine the point of the curve that needs to be corrected. Since the human eye is most sensitive to bright tones, it is best to set a few more points at the bright tones.
e. Correct the NPDC curve
Do the following at each calibration point (see Figure 8):
· Draw a vertical line from bottom to top to intersect the target line · Draw a horizontal line from the intersection point (left or right) to intersect the standard line · Draw a vertical line from the intersection point down to the coordinate axis To get a new target value. Record the value on the drawing. Repeat the above steps at each curve point, do not move at 0% and 100%.
As shown in Figure 8, the original new value at 50% of the dots should be 45%.
(Figure 8)
Figure 9 is a calibration legend in actual production. The green curve in the figure is the actual NPDC curve drawn in step b, and the orange curve is the standard curve determined in step c. After the calibration is completed, the new values ​​of several calibration points 20%, 50%, 75%, and 90% are 10%, 26%, 47%, and 68.5%, respectively.
(Figure 9) [next]
(2) If the gray balance is not reached
a. GrayFinder
We have arranged GrayFinder in the standard proofs (Figure 10). If the desired gray balance cannot be achieved during printing, we can use it to help complete the calibration.
Using a spectrophotometer, measure the middle of the color patch where the cyan color is 50% (actually 49.8%). It is also necessary to measure adjacent color patches and find a neutral gray value that is closest to the target (ie 0a *, -1b *).
If the middle color patch is closest to the target gray, then the device is already gray balanced (at 50% C) and no correction is required, see Figure 11 left.
(Figure 10)
(Figure 11)
If the a * b * value closest to the target is not the middle color patch, pay attention to the percentage values ​​listed next to M and Y. For example, if the best measurement is between +2 and +3 M, -3Y, then the desired good gray balance is + 2.5M and -3Y, see Figure 11 right. Repeat this step to find the actual gray balance value for 75%, 62.5%, 37.5%, 25% and 12.5% ​​color blocks.
b. Determine the NPDC curve of monochrome C, M, Y
On the CMY chart, first draw the curve of the fourth column of the P2P value, which is the C version of the curve, and then draw the monochrome M and Y version of the curve based on the percentage found on GrayFinder. The original curve (C version) To the left or right of Figure 12.
(Figure 12)
c. Determine the standard NPDC curve, correction points and draw a new NPDC curve
In the NPDC fan chart, determine the standard curve that is closest to the actual printed solid density value. Then, find the point that needs to be corrected. Correct each calibration point, see Figure 13. In the figure, the black line is the standard curve.
· Draw a vertical line from bottom to top and intersect the target line · Draw a horizontal line at the intersection point and intersect with the C, M and Y lines · Draw a number of vertical lines from top to bottom at the intersection point Intersect to get three new target values ​​of C, M and Y.
· Record the target value of CMY in the new value column of the drawing · Repeat each curve point, 0% and 100% will never change
(Figure 13) [next]
2. Correction of monochrome black version
On the black version of the special drawing, draw the value of the fifth column of P2P. Practice can refer to the NPDC correction of three-color CMY.
3. Assign values ​​to RIP
Use the NPDC correction results of the above CMY and K to assign new target values ​​to the RIP or correction equipment. Some RIP devices need to enter the "after measurement" value instead of the "required" value, and some RIP will require the delta value. In any case, remember that the new target value is the value that should be obtained for each curve point after correction.
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