Test and Calibration of Printing System Regulated by GRACoL7 (II)

It can be calculated by IDEAlink curve software. Brightness contrast is independent of dynamic range. It is a constant:
HC_cmy=ND(25c, 19m, 19y)-ND(paper)=0.25HC_k=ND(25k)-ND(paper)=0.22 The introduction of these parameters brings many benefits to print quality control:

(1) The calculation methods of these parameters and dot gain rates are different. They are independent of the density of the field. Therefore, they directly reflect the light and darkness, and make the subjective and objective quality evaluation more consistent.

(2) It has nothing to do with the screening parameters, so no matter what kind of dot shape or number of screening lines are used, comparison can be made under the same standard;

(3) Due to the reduction of necessary measurement data, the efficiency of the printing press is greatly improved through the adjustment of the measurement results.

2. Preparation

(1) According to the instructions of GRACoL7, it takes about 6,000 to 10,000 sheets of paper to test and calibrate the printing press. The paper and ink must meet the standard of GRACoL7 GRACoL7 neutral printing density graph specified in ISO12647-2. .

(2) Print layout. Layout text is the main test object of this test, and its necessary layout content includes:

a. Two sets of GRACoL7P2P color marking Kunming printing, reverse 180 degrees typesetting respectively;

b. A set of GRACoL7 grey balance guides;

c. Two IT8.7/4 characterization targets, reverse 180 degree layout;

d. about 1cm wide three-color overprint gray balance color bar: 50c, 40m, 40y;

e. Monochrome black and gray balance color bar about 1cm wide: 50k;

f. Necessary printed strips and subjective evaluation images.

(3) Clear all the calibration curves on the photoprinting or CTP platesetter (but ensure that the exposure system is in a normal stable working state) and measure the dot size on the plates produced in this state.

(4) Ensure that the performance of various color measuring equipment and observation equipment is stable, and then print the document under the normal operating conditions of the printing press.

3. Printing

GRACoL recommends that the printing color sequence be k-cm-y; the solid density that should be achieved during printing is c=1.45±0.10, m=1.45±0.10, y=1.0±0.07, and k=1.65-1.9.

4. Generate a neutral print density curve

After the proofs are dried, measure the monochrome black and three-color overprint black ash scales on the P2P color scale respectively. To eliminate the error, measure the two sets of color patches patterned with 180 degrees each other, and then take the measurement results as average values. The measurement results are plotted on the GRACoL self-made coordinate graph, then the closest point to the maximum density point of the preset curve and the plotted curve and some inflection points of the curve are found. According to the mapping rules, the key points of the RIP calibration curve are found in the figure. The value is entered in the RIP calibration curve.

5. Generate gray balance curve

The gray balance curve is obtained from the gray balance guide and the GRACoL7 plot. The process is as follows: First, a spectrophotometer is used to measure the patch color value at the gray balance guide portion C50 on the printed sample sheet. If the gray level of the center patch reaches a standard (a=0,b=-2), then The gray balance of the output device is very accurate, and there is no need to establish a new gray balance curve. If the gray balance point is not a preset central color block, then the output device needs correction of the gray balance curve.
If gray balance calibration curve is needed, firstly make CMY tri-color curve on GRACoL7 graph, use reference line as basis to print material, find the difference of gray balance achieved by three colors, and then find the data of gray balance correction in RIP. , and then enter the data into the RIP. Finally, use the newly added RIP calibration curve to reprint the test target plate and check whether the corresponding parameters are up to standard.

6. Conclusion

This paper briefly describes the steps of the printing system test and calibration as specified by GRACoL7. The core mechanism of this method is to adjust the output calibration curve of CTP or CTF in order to achieve the purpose of improving the quality of the final print product; there are two prerequisites for implementing this test and calibration process. Ignoring, first, all output devices must be stable and output repeatable throughout the entire plate making and printing process. Otherwise, the parameters obtained will be meaningless to actual production. Second, the materials used must comply with relevant ISO standards. Because this is the basis for measurement, calculation, and judgment.