Process Connection and Image Quality (3)

3. Calibration of the color proofing system. Regardless of which kind of proofing system is used, it must be calibrated against the printing system. This is called the calibration of the proofing system. This calibration is required to be done by the following process. To simulate color printing, proofs can be printed on a set of acceptable color separation sheets, and the proofs can be color matched by correcting the exposure time, toner, ink, paper, and handling methods. When the calibrated proofs match the print results, the proofing system achieves an ideal calibration.
Whether the color proofing has been calibrated to the printing system can be tested by Francis L. Cox in the method reported by the Printing Technology Foundation of the United States. That is, the same density meter is used to measure the color proof proofs and printed proofs, and then the measurement results (Table 3-3) are drawn on the GATF color hexagon (Figure 3-12), and the resulting two hexagons are processed. In comparison, if they are the same, then the proofs and print presses are matched, indicating that the color proofing system has been properly calibrated.

Fig. 3-12

Table 3-3

Proofing... Printing -

R G B R G B

Yellow 0.13 0.18 1.02 0.5 0.11 0.92

Magenta 0.12 1.67 0.63 0.12 1.21 0.64

Green 1.34 0.52 0.24 1.32 1.19 1.35

Red 0.22 1.29 1.51 0.12 1.19 1.35

Green 1.36 0.58 1.12 1.32 0.57 0.94

Blue Violet 1.40 1.52 0.70 1.28 1.40 0.65

Tricolor

Post-processing of proofing. Proofing proofs may also require necessary post-processing. When the gloss of the proofing paper is different from the printed paper, it is often necessary to use specialized techniques to simulate the effect of the proofs, which is particularly suitable for packaging printed paper jams or certain newspapers, such as It may be necessary to change the gloss of the proof using a film or toner.

The increase in the number of outlets on the press must be taken into account. The dot gain on the lithographic printing machine has a greater effect on color reproduction than any other variable. If there are several printers in the same factory, this makes it difficult to calibrate the proofing system to several printers. In 1984, the Printing and Communications Association of the United States (GCA), with the help of 3M, DuPont, and Kodak, specified that when printing newsprint on a web offset printing press, the average dot gain for negative chart printing was 22%. The increase rate is 18%.

How to calibrate the color proofing to changing conditions? The answer is: Calibrate color proofing to standard printing conditions. In order to make the readings of different densitometers comparable, the Print Communications Association of the United States manufactured standard reflection densitometer color standard calibration plates to calibrate different densitometers. Using this standard plate allows the densitometer to have comparable readings, thereby improving the proofing process.

Controlling the consistency of color proofing is a very necessary step. After all the proofing methods have been calibrated, proofing can be done with a color separation sheet that has not yet been sampled. The proofing method should be used when proofing, so that every proofing done has the correct quality standard, and the proofing process must ensure that the color separation sheet used in the next working procedure is correct. This is an important duty for proofing personnel. If it is found in the printing process that the color separation sheet is not correct and the printed matter cannot be consistent with the proof sheet, then it is too late, which causes great waste. The quality-related variables of each proofing system should be mastered and controlled. Whether it is mechanical proofing or pre proofing; it is necessary to measure exposure, ink density or dot gain and control them. Exposure control devices and print control strips should be used during the proofing process to obtain more accurate results. Finally, the quality of proofing is evaluated to determine whether the proofs made are correct.

3M is equipped with a 3M Matchprint Color Element control strip on its Matchprint pre-proofing system to monitor and control pre- proof quality. The control bar contains real blocks, dot blocks, and some micro line exposure control blocks. Changes in the micro lines indicate that the dot gain has changed, but it does not indicate how much the dot gain has changed.

DuPont's Cromalin Pre-proofing System is equipped with a color proof exposure control strip called "Cromalin Offset Com Guide Eng". It contains micro-line blocks, solid blocks, and dot-blocks for measurement purposes, as well as Brunel's thick and thin line dot gain blocks, which are useful for detecting exposure and tuning values.

The "Off-Press Proof Comparator" pre-proofing control bar designed by the Print Communications Association of America and the Printing Technology Foundation of the United States is a high-quality control tool. It contains a collection of field, dot, overprint, overprint dot, and dot gain blocks, as well as visual comparison elements that can be compared to the standard. It is easy to see if a match between a color proof and a standard proof (with the same element) is made.

If you have already calibrated the color proofing to the printing conditions, then the control of the proofing is consistent, but sometimes the printing effect is always inconsistent with the effect of the color proofing. What is the reason?

The difference in surface gloss is one of the reasons. The surface of the material used for proofing is not exactly the same as the surface of the printed paper, which causes differences in the appearance of the two. For example: 3M Transfer-Key color proofing is done on plastic film, 3M Matchprint proofing is made by laminating plastic film onto paper, Cromalin pre proofing is made on very smooth special proofing paper, all these The surface of the pre-proof material is not the same as the ink printed on the paper, and their smoothness is better. On the other hand, a glossy surface gives a more saturated color, and a rough surface reduces the color density. The situation of pre-proofing and newspaper printing is extremely different. This appearance difference is the reason why many people prefer to use mechanical proofing rather than pre-proofing. Mechanical proofing and printing can use the same paper and ink, so that the difference in glossiness To a minimum.

One way to compensate for the difference in pre-dye gloss is to post-process the pre- proofed surface. Proofing Cromalin often requires a reduction in gloss, whereas a 3M proof is often covered with a film to increase gloss.

There are differences in dot gain. Another reason for the mismatch between proofing and sheet color is that the two dots have different dot gains. Most proofing systems have a certain degree of dot gain, and the increase is close to 20%, which is larger than the dot gain of some new sheetfed printers because the dot enlargement of the new printer is about 12%. The printer must increase the density of the ink in order to simulate the increase of the pre-proofing; the opposite situation is that the printer has a larger dot, the printed part is obviously deeper than the color proofing, and the printing personnel should reduce the ink density. The reduction in midtone dots produced by such corrections is not sufficient to match prints with proofs.

Difference in exposure. Differences in the amount of pre- proofing affect the dot gain of the color proof, so the exposure of the pre-proofing must be controlled, and the exposure of a batch of new proofing materials may be slightly different, if a new batch of material is overexposed, proofing The increase in the number of outlets will change.

Some pre-proof materials tend to have metamerism. When viewed under different light sources, the color changes, and attention must be paid to the observation of color proofs. Specimen proofs should be placed side-by-side with the printed sheet under a standard light source with a color temperature of 5000K. Observing with other light sources will make the proof sheet look the same as the printed sample, but it is different under standard lighting.

A workshop that strictly manages the quality of proofing should follow the standardized operations described below. After a period of time, a set of standard color separations should be used to make a color proof. Compare this experimental proof to the previous proof of the experiment to see how the color consistency is. If deviations occur, use proofing control. Check for any problems.

Color proofing is a high-quality printing control method and is an effective measure to save raw material consumption and man-hour consumption. In order to increase the productivity of proofing, it is important to understand what kind of proofing is used. The use of soft proofing, digital hard proofing. Pre-proofing, or mechanical proofing? Each proofing meets certain proofing needs. Soft proofing is currently less used for "contract proofing," but it also requires efforts to improve quality. In the past, it was considered that pre-proofing was not good enough, but now, some foreign users prefer to use pre-proofing instead of mechanical proofing. They believe that pre-proofing is easier to control consistency than mechanical proofing. Basically, domestic use of mechanical proofing.

Quality Control "print quality inspection and quality control" · Chapter III of replication process: Author: Shichang source