The purpose of controlling color is to obtain products with no deviation in hue and saturation throughout the printing process. In order to maintain the consistency of product quality, it should be effective to control the consistency of field density, but this method is only a method of early color reproduction with sheet-fed printing presses, which is wrong for four-color printing presses. . In fact, on the contrary, the operators of four-color printing machines tend to control the hue, saturation of colors, and compensate for many variables in the printing process by changing the solid density.
There are many variables that affect the quality of printing during the operation of the printing press, and more than 100 printing parameters are taken into consideration. In order to properly control a printing press, each printing unit can set adjustment knobs for dot increase, dot deformation, ghosting, overprinting, and other important variables, so that the console of the printing press can be easily viewed and adjusted like the control panel of an aircraft. . In fact, each printing unit has only two main controls, one for controlling ink delivery and one for controlling water delivery.
At present, the method for correct reproduction of printed colors is to follow the following sequence:
â‘ Control the printing conditions, determine the correct printing color sequence, and optimize the solid density of the printing in order to achieve the maximum printing contrast; when the amount of ink conflicts with the increase of dots and overprinting, the ink should be adjusted while considering the compromise The amount makes the overprint color balance.
â‘¡ Calibration color proofing system: Use the visual observation method to calibrate the color proofing so that it can consistently simulate the printing control conditions already determined above.
â‘¢ Color proofing for a set of color separation film: use the proofing method and control means after calibration to make correct and consistent proofs.
â‘£Determine standard proofing proofs: select proofing proofs with visual appearance that meets the expected requirements as standard proofs.
⑤ Adjust the four-color offset printing machine: the screen of the overprint screen produced on the normal printing machine should be visually consistent with the hue and saturation of the standard proofing proofs.
⑤In order to obtain a visually consistent product, adjust the solid density during operation: even if dots increase, overprint, dot distortion or ghosting may occur, the hue and color saturation of each print should be the same. The appearance of each printed matter should be visually the same as the standard printed proofs.
3. The importance of color proofing
In order to make the printing effect and the proofing visually consistent, the proofing method should be calibrated to the printing machine in the printing production, and then the printing machine is adjusted to match the printing effect with the proofing proof. This gives the reader the impression that the attempt to control color is like we are chasing our own tail around the circle, but practice tells us that this method is successful.
Califrid once pointed out in 1983: "For a color, we can use language to describe it and measure it to quantify it, but we must also use a sample as a reference to describe the color characteristics." In practice, Color proofing is a contract between the customer and the printing house. It is a visual expression of the customer's product requirements and the printing plant's promise of quality. The effectiveness of using color proofing as a benchmark to control printing colors has been proven in practice. It is an actual sample that does not require verbal description and an effective communication tool that effectively conveys color information from this person to that person and can actually be implemented.
In order for a color proof to be practical, it must match the printed product and the quality must be stable. Color proof is an important means of color communication in the printing industry.
In fact, it can be seen that good prints are sometimes heavier in density and sometimes lighter in density; bad prints are usually due to the increase in dots. The direct reason for the increase in dots is that excessive ink is received on the paper, and the The result is that the printed sheet appears too full or produces a wrong hue. Practice has concluded that there must be more important variables than density on the ground.
With the popularization of four-color printing presses, density meters have been widely used, but printing press operators always have to visually keep the printing effect consistent with the standard proofing. They adjust the printing press just like adjusting the color TV to make the printed sheet reach Color balance. This technique has attracted the attention of printing researchers. Why the eyes of the printing operator can beat the density meter? To answer this question, one must observe how a printing machine operator works.
Lexmark Moller used different magenta and yellow solid ink densities for printing experiments, and adjusted the tone reproduction of the dichroic film in order to obtain the desired printing effect. The experiment found that matching color is more important than matching ink volume. Because the prints with good reproduction effect are obtained with a small amount of ink supply, it shows that printing with high ink volume is not beneficial at all.
Other studies have also pointed out that print quality control personnel pay more attention to changes in hue, rather than changes in solid density. When controlling the color of the printed image, it is more effective to combine the measurement data of the density meter with the visual evaluation of the printing personnel. Studies have also found that the relationship between field density and the hue change of the mesh tone image is not so great. In the printing process, in order to obtain uniform and stable printing quality, the magenta density can even change as large as 0.20.
The famous printing expert Felix Brunner pointed out: The effect of the increase in dot value on color reproduction can be much greater than the density on the ground. He believes that the increase in network points is much more important than the density of the field control data.
The American Press and Communications Association once did an experiment: 16 web offset presses, each of which printed a set of test plates, and also printed an advertising image under the same printing conditions as the test plates. The printing conditions should be the same as The actual production conditions are usually the same, and the printing staff did not use the proof of the trial version to match. The printing results were analyzed with colored hexagons, and the analysis results concluded that each operator produced similar overprint colors through overprinting, dot gain, and other printing parameters. This fact makes people believe that in order to obtain the correct hue and color saturation, the control of the tone of the overlay screen is the most important control target.
Print image reproduction is important for color matching, not ink volume matching, and hue matching is more important than color saturation matching. This is the theoretical basis for the transition from density measurement and control to chromaticity measurement and control.
V. Application of density meter in color control
In order to make the printed matter produce a stable and consistent hue, the printer operator seems to adjust the inking to compensate for the changes in printing parameters. In the middle of printing, when the dots increase and change, the overprint color will change significantly. The operator of the printing press will maintain or reconstruct the matching relationship between the printed matter and the standard proof by adjusting the amount of ink. The amount of ink printed on the printed sheet will affect the amount of dot increase. Conversely, the amount of dot increase can also be controlled by changing the solid density.
Automatic adjustment can obtain better process control. The printing press is controlled by the amount of inking. Based on this premise, quantify the expected mesh tone value (in the image, the observer sees the hue and color saturation), and measure the mesh tone density (not the solid density) It is possible. Due to the synthesis of colors, the observer sees the red, green, and blue-violet light reflected by the printed sheet, which reaches the eye to form a comprehensive color vision. Changing the dot increase amount and ink overprinting has a significant effect on the printing color effect. The amount of red, green, and blue-violet light that forms the printing color can be displayed on the fluorescent screen and compared with the standard printing sample, so that it is possible to control the printing color consistency.
The densitometer can effectively measure the reflectance of red, green, and blue-violet light on a given surface. Therefore, a new type of density meter can be used to measure the color adjustment surface specified on the color proofing or standard printing sheet, and use the measured value as the control value or target value when the surface is printed. When the printed matter passes through the printing machine, the corresponding part on the printed sheet is measured, and the measured value is compared with the target value, and the automatic control of the printing quality can be realized.
The density meter is to adjust the density on the standard printed sheet, then measure the density on the production printed sheet, and then compare it with the density reading of the same part on the standard printed sheet. The measured value can indicate whether the contents of yellow, magenta and cyan are equal. If the value of the production sheet deviates from the zero value, this indicates that the printed image no longer matches the standard sheet, and it may have to be corrected. The three density readings will show the necessary correction function. The densitometer reading does not indicate a change in printing conditions, but indicates a change in the thickness of the ink layer. Compensating for changes in printing conditions will return the measurement surface to the balance of red, green, and blue-violet colors.
The production sheet may have the correct hue without the correct saturation, in which case all three density readings will be wrong. According to the size and balance of the reading, the necessary correction can be pointed out. Maintaining color balance is more important than maintaining correct color saturation.
What is not known at this moment is the correct amount of changes in qualified hue and color saturation. If these amounts are determined, algorithms, programming, and adding them to the system can be determined. Past experience has pointed out that it is more accurate if the density meter is zeroed on paper instead of zeroing on pre-proofing, which needs to be determined by experiment.
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