Printing image color control method

Color control is a topic that is often involved in print image replication technology. The accepted fact is that color proofing is used as a quality control tool to obtain acceptable standard printed proofs, and then the printer is controlled to produce a stable hue. Although the printing parameters may change significantly, as long as the printer can be controlled, the density of the field can be changed to make the hue of the printed image stable. The control of the printing press can be performed by using a density meter rationally. Although the density meter cannot measure the color, But it can be used to control the stability of color.

First, the method of controlling color

The purpose of color control is to obtain products with no deviation in hue and saturation throughout the print production process. In order to maintain the consistency of product quality, consistency of control of density in the field should be effective, but this method is only a method used in the early color reproduction using a sheet-fed printing press and is wrong for four-color printing presses. . In fact, on the contrary, operators of four-color printing presses tend to change the density of the ground to control the printer's hue, color saturation, and compensate for many variables in the printing process.

There are many variables affecting the quality of printed products 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 gain, dot distortion, ghosting, overprinting, and other important variables, so that the console of the printing press can be viewed and adjusted as easily as the control panel of the aircraft. . In fact, there are only two main controls for each printing unit, one for controlling inking and one for controlling water delivery.

Second, the steps to control color

At present, in order to make the correct reproduction of printing colors, the following sequence is followed:

1Control the printing conditions, determine the correct printing color sequence, and optimize the density of printing on the spot so as to achieve the maximum printing contrast. When the amount of ink affects the dot gain and the overprint, the ink should be adjusted at the same time. The amount balances the overprint color.

2 Calibrate the color proofing system: Use visual observation to calibrate the color proofing so that it can consistently simulate the above-defined print control conditions.

3 Make a color proof for a set of color separations: use the calibration proofing methods and control methods to make correct and consistent proofs.
4 Determine the standard proofs: Select proofs that meet the expected visual appearance as standard proofs.

5 Adjusting the four-color offset press: The superimposed halftone screen produced on a normal printing press should be visually consistent with the hue and saturation of the standard proofs.

5 In order to obtain a visually consistent product, adjust the density of the field during operation: Even if the dots increase, overprint, dot distortion or ghosting may occur, the hue and color saturation of each print should also be the same. The appearance of each print should be visually the same as the standard printed proofs.

Third, the importance of color proofing

In order to make the printing effect consistent with the proofing visually, the proofing method should be calibrated on the printing press during printing production, and then the printing presses should be adjusted to match the printing effect with the proofs. This gives the reader an impression that the steps to control color are like we are chasing our own tails around a circle, but practice tells us that this method is successful.

Califride once pointed out in 1983: “For a color, we can describe it in terms of language, quantify it with measurement methods, 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 representation of the customer's product requirements and the printer's commitment to quality. The effectiveness of using color proofing as a benchmark to control printing colors has been proved by practice. It is an actual sample that does not require verbal descriptions, and an effective communication tool that effectively conveys color information from this person to that person and can actually implement it.

In order for a color proof to be practically available, it must match the printed product, the quality must be stable, and color proofing is an important means of color communication in the printing industry.

Fourth, the main control parameters

In practice, it can be seen that good prints are sometimes denser and sometimes lighter in density; poor prints usually have too large dot gains, and the direct cause of dot gain is that the paper receives an excessive amount of ink, and the dot gain is increased. The result is that the printed sheets are too full or have the wrong hue. Practice has concluded that there must be a more important variable than the density of the field.

With the popularity of four-color printing presses, densitometers have been widely used, but press operators always have to visually match the printing results with standard proofing. They adjust the presses just as they do with color televisions. Color balance. This technique caught the attention of the printing researcher and why the operator of the printing operator could outcompete the densitometer. To answer this question one must observe how a press operator works.

Lee De Moller used different magenta and yellow solid ink densities for printing experiments to adjust the tone reproduction of the color separation sheet in order to obtain the desired printing effect. Experiments have found that matching colors are more important than matching ink volumes. Since good copying results are obtained with a smaller amount of ink supply, this means that printing with high ink volumes has no benefit at all.

Other studies have also pointed out that the print quality control personnel pay more attention to changes in hue, rather than more emphasis on changes in the density of the field. When controlling the color of the printed image, it is more effective to combine the measurement data of the densitometer with the visual evaluation of the printer. Studies have also found that the relationship between field density and hue changes in halftone images is not that large. In the printing process, the density of magenta can even be changed by as much as 0.20 in order to obtain a uniform and stable print quality.

The famous printing expert Felix Bruner pointed out that the effect of changes in dot gain value on color reproduction can be much greater than the density in the field. He believes that dot gains are much more important control data than the density of the field.

The Printing and Communications Association of the United States once conducted an experiment: 16 web offset presses, each printing a set of test plates, and printing an advertising image under the same printing conditions as the experimental version. The printing conditions should be followed. As usual, the actual production conditions, the printing staff did not match with the trial version of the proof. The print results were analyzed using color hexagons. The analysis results showed that each operator produced a similar overprint color through overprinting, dot gain, and other printing parameters. This fact makes people believe that in order to get the correct hue and color saturation, the control of the superimposition mesh is the most important control target.

It is important to match the color of the printed image, not the amount of ink, and the hue match is more important than the color saturation match. This is the theoretical basis for the transition from density measurement and control to color measurement and control.

Fifth, the application of density meter in color control

In order for the prints to produce a consistent hue, the press operator appears to compensate for changes in printing parameters by adjusting the ink transfer. In the middle of the printing, when the dots increase and change, the overprint color will change significantly. The press operator will maintain or reconstruct the matching relationship between the print 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 gain. Conversely, the dot gain can also be controlled by changing the density on the ground.

Automatic adjustment can get better process control. The press is controlled by the amount of ink delivered. Based on this premise, the expected halftone value is quantified (in the image, the observer sees the hue and color saturation) and the mesh density is measured (rather than the density in the field). It is possible. Due to the color synthesis, the viewer sees the red, green, and blue light reflected by the printed sheets, and these lights reach the eyes to form a comprehensive color vision. Changing the dot gain and ink overprinting have a significant effect on the effect of printing color. The amount of red, green and blue violet that forms the printing color can be displayed on the fluorescent screen and compared with the standard printing sample, thus it is possible to control the printing color. consistency.

Densitometers can effectively measure the reflectance of red, green and blue violet light on a given surface. Therefore, a new type of densitometer can be used to measure the color tone or the specified mesh surface on the standard sheet, and the measured value is used as the control value or target value when the surface is printed. When the printed matter passes through the printing press, the corresponding part on the printed sheet is measured, and the measured value is compared with the target value to realize the automatic control of the quality of the printed matter.

Densitometers are zero-adjusted on a standard sheet and then measure the density on the production sheet and compare it with the densitometric reading on the same part of the standard 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 zero, this indicates that the printed image no longer matches the standard sheet and may have to be corrected. The three density readings will show the necessary corrections. Densitometer readings do not indicate a change in printing conditions, but indicate a change in the thickness of the ink layer. Compensation for changes in printing conditions will return the measurement surface back to the red, green, and blue color balance.

There may be a correct hue on the printed sheet without the correct saturation, in which case all three density readings will be wrong. Depending on the size and balance of the readings, the necessary corrections can be indicated. It is more important to maintain the color balance than to maintain the correct color saturation.

What is not known at this moment is the correct amount of qualified hue and color saturation changes. If these quantities are determined, algorithms can be determined, programs can be created and added to the system. Past experience has shown that if the densitometer is zeroing on paper rather than zeroing on pre- proofing, it will be more accurate, which needs to be determined experimentally.