Discussion on Quality Control Technology of Printing Image

The desktop publishing system consists of a number of hardware devices and application software such as scanners, work hosts, monitors, inkjet printers, imagesetters, scanning software, Photoshop, Pagemaker, and others. Any of these hardware and software will have an effect on image quality. For example, if the display is too bright, you will think that the printed image is brighter and the tone level is very rich. However, when you saw proofing, you were surprised to find that the original image was black and could not be read. If there is some deviation in each process, then the system will be a mess, and the standardized operation and quality control will be impossible to talk about. In my practical work, the author feels that there are indeed some units and individuals who do not have a clear understanding of how to make good use of the prepress system and how to control the system quality. The author has been engaged in color work for many years and has accumulated some experience in the hope of helping readers. He also hopes that everyone will work together to improve the quality of printed materials and beautify our lives.

First, image scanning

Scanning is a process in which the desktop system's portal analog images (photos, reversals, etc.) must be scanned into digital images before entering the prepress system. Scanning devices mainly include flatbed scanners, desktop roller scanners and electric extensions. Although the performance of different image input devices is not the same, from the point of view of use, the standardization and flow of operations are not much different. Here we take the most used platform scanner as an example to illustrate the scanner's correction.

The core part type CCD (Charge Coupled Device) of the flatbed scanner mainly relies on the conversion of light signals into digital signals. In addition, the quality of the scanning light source, mirrors, and analog-to-digital converters can also affect the scanned electronic image. Therefore, the first step in quality control of the prepress system is to calibrate the scanner.

The principle of calibrating the scanner is to adjust the scanner to be able to faithfully copy the original's tone level information, color change, and gray balance. The specific method is to use dedicated reflection or transmission color scales to adjust the highlight, dark tone, and midtone gamma values ​​in the scanning software. If necessary, adjust the red, green, and blue (or yellow, magenta, cyan, and black) single channel values. , so that the electronic image of the tone, color and gray balance with the color standard.

The specific operation is as follows:

1. After the scanner has been turned on for 10 minutes, place the standard color mark in the scanning area, start the scanning software, and scan with the system's default scanning parameters (scanning RGB as an example).

2, After the scan is complete, measure the color data of the gray scale with a probe in Photoshop, and adjust the highlight value and the dark tone value in the scan software as required so that the first level of the gray scale is between 250 and 255. , Darkness 22 is between 0 and 5. For example, if the dark tone value obtained after the scan is 50, indicating that the dark tone is not black enough, the dark tone value may be increased from the original value of 0 to 15, so that the value of the 22-step gray scale may be about 5 after the scan. The lighting adjustment method is similar. Now, readers may find that there is not much difference between the 19th and 22nd levels of the gray ladder ruler, which is roughly 10 to 5, and the dark field level is not pulled apart. The tone is compressed and lost. This is determined by the performance of the scanner itself, and has little effect on most manuscripts. For originals with extremely rich dark levels, it is recommended to use a roller scanner.

3, adjust the midtone Gamma, so that the gray scale 11th level value is about 125. If the 11-level value is too small, you can increase the Gamma value, and pay attention to modify the brightness and dark tone values ​​to ensure that all three can meet the requirements.

4, according to the size of the relationship between the level of each level grayscale feet RGB, single-channel adjustment of the three values, so that each ladder red, green and blue values ​​are roughly equal, as shown in Figure 1. For example, if the intermediate tone 11 is R=118, G=B=124, we can increase the gamma value of the red channel, increase the intensity of the red channel, and make it equal to the green and blue, to ensure the scanned image and score. The colored images are not blue.

If you can do the above steps according to the required data, it means that the scanner has been basically calibrated, and the adjusted scanning parameters are valid parameters after calibration. For most manuscripts, scanning with this parameter is sufficient. It can be said that the scanned electronic image basically restores the gray component information of the original document. For manuscripts with higher saturation and darker density, readers can also make adjustments based on this parameter. For example, pre-scan tone distribution histogram adjustment and the like are used.

Second, color separation correction

The scanned RGB image must be converted to CMYK after color separation before the film can be produced and proofed. Color separation correction is mainly carried out in Photoshop, Photoshop software has a strong color separation function, adjust the parameters more, here mainly talk about two issues: built-in color separation parameters and color adjustment.
1. Setting of color separation parameters

The setting of color separation parameters mainly includes two items, that is, printing ink setting and color separation setting.

(1) Printing Inks Setup

In the 4.0 version of File/Preference, there is the Printing Inks Setup dialog box. This item is used to set the printing ink type, gray balance data of the back-end, and the dot expansion value in the printing, as shown in Figure 2.

a. If you use copperplate paper printing, Ink Colors can use Toyo Inks (Coated), other parameters need to be provided by the proofing company, readers can also calculate their own. b. Dot Gain refers to the enlargement of outlets at 50% in the middle tone. If printed on copperplate paper, it is recommended to take 20% to 25%, offset paper to be 30%, and newsprint 35% to 40%. The larger the value is set, the smaller the CMYK data after color separation. c. Gray Balance refers to the value of ink gray balance for printing proofing. Each proofing company has its own gray balance data, which readers must memorize before proceeding with image processing. When the CMYK data obtained after separation of the gray ladder rule deviates from the desired value, these settings need to be adjusted. For example, after the separation of the 11th level of the ash ladder data for the C55, M53, Y46, K7, it shows that after the color separation image magenta, M need to be down, can be reduced from 1.0 to 0.95, as shown in Figure 3. d. Use Dot Gain for Grayscale Images for grayscale images. With the item in the dot, the grayscale image display will change according to dot gain compensation. The larger the dot gain value, the darker the display. This option will also affect conversion data from other modes to grayscale mode. For example, when converting from RGB to grayscale, the selected value is used to compensate the value of the grayscale. The larger the dot enlargement value, the grayscale, the smaller the software will automatically call the dot enlargement data, the reader is recommended to set the value as valid. .

(2) Separation Setup

Select the Separation Setup item under File/Preference to bring up the dialog box shown in Figure 4. a. Select Separation Type There are two types of Separation Types, GCR and UCR. You can choose one method at a time: GCR is a gray component substitution, and UCR is a background color removal. The background color removal here is different from the previous background color removal on the extension, so the reader is advised to use the GCR method. b. Determining the black version of the black version The distribution of the gradation of the black version of the image has a very important influence on the image, and the reader must modify it based on the essence of the image. Specifically, when the gray content in the image is not very large (such as the daily-used landscape pictures, portraits, etc.), the black edition is usually set as the medium tone black edition. If the image is a high-saturation, high-contrast art photography, it can be The black version is set as a short-cut, high-contrast skeleton blackboard. When there are many gray components in the image (such as some ancient paintings), in order to achieve better gray balance, we can use long blackboards, or even full blackboard copy. c. Black Ink Limit. Different types should have different values, from 70% to 90%, usually 85% to ensure that there is enough contrast in the black field of printed matter. d. Tolol ink limit. Indicates the maximum value of the sum of four-color dots. For offset printing, the value may be 340% to 380%, and newsprint may be less than 260%. e. UCA Amount. This value is mainly to increase the values ​​of blue, yellow, and yellow in the dark tone. For darker originals with rich color hierarchies, the value can be larger, for example, 40% can be taken. At this point, the setting of separation parameters is basically completed.

2. Color correction

Separated images often require a certain amount of correction to meet the requirements. There are many correction tools in Photoshop software, we usually use the following:

(1) Use the Curves curve to correct the high and low tone values ​​of the image. Taking into account the tone and gray balance of the entire image, the Curves curve is powerful, and the loss of the tone of the image is very small. I hope readers like it. For example, the midtones of the image are dark and we can use curves to adjust.

(2) Use Selective Color to select the color correction tool to make necessary adjustments to the local color patches in the figure.

For example, we want to increase the saturation of the blue block in the image, which can be adjusted as shown in Figure 7. For continuous manuscripts with rich color hierarchies (more transitional gradients), readers are expected to use this item with caution. The amount of adjustment should not be too large. Otherwise, faults will occur and the continuity of the tone will be undermined.

(3) Use sharpening tools to improve the sharpness of the image. There are 4 sharpening methods under Filter/Sharpen. Among them, Unsharp Mask has the strongest function. Amount represents the amount of sharpening (intensity), Radius is the number of pixels that participate in sharpening, and Threshold represents the starting point of sharpening. The reader can modify the 3 parameters according to the screen display. Ensure that printed images are clear, natural, and properly grained. Of course, the reader must first ensure that his monitor is focused properly.

The school's finished image should be bright and dark in the normal range, and the middle tone should meet the visual needs of the human eye, faithfully restore the gray balance. By doing this, you can say that the map is done.

Three. Output correction

The output correction mainly refers to the correction of photosetters or color printers. The adjustment of the image-setter is mainly to look at the linearization of the image-setter. There are many factors that affect linearization, such as the imager's laser performance, the image-setter's ground voltage, film, processing solution, temperature, and speed. We can output a 0 to 100% dot scale to see if the number of dots on the film corresponds to the number of dots in the electronic image. If there is a basic correspondence, the linearization of the image-setter is better under this condition; otherwise, the influencing factors mentioned above will be changed, and even the linearized data in the image-setter software will be changed until the electronic image data is reached. After we control the quality of scanning, color separation, and output, the quality of the prepress system is guaranteed. Operational norms are the prerequisite for quality control. I hope everyone will have a comprehensive understanding of the prepress system and carefully control each step as required to achieve a high level of image quality. (Text / Zhou Baogen)