Standardization of pre printing operation system a

Prepress operating system standardization and image quality control

desktop publishing system is composed of several hardware devices and application software, such as scanner, working host, display, inkjet printer, Imagesetter, scanning software, Photoshop, PageMaker, etc. Any of these hardware and software will affect the image quality. For example, if the monitor is too bright, you will think that the printed image is also bright, and the level of gradation is very rich. However, when you see the proofing, you are surprised to find that the original image is black and can't be seen. If there are some deviations in each process, the system will be a mess, and standardized operation and quality control are out of the question. The author feels in actual work that some units and individuals do not have a clear understanding of how to make good use of the prepress system and how to control the quality of the system. The author has been engaged in color work for many years and has accumulated some experience. I hope it will be helpful to readers, and I hope you can work together to improve the quality of printed matter and beautify our life

I. image scanning

scanning is the portal of desktop system. Analog pictures (such as photos, reverse films, etc.) must be scanned into digital images before entering the prepress system. Scanning equipment mainly includes platform scanner, desktop roller scanner and electric extension. Although the performance of different image input devices is different, from the perspective of use, the standardization and flow of their operation are not different. Let's take the most used platform scanner as an example to illustrate the correction of the scanner

CCD (charge coupled device), the core component of the platform scanner, mainly relies on it to convert light signals into digital signals. In addition, the quality of scanning light source, reflector and analog-to-digital converter will also affect the electronic image after scanning. Therefore, the first step of prepress system quality control is to calibrate the scanner

the principle of calibrating the scanner is to adjust the scanner to be able to faithfully copy the gradation information, color changes and gray balance of the original. The specific method is to use a special reflection or transmission color code to adjust the highlight, dark tone value and gamma value of intermediate tone in the scanning software, and if necessary, adjust the red, green, blue (or yellow, magenta, cyan, black) single channel value, so that the order, color and gray balance of the electronic image are consistent with the color code

the specific operations are as follows:

1. After the scanner starts and stabilizes for 10 minutes, place the standard color code in the scanning area, start the scanning software, and scan with the default scanning parameters of the system (taking scanning RGB as an example)

2, after scanning, use a probe to measure the color data of the gray scale in Photoshop, and adjust the highlight value and dark value in the scanning software as needed, so that the first level of the gray scale is between 250 and 255, and the dark level of 22 is between 0 and 5. For example, if the darkening value obtained after scanning is 50, it indicates that the darkening is not black enough. You can increase the darkening value from 0 to 15, so that the value of the 22 grade gray ladder after scanning can be about 5. The light adjustment method is similar. Now, readers may find that the values of the 19th to 22nd levels of the gray ladder are not much different, about 10 to 5. The dark field level is not opened, and the tone is compressed and lost. This is determined by the performance of the scanner itself, which has little impact on most originals. For originals with extremely rich dark levels, it is recommended to use a roller scanner for scanning

3, adjust the intermediate gamma so that the value of the 11th level of the gray ladder is about 125. If the value of level 11 is too small, you can increase the gamma value, and pay attention to modifying the bright and dark values to ensure that all three can meet the requirements

4, according to the relationship between the RGB values of each gray ladder, adjust the values of the three in a single channel to make the red, green and blue values of each ladder roughly equal, as shown in Figure 1. For example, if the intermediate adjustment level 11 is r=11, the tensile property of high molecular polymer is 18, g=b=124, we can increase the gamma value of the red channel, increase the intensity of the red channel, make it equal to green and blue, and ensure that the scanned image and the color separated image are not biased towards cyan

if the above steps can be completed according to the required data, it indicates that the scanner has been basically calibrated, and the adjusted scanning parameters are the effective parameters after stress deformation calibration. For most manuscripts, scanning with this parameter is OK. It can be said that the scanned electronic image can basically restore the gray component information of the original. For manuscripts with high saturation and high dark density, readers can also make adjustments based on this parameter. For example, adjust the histogram of gradient distribution after pre scanning

II. Color separation correction

the scanned RGB image can be produced and proofed only after color separation and conversion to CMYK. Color separation correction is mainly carried out in Photoshop. Photoshop software has a strong built-in color separation function and many adjustment parameters. Here we mainly talk about two problems: built-in color separation parameters and adjustment after color separation

1. Setting of color separation parameters

the setting of color separation parameters mainly includes two items, namely, printing ink setting and color separation setting

(1) printing inks setup

there is a printing inks setup dialog box under file/preference in version 4.0, which is used to set the printing ink type, gray balance data and printing midpoint expansion value at the back end, as shown in Figure 2

a. if copper paper is used for printing, ink colors can choose Toyo inks (coated), and other parameters need to be provided by the proofing company. Readers can also calculate by themselves. b. Dot gain refers to the enlargement of the dot at 50% of the middle adjustment. If it is printed on copper paper, it is recommended to take 20% to 25%, offset paper is about 30%, and paper is 35% to 40%. The larger the value is set, the smaller the CMYK data after color separation. c. Gray balance refers to the ink gray balance value of printing proofing. Each proofing company has its own gray balance data, which readers must memorize before image processing. These settings need to be adjusted when the CMYK data obtained by the gray ladder after color separation deviates from the value we want. For example, the 11th level gray ladder data after color separation are C55, M53, y46, K7, which indicates that the image after color separation is magenta, and M needs to be reduced from 1.0 to 0.95, as shown in Figure 3. d. Use dot gain for grayscale images. Click this item, and the display of grayscale images will change according to the compensation of dot gain. The larger the dot enlargement value, the darker the display. This option will also affect the converted data from other modes to gray mode. For example, when converting from RGB to gray-scale image, select the value to compensate the value of gray-scale image. The larger the point enlargement value is, the smaller the gray-scale item will be automatically called by the software. It is recommended that readers set this item as an effective state

(2) separation setup

select the separation setup item under file/preference to call up the dialog box shown in Figure 4. a. Choose the separation type there are two types of color separation, GCR and UCR. You can choose one method each time: GCR is gray component substitution, UCR is background color removal. The background color removal here is different from the previous background color removal on the electric extension, so readers are advised to use GCR. b. Determine the gradient curve of the black version. The gradient distribution of the black version of the image has a very important impact on the image. Readers should modify it on the basis of understanding the essence of the image. Specifically, when there are not many gray components in the image (such as daily landscape pictures, portraits, etc.), the black version is usually set as the middle note black version. If the image is an art photography with high saturation and high contrast, the black version can be set as the short tone, high contrast skeleton blackboard. When there are many gray components in the image (such as some ancient paintings), in order to achieve a better gray balance, we can use the long tone blackboard, or even the full tone blackboard to copy. c. Black ink limit. Different types should have different values, ranging from 70% to 90%, usually 85% to ensure sufficient contrast in the black field of the printed matter. d. Total ink limit. Represents the maximum value of the sum of four color points. For copper sheet offset printing, the value can be 340% to 380%, and the paper is less than 260%. e. Base color gain (UCA amount). This value is mainly to increase the value of cyan, magenta and yellow in the dark tone. For originals with rich dark tone color levels, you can take this value higher, such as 40%. So far, the setting of color separation parameters is basically completed

2. Color correction

images after color separation often need to be corrected to meet the requirements. There are many correction tools in Photoshop software. We usually use the following:

(1) use curves to correct the high-key and dark key values of the image. Taking into account the tone and gray balance of the whole image, curves curve is powerful, and the tone loss of the image is very small. I hope readers like it. For example, if the image is dimmed in the middle, we can use the curve to adjust

(2) use the selective color selection and correction tool to make necessary adjustments to the local color patches in the figure

for example, if we want to increase the saturation of blue blocks in the image, we can make adjustments as shown in Figure 7, which we should all check regularly. For originals with rich color levels of continuous tones (more transitions and gradients), readers are expected to use this item carefully and the adjustment amount should not be too large, otherwise there will be faults, which will destroy the continuity of tones

(3) use sharpening tools to improve the clarity of the image. There are four sharpening methods under filter/sharpen. Unsharp mask is the most powerful. Amount refers to the amount (intensity) of sharpening, radius refers to the number of pixels participating in sharpening, and threshold refers to the starting point of sharpening. Readers can modify the three parameters according to the screen display. Ensure that the printed image is clear, natural and the granularity is appropriate. Of course, readers should first ensure that their monitors focus normally

after color correction, the light and dark tones of the image should be within the normal range, and the middle tone should meet the visual needs of the human eye, faithfully restoring the gray balance. If you do these things, you can say that the drawing is finished

III Output correction

output correction mainly refers to the correction of imagesetters or color printers. The calibration of imagesetter mainly depends on the linearization of imagesetter. There are many factors that affect the linearization, such as the laser performance of the Imagesetter, the grounding voltage of the Imagesetter, film, developing solution, temperature, speed, etc. We can output 0 to 100% point ladders to see whether the points on the film correspond to the points in the electronic image. If it can be basically corresponding, it means that the imagesetter is better linearized under this condition; Otherwise, it is necessary to change the above influencing factors, and even change the linearized data in the imagesetter software until it reaches the data of the electronic image. After we control the quality of scanning, color separation and output, the quality of the prepress system is guaranteed. Operation specification is the premise of quality control. I hope you have a comprehensive understanding of the prepress system, and carefully control each step as required to achieve a high level of image quality

from printing technology by zhoubaogen