The first step of scanning and color separation be

2022-07-27
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The first step before printing: scanning and color separation

release date: Source: China paper

in the packaging and printing industry, it is often necessary to scan the pre printing data in detail. Today, China's paper industry starts with printing and scanning. Let's introduce the relevant contents in detail

1. scanning color separation

at present, there are mainly two types of scanners in practical application: flat panel scanners and roller scanners. The core component of flat-panel scanner is CCD, namely photoelectric coupler; The core component of drum scanner is PMT, namely photomultiplier tube. The scanner relies on its core components to convert the scanned image optical signal into electrical signal; Then the electrical signal is converted into digital signal through analog-to-digital converter and transmitted to the computer. Obviously, the core components of the scanner have a great impact on the results of scanning and color separation. Therefore, the quality of scanning light source, reflector and analog-to-digital converter will also affect the electronic image. Therefore, the first step of prepress system quality control is to calibrate the scanner

the calibration principle of the scanner is to adjust the scanner to be able to faithfully copy the gradation information, color change 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 the gamma value of the intermediate value in the scanning software. If necessary, adjust the r/g/b or c/m/y/k single channel value to make the gradient, color and gray balance of the electronic image consistent with the color code

2. key technology of scanning and color separation

the quality of image scanning determines the final output quality of the image. How to get the best quality image is the long-term common goal of the industry. The quality of scanning is closely related to the scanner, scanning software and the actual experience of the operator. The quality control is nothing more than the originals, scanning equipment, scanning parameter settings and scanning skills

2.1 original type and analysis

faithfully reproducing the original is the eternal pursuit of the industry. Therefore, the original is the prerequisite and foundation to ensure the quality of color reproduction

2.1.1 color reproduction requires high definition for the manuscript when the well-off society is fully completed by 2020. It is the basis for recording the price adjustment, color, texture, three-dimensional sense, perspective sense, etc

② density contrast. The density contrast of the original must be normal. Generally, the contrast density of the original is required to be between 1.8 and 4.0. This kind of original picture has rich levels of bright, middle and dark tones, and the distribution of coordination levels is normal. Good color saturation and realistic texture (if there are rich dark tone levels and few middle tone levels, you must open the dark tone level and compress the middle tone level during scanning)

③ color saturation. Check the original under the standard light source (50o0k). The color saturation is the real response to the subject. Only the original with large color saturation can be the original with normal hue

④ image granularity. The granularity of the original affects the color, level and texture. The original with fine granularity is conducive to the reproduction of layers and tones (especially the texture of bright tones) and the true performance of colors

2.1.2 originals are usually divided into transmission, reflection and secondary originals

① secondary originals: washout products; The density range is small and the level is not rich. Generally, it can only be scanned in the original large scale, and it is not suitable for amplification. Generally, the density range of high-quality secondary originals is only about 0 to 1.6. It is far lower than the density recognition ability of the scanner. Scanning such originals can be done only with a general flat-panel scanner. Secondary originals often have color deviation problems, which require proper color correction during scanning. It should be noted that the scanning of secondary originals must be processed

② reflection draft:

a. photos. The density range of high-quality photos is only o-2.2, which basically meets the pre press processing requirements and printability; It is the most common manuscript in our daily work

b. real object

③ transmission manuscript:

a. negative film. Generally, negative film is not scanned directly. It is better to develop the negative film into a photo before scanning. This is because, first, the negative film can not see the effect. Not easy to restore color; Second, the film base of the negative film has a layer of colored light film, which will cause the negative film to have no white field and difficult to calibrate

b. positive film (slide). It is developed through negative reprocessing, such as 135 slides. Its film base is not as good as the reverse film. Its density range is small (0-2.8), and the particles are rough. The scanned image is also dirty. Its magnification can only be controlled at times. However large, the image will be distorted

c. the reverse film, also known as positive negative film, is different from negative film and positive film in terms of negative film material and processing technology. Its quality is the best among the originals, with a density range of 0-3.8. It can be enlarged more than ten times during scanning. It is the most ideal original

2.1.3 analysis of originals

the operator should have certain experience and the ability to analyze the low price of originals, so that he can set the scanning parameters according to the actual situation of the originals and obtain the best scanning quality. During the actual operation, special attention should be paid to avoid automatic adjustment of any parameters, which can only make the picture quality flat and even fail to meet the printing requirements. For color biased pictures, if you have rich experience in color correction, it is better to correct them in different channels during scanning than in Photoshop after scanning.

2.2 scanning preparation

to obtain an ideal scanning image, you should do the following preparations:

① preheating. The scanner needs several minutes to warm up. Power on the scanner 30 minutes before starting the scan

② regularly clean and maintain the scanner that can be operated on the side 1 that exactly blocks the reinforcement. Regularly clean and maintain the scanner according to the requirements of the scanner manual. When the scanner is not in use, it should be covered with a dust seal. Do not let the scanner get wet

③ wash your hands before scanning, so as to avoid the pollution of fingerprints on the scanned image. Do not blow dirt and dust on the scanning platform glass with your mouth

④ clean the surface of the object or image to be scanned. Air bags can be used to blow. According to the different scanned objects, the surface can also be cleaned with water. However, it is necessary to avoid damage to the objects, especially not to leave scratches on the surface of photos and films. Pay more attention to protection when cleaning some original artworks

⑤ flatten the original. If you use software to trim the traces caused by the curling of the original. Will reduce the clarity and quality of the image. Although Photoshop has many functions (such as virtualization and sharpening) to process images, this is not the best way to solve the defects in scanned images

⑥ approval. The calibration software can adjust the image and improve the scanning quality. Most scanners come with a variety of approved options to check the light source and stepper motor. Running the calibration option once a day will stabilize the scanner

2.3 scanner input indicators and parameter settings

the difference in scanning image quality largely depends on the input indicators of different scanners (such as bit depth, resolution, etc.) and the setting of each input indicator. Practice has proved that only by correctly setting the scanning parameters of the scanner for different original types can we get the best color, level and clarity of the image and ensure high-quality color reproduction. Among several input indicators of the scanner, parameters such as bit depth, black/white field calibration and gamma mainly affect the image level, while tools such as resolution and filter mainly affect the fine level of the image. Although the menus of most scanners have different forms, the basic control parameters are the same, including black-and-white field, gamma value, gradient and analysis of originals, which are the key factors for the quality of scanning

2.3.1 resolution

resolution reflects the richness of image details recorded by the scanner, which can be divided into optical resolution (physical resolution) and interpolation resolution (maximum resolution). Optical resolution is the actual resolution of the scanner. It is the real resolution (vertical and horizontal values) obtained by the scanner during hardware scanning. It is the key factor to determine the clarity of the scanned image. The interpolation resolution is improved by software operation to strengthen the point filling. It can not add new information to the image, but it is very useful for the image with specific requirements or scanning line graph. In order to obtain proper image quality, the scanning resolution should match the number of lines added. For original size reproduction, the best scanning resolution can be determined by multiplying the number of lines added (LPI) by a quality coefficient K (1.5-2.0). For example, when the number of lines added is 175lpi; If k = 2, the scanning resolution is 350dpi

2.3.2 bit depth

description the bit depth of color information of each pixel in the scanning input image is also called sampling depth. It is generally 24 bits, that is, R, G and B each occupy 8 bits, and each has 256 gray levels; It can indicate that 2 (8+8+8) = 16.78 million colors are true colors, and professional scanners should have at least 36 bit depth. At present, Photoshop can only process 24 bit images. Images obtained by scanners with bit depth higher than 24 bits need to be converted into 24 bit images before they can be processed in Photoshop

2.3.3 select the appropriate black-and-white field

① the brightest point in the figure is the white field, which directly affects the tone level of the bright tone and the middle tone, while the human eye is extremely sensitive to the change of the bright tone. The setting of the highlight points will also affect the neutral gray balance of the color and improve the coal quality. When the white field is set too bright, it will cause the loss of some levels of the bright tone; Set too dark, the overall level of the picture is dark. For general pictures, when the fixed point of the white field is neutral gray, the CMYK value of the white field can be 25%, 0%, 0% and 0%. For the case that the white field is biased, it must depend on the specific situation, and the judgment ability can be improved through continuous experience accumulation

② the darkest point in the figure is the black field. The setting principle of black field is basically the same as that of white field. The difference is that the highlight can be determined through the eyes, while the black field of the image requires the operator to have a certain judgment. Some scanning software can automatically identify the black field, and some also provide certain tools (such as histogram adjustment) to determine the black field. If the black field is set too dark, the dark tone level will be lost, resulting in the "union" phenomenon. If the black field is set too bright, the dark tone will be too bright. For most normal pictures, the K content of CMYK in the black field shall not be less than 70% and generally not more than 80%

generally speaking, during scanning, compare the difference between black field and dark tone level, white field and bright tone level. Judge the scanning effect by comparing the values indicated by the eyedropper. Do not easily believe the color effect displayed on the screen

2.3.4 adjust gamma value

① gradient: generally, pictures are divided into bright tones, middle tones and dark tones. After setting the black-and-white field, the tone range of the whole picture is determined, but how to locate the intermediate tone within this range, that is, how to distribute the level, is determined by the gamma value. Generally speaking, the tone of a picture should be distributed in the middle tone as much as possible, because people are used to seeing pictures with rich levels of middle tone

② gamma value: for most scanners, there are their own default gamma experience values suitable for most pictures. It must be noted that the gamma experience values of different scanners are different. When adjusting the gamma curve, it has a great impact on the gradient of the whole picture. Only people with certain scanning experience can make beneficial adjustments to specific pictures through the curve, otherwise they will

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