Discussion on Printing Color Model Used in Printing, Dyeing and Separation

For a long time, the color separation method used in printing and dyeing is different. From the process point of view, printing is to combine the four colors of cyan (C), magenta (M), yellow (Y), and black (K) into one color picture; Represents the color. Because printing and printing and dyeing processes have many things in common, and the extensive use of pre-press processing systems, people try to use computer color separation and plate-making process to find different ways to apply four-color printing method to printing and dyeing and plate-making.

Printing and dyeing products give us the impression that there are too many mud spots on the printed product, the picture is too dark, and the color is distorted. The main reason is that the printing process and parameters are directly applied, causing the actual layout of the Internet site to expand too much. In this regard, I would like to know some experiments of printing methods for printing and dyeing from the following three aspects.

The computer is used for color separation of printing plate making. The processing of the image mainly uses Photoshop image color separation software. It needs to change the PrintingInksSetup in the File / Preferences item in its work menu for different printing inks, printing media, and printing processes; The two sub-items, which cannot be done with Photoshop image software, are completed by special items in the RIP interpreter. The implementation plan is as follows:

First, the ink settings. Change the color tone of the image produced on the screen through the setting of printing ink to keep it as consistent as possible with the sample printed by the dye,

Second, the color separation settings, by adjusting the curve of the image RGB color mode to CMYK color mode, to meet the printing and dyeing process requirements.

Third, compensation in RIP. The main compensation is established because the settings in Photoshop cannot meet the requirements of the printing and dyeing process. [next]

1 Printing ink settings

The printing ink setting is set for the medium (satin) and dye of the printed product, so that the color tone seen on the screen and the sample are consistent.

We understand the requirements for printing and dyeing from the following parameters:

1.1 Ink setting
Swop (coated) is usually used for printing. This is a standard printing method for offset web printing in the United States. The chromaticity coordinates and brightness factor of the ink can be easily obtained from related materials. From the chromaticity coordinates, the x and y coordinate values ​​of the color can be found, and the brightness factor Y is measured upward at the white point.

If the printed ink CMYK chromaticity coordinates and brightness factors are different from the CMYK value of the printing and dyeing materials, this will cause the difference between the screen color and the color of the printed product. For this, the following two methods can be used.

(1) Choose an ink parameter in the ink setting item for printing and dyeing. If the color difference on the sample is small, it can be corrected when the dye is prepared. For the case where the chromaticity coordinates and brightness factors of the printing and dyeing materials are not familiar, we can proceed from the idea that the printing medium is similar to the printing and dyeing medium. It is recommended to select an ink as the Swop (NewPrint) method first. The main consideration is that the water absorption of newsprint is similar to that of silk.

(2) If the difference in color difference is too large using the above method, use this solution: In the InksColors item, select Other, and fill the Custon column with the chromaticity coordinates and brightness factors of the actual dye. For specific operations, the tedtprint file in Photoshop (in the Calibrat subdirectory of Photoshop) can be used to output the film, which is then printed on silk samples, and then the reflection curve of each spectrum and its calculation x, Y, z can be measured with a color separation instrument Value, fill the obtained value into the Custom column.

1.2 point gain

When the dye is printed on the silk cloth, it will bleed and diffuse, which is unavoidable. Since the expansion of the dots has a greater impact on the midtone and darker areas, the expansion rate of the dots is different for different fabrics. If the dots are enlarged, the image becomes darker. Judging from the existing samples, the image on the sample is obviously dark, which means that the original 20% gain of the printed dots is not enough to compensate for the actual dot expansion rate. In Photoshop, the point gain can be selected up to 40% (relative increment at 50% gray scale), so the point gain can be selected around 35%.

1.3 Gray balance

Usually, the image we see on the display will have some differences in lightness and darkness compared with the printed samples, and some colors will also have large differences. For the minimum requirements, at least the brightness should be as consistent as possible. If the sample is darker than the display image, the CMYK value in GrayBalance can be reduced, otherwise it can be increased (in the case of fixed dot gain). Secondly, if the mid-tone is a bit biased, move the gray output slider in Images / Adjust / Lev-els to control, and count this r value in the GrayBalance column.

The above three parameters are useful when converting RGB mode to CMYK color mode in Photoshop. The parameter change does not affect the RGB image display, only the CMYK mode display image on the screen is changed, and the CMYK mode data is not changed. In other words, if the picture on the screen is dark, you can brighten the picture by changing the dot gain and gray balance, while the CMYK value remains unchanged. If you adjust the Image & / Adjust / Levels value to brighten the picture, the CMYK value will change. [next]

2 Separation settings

The scanned image is generally RGB color mode, when it is converted to CMYK mode, it needs to follow certain conversion rules, that is to say, there needs to be a group of color rules.

2.1 Color separation type

In the printing industry, the "gray component replacement" GCR or "undercolor removal" UCR separation process is commonly used.

(1) GCR is also known as the "achromatic structure" process. That is, the achromatic components in the picture (composited by multiple colors) are all formed by printing with black ink. However, it has no effect on monochrome and intermediate colors. People call the black version the "long-tune version".

Because the blackness of black ink is not enough today, the density is low in the dark tones of the image. This requires the use of color inks in the dark of the non-color components. This process is called "Underground Color Gain" (UCA).

The GCR method is less affected by the amount of ink supply for the double color, dark color, and neutral gray replaced by ink, and the partial color of the picture composed of the primary color and the interrogation color will be affected by the amount of ink. Therefore, the process requirements are relatively higher when using the GCR method.

(2) UCR is also called "color structure" process. The achromatic and color components in the picture are all composed of YMC three primary colors. Because the blackness of the achromatic component synthesized by the three primary colors is not enough, the amount of black ink is increased in the dark tone part while the amount of ink of the three primary colors is reduced. People also often refer to the black version as the "short version" or "outline version".

To sum up the two color separation methods, from the point of view of printing and dyeing process, the UCR method is better than the GCR method. This is mainly to reduce the unevenness in the screen making process and the unevenness of the dye amount in the printing and dyeing process. From the color effect, GCR is more delicate and expressive than UCR, so under the premise that the process is relatively guaranteed, GCR is better. In addition, GCR is usually used for materials with a coating layer, and the dye dip is relatively small. UCR is used for uncoated cover materials (mostly printed and dyed fabrics belong to this category). In this way, different color separation methods should be selected for different fabrics.

Under normal circumstances, the pictures created with the UCR and GCR methods are indistinguishable to the naked eye. At the beginning of the trial, it is recommended to use the UCR method to obtain other process parameters.

2.2 Black Synthesis

This item is only valid for the GCR method. Because the GCR method replaces one or all of the C, M, and Y colors with black, the degree of substitution is selected by this item. From a printing point of view, Medium is preferred. For printing and dyeing, when the black is larger (Heavy item), the chromaticity of C, M, and Y will be reduced, it can show a softer effect, which is beneficial to the clarity of the picture.

2.3 Black ink limit

Control the total black ink volume. For printing, 90% to 100% is used; for printing and dyeing, it can be selected to be a little lower than the printing parameters (such as 70% to 80%). If it is found that the selected amount of black ink is too large, the black ink limit value can be lowered a little.

2.4 Total ink limit

The total ink limit indicates the total amount of C, M, Y, and K. If the ink is limited, the remaining values ​​are the total amount of oil in C, M, and Y. In printing, the values ​​of c, M, Y, and K are generally not less than 300%. For printing and dyeing, it is more suitable between 250% and 300%. Because the ink absorption of printed silk is larger than that of printed paper, the total amount of ink can be set lower for printing and dyeing.

2.5 additional background color

For the GCR method, the black ink cannot meet the requirements of the dark part of the image alone (the purity of the black ink is not enough). At this time, it is necessary to add color ink to the dark part of the image of the non-color component. It only works on polychromatic colors, but not on primary and intermediate colors. For printing, this item is generally not used, so it is not necessarily used for printing and dyeing. [next]

3 Compensate the printing and dyeing process in RIP software

Although the image seen on the screen has achieved satisfactory results, the actual sample is not satisfactory. In addition to the selection of some of the above parameters to meet the printing and dyeing process as much as possible, in addition, the linearity of the film when the film is released, the expansion rate of the dot during the screen making, and the tension during printing and dyeing, etc., will cause the change of the dot, so it is necessary make up. There are actually many ways to compensate. The commonly used method is: first, through the test, measure the dot expansion rate of each step on the actual sample, and then add a point to the dot gain in the aforementioned Photoshop software, and also add a point when RIP rasterization, so that the actual sample three The stimulus value is as consistent as possible on the screen. Therefore, in order to obtain the correct printing and dyeing parameters, the following steps should be started:

(1) Adjust the focus of the imagesetter to reduce the expansion or contraction of the photosetter's light spot, which affects the expansion rate of the film dots and the appearance of interference fringes.

(2) Adjust film linearity. Different films have different development, fixing and other characteristics, so that the same size of light spots on the film will have different sizes. In order to compensate for the expansion rate of each tone halftone dot on the film, the compensation parameters should be false in RIP.

(3) Compensation for dot expansion in the printing process. In the previous description, we have defined a point expansion of 35% to 40%, which only compensates for the 50% tone. The compensation of each point is determined by the compensation curve set in Photoshop. That is to say, this compensation has a certain difference from the actual. In addition, if the actual expansion rate of the 50% tone point is 40%, Photoshop will be powerless. At this time we can only make another compensation in RIP.

Another method is to define the point gain as 0 in Photoshop, and the dot expansion of each tone is compensated in RIP. The compensation value is the difference between the measured value of the sample and the set value.

(4) RIP parameter selection. For printing requirements, each color can represent 0 to 255 levels, and printing and dyeing need not have such high requirements, and can generally be controlled between 64 to 128 levels. Screen mesh: For the printing and dyeing requirements, the screen mesh of the screen version is between 98-118 / cm. For the printing and dyeing process, the mesh size of 24 ~ 39 / cm can meet the requirements. In order to meet a certain number of gradations, the output of up to 101 layers can be achieved by using 600dpi phototypesetting with 60 mesh / inch mesh.

Screening angle: for C, M, Y, and K color screens, the recommended screen angle is l5. , 75. , 90. , 45. . If there is a conflict between the silk screen pattern and the color pattern, the screen will be hit, and you can choose to adjust the cloth angle and mesh number of the screen (generally, the mesh number should not be changed).

From the above analysis, we can see that the color separation settings are related to the printing and dyeing process. In order to make the printing and dyeing effect close to the printing effect, when using the color separation settings of Photoshop, you can make appropriate choices in its various parameters to make it meet the printing and dyeing process requirements, supplemented by RIP compensation, and then look at the sample effect, from the sample To gain experience and data.

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