Outlets are the basic units that represent print hierarchy, tone, and color. Changes in printing outlets often lead to quality problems such as color reproduction distortions and reduction in tone levels. The common network transfer problems in the printing process include network expansion, slippage, and ghosting. The most common problem is the expansion of outlets. The main factors that lead to network expansion are two major aspects: the non-linear characteristics of the image-setter or CTP, the transfer characteristics of the exposure system, and other factors; printing process conditions such as type of printing presses, paper and ink, and printing pressure.
1 Dot gain compensation during the output stage
Under ideal conditions, after the PS or ONE-BIT-TIFF file is received by the imagesetter or CTP platesetter, the amount of laser output should be proportional to the dot area of ​​the different graphic parts of the document. Each pixel of the electronic image is output as a dot, and the gray value of the pixel is in one-to-one correspondence with the dot size. However, due to the process precision of machine manufacturing and other reasons, the actual output of most devices is not the case, and it tends to exhibit a certain degree of non-linearity. In addition, due to the influence of other factors such as optics and development, it will result in a film or CTP print. The gradation of the dot image output on the plate deviates from the original pixel value.
In order to obtain the required dot size, measures must be taken to correct the deviation so that the pixel value maintains a linear relationship with the final output. The correction of this kind of deviation can be achieved by using the transformation function in the PS page description language. This compensation needs to be performed before the RIP. By correcting the PS color separation file, it can achieve the purpose of compensating for dot gains generated during the plate making stage. This process generally has the following two means of realization.
1.1 Compensation through Linearization of the Image-setter
RIP manufacturers provide custom transform functions in their products, that is, they can call Postscript language transform function operators in their RIP. This dot gain compensation method is often called linearization. Linear adjustments must be made before the new photosetter is officially put into production; linearization in the current state should also be done after replacing the different developer or film. For CTP platesetters, for the same reason, when changing the plate or plate conditions and changing the FOCUS, ZOOM, or laser power values ​​of the platesetter, precise network point control should be done, and corresponding conditions should be re-doed. Linearization. The specific method of linearization compensation is:
(1) Firstly, a test piece is called from the RIP. The film is output by an imagesetter. The film includes four color patches with different dot area rates. Figure 2 and Figure 3 are the black version correction test strips of the Founder RIP and the Harlequin RIP. The Founder RIP test strips have 24 different density levels. The Harlequin RIP provides 18 different densities.
(2) Use the transmission densitometer to measure the dot area ratio on the film, and then input the measured film density value into the RIP corresponding box as shown in Figure 4 and Figure 5.
(3) Click OK to generate the linearization curve to complete the linearization operation; the CMYK four-color curve can be established separately, or the same curve can be used for four colors.
If the CTP plate-making machine is linearized, special dot-measuring devices are used to measure the dot area rate. For example, X-RITE's CC-DOT plate has large dot deviations, especially the dot area rate is less than 10%. In the outlets, the values ​​measured before and after two measurements will change greatly. Therefore, when measuring, it is necessary to measure several times for the same inspection point, remove the singular value, and find the average value.
Linearization should be performed under normal conditions of development. For CTP positive plates, the dots on the plate will be significantly enlarged when the development ability of the developing solution is severely attenuated. Generally, 50% of the dots are expanded to 53%. It is necessary to consider replacing the developer solution.
1.2 Using Photoshop's Transfer Function
Compensating for dot gain caused by improper image-setter calibration, in addition to the linearization of the output device, using Photoshop's built-in transfer function to create a custom transfer function curve can also achieve this goal.
The use of dot transfer function in Photoshop for dot gain compensation:
(1) The test piece is output by the photo-composing machine, and the density value of the different dot area ratio on the film is measured by a transmission densitometer.
(2) Execute the "Page Setup/Transfer Function" command in the File menu. The dialog box shown in Figure 6 is displayed.
(3) Calculate the required adjustment value and enter the calculated value into the corresponding box in the transfer function dialog box. The so-called calculation means that if 50% of the outlets are specified to be output, the screensetter outputs 52% of the outlets, and thus the output is expanded by 2%. To compensate for this widening value, enter 48% in the dialog box 50% text box. Thus, when outputting a separation file containing this transfer function, the desired 50% dot is obtained (note the "override printer default function" at the bottom of the dialog box). The setting of the transfer function can be different for each color version, and all color versions can also call the same curve. If you use this compensation method, save the separation file as a DCS or EPS and check the "Include transfer function" option. As shown in Figure 7.
It should be pointed out that Adobe does not encourage users to set the transfer function in Photoshop. Instead, users are advised to use the imagesetter's own calibration program to do this step when exporting on the imagesetter. (See Adobe Systems Incorporated.) Postscript Language Reference Manual).
2 Printing Dot Expand Compensation
Dot gain in printing is the increase in dot gain due to diffusion when the ink is absorbed by the paper during the printing process. The dot expansion of printing makes the printed dot area different from the actual required dot area, resulting in inaccurate color reproduction.
Different presses and different paper combinations have different dot gain values. It is worth mentioning that the offset compensation for printing is not to subtract the measured value from the theoretical value to obtain the compensation amount, but to expand the curve from the dot, and obtain the compensation value according to the inverse function relationship. Take the dot A at 50% as an example. The dot area after dot gain is 75% (point C), but actually it is not compensated by the dot area of ​​75%-50%=25%, but by the dot. B is perpendicular to the Y-axis, and the point E (30%) on the X-axis corresponding to the other intersection D of the curve is the dot area after dot gain compensation.
Dot gains generated during the printing process are generally compensated during color separation in Photoshop. Dot gain compensation settings can be set for the overall tone of a color image or for four independent channels of CMYK. In addition, Photoshop also provides grayscale images (in points It is only necessary to convert to gray scale when color is used) and dot gain compensation for spot colors.
When using Photoshop to separate colors, the procedure for setting up dot gain compensation is as follows:
(1) Make color patch test files with different dot area rates in CorelDraw; make CMYK monochrome dot patches and four-color overprint patches.
(2) After the prepared test file is printed under the specified printing conditions, the actual dot area of ​​each color block is measured by a reflection densitometer.
(3) Open the test file in Photoshop, select "Edit/Color Settings/Custom CMYK", select "Curve" at "Network Dot Gain", and input the measured dot area to the "Dot Zoom Curve" dialog box. Corresponding location.
(4) Photoshop will automatically calculate interpolations and generate dot expansion curves for the printing conditions (ink, paper, printing pressure, etc.). After printing under these conditions, the printed dot expansion curve created for the current dot Expansion has the best compensation effect.
In addition, you can also select “Standard†in “Custom CMYKâ€. By modifying the “Ink Colorâ€, you can call the already set dot gain compensation value. Use different inks and paper combinations to call up different dot gain values. For example, when the European ink standard is used with coated paper, offset paper, and newsprint, dot gains are 9% (Eurostandard coated), 15% (Eurostandard uncoated), and 30. % (Eurostandard newsprint). In addition, if an ICC file under a certain print bar is created, besides the expansion of the self-defined network point, the ICC file can also be called in “Loading CMYK†to perform color separation, and the purpose of network point compensation can also be achieved.
3 Dot expansion compensation for text and graphics
The above-mentioned dot gain compensation is performed in Photoshop, but in fact many colors are edited in graphic software (such as CorelDraw) and typesetting software (such as Founder Feiteng), but such software does not have dot gain compensation. Functionality, which requires the color of printed dots to be taken into account when setting the color, the professional practice is to have a reference chromatogram (color scale) for reference when performing such color settings, such as the PANTONE color scale.
Source: "Printing World" Author: Li Chao