How to effectively control image tone shift
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Continuously copying pictures into halftone prints requires many links, such as scanning, display, RGB mode to CMYK mode conversion, filming, proofing, printing, printing, etc. There are many variables that affect the color reproduction. Any mistake in the link will lead to the elimination of the previous work. To control the image tone reproduction has a perfect effect, it is necessary to eliminate many unfavorable factors, and take necessary parameter detection and correction for each link. In this paper, the relevant settings are made on the PC to make the prepress production meet the actual production requirements.
First, the correction of the display
Image correction was performed in Photoshop 6.0. In order to ensure that the color tone displayed on the display screen is predictable, to correct the display, two things to pay attention to when correcting the display: the ambient light source should be standard, and the professional fluorescent tube can be used, which should be the same as the sample light source in the printing workshop; It takes a period of time to wait for at least half an hour to wait for the display to stabilize and then correct it.
1. Determine the screen color space with Gamma settings
A Gamma program is bundled with Photoshop 6.0 and can be found in the Control Panel. The Gamma settings can be used to generate a device description file that conforms to the ICC standard, either in a centralized setting or in accordance with the wizard. As shown in Figure 1 Gamma control board, we can load the known ICC profile as a starting point, or rename a file. We can adjust the brightness and contrast in this palette, select the model of the display phosphor, and set the Gamma value. And fine-tuning, white field settings.
If you follow the wizard to set it up, follow the steps below:
1.1 Adjust the brightness and contrast of the display. Adjust the contrast to maximum, then adjust the brightness so that the small squares in the middle are as dark as possible.
1.2 Select the model number of the phosphor used in the monitor. Most monitors use the Triniton standard.
1.3 Gamma values are set and fine-tuned. The Gamma value is essentially the overall correction for the contrast of the display, and if used for the printing setup, it is set to 2.2 because the density value reflected by the overprinting of the printing ink on the paper is basically this value. Fine-tune the slider so that the gray value of the center of the square looks like the gray value of the surrounding. When fine-tuning, you can display a single grayscale block or display RGB three-color grayscale blocks to adjust each color.
1.4 Determine the white field color temperature of the display hardware. The white field value essentially refers to the appearance of white in the display. The white field color temperature of the display should be the same as the ambient light source color temperature, that is, D65. You can also click on “Measure” to select by vision. The screen will turn black and three squares will appear, at which point the most neutral one will be selected. Setting the left or right square will set the display to a cooler or warmer color temperature; when you click on the middle square, the setup is complete.
1.5 color temperature adjustment of the working white field. You can choose a value that is the same as the display hardware settings to match the hardware settings or choose a different value to adjust how the display is displayed.
1.6 Complete the Gamma settings, select Before or After to observe the display changes before and after the Gamma setting. Enter the file name and tap Done to save your settings for later recall.
2. RGB settings determine the photos hop working color space
The most basic parameters of some monitors can be set by editing the Photoshop 6.0 - color settings - custom RGB settings. The setting parameters are the same as those set by Gamma, so that the Photoshop color space is consistent with the display color space.
Second, the scanner test
Most scanners are calibrated at the factory, but due to differences in manufacturing conditions, changes in the color temperature of the source, age and so on will affect the scanner's specifications. When purchasing a flatbed scanner, the manufacturer generally provides a standard transmissive or reflective original that includes a neutral gray scale and partial color patches. After normal scanning of the standard original, observe a certain line image in the image in Photoshop 6.0 (observe the white line or the black line is particularly obvious) whether the RGB three-channel pixels overlap, so as to check the scanning accuracy (the magnification should be selected at the time of observation) ). The corresponding pixels of the RGB channel do not coincide, which will affect the sharpness of the scanned image on the one hand, and affect the color reproduction effect during the copying process on the other hand.
The neutral gray scale ruler was used to test the ability of the flatbed scanner to restore the neutral gray color and the performance of the ladder density level. The correct restoration of neutral gray color by the scanner is the premise and basis for identifying color and reducing color. Scanning color cast is caused by the difference in color perception performance of the scanned components from the standard components during the scanning process. Although the device has been calibrated before leaving the factory, there are inevitably some color casts in use. In the actual process, the display screen is often corrected so that it can display the color correctly, and then correct the scanning color cast.
At present, some pre-press production centers use medium-to-high-end scanners, and most of the software can customize the output characteristic files. For example, the Heidelberg scanning software NewColor can define the RGB output characteristic culture. Just set the output property file in the scanning software to match the "RGB setting" in PhotoShop, so that the image in PhotoShop is almost the same as the screen display in NewColor. For lower-grade scanners, scanning profile files are not supported and PhotoShop features can be used.
When correcting the scanning color cast, the scanned neutral gray scale is placed into Photoshop6.0 image processing software, and the information palette is used to display the RGB values of the scales in the image, and the Curves curve is called to adjust the RGB three channels. Until the CMYK display value of each step in the image reaches the required value in neutral gray, and the visual neutral gray scale can be visually observed on the display screen. The Curves curve at this time is the color curve after the scanner corrects the color cast. When scanning the image, the scanned color should be properly compensated according to the curve, and the curve can be saved for calling. By scanning with color patches in a standard original, it is possible to judge the reproduction characteristics of the scanner for each color. The visual contrast between the original color patch and the scanned image can qualitatively recognize which colors the scanner reproduces better and which colors are different, so that in the future, in the scanned image, the color with poor reproducibility is corrected by the image processing software. In addition, the scanner should be calibrated regularly.
Third, the printing parameters set and measurement
It is very important to set the printing parameters. The work that pays attention to the pre-press production without considering the printing conditions must be a failure. Only the objective conditions of printing, such as the quality of the ink paper and the plate, are rejected. Factors such as resolution, printer performance, etc., can make our replicas achieve the desired results. Therefore, some measurements must be made on the print before copying the original. The effective method is to copy the GATF on the stable Heidelberg, Roland and Beiren four-color printing press through the determined ink and paper. From this we can get: the dot enlargement rate, the CMYK overprinting tristimulus value, suitable The total value of overprinted ink.
The following is a brief introduction to the GATF color copy test chart. It is divided into six areas according to the map function: 1 simple color wheel map for discriminating color and detecting color reproduction; 2 contact copy control standard section, which can be used to detect drying The change of the dot in the process of the version; the comparison chart of the number of dots of the dot, which is used to select the matching reference of the number of screen lines and the suitability of the paper, and also reflects whether the reproduction of each color tone is normal; 4 the neutral color balance control chart of the three primary colors; 5 gray balance card, according to this, calculate the ratio of Y, M, C; 6 proof control bar. Through the printing and printing reproduction of the GATF test chart, we can know the transfer of the dots from the film to the PS version, the appropriate number of screen lines, the neutral gray balance and the performance of the machine. This ensures that the printing factors are deterministic from the printing to the printing process.
Next, in the custom CMYK, the ink color, the color separation parameter, and the dot gain ratio can be set.
1. Determination of the color of the ink used and the whiteness of the paper
In Photoshop 6.0, use the file drop-down menu to enter the "custom CMYK" color, select the custom in the ink color box, copy the solid block on the map by GATF, the three primary colors yellow (Y), product (M) , cyan (C), three color red (Y+M=R), green (Y+C=G), blue (M+C=B), synthetic black (Y+M+C), black (K) Eight color blocks and paper white, use the spectrophotometer to measure the tristimulus value XYZ of each color block and paper used, and then use the formula x=X/(X+Y+Z) and y=Y/(X+Y +Z) Calculate the chromaticity coordinates x and y respectively, where Y is the brightness coordinate, the value range is 0 to 100, and the range of x and y is 0 to 1. Then fill this in the box corresponding to "Ink Color".
After filling in the data, the color of each color block will change, and the nine color blocks printed will be compared with the colors displayed on the screen. If the measurement calculation is correct, the colors should be basically the same. If there is a difference, the measurement calculation may be different. The measurement calculation is to be repeated.