Consecutive transfer of pictures into halftone prints requires many links, such as scanning, display, conversion from RGB mode to CMYK mode, filming, proofing, printing, printing, etc. There are many variable factors that affect the effect of tone reproduction. Errors in any one of the links will lead to abandoning the previous work. To control the tonal reproduction of the image, there is a perfect effect. It is necessary to eliminate many bad factors and take necessary measures such as parameter detection and correction for each link. This article on the PC on the relevant settings, so that pre-press production meet the actual production requirements.
First, the display calibration
Image correction in Photoshop 6.0. In order to ensure that the hue displayed on the monitor screen is predictable, two things need to be taken care of when correcting the monitor: the ambient light source should be standard, professional fluorescent lamps should be used, and the sample light source of the printing shop should be the same; after the monitor is turned on , After a period of time, at least half an hour after the display is stable and then correct.
1. Determine the screen color space with Gamma settings
A Gamma program is bundled in Photoshop 6.0 and can be found in the Control Panel. Through the setting of Gamma, a device description file conforming to the ICC standard can be generated, and there are two ways of centralized setting or following the guide. Figure 1 Gamma control panel, we can load the known ICC profile as a starting point, we can also 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, follow the steps below:
1 Adjust the brightness and contrast of the monitor. Adjust the contrast to the maximum and adjust the brightness so that the middle cube is as dark as possible.
2 Select the type of phosphor used by the monitor. Most monitors use the Triniton standard.
3Gamma numerical settings and fine-tuning. The gamma value is essentially the overall correction value of the display contrast. If it is used for print setting, it is set to 2.2, because the density value reflected on the paper according to the printing ink is basically this value. Then fine-tune the slider so that the gray value of the central area of ​​the square appears to be consistent with the surrounding gray values. When performing fine adjustment, a single grayscale block or RGB three-color grayscale block can be displayed to adjust each color.
4 Determine the white field color temperature of the displayed hardware. The white field value essentially refers to the white appearance of the display. The white field color temperature of the display should be the same as the ambient light source color temperature, ie D65. You can also click on "Measure" to select it visually. The screen will turn black and three squares will appear. At this time, select the most neutral one. Setting the display to a cold or warm color temperature by tapping the left or right squares; setting is complete when you tap the middle square.
5 Work color temperature adjustment of the white field. You can select a value that is the same as the hardware setting of the monitor to make it match the hardware setting or select a different value to adjust the display of the monitor.
6 Complete the gamma setting. Select Before or After to observe the display changes before and after the gamma setting. Enter a file name, tap Done to save the settings for later recall.
2.RGB settings determine Photos hop work color space
Through the Photoshop 6.0 editor - color settings - custom RGB settings can be set to some of the most basic parameters of the monitor. The setting parameters you make are the same as the ones set by Gamma, making the Photoshop color space and monitor color space consistent.
Second, the scanner test
Most scanners have been calibrated at the factory, but due to differences in manufacturing conditions, changes in the color temperature of the light source, newness, etc. will affect the scanner's specifications. When purchasing flatbed scanners, manufacturers typically provide a standard transmissive or reflective original that includes a neutral grey scale and partial color patches. After normal scanning of the standard manuscript, observe the image in a certain line of the image in Photoshop 6.0 (observe the white line or black line is particularly obvious) whether the RGB three-channel pixels coincide, in order to check the scanning accuracy (observe should choose at high magnification ). The RGB channels do not correspond to pixels, which will affect the sharpness of the scanned image on the one hand, and affect the color reproduction effect on the other hand during the copying process.
Neutral ash scales were used to examine the ability of the flatbed scanners to restore neutral gray color and to express changes in the density level of the ladder. The correct restoration of the neutral gray color by the scanner is the premise and basis for identifying colors and restoring colors. Scanning color deviation is caused by the difference in the color performance of the scanning components and the standard components during the scanning process. Even though the scanner has been calibrated before shipment, some color casts are inevitable in use. In the actual process, the display screen is often corrected so that it can correctly display the color, and then correct the scanning color cast.
At present, pre-press production centers use middle-to-high-end scanners. Most of the software can customize the output profile. For example, the RGB output characteristics culture can be defined in the NewColor software of Heidelberg. Just set the output profile in the scanning software to match the "RGB setting" in PhotoShop, so that the picture in PhotoShop closely matches the screen display in NewColor. Scanner profiles are not supported for lower profile scanners and PhotoShop features can be used.
When correcting the color cast of the scan, the scanned neutral gray scale is placed in the Photoshop 6.0 image processing software. The information palette is used to display the RGB values ​​of the scales in the image, and the Curves curve is used to adjust the RGB three channels. Until the CMYK display values ​​of the steps in the image reach the required values ​​for neutral gray, a 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 the image is scanned, the color after scanning should be properly compensated according to the curve. The curve can be saved for calling. By scanning with the color patches in the standard document, the scanner can determine the reproduction characteristics of 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 worse, so that in the future, in the scanned image, the color of poor reproducibility is corrected by the image processing software. In addition, the scanner should be calibrated regularly.
Third, the printing parameter setting and measurement
The setting of printing parameters is a very important issue. Attention must be paid to pre-press production regardless of the printing conditions. It must be a failure. Only respect the objective conditions of printing, such as the quality of ink paper, and the quality of printing materials. The resolution, the performance of the press, and other factors can make our reproduction ideal. Therefore, some printing must be done before copying the original. The effective method is to copy GATF on stable Heidelberg, Roland, and Beiren four-color printing presses with certain inks and papers. From this we shall draw: the dot gain rate, the CMYK overprint tristimulus value, and the appropriate Overprint ink total value.
The following is a brief introduction to the GATF color copy test chart, divided into six regions according to the map function: 1 simple color wheel chart for identifying colors and detecting color reproduction; 2 contact copy control standard paragraph, can be used to detect the sun The changes in the printing process during the printing process; the comparison of the line counts of the three outlets is used to select the matching reference of the screen lines and the suitability of the paper, and it can also reflect whether the reproduction of each color version is normal or not; 4 Neutral gray balance control chart of the three primary colors; 5 gray balance card, according to calculate the Y, M, C ratio; 6 proofing control bar. Through copying and copying of the GATF test chart, we can know the transmission of the dots from the film to the PS, the appropriate number of screens, neutral gray balance, and the performance of the machine. This ensures that printing factors are deterministic from printing to printing.
Next, in the custom CMYK, you can set the ink color, color separation parameters, dot gain rate.
1. Determination of the color and paper whiteness of the used ink
In the Photoshop 6.0 use the file drop-down menu to set the color to enter "Custom CMYK", select the custom in the ink color box, through the real block on the GATF copy map, three primary colors yellow (Y), product (M) , Blue (C), Three Color Red (Y+M=R), Green (Y+C=G), Blue (M+C=B), Synthetic Black (Y+M+C), Black (K) Eight color patches and paper white, respectively using a spectrophotometer to measure the tristimulus value XYZ of each color block and the paper used, and then use the formula x = X / (X + Y + Z) and y = Y / (X + Y +Z) Calculate the chromaticity coordinates x, y, respectively, where Y is the luminance coordinate, the value range is 0 to 100, and the x and y values ​​are 0 to 1. Then fill this in the box corresponding to "ink color".
After the data is filled in, 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 is calculated correctly, the colors will be basically the same. If there is any difference, the calculation may be different. To re-measure calculations. (To Be Continued) (Author: Wu Ming root)