How to calibrate your system for color management
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With the development of calculators and external devices, color desktop systems have emerged, from image input devices and displays to final output methods. High-resolution scanners, digital cameras, color printers, and digital color presses for the short-run market have become commonplace. The types of originals have also evolved from traditional reversal films, negative films, and reflective documents to photo CDs. The calculators and displays used for image processing are also different. In addition, color output devices that adapt to various needs are also increasing, such as color inkjet printers, hot wax printers, and sublimation printers. The emergence of the Internet has opened up a new field for image output. Since the color image processing system is an open system, the composition of the system is flexible, that is, the scanner produced by Party A, the display produced by Party B, the imagesetter produced by Party C, and the printer can be used. The equipment produced by different manufacturers is combined into one system. How to maintain the color consistency from the original input of the scanner to the proofing and finished product output, the quality control of the image color is crucial.
First, the essence and task of color management
The essence of color management is to accurately control and manage color transfer in the entire desktop publishing system to achieve perfect color reproduction. Color management enables color reproduction and reproduction of colors to be independent of the device used, that is, the same color data. With any system output, you get the same color effect.
All hardware devices (scanners, displays, printers, laser imagesetters, printers, and other output devices) in the production process are interpreted and defined in different ways, and color information conversion between different types of devices is prone to errors. There is no guarantee that the color of the files exchanged between the systems will be the same. The main task of color management is to solve the problem of image conversion of images in various color spaces, so that the color of the image is minimized during the entire production process. The basic idea is to select a color reference space that is independent of the device, then characterize each device of the entire system, and finally establish a certain correspondence in the color space of each device.
Second, the content of color management
Color management includes device calibration, characterization, and color conversion. Equipment calibration is the premise to ensure the normal operation of the system, but it does not guarantee that the image is consistent with the original, but rather whether the device reaches the original design pointer; the description of the device feature is the color expression ability of the different devices and the color range presented. Description; color conversion is to convert the color space of one device to the color space of another device under consistent conditions, and establish a color image of the mapping between different color spaces.
Feature description refers to the detailed description of the color rendering performance of color media such as scanners, displays, color printers, inks, etc. using a digital method, that is, the color space of each medium is described by an appropriate color scheme. Feature description is a method for measuring and determining the color gamut or replicable color set of various input and output devices. It can define the color gamut. The color data describing the color range of the device after feature description is called device color. Profiles, which are usually provided by the manufacturer at the time of shipment from the factory, and sometimes can be used with certain tools (such as Color Source Color Gauge from Light Source, USA) and corresponding color management software (such as Haier). Printopen, Scanopen, etc.) create Pro-files on their own.
Color conversion refers to converting a color in a color space on a device to a color space under another known condition according to a one-to-one mapping relationship between different color spaces of different colors. Since the color gamut of the output device is generally smaller than the color gamut of the original, the scanner, and the display, it is necessary to compress the original gamut in the color gamut of the output device. First, the RGB color information obtained from the scanner is converted into a device-independent CIE-Lab color space, and then the CIE-Lab color space is converted into the device's own color space according to the ICCProfile of the next device. Since the CIE-Lab color space has a wide color gamut containing other color spaces, color quality is not lost.
Third, the display calibration (calibration)
The device color correction is to make the input and output devices use the color description file to formulate and describe the color according to the respective characterization curves. It is the basis for the device color rendering as the feature description. Before converting color data between different media, be sure to calibrate the device. The deviation of the color rendering performance of the same device under different conditions is determined by calibration, so that the color information has temporal coherence in the process of acquisition and transmission. The purpose of calibrating the display is to make the displayed image and the final output image color as close as possible. The steps of the calibration are as follows: 预 preheat the display for half an hour after the display is turned on, so that the display is in a stable state; 调整 adjust the indoor light source to a level that can be maintained frequently, turn off the extra light source to prevent these dynamic changes from affecting the display, and set the display. Brightness and contrast: Turn off all desktop patterns, change the background color of the display to neutral gray, so that it will not affect the visual during the calibration process, help adjust the gray balance; set the gamma value, first adjust From the Gamma control panel, select the appropriate Target Gamma at the top of the dialog box, which is the target gamma value. The general image is recommended to use 1.8. If you want to record with a video recorder or film to output the image, Gamma is only set to 2.2; To correct the white field, first create a blank new file in Photoshop, then select a piece of paper with the same whiteness as the print, click the WhitePoint button, and drag the triangle slider until the white in the display matches the white in the pattern as much as possible. Correct the gamma value and adjust it with GammaAdjustment until the two color blocks in the two-color gray bar above the triangle slider have visual effects Nearly, there is no obvious limit; correct the color balance and gray balance, click Balance, adjust the RGB three-color slider, until there is no color in the gray scale below the slider, it is a gray level; correct the black field, Click on BlackPoint and drag the RGB tri-color slider until the dark portion of the gray gauge below the slider feels the same as the dark portion of the gray scale in the print. After the above steps, the color correction process of the display is completed. The color correction result will immediately affect the display of the display. At this time, the Gamma window is closed, and the Gamma window setting will take effect each time the computer is activated. Of course, we can also click the Save Setting button to store several Gamma files in the Control Panel for different papers and different displays. After setting the storage, reactivate the machine, click the Load Setting button in the Camma dialog box, and select the appropriate Gamma setting.
Fourth, the scanner calibration
For scanners, products from different manufacturers have their own color correction system, such as: MICROTEK's DCR (dynamiccolor rendition) dynamic color correction software, Agfa's FotoTune color management software, Tsinghua Unisplendour's ImageCalibration color management software, etc. Through these color management software, color compensation is automatically performed, which effectively solves the color distortion problem of the scanned image, so that the color image has the best color effect. The use of scanner color management software is very simple. Take DCR as an example. First, the standard color table AGFA-IT8 (for scanning reflection) and KODAKQ-60 (for scanning transmission) provided by MICROTEK are placed in the scanner. Execute the MICROTEK CALIBRATION program, select CALIBRATE, and activate the DCR color correction system to complete the operation. When scanning a general image later, simply check to see if DCR is selected, and once selected, DCR will automatically be applied to all scanned color images.
Fifth, the system color
Under normal circumstances, most systems are equipped with scanner input images, image processing in the calculator, graphics and text, etc., and then transferred to the output center output film through MO disk, Zip disk or network, and then to plate making , proofing, printing, etc., so when the scanner scans the image, you should know what it is, such as where the scanned image is used, which type of printer, ink, paper, etc. to use. Although it is not possible to directly adjust the graphic output machine or printer, you can use image processing software such as Photoshop for ink parameters and color separation settings.
1. Printer ink settings (Printinglnk Setup)
Printinglnk Setup in Photoshop is an advantageous tool for back-end printing, where we can set variables such as the type of ink to be printed, dot gain and color cast, and these settings are only between color modes. Does not work when converting. In the Printinglnk Setup dialog, tap Ink Color to select a suitable ink standard and printing paper, because the ink's absorbency of the ink will have a big impact on the final result. The default setting is US Standard Ink on coated paper. (Swollen paper) on the SwapCoated standard, this setting is more suitable for most of China's printing requirements: the dot enlargement preset represents the dot enlargement rate of the image in the specified paper, Photoshop then adjusts the dot expansion curve based on this data to adjust The dot gain rate of each tone of the image. Set the dot gain to a smaller value, the image is brighter, set a larger value, and the image is darker. Use the reflection densitometer to measure the color correction bar in the proofing, and adjust the dot gain value according to the test result; the gray balance setting can separately control the CMYK color to compensate the printing color shift; if the image is grayscale or a color image Channel, you want to see the effect of using dot gain on the screen during image processing, you can choose the Use Dot Gain for Grayscalelmage option.
2. Separation Setup (SeparationSetup)
In the color separation setting, how to control the generation of CMYK four-color version is provided, including: Black Generation method, Under color removal UCR (Under ColorRemoval), Total lnk Limit control and Under color gain UCA (Under Color Addition). In the Separation Settings dialog box, you can select the color separation type according to the needs and the production quality of the printing house – UCR/GCR. Due to technical factors, GCR often causes dirty spots in the highlights, while UCR is only for the dark part. Alternative, there will not be too much deviation, more use: the degree of black version produced by Light (light), Medium (medium), Heavy (high), Maxium (large), Custom (custom), etc. A choice, custom can adjust the black curve in the image, other color version will automatically calculate its own replacement; black ink limit is the amount of ink used in the black version of UCR / GCR, lack of Photoshop The provincial setting is 100%, usually set to 80%-90%, it can basically suppress other colors; total ink limit means the maximum ink density that the printing machine can support, the default setting in Photoshop is 300%; UCA (Undercolor Addition) refers to the use of the GCR black version to increase the amount of green, yellow, and yellow in the dark area of the image. If only the black version is used in the dark area, it may cause Too dull Tune, appropriately adding yellow green color ink products, will make dark portions of images richer. In addition, this option can only be adjusted when the GCR black version generation mode is selected. The range in which UCA can be adjusted is 0%-100%, and as the value increases, the amount of CMY gradually increases.
Color management is the most difficult and important part of the color image reproduction process. As users' demand for color continues to increase, the position of color management in the prepress publishing system will become more and more important. The international color organization ICC has developed a standard ICC Profile that describes the color expressiveness of the device. According to the standard, the color description files created by different color management systems can be freely converted to each other, enabling cross-platform color communication.