Color management runs through the entire printing process (2)

2. Reasons for introducing color management in the prepress area

In print copying, we often refer to the phenomenon of “color device correlation”, that is, the same color is input on a scanner provided by different manufacturers that are identical in both modes, and the color displayed on the display is obvious. Difference; Similarly, in the output of printers provided by two different manufacturers with exactly the same pattern, there are obvious color differences in the output results, not to mention that in the entire color reproduction process, the same color should be different. Transfer between hardware devices, manuscripts (mostly subtractive coloring principles, digital manuscripts for additive coloration principles), scanning, image processing (additive coloration principle), final output printing, print (subtractive coloration principle) .

Because there are essential differences between the additive coloration principle and the subtractive coloration principle, it is necessary to introduce the concept of color management in order to ensure the consistency of the color of the respective process links in the printing and copying process so as to achieve the purpose of controlling the color reproduction quality. Color management includes:

1) Color matching between different input devices; for example, the same original is processed with different input methods.
2) Match between the original color and the display color;
3) Color matching between output devices;
4) Match between display color and print color;
5) The color matching between the original and the print. Color management is to solve the problem of color conversion and matching among various devices. The main task of color management is to solve the problem of data conversion of images in various color spaces, and minimize the distortion of the colors of the images throughout the entire production process.

The basic idea is to choose a device-independent color reference space, then characterize and describe each device in the entire system, and finally establish a definite correspondence relationship in the color space of each device.

3. The steps of color management

Color management generally takes three steps. The first step is Calibration, which means "calibrate the instrument"; the second step is Charactersation, which is the "characterization process"; and the last is Conversion, which means "convert the color space." These three steps are abbreviated as "3C".

1) Calibration refers to adjusting the instrument to achieve the standard state. All instruments must be calibrated before use to ensure proper instrument performance. Equipment calibration is to make the input and output devices use color description files and formulate and describe colors according to their respective characteristic curves. It is the basis for characterizing the device's color rendering method. Equipment calibration includes the following aspects: scanner calibration, calibration of the display, and calibration of the output system. After the device is calibrated, a device profile is generated.

General scanner providers are equipped with their own color correction system when launching scanning equipment. Through these color management software, color compensation is performed automatically, which effectively solves the problem of color distortion in scanned images, so that the color images have the best color effect. The operator can complete the calibration process of the scanner step by step according to the management software prompts.

Computer monitor calibrations use built-in hardware and software to calibrate or use built-in software and calibrate with monitor assist software. Calibration steps: first, preheat the display, so that the display light is in a stable state; second, set the brightness and contrast of the display, adjust the display gray balance reproduction; third, set the gamma value, usually the image is recommended to use 1.8; Fifth, correct white field and black field; sixth, calibrate gamma value and gray balance; seventh, save the setting value.

Output system calibration includes: ink parameter settings and separation settings. The ink parameter setting is an important setting for the back-end printing. The operator can set the ink type, dot gain, and color shift and other variable factors during printing, and these settings only take effect when the color mode is switched.

Separation settings provide control of YMCK four-color version output, such as Black Generation, Under Color Removal (UCR), Gray component replacement, Total Ink Limit And background color gain UCA (Under Color Addition).

2) Characterization refers to each color input or color output instrument, even color materials (such as ink, ink, etc.) have a certain range of color or color performance. The purpose of this step is to establish the range of color representation of the instrument or material and record its characteristics in a "data map" for color conversion purposes. The working process of the color profile is to convert any input color information into the colors in the CIE Lab color space. In turn, it can convert the color of the CIE Lab color space into the color reproduction space of the output device. The algorithm idea of ​​the color profile is to realize the conversion of the color space.

Scanner profile: The scanner user uses a color management program and an industry-standard IT8 scanner color scale, calibrates the scanner and stores this information in an electronic file, called the "scanner color profile". The file can ensure that uniform colors get similar results in different scanners.

The display profile is also referred to as soft proofing. The calibration display requires a colorimeter or chromatograph. Then, storing the color profile file enables the display of different color displays to be synchronized under their individual performance. The match between the screen and the display.

Printer profile: The use of a color management program can be characterized by a printing process. A color profile can create a color press or a printing process. The printer's color profile is important for the match between the screen and the print. In order to obtain the color matching between the sample and the print or the color matching between the prints printed by different printers, the color profile of the device is required between the proofer and the printer or between different printers.

3) Conversion refers to the color conversion between instrument and instrument or instrument and material or material and material. Each instrument or material has a different color range, for example, the color display is RGB color, and the color printing is CMYK color. How to establish a connection between them is to use the device-independent color space Lab to accomplish the color conversion between different devices. The specific conversion process applies a color matching mode (CMM). The CMM contains a collection of all functions and algorithms used in the color conversion process. It converts color information from one hardware device to another through a separate color space. The color information, CMM from the data map to obtain the necessary information to ensure that the same color display results between different hardware devices.