How to achieve consistent color reproduction

There are many factors that affect the color reproduction. Looking at the process of color formation, we can see that the first condition is the light source. If the spectral composition of the light source changes, the absorption spectrum of the corresponding object can change, which will affect the color of the object. This light source is called a white point on the chromaticity diagram. It should be noted that the chromaticity diagram at this time is the chromaticity diagram of the color produced under this light source. There are many sources of light, and a standard should be developed so that there is a reference when doing color measurements and calculations. Therefore, CIE has developed several standard light sources, namely A, B, C, D65, D50. They respectively correspond to different stages of daylight during the day. Now the B and C standards have been abandoned. In terms of printing, since the color reproduction is performed on a certain device with a certain pigment, the appearance of the generated color has a certain relationship with the equipment used and the corresponding pigments and printed carriers. Different printing processes and equipment such as offset printing, screen printing, ink-jet printing, dot-matrix printing, the appearance of the colors produced are different, but because the pigments are all used for printing, the difference is not great. Display devices, different display principles of equipment, such as CRT display, liquid crystal display, plasma display, etc., the displayed colors also have some differences; scanner, different scanners, sampling, quantization process is different The resulting color values ​​are also different.

The most discussed in color is the consistency of color reproduction. The process of color reproduction is actually the process of transferring color data in the system. The basic requirement for color reproduction is the same as the original. Each device in the system has its own way of expressing colors and applies different software for processing. Therefore, there is an objective situation in which the consistency of color between originals, prints, and soft samples is subject to these limitations. Consistency has a certain scope. The difference between manuscripts, prints, and proofing exists. It must be acknowledged. However, this difference should be controlled within a certain range, at least within the scope of the customer's approval. Of course, if you can make the user very satisfied it is best. The most basic is to be able to minimize the color difference between printed products and originals under objectively limited conditions. In addition, consistency between soft samples and prints is now becoming more and more important. Under normal circumstances, manuscripts are produced and processed on a computer, so they must be processed on the screen before printing. Therefore, the color management of the screen is very important.

In terms of printing presses, the color reproduction of the first printing and the printing of the hundredth printing may not be the same. Therefore, the equipment needs to be corrected and managed so that the color reproduction of each batch of products is consistent. The input, display, and output of the three different types of device color management content are different. Different corrections can be made depending on the different color reproduction characteristics of different devices. There are usually two methods for calibration, one is hardware adjustment, and the other is software adjustment, which can be performed in the application program or in the device driver. To achieve consistency in color reproduction, or to correctly convey color information, color data must be accurately described during color transfer. Accuracy is the same as the information expressed by the original data. Therefore, the color must be mapped in the color space represented by different devices so that it is consistent with the color of the previous level. In this way, after an accurate mapping at the first-level level, the colors of the original and the printed product will be in good agreement. Then, how should this precise mapping be implemented in the implementation? This is what is commonly referred to as color management technology.

The color management technology realizes accurate mapping of various device colors through a color management mechanism. For example, to print on the print the same red as the original, then the red of the scanner and the red of the display and the red of the printer must match, that is, must be performed between the color spaces of the three devices. The color of the map. To do this mapping, you must know what the color space of each device is, that is, how large the device's color space is, and how the specific color data is distributed. This can be described by the corresponding device's profile.

Color management mainly includes two aspects, one is the description of the device's color characteristics, and the other is a color matching algorithm (CMM). The color image file is actually a color data file. The copy of the color is the copy of the color image, and it is the process of inputting, processing, and outputting the color image file. Therefore, the reproduction of the color is the reproduction of the image, that is, the image is the carrier of the color. Therefore, the color management mechanism is tightly combined with the image file. In the image file, the color description file of the corresponding device is attached, and the color matching algorithm can be placed in the device driver for execution. This is usually called The application-level color management. In order to save the calculation cost, reduce the memory consumption, improve the speed and quality of image reproduction, the color management algorithm can be used as part of the operating system, which is system-level color management. The color management based on the ICC standard is this type of color management, and the format of its device description file is recognized by the operating system and the driver of each device manufacturer, and is embedded in an image file. There are many discussions about the color management of ICC standards. The most important thing in color management is the precise mapping between device color spaces. This is also the most discussed aspect at present, and various color mapping algorithms are proposed.

The consistency of color reproduction is the most basic requirement for color reproduction. As a management tool for realizing this requirement, the color management technology is gradually improving. I believe in the near future, there will be a more satisfactory method of color management. (Author: Hu Tao Tang Zhaoju Xi'an University of Technology)