Technical Principles for Color Quantification of High-Fidelity Printing

In the early days of researching high-fidelity color printing technology, experts had heated arguments over the possibility of forming neutral gray for high-fidelity primary inks with more than three primary colors of CMY and the difficulty of control in practical applications. Whether color printing will hit the net or not will cause great doubts. With the knowledge of experimental proof production and related theories, these are now standard technical specifications.

At present, one of the outstanding issues is the feasibility of expressing colors in RGB mode. The printing industry is accustomed to viewing RGB parameters as device-related. It is considered that the colors represented by RGB parameters cannot be determined if they are separated from specific devices. Therefore, it is concluded that RGB parameters cannot be used to represent printed color space. Whether RGB parameters can be used like CIELab or other chroma parameters, or under what conditions RGB parameters can be used as chromaticity parameters, clarifying these issues becomes the key to solving the problem.

The RGB color mode uses additive addition of three primary colors to produce different colors. Its color space is as follows:

The CIE RGB spectral tristimulus value introduced by the International Commission on Illumination (CIE) first determines the wavelength and amount of light of the three primary colors, and then the equal-energy spectrum color of each wavelength in the visible spectrum color can be expressed by a certain amount of RGB values. , which includes the negative number of tristimulus values. These values ​​are called spectral tristimulus values ​​and all spectral tristimulus values ​​are called "standard chroma observers." As shown below:

This experimental method and data is the quantitative basis of the modern CIE standard colorimetric system, and it is also the original basis for industrial color calibration, color measurement and calculation. In order to eliminate the inconvenience of using negative numbers, CIE later adopted a hypothetical method of XYZ primary colors to establish a new chromaticity diagram and normalize the trichromatic chromaticity values ​​of the matching isoenergetic spectrum, and called this method and data the CIE 1931 standard. The colorimetric system is the 1931 CIE XYZ system. The CIE XYZ color coordinates are as follows:

The relationship between RGB color mode and chroma can be divided into two manifestations. One is a photoelectric conversion device such as a scanner and a digital camera. The chromaticity value corresponding to the generated RGB color is generally indirect, that is, determined by the performance of a color filter or a CCD device. There is no direct correspondence between early image electronic files. The other is image processing software represented by Photoshop. In recent years, the color RGB value has been added to the function corresponding to the color space selection. The image electronic file can carry the profile parameter table conforming to the ICC standard. The main function of the parameter table is to describe the correspondence between the color space and the chromaticity of the image color. Under this condition, any color represented by RGB can be applied to the matrix operation and converted to the CIE XYZ color tristimulus values. Therefore, the corresponding CIE Lab color values ​​can be obtained for each RGB color value. In this sense, RGB parameters have chromaticity uniqueness and can be regarded as chromaticity parameters. RGB is related to the device in the traditional sense and has evolved to relate to the Profile parameter table. The RGB mode can be used more widely in the printing field, including printing colors that represent primary color inks.