Density measurement has always been the most commonly used measurement method in the printing industry, but densitometers cannot provide psychophysical quantities that are related to human eye sensitivity, so their analytical measurement capabilities are limited, coupled with density measurements that cannot be in some form and CIE's The color system is correlated (CIE's color system is a recognized color representation language). Therefore, color measurement plays an increasingly important role in the detection and evaluation of printed materials. In colorimetry, there are several important factors that must be taken care of, otherwise the measured data will not accurately reflect the quality of the printed matter. The following uses X-Rite 528 as an example to explain the factors that affect the measurement results of colorimetry. 1 Whiteboard correction Each instrument comes with a standard whiteboard that must be calibrated before each use of the instrument. 2 measuring light source The light source is a very important factor influencing the color reproduction. The colors of the same color object presented by different light sources are different. For example, a blue dress under the sunlight looks black under the red light; a yellow object is red under the red light, green under the green light, and black under the blue light. Therefore, the choice of light source in color measurement should be paid attention to. The light sources that can be selected in X-Rite 528 are A, C, D50, D55, D65, D75, F2, F7, F11, and F12. The F-series light source is a light source for measuring fluorescent products. The color temperature of A light source is 2856K. The color of the light source is yellow, and it is rarely used now. The color temperature of the C light source is 6744K, which means the sunlight of the northern sky, and the D65 light source means the average daylight. A light source and other light source measurement results are very different, D series of light because of its color temperature in the 5000K ~ 7500K, close to white, so its display is good, in the printing industry, the observation of the original type of transmission samples recommended D50 light source It is recommended to use the D65 light source when observing reflective samples such as prints. Therefore, the measured data must indicate what light source was measured. 3 measured field of view The ability of the human eye to observe the details of an object is related to the size of the field of view during observation. Similarly, the resolution of the human eye to color is also affected by the size of the field of view. Experiments show that the ability of the human eye to discriminate differences when viewing colors with a small field of view (<4°) is low, and when the viewing field of view is increased from 2° to 10°, the accuracy of color matching and the ability to discriminate color differences increase. However, when the field of view is further increased, the accuracy of color matching is not improved. The X-Rite 528 supports both 2° and 10° fields of view. In the 2° and 10° fields of view, the same color presents different colors. Therefore, in the color measurement, the field of view selected for measurement must also be marked. 4 color space X-Rite 528 has 6 color spaces to choose from, namely XYZ, Yxy, L*a*b*, L*C*H*, L*U*V*, Yu*v*. In fact, both XYZ and Yxy are different representations of the CIE 1931 XYZ standard colorimetric system in color theory, but X-Rite uses them as two color spaces. The choice of color space is actually the choice of the expression of the measurement results. 5 Measurement of color difference Chromatic aberration is an important parameter indicator for quality inspection and evaluation of printed matter. The size of color difference directly affects the quality of the product and its grade. Commonly used in the printing industry is the color difference formula ΔE*ab based on the CIE 1976 LAB uniform color space. With the deepening of research, it has gradually been found that the color difference formula also has a problem, that is, its data expression can not match the visual perception of the human eye, so the experts in color science have successively introduced the CMC (1:c) color difference formula, CIE94 color difference formula As well as the latest CIEDE2000 color difference formula, the CMC (1:c) color difference formula is listed as an international and national standard in the textile industry. X-Rite 528 integrates CIE 1976 LAB color difference formula, CMC (1:c) color difference formula and CIE94 color difference formula, and is freely selectable in color difference measurement. Table 1 shows the results of measuring the two pairs of colors using the above three color difference formulas, respectively. The results of measurement using different color difference formulas are quite different. The color difference of the qualified products in accordance with GB7705-87 on-site colorimetric inspection standards is less than 6.0, and the color pair 1 in Table 1 has not reached the requirements for qualified products, but Measured by the CMC and CIE94 color difference formulas, the color difference is significantly less than 6.0. Therefore, the selected color difference formula must also be specified in the measured color difference data. We know that CMC (1:c) has two parameters 1 and c, which can only be 1 = 1 and c = 1 under the reference conditions specified by the CIE, 1 = 2 in the textile industry, and c = 1 that is CMC ( 2:1) Formula, CIE94 has 3 parameters, namely KL, Kc, KH. Similarly, KL=KC=KH=1 under the standard reference conditions, otherwise it will not work, and it is not told in the X-Rite 528 instrument. What is the setting of each parameter when using these formulas for color difference measurement? 6 Conclusion Chromaticity measurement is a very important measurement method in the printing industry. In the process of measurement, attention must be paid to the selection of light source, color difference, color space, etc., and be noted in the measurement results, otherwise it may meet the customer's parameter index requirements. Inconsistently, the products that should have been qualified are turned into unqualified products, and it is also not conducive to the implementation of data processing and standardization of the production process. Source: "Printing World" Author: Lee and Wei Zheng Yuanlin