Density measurement has always been the most commonly used measurement method in the printing industry, but the density meter can not provide psychophysical quantities related to the sensitivity of the human eye, so its analytical measurement ability is limited, coupled with the density measurement can not be associated with the CIE color system in some form (the CIE color system is the recognized color expression language), so, Colorimetric measurement plays an increasingly important role in the detection and evaluation of printed matter. In chrominance measurement, there are several important factors that must be paid attention to, otherwise the measured data will not correctly reflect the quality of the print. The following takes X-Rite528 as an example to explain the factors that affect the colorimetric measurement results.

1. Correct the whiteboard

2. Measure the light source

The light source is a very important factor affecting the color reproduction, and the color of the same color object is different under the irradiation of different light sources. For example, clothing that looks blue in the sun becomes black at a red light. Yellow objects appear red at a red light, green at a green light, and black at blue light. Therefore, the choice of light source in color measurement should be paid attention to. The light source that can be selected in X-Rite528 includes A, C, D50, D55, D65, D75, F2, F7, F11 and F12. F-series light source is the light source used to measure fluorescent products. The color temperature of A light source is 2856K, the color of the light source is yellow and rarely used now. It represents daylight in the northern sky, and the D65 light source represents average daylight.

A light source and other light source measurement results are very different, D series light source because of its color temperature in 5000K ~ 7500K, close to white, so its display is better, in the printing industry, the observation of the original transmission samples recommended to use D50 light source, observation of prints and other reflective samples recommended to use D65 light source. Therefore, the measured data must indicate what light source is measured under.

3. Measured field of view Angle

The resolution of the human eye when observing the details of the object is related to the size of the field of view. Similarly, the color resolution of the human eye is also affected by the size of the field of view. The experiment shows that the small field of vision (< 4 degree ) The ability to distinguish difference when observing color is low, when the field of view increases from 2 degree to 10 degree , the accuracy of color matching and the ability to distinguish color difference are increased; However, when the field of view is further increased, the accuracy of color matching is not much improved. The X-Rite528 supports both 2 degree and 10 degree fields of view. In the 2 degree and 10 degree field of view, the same color is not the same color, therefore, in the color measurement, it is also necessary to mark the field of view selected when measuring.

4. Color space

The X-Rite528 has 6 color Spaces to choose from: XYZ, Yxy, L*a*b*, L*C*H*, L*U*V*, Yu*v*. In fact, XYZ and Yxy are both different representations of the CIE 1931 XYZ standard colorimetric system in color theory, but X-Rite treats them as two color Spaces. Choosing a color space is actually choosing the expression of the measurement result.

5. Measurement of color difference

Color difference is an important parameter for the quality detection and evaluation of printed matter, and the size of color difference directly affects the conformity of the product and its grade. Our printing industry commonly uses the color difference formula △E*ab based on the CIE 1976 LAB uniform color space. With the deepening of the research, it is gradually found that the color difference formula also has problems, that is, its data expression can not match the visual experience of the human eye, so the color research experts have introduced the CMC(1∶c) color difference formula, CIE94 color difference formula and 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.

The X-Rite528 integrates the CIE 1976 LAB color difference formula, CMC(1∶c) color difference formula and CIE94 color difference formula, which can be freely selected when measuring color difference.

According to GB{{0}}, the color difference of qualified products is less than 6.0, and the color pair 1 in Table 1 has not met the requirements of qualified products, but the color difference measured by CMC and CIE94 color difference formula is significantly less than 6.0. Therefore, the color difference formula selected must also be indicated in the measured color difference data. We know that CMC(1∶c) has two parameters 1 and c, only under the reference conditions stipulated by CIE can be 1=1 and c=l, in the textile industry 1=2, c=l is CMC(2∶1) formula, CIE94 has three parameters, namely KL, Kc, KH, similarly, under the standard reference conditions KL=KC=KH=1, Otherwise, it is not possible, and the X-Rite528 instrument does not tell it what parameters to set when using these formulas to measure color difference.

6. Conclusion