Density measurement has always been the most commonly used measurement method in the printing industry, but the density meter cannot provide psychophysical quantities related to human eye sensitivity, so its analytical measurement ability is limited, plus density measurement cannot be in some form and CIE ’s The color metering system is associated (CIE's color metering system is a recognized language for expressing colors). Therefore, colorimetric measurement plays an increasingly important role in the detection and evaluation of printed matter. In chromaticity measurement, there are several important factors that must be noted, otherwise the measured data will not correctly reflect the quality of the printed matter. The following uses X-Rite528 as an example to explain the factors that affect the color measurement results.
1. Whiteboard calibration
Each instrument comes with a standard whiteboard, which must be used for calibration before each use of the instrument.
2. Measuring light source
The light source is a very important factor that affects the color reproduction. The colors of objects of the same color under different light sources are different. For example, clothes that look blue under the sunlight become black under a red light; yellow objects become red under a red light, green under a green light, and black under a blue light. Therefore, the choice of light source in color measurement should attract attention. The light sources that can be selected in X-Rite528 are A, C, D50, D55, D65, D75, F2, F7, F11, F12. F series light sources are used to measure fluorescent products. The color temperature of A light source is 2856K. The color is yellowish, and it is rarely used now. The color temperature of the C light source is 6744K, which represents the daylight in the northern sky, and the D65 light source represents the average daylight.
The measurement results of A light source and other light sources are very different. The D series light source has a good display because its color temperature is 5000K-7500K, close to white. In the printing industry, it is recommended to use the D50 light source when observing the transmission samples of manuscripts. It is recommended to use D65 light source when observing reflective samples such as printed matter. Therefore, the measured data must indicate under which light source it was measured.
3. Measured field angle
The resolution of the human eye when observing details of an object is related to the size of the field of view at the time of 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 human eye has a low ability to distinguish differences when observing colors with a small field of view (<4 °). When the field of view is increased from 2 ° to 10 °, the accuracy of color matching and the ability to distinguish chromatic aberrations are increased; However, when the field of view is further increased, the accuracy of color matching is not improved. X-Rite528 supports two fields of view, 2 ° and 10 °. In the 2 ° and 10 ° fields of view, the same color presents different colors. Therefore, in the color measurement, the field of view selected during the measurement must also be marked.
4. Color Space
X-Rite528 has 6 color spaces to choose from, namely XYZ, Yxy, L * a * b *, L * C * H *, L * U * V *, Yu * ​​v *. In fact, XYZ and Yxy are different expressions of the CIE 1931 XYZ standard colorimetry system in color theory, but X-Rite uses them as two color spaces. Choosing the color space is actually choosing the expression form of the measurement result.
5. Measurement of color difference
Color difference is an important parameter index for printing quality inspection and evaluation. The size of color difference directly affects the product's qualification and its grade. The color difference formula â–³ E * ab based on the CIE 1976 LAB uniform color space is commonly used in our printing industry. With the deepening of research, it was gradually found that the color difference formula also had problems, that is, its data expression could not match the visual perception of the human eye, so color research experts successively launched the CMC (1: c) color difference formula and the CIE94 color difference formula And the latest CIEDE2000 color difference formula, of which CMC (1: c) color difference formula is listed as international and national standards in the textile industry.
X-Rite528 integrates CIE 1976 LAB color difference formula, CMC (1: c) color difference formula and CIE94 color difference formula, which can be freely selected during color difference measurement.
The results measured using different color difference formulas vary greatly. The color difference of qualified products in accordance with the GB7705-87 standard for the on-site chromaticity test of printed matter is less than 6.0. For the colors in Table 1, the 1 has not met the requirements of qualified products, but Measured with 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 indicated in the measured color difference data. We know that CMC (1: c) has two parameters 1 and c, which can be 1 = 1 and c = l only under the reference conditions specified by CIE. In the textile industry, 1 = 2, c = l is CMC ( 2: 1) The formula, CIE94 has 3 parameters, namely KL, Kc, KH. Similarly, under the standard reference conditions KL = KC = KH = 1, otherwise it will not work, it is not told to use in X-Rite528 What are the parameters set by these formulas when measuring the color difference?
6 Conclusion
Chromaticity measurement is a very important measurement method in the printing industry. During the measurement process, attention must be paid to the choice of light source, color difference, color space, etc., and it must be noted in the measurement result, otherwise it may be required by the customer's parameter index. The inconsistency makes the originally qualified products into unqualified products, and is not conducive to the implementation of data and standardization of production processes by enterprises.
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