Color difference is an important technical index for printing quality evaluation and detection. The leading national standard GB / T 7705-2008 (lithographic decoration printing); GB / T 7706-2008 (letterpress decoration printing); GB / T 7707-2008 (intaglio decoration printing); CIELab color difference formula is mostly used in GB / T 17497-1998 (flexographic decoration printing).
However, according to practice, CIELab color difference formula can not match the visual feeling of human performance well, and can not accurately reflect the visual feeling of human eyes. There are many reasons for the color difference of human eyes, as shown in the figure below.

Our ideal color difference formula should be able to accurately describe the visual feeling of human eyes, but CIELab color difference value gives people different visual feeling under different brightness, saturation and hue.
In the printing industry, CIELab color difference formula is still used in the current international standards and industry standards. Some enterprises found the shortcomings of this color difference formula in actual production and began to adopt CMC color difference formula in enterprise standards.
CMC (L: C) color difference formula is recommended by the society's color measurement Committee (CMC) of the society of Dyers and colorist in 1984. It overcomes the defect that the calculated value of jpc79 color difference formula in dark and neutral color areas deviates greatly from the visual evaluation results, The lightness weight factor L and chroma weight factor C are introduced to meet the needs of different applications.

CMC chromatic aberration ellipse

In CIELab color space, CMC (L: C) formula defines the visual width around the standard color as an ellipse. The color inside the ellipse is visually the same as the standard color, while the color outside the ellipse is different from the standard color. In the whole CIELab color space, the size and eccentricity of ellipses are different. The characteristics of an ellipse centered on a given standard color are determined by the length of the two half axes in the directions of ⊿ L *, ⊿ c * AB and ⊿ h * AB relative to the standard color. The color difference formula ⊿ ECMC (L: C) defined by elliptic equation is as follows:

l. C is the factor. The length of the relative half axis can be changed through L and C, and then the relative tolerance of ⊿ L *, ⊿ c * AB and ⊿ h * AB can be changed. For example, in textile, l is usually set to 2, allowing a relatively large tolerance on ⊿ L *, which is the CMC (2:1) formula.
Because CMC color difference formula has better visual consistency than CIELAB formula, the "single number tolerance" independent of color region can be used for the quality control of different color products, which brings great convenience to the instrument evaluation of color measurement and color difference. In China, CMC color difference formula is also adopted in international standards GB / T 8424.3-2001 (calculation of color difference in textile color fastness test) and GB / T 3810.16-1999 (experimental methods of ceramic tiles Part 16: Determination of small color difference). In the printing industry, CIELab color difference formula is still used in the current international standards and industry standards. Some enterprises found the shortcomings of this color difference formula in actual production and began to adopt CMC color difference formula in enterprise standards. For example, a Paper Co., Ltd. in Xinxiang, Wenzhou adopts CMC (1:1) formula when measuring spot color prints (as shown in the figure) and a color printing Co., Ltd. in Zhejiang when measuring the color of cigarette labels.

packing box

Some manufacturers of testing equipment also comply with the trend and begin to integrate CMC color difference formula into their equipment. At present, among the domestic color measuring instruments, the instruments supporting CMC (L: C) color difference formula include the spectrophotometer series of CHNSpec technology (as shown in the figure below).

Test packing box

With the continuous development of printing and packaging technology, its products are becoming more and more exquisite. In order to better detect and control printing and packaging products and make the measurement data of color difference match the vision of human eyes as much as possible, we might as well try CMC (1:1) color measurement with D / 8 structure. CHNSpec helps domestic printing manufacturers go to the world!