When comparing the colors between two color samples, the colorimeter can not only give the excellent value, but also the color difference value after colorimetry and the total color difference value calculated by the color difference formula. At present, there are many color difference formulas used for colorimeters, among which the common one is CIE94 (e*94) color difference formula. This article introduces the characteristics of CIE94 (E*94) color difference formula and the color difference calculation formula, interested friends can learn about it!
In 1989, the CIE established the Technical Committee TC1-29 (Industrial Chromatic Aberration Evaluation), whose main task was to examine the standards currently used in industry for the evaluation of the chromatic aberration of objects under daylight illumination. The final proposal was published in 1995 as a technical report from the CIE. The report details the previous work done on chromatic aberration for the new chromatic aberration formula. The full name of the new formula is "CIE1994 (L*, C*ab, H*AB) Chromatic Aberration Model", abbreviated as "CIE94", or the chromatic aberration symbol E*94.
Many factors affect the visual evaluation, such as the characteristics of the sample and the observation conditions. On the basis of TC1-28 (Factors Affecting the Evaluation of Color Difference), another technical committee of the CIE, TC1-29 fully recognized the factors that affect color difference and studied them in detail. Some factors were taken into account in the CIE94 formula, but now it is not possible to consider the influence of all factors, and the two technical committees jointly define a series of reference conditions under which the parameters are given default values, and the CIE94 formula performs well. Under other conditions, the determination of parameter values is considered to be part of the formulation improvement work. These reference conditions suitable for the evaluation of industrial color aberration are:
1) Homogeneous samples;
2) Chromatic aberration between samples e*ab 5;
3) Samples are placed side by side;
4) The visual acuity of the color evaluator is normal, and the field of view formed by the sample is greater than 4°;
5) The illuminance is 1000lx, the background is L*=50 gray, and the illumination source is D65.
The new chromatic aberration formula is still based on the CIHLab color space. TC1-29 believes that the color difference formula is widely accepted in the dyeing industry and that the difference in lightness, saturation, hue and human feeling are extremely important. This color difference formula replaces the previously recommended color difference formula when calculating small and medium color differences in colored materials. But it is not available as a color space replacement for cihlab and cieluv.
The CIE94 formula contains a new term (v), which is the visual quantification value of chromatic aberration:
KE is not used as a commercial chromatic aberration measurement, but as a total visual factor (visual sensitivity coefficient) set to 1 unit under industrial qualification, i.e. v = e*94.
The CIE94 formula is as follows:
The variables kl, kc, and kh are called parametric factors, and they can change the length of the relative half axis, which in turn changes the relative tolerance of l*, c*ab, h*ab. Under the reference condition, kl=kc=kh=1, when the usage condition and the reference condition deviate, it will cause a visual change in each component (luminance, saturation, hue), so that the individual chromatic components in the chromatic aberration formula can be adjusted separately to accommodate this change. For example, when evaluating textiles, it is recommended to use kl=2 and kc=kh=1 for the reduction of brightness, and the calculation results of the CIE94 formula are relatively close to the visual evaluation of textiles.
As done in the CMC(L:C) formula, the length of the elliptical semi-axis (SL, SC, and SH) called the "weight function" in CIE94 allows for their respective adjustments in the CIHLab color space according to the regions, which are defined as follows:
When the standard color and the sample color in a pair of colors are obviously different, then c*ab,1=c*ab.2。This asymmetry of the optimized equation leads to the inconsistency of the color difference between a pair of sample colors, i.e., color samples A and B, the results calculated with A as the standard and B as the standard are not the same. Logically, if no sample is available as the standard color, the cab can be expressed as the geometric mean of the chroma of the CIELAB for 2 colors, as follows:
The CIE94 chromatic aberration formula is actually a modified version of CMC (1:C), and its advantages are: ** The uniformity of the chromatic aberration distance and the influence of parameters on the perceptual chromatic aberration are applied to correct the uniformity of the CIELAB space, and at the same time a set of basic conditions are defined, and there is no need to correct the parametric effect when applied under the basic conditions. The proposed draft has achieved the expected success in improving the uniformity of the CIELAB space, and the weighting function of the color difference formula is much simpler than that of the FMC-2 and ECMC color difference formulations. Therefore, the CIE94 color difference formula has been widely used in the textile industry.
In the printing and packaging industry, scientific researchers and production quality inspectors have realized the shortcomings of the CIE1976LAB color difference formula, and began to consciously choose the CIE94 color difference formula. Some manufacturers of production and testing equipment have also followed the trend and begun to integrate the CIE94 chromatic difference formula in their production equipment, and there are many instruments that support CIE94, such as: X-RITE 530 spectral density meter, X-RITE SP60 portable spherical spectrophotometer, TS7036 spectrophotometer, NH310+ portable colorimeter, etc.
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