In the current colorimeter detection procedures, only the verification method of the tristimulus value is given. However, with the wide application of the CIELAB color space, the colorimeter produced by some manufacturers only provides the measurement results in the CIELAB color space, and the measurement and calibration results in the CIELAB color space will not be able to be given according to the current verification procedures. So, how to measure and calibrate the color colorimeter in the CIELAB color space? This is described below.
Colorimeter is a device for measuring the color and color difference of objects, from the structural point of view, the colorimeter on the market is divided into two kinds, one is the spectroscopic type, the basic principle is to calculate the color of the object by measuring the spectral data reflected on the surface of the object. The other is the filter type, in which the manufacturer matches the response curve of the instrument with the standard illuminator and the human eye through the combination of several color filters, so as to obtain the surface color of the measured object.
At present, there are several methods of expressing colors by numbers in the world: CIE tristimulus value and color coordinates (y, x, y) representation method, L*, U*, V* in CIELUV uniform color space, L*, A*, B* in CIELAB uniform color space. In the fields of printing and dyeing, textile, color matching, etc., the use of L*, A*, B* in the CIELAB color space and the color difference E in the color space to express the color and color difference of the object has been widely used. However, the colorimeter produced by some manufacturers only provides the measurement results in the color space of CIELAB, but does not provide the measurement results of CIE tristimulus value and its color coordinates (Y, X, Y).
In the current JJG 595-2002 "Color Colorimeter" verification regulations, only the measurement methods and tolerances of the tristimulus value and its color coordinates (Y, X, Y) are specified, and the instruments that only provide L*, A*, B* and the color difference E in the color space do not meet their measurement needs, and are not conducive to the qualification evaluation of the instrument by the enterprise. Therefore, it is particularly important to study the measurement and calibration method of colorimeter in CIELAB color space.
According to JJG 595-2002, the verification item includes the measurement of the accuracy of y,x,y for the white, red, green, blue, and yellow color plates, and gives the tolerance of y,x,y y, x, y.
According to the description and definition of the CIELAB color space by the International Commission on Illumination, a strict algebraic transformation relationship between l*, a*, b* and the tristimulus values x, y, z can be obtained as follows:
where: x, y, z are the tristimulus values of the object color; xn, yn, zn - tristimulus values of standard illuminators; l*, a*, b* – the luminosity index and chromaticity index of the object's color.
Therefore, for the instrument that needs to be calibrated in the uniform color space of CIELAB, or if the instrument only has the measurement results in the uniform color space of CIELAB, the L*, A*, and B* of the five color plates can be calibrated, and the L*, A*, and B* in the standard color plate traceability certificate can be compared, and the presentation error L*, A*, B* is given, so that the instrument user can judge the qualification of his equipment according to the results.
For the instrument that needs to be qualified, because the algebraic relationship between y,x,y and l*,a*,b* is nonlinear, therefore, the qualification judgment needs to convert the measured l*,a*,b* to y,x,y, and calculate y, x, y, and then according to the tolerance requirements in jjg 595-2002 (level 1: y 1.).5，△x，△y≤0.020；Level 2: y 3, x, y 0025).
For instrument users who are concerned about the accuracy of color difference, the accuracy of the measurement of color difference between two colors is also concerned about the accuracy of the measurement of color difference between two colors, i.e., the accuracy of e*ab measurement.
According to JJG 595-2002, the quantity associated with the color aberration e*ab in the verification item is only the repeatability and reproducibility of the color aberration e*ab, and does not involve the measurement of the accuracy of the e*ab. According to the use scenario of the colorimeter, it is understood that the user of the instrument pays more attention to the color difference of similar colors rather than the color difference between different colors.
Therefore, a measurement method for the accuracy of color difference can be adopted: using two sets of five-color standard color plates, and the standard values in the CIELab color space given by the two sets of standard color swatch traceability certificates are l*1s, a*1s, b*1s and l*2s, a*2s, b*2s, respectively, the color difference value between the two sets of color swatches can be calculated as the color difference standard value e*abs, as shown in the following formula:
The color difference between the same colors in two sets of standard color swatches was measured separately with a colorimeter to be measured. Then calculate the difference between the standard value and the measured value, that is, calculate the difference of color difference as an index to evaluate the accuracy of color difference, as shown in the following formula:
Using two sets of five-color standard color plates, the color difference indication error of five colors can be obtained. Then, according to the actual use scenario of the instrument user, the tolerance of the color difference indication error can be specified as the basis for evaluating the accuracy of color difference. So as to complete the measurement and calibration of color difference accuracy.
To sum up, for the metrological calibration in the color space of CIELAB, the characteristics of the instrument can be judged from two aspects, one is to provide the accuracy data of the instrument to measure the surface color of the object, that is, by measuring l*, a*, b*, and converting it into y, x, y, and then making a qualification judgment, so as to help the instrument user to make a judgment when measuring the color of the object. The second is to provide the accuracy data of the instrument to measure the color difference, that is, to measure the indication error of the color difference of the two standard color plates, so as to help the instrument user to judge when paying attention to the color difference of the two colors.
Guangzhou Baolaifa Instrument Co., Ltd. products are widely used in domestic printing, coatings, auto parts, metals, home appliances, gold standard test strips, food safety, medical testing and other industries and scientific research institutions, while the industry application range is still expanding, the company unremittingly carries out technology research and development and brand service to ensure the stability of the company's products, and is committed to providing users with high-quality products and services with technology. The main color colorimeter, spectrophotometer, hyperspectral camera, standard light source box, gloss meter, haze meter, thickness gauge, resolution test card, image test card, test software and other color matching system products and provide solutions.
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