In order to unify the methods of color measurement and calculation, CIE has specified a number of standard chromaticity systems for color measurement and calculation, among which the common ones are CIE1931RGB standard chromaticity system, CIE1931XYZ standard chromaticity system and CIE1964XYZ supplementary standard chromaticity system. In view of the fact that many friends do not know much about the CIE1964XYZ supplementary standard chromaticity system, this article introduces the color measurement system, and friends who are interested in it can learn about it!
After the establishment of the CIE1931 standard chromaticity system, it has been proved in practice for many years that the data of the CIE1931 standard chromaticity observer represents the average color level characteristics of the 2° field of view of the human eye. However, when the field of view increases to more than 4°, some researchers find that the spectral tristimulus value is low in the wavelength range of 380 460nm. This is due to the participation of rod cells and the influence of yellow pigment in the fossa under the condition of large-area field of view, and the color vision will undergo certain changes. The field of view often exceeds the range of 2 ° when observing objects in daily life, therefore, in order to meet the needs of color measurement with a large field of view, CIE specified a set of "CIE 1964 Supplementary Standard Chromaticity Observer Spectral Tristimulus Value" in 1964 referred to as "CIE1964 Supplementary Standard Chromaticity Observer", which is now renamed "CIE1964 Standard Chromaticity Observer", and this system is called "CIE1964 Standard Chromaticity System", also known as 10° field of view X10Y10Z10 Chromaticity System.
From the perspective of the development history of the chromaticity system and the chromaticity formula, before 1964, the 1931CIE-RGB standard colorimetry system and the 1931CIE-XYZ colorimetric system were experienced respectively. However, with the continuous improvement of scientific research level and the continuous expansion of application, the shortcomings of the above two colorimetric systems have been gradually exposed, and they are less and less suitable for the current engineering application and the determination of corresponding standards. Therefore, in 1964, CIE proposed the CIE1964 supplementary standard chromaticity system, which will be described in detail below
The light emitted by the light source (light source color) or the spectral energy of the light source that enters the human eye after being reflected or transmitted by the object (object color) to produce color sensation is called the color stimulation function, and according to the provisions of CIE, the tristimulus value caused by the color stimulation function is:
where x10, y10, and z10 are the tristimulus values of the CIE1964 supplemental standard chromaticity system. X10( )Y10( )Z10( ) is the Spectral Tristimulus of CIE1964 Supplemental Standard Observer. x10( )y10( )z10( ) is a set of values specified by CIE, and three stimulus curves can be simulated with this set of values, as shown in the figure below.
Considering that the actual color stimulus function is often difficult to accurately express in algebraic form, the sum is usually used to approximate the integration in the above equation in the actual chromaticity calculation, i.e.,
The reason why the CIE1964 supplementary standard chromaticity system replaces the previous colorimetric system is that the previous chromaticity system 1931CIE-RGB and 1931CIE-XYZ colorimetric systems have a low value in the spectral range of 380nm and 460 nm when the field of view exceeds 4°, which is because under the observation conditions of a large field of view, the participation of rod cells and the influence of **fossa pigment make certain changes in color vision. The field of view when observing objects on a daily basis is usually more than 4°. Therefore, in order to meet the needs of color measurement with a large field of view, CIE recommends a set of "CIE Supplemental Standard Chromaticity Observer Spectral Tristimulus Values" suitable for 10° field of view conditions, so that the "CIE1964 Supplementary Standard Chromaticity System" is formed, and the corresponding spectral tristimulus curves are represented by XN, YN and ZN respectively. The CIE1964 supplementary standard chromaticity system constitutes the standard for large field of view measurement, and as the basis for a series of color difference formulas developed in the future, it has been continuously improved and revised in various color difference formulas, and has been widely used.
CIE1964 Supplementary Standard Chromaticity System The measurement of color is divided into two types: object color measurement and light source color measurement, object color and lighting conditions are related to the spectral characteristics of the object itself, so the stimulus function of the color that actually enters the human eye should be the product of the relative spectral power distribution of the illumination source and the spectral characteristic function of the object itself, if it is a reflective object, then:
where ( and ( are the spectral reflectance and spectral radiance factor of the opaque object, respectively, which can be selected and used according to the geometric conditions during the actual measurement; p( ) is the relative spectral power distribution of the illuminator, which generally adopts the standard illuminator recommended by CIE, which is determined by the actual situation of the measured object; (is the color stimulus function.) Standard illuminators b, c, d65 can be used when observing objects under daylight, and standard illuminators a should be used when observing objects under light. In the formula for calculating the tristimulus value, the constant k10 is called the normalization coefficient, and it is calculated as:
In addition, in the calculation of the above tristimulus value, the selection of wavelength interval a should be determined according to the spectral characteristics of the measured color and the required calculation accuracy, and the wavelength interval that can generally be selected is 10nm, 5nm, 1nm, etc. In most cases, using 5nm, precise results can be given. If the accuracy of the calculation is not high, =10nm is sufficient, but when a very accurate calculation is required, δ =1nm can be used.
The international standard stipulates that when measuring the color of an object, the object should be measured under the standard illuminator D65 recommended by the CIE. The CIE standard illuminator D65 is based on a large number of data on the spectral radiation detection of sunlight in different places on the earth, and summarizes a set of relative spectral power distribution data, and its correlated color temperature is 6500K. The standard illuminator D65 approximates the mean natural daylight and is in good agreement with the scattered light and sunlight of the entire sky on a horizontal plane at the same time. Its correlated color temperature is the average of "global daylight" in the range of 6000 k-7000 k. "Global daylight", on the other hand, is generally considered to be the average relative spectral power distribution of sky light from two hours after sunrise to two hours before sunset and from cloudy to clear in any place. At present, the relative spectral power distribution of the actual light source developed for the standard illuminator D65 is not ideal, but as long as the overall spectral response characteristics of the light source, optical system and detector when detecting the color of the object conform to the tristimulus value of the standard illuminator D65 and the 10° field of view color matching function. i.e. luther condition is met
where s( is the relative spectral power distribution of the light source: sd( ) is the relative spectral power distribution of d65; x10( )y10( )z10( ) is the color matching function of the 10° field of view; kx, ky, kz are wavelength-independent constants; (is the relative spectral sensitivity of the detector before correction; X, Y, and Z are the transmittance of the instrument-specific filters. According to the relationship between the terms in the above equation, the relative spectral power function using the D65 light source is simulated. That is, the chromaticity value measured with the standard A light source is the same as the chromaticity value measured with the standard D65 light source.
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