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2024-12-13
Colorimeter is an optical analysis instrument developed based on colorimetry. Therefore, various colorimetric parameters are involved in the process of color measurement, among which the common ones are: 45/0 and d/8, SCI and SCE, 2° and 10°, L*a*b* and Lab, XYZ and Yxy. Initial users are generally not very clear about these similar parameters, which are introduced below.
CIELab color space
A colorimeter is an instrument that simulates the human eye's color detection of an object. It absorbs the three natural light colors of red, green, and blue and converts them into color data and displays them through the instrument. Here we need to talk about the three elements of color recognition - light source, object, and detection head. The data detected by different light source angles and different detection heads are different, so we generally specify two angles: 45/0 and D/8.
45/0 means that the detector receives the light perpendicular to the material being tested on the horizontal plane, and the light source at a 45-degree angle to the detector shines on the object on the horizontal plane. The detector then receives the light reflected from the object, generates RGB/LAB values and displays them on the colorimeter screen to complete the test.
Combined with the previous principles, the difference of the D/8 colorimeter lies in the integrating sphere. Simply put, there is a measuring aperture (incident light) on the integrating sphere . The incident light illuminates the integrating sphere, and the inside of the integrating sphere is reflected and refracted to make the light source uniform. The receiving hole receives the light at an angle of 8° away from the central axis and calculates it through the color difference formula. The integrating sphere is a key part of the colorimeter. The color measurement accuracy has a lot to do with the integrating sphere. If the quality of the integrating sphere is not good, it will affect the service life of the color meter, thereby greatly increasing the maintenance cost. Therefore, using a high-quality integrating sphere will extend the service life of the instrument.
SCE: Specular Component Exclude. SCE refers to a color measurement method that excludes specular reflected light. This type of measurement result is similar to that observed by the naked eye.
The SCE method eliminates specular reflections, and the measurement results show the colors that the human eye actually sees. When observing an object, the human visual system receives information about the diffuse reflection of the object under most conditions, and does not pay attention to specular reflections. If the reflected light directly hits your eyes, it will only blind you, and you will not be able to see the colors.
Since the specular reflected light is filtered out, the properties of the object in the three elements of surface color (light source, object, observer) cannot be truly reflected, so the calculated results are different from the SCI mode. And it is greatly affected by the structure and roughness of the object surface. (Because the surface properties of the object will affect the reflected light)
SCI: Specular Component Include, SCI refers to a method of expressing color that includes specular reflected light; to this end, it minimizes the influence of the sample surface and is particularly suitable for color quality monitoring and computer color matching.
The SCI method including specular reflection light is adopted, and its measurement results completely include all surface reflection light of the object (including specular reflection and diffuse reflection). Therefore, its results can objectively represent the surface color of the object, regardless of the structure and roughness of the surface of the object, and are widely used in computer color matching, etc. That is, the brightness of the object will not affect the measurement results of the color data.
That is to say, the SCI mode includes all the light reflected by all objects, and the measurement results can truly display the true color properties of the object, which conforms to the definition of the three elements of surface color (light source, object, and observer).
Standard observer is a special term. If you understand the development process of the color system, it is not difficult to understand the 2° standard observer and the 10° standard observer (hereinafter referred to as observer). The essence of the 2° observer and the 10° observer is a set of color matching functions of standard observers. This set of color matching functions is obtained through statistics of some normal color observers and is related to the structure of the human eye.
The color matching function of the 2° observer was established in 1931, while the color matching function of the 10° observer was developed in 1964 based on the statistical data of the 2° observer. The field of view of the 10° observer includes the field of view of the 2° observer. The 10° observer has a more rigorous statistical basis because it is based on the statistical data of more normal color observers.
All parameters of the CIE1931-XYZ standard observer are applicable to the central observation condition of 2° field of view (applicable to 10-4° field of view). When observing objects under this field of view, the foveal cone cells of the human eye are mainly responsible. Therefore, the CIE1931-XYZ standard colorimetric observer is not applicable to color observation of extremely small fields of view less than 1° and color observation conditions of fields of view greater than 40°. Therefore, in order to adapt to color observation of large fields of view, people established the "CIE1964-XYZ colorimetric system" based on a large number of experiments.
Observing the measured object in the "CIE1964-XYZ complementary colorimetry system" covers both the cone cells in the center of the retina and the rod cells around the fovea of the retina, which is suitable for a large field of view of 10°. The human eye has a low ability to recognize the color of an object under a field of view of 2°, but has a higher accuracy and reproducibility in judging color under a field of view of 10°. Currently, most color measurements use a field of view of 10°.
CIEL*a*b* (CIELAB) is the most complete color model commonly used to describe all colors visible to the human eye. The asterisk (*) after L, a, and b is part of the full name because they represent L*, a*, and b*. Lab refers to Hunter Lab, which is a three-dimensional rectangular coordinate system based on the theory of color oppositions. It does not have an asterisk, unlike L*, a*, and b*. These two color systems are both based on the theory of color oppositions and are widely used. It is still necessary to distinguish between these two color systems.
Hunter Lab and CIELAB color systems have different calculation formulas. Although they are both derived from X, Y, and Z through mathematical calculations, the values of chromaticity are different. For example, for the same tile color - yellow, the color parameter values of the Hunter Lab system are L=61.42, a=+18.11, b=+32.23, while the color parameter values of the CIELAB system are L*=67.81, a*=+19.56, b*=+58.16.
There are differences in the uniformity of the two chromaticity spaces and visual perception. For example, although the CIELAB color space is a uniform color space recommended by CIE, in fact, the CIELAB space is still uneven for the color perception of the human eye. Take two color sample points in a certain area of the space (such as the red area) and compare them with two color sample points of the same distance in another area (such as the green area). It will be found that the visual difference between the two color samples in the red area is different from the visual difference between the two color samples in the green area, that is, the color width capacity values are not equal in different color areas.
CIEXYZ color space, also known as CIE1931 color space. CIE hopes to be able to describe any color that can be perceived by the human color vision system through the three components (X, Y, Z) in this color space, where X and Z are defined as the chromaticity of the color and Y is the brightness of the color.
The CIEXYZ color space is mainly used in analytical instruments such as colorimeters and digital color analyzers. It provides these instruments with the three-color light signals of transmission or refraction required for the samples to be analyzed. Although the CIEXYZ color space is also widely used in analytical chemistry, the colors it represents are not consistent with human eye perception, resulting in certain defects in the contrast of different colors. Therefore, this color model is often only used as a transitional color space for linear conversion to other color spaces.
The tristimulus values XYZ are very useful for defining colors, but it is not easy to directly visually observe the results. For this reason, CIE defined the Yxy color space in 1931, which describes colors on a two-dimensional graph but has nothing to do with brightness. Y is the reflectance of an object, expressed in % (compared with an ideal diffuse reflectance of 100%). It corresponds to the brightness of the three elements of color, and its value is equal to the Y value of the tristimulus value. The chromaticity coordinates x and y correspond to the hue and saturation of the three elements of color. The x and y chromaticity coordinates can be calculated by the following formula: x=X/(X+Y+Z)y=Y/(X+Y+Z) (where X, Y, and Z are the tristimulus values) The X-axis chromaticity coordinate is equivalent to the proportion of the red primary color; the Y-axis chromaticity coordinate is equivalent to the proportion of the green primary color. There is no Z-axis chromaticity coordinate (i.e. the proportion of the blue primary color) in the figure. Because the proportionality coefficient x+y+z=1, the coordinate value of Z can be deduced, i.e. z=1-xy.
