When working, the colorimeter generally relies on the CIELab color space, and outputs L, a, b values, and color difference values such as △L, △a, △b, △E after colorimetry by comparing the color difference values between the sample and the standard. This article introduces the range of L, a, b of the colorimeter and the meaning of Lab color difference values.

Colorimeter L, a, b values

Colorimeter L, a, b range:

As a photoelectric colorimeter, the measurement principle of the colorimeter adopts the CIELab colorimetric system of the International Commission on Illumination, which is the most widely used to measure the hue of an object. With the help of the stereoscopic representation method of uniform color, all colors are defined by the coordinates of the three axes of L*, a*, and b*. The CIELab color space uses three different coordinate axes of L*, a*, and b* to indicate the position and code of colors in the geometric coordinate diagram. It is based on the theory that a color cannot be both green and red at the same time, nor can it be both blue and yellow at the same time. When a color is represented by CIEL*a*b*, the L* axis represents lightness, black is at the bottom, and white is at the top. +a* represents red, -a* represents green, +b* represents yellow, and -b* represents blue. The color change of any color can be represented by the a* and b* values, and the level change of any color can be represented by the L* value. The three values of L*, a*, and b* can describe any color in nature.

The parameter L* is the lightness value of the color. Lightness refers to the brightness of the color. The lightness and darkness of various colored objects are caused by the difference in the amount of light they reflect. There are two types of color lightness: one is that the same hue has different lightness. For example, the same color appears bright under strong light, but appears dark and blurred under weak light; the other is that the lightness of pure colors of various colors is different. Each pure color has a corresponding lightness, such as yellow has the highest lightness, blue-purple has the lowest lightness, and red and green have intermediate lightness. The lightness value varies in the interval (0, 100). L*=0 indicates black, L*=100 indicates white, and the closer to 100, the brighter it is.

The color range displayed by the a* channel is: green - 50% gray (neutral gray) - red. In the grayscale image of this channel, the dark part, that is, less than 50% gray (the color scale value is 128), represents green. Moreover, the darker the color in the grayscale image, that is, the lower it is than 50% gray, the higher the saturation of green; the closer the grayscale value of the color is to 50% gray, the lower the saturation of green. At the same time, the bright part, that is, greater than 50% gray (the color scale value is 128), represents red. The brighter the color, the higher the saturation of red, and vice versa, the closer it is to 50% gray, the lower the saturation of red.

The color range displayed by the b* channel is: blue - 50% gray (neutral gray) - yellow. In the grayscale image of this channel, the dark part, that is, less than 50% gray (the color scale value is 128), represents blue. Moreover, the darker the color in the grayscale image, that is, the lower it is than 50% gray, the higher the saturation of blue; the closer the grayscale value of the color is to 50% gray, the lower the saturation of blue. At the same time, the bright part, that is, greater than 50% gray (the color scale value is 128), represents yellow. The brighter the color, the higher the saturation of yellow. Conversely, the closer it is to 50% gray, the lower the saturation of yellow.

Colorimeter Lab color difference value

The meaning of the Lab color difference value of the colorimeter:

Color difference refers to the difference in color perception between two colors expressed in numerical form. If two colors are calibrated according to L*a*b*, the total color difference and each individual color difference between the two can be calculated using the following formula.

Lightness difference: △L*=L*1-L*2

△L*＞0 means that the sample color is lighter than the standard color and has a higher brightness; if △L*＜0, it means that the sample color is darker than the standard color and has a lower brightness.

Chromaticity difference: △a*=a*1-a*2; △b*=b*1-b*2

△a*＞0 means that the sample color is redder than the standard color; if △a*＜0, it means that the sample color is greener than the standard color.

△b*＞0 means that the sample color is yellower than the standard color; if △b*＜0, it means that the sample color is bluer than the standard color.

Total color difference: ΔE = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2

△E (or △Eab) is the total color difference, which indicates the direction of color difference deviation. The larger the value, the greater the color difference.

Chroma difference: △C*ab=C*ab,1-C*ab,2

△C*ab＞0 means that the sample color is higher than the standard color and contains less "white light" or "ash"; if △C*ab＜0, it means that the sample color is lower than the standard color and contains more "white light" or "ash".

Hue angle difference: △h*ab=h*ab,1-h*ab,2

△h*ab＞0 means the sample color is in the counterclockwise direction of the standard color; h*ab＜0 means the sample color is in the clockwise direction of the standard color. According to the position of the standard color, it can be judged whether the sample color is green or yellow.