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2024-12-20
As a tool for color quantification, the colorimeter uses the internationally accepted CIELab color space when measuring color, and digitally expresses the color in three dimensions: L, a, and b. This article introduces the range of the colorimeter Lab value and the meaning of the Lab color difference value.
Colorimeter Lab Value
A colorimeter is a device used to measure color differences and is widely used in multiple industries to ensure the accuracy and consistency of product color. In the measurement results of a colorimeter, the Lab value is an important parameter that represents the three dimensions of color and is the scientific code behind color.
Lab values are based on the Lab color space recommended by the International Commission on Illumination (CIE). The Lab color space is a three-dimensional color model in which L represents brightness, a represents the color change from green to red, and b represents the color change from blue to yellow. In this model, each color is defined as a point containing three coordinates, which correspond to the values of L, a, and b respectively.
First, the L value represents the brightness of the color, ranging from 0 to 100. The larger the L value, the brighter the color; the smaller the L value, the darker the color. Brightness is an important factor in color perception, which determines the lightness or darkness of the color.
Secondly, the a value represents the position of the color on the red-green axis. Positive values represent red, and negative values represent green. The larger the absolute value of the a value, the higher the saturation of the color. For example, a color with a large positive a value may be a bright red, while a color with a large negative a value may be a dark green.
Finally, the b value represents the position of the color on the yellow-blue axis. Positive values represent yellow, and negative values represent blue. Similar to the a value, the larger the absolute value of the b value, the more saturated the color. For example, a color with a large positive b value might be a bright yellow, while a color with a large negative b value might be a dark blue.
Through these three Lab values, we can fully describe the characteristics of a color. For example, if we say that the Lab value of a color is L=50, a=30, b=20, then it means that the color has medium brightness, a hue that tends to be red, and a saturation that tends to be yellow. Such a description is more accurate and specific than simply using adjectives to describe the color.
In a colorimeter, by measuring the light reflected by an object, the Lab value of the object's color can be obtained. These Lab values are compared with the Lab values of the standard color to calculate the color difference. In this way, we can accurately determine whether the product color meets the expected requirements.
In general, the Lab value of a colorimeter is the key to deciphering the scientific code behind color. It provides a comprehensive description and analysis of color, allowing us to measure and compare colors more accurately and objectively. Whether in industries such as plastics, coatings, textiles or printing, mastering and understanding Lab values is an important part of ensuring product quality and customer satisfaction. By using a colorimeter and the corresponding Lab value judgment standard, we can achieve more accurate color matching and control, thereby improving product quality and competitiveness.
Colorimeter Lab Color Space
The L* value range is from 0 to 100, and when L=50, it is equivalent to 50% black; the value ranges of a* and b* are both from +127 to 128, where +127a is magenta, and gradually transitions to -128a when it turns green; based on the same principle, +127b is yellow and -128b is blue.
From the concept of Lab mode, we know that a*: dark green → 50% gray (neutral gray) → magenta. In the grayscale image of this channel, dark represents green: gray less than 128 (ie 50%) is green, the closer the grayscale value is to 50% gray, the lower the saturation of green, the farther the grayscale value is less than 50% gray, the higher the saturation of green. The bright end is magenta, and the value greater than 128 (ie 50% gray) is magenta. The brighter it is, the higher the saturation. Conversely, the closer the value is to 128 degrees gray, the lower the saturation.
The b* channel displays from cyan → 50% gray (neutral gray) → yellow. The bright area of the channel grayscale image is the yellow area. The higher the brightness, the higher the saturation, the closer it is to 50% neutral gray, and the lower the saturation. The dark area of the channel grayscale image is the blue area. The darker the display area, the higher the saturation, the closer it is to 50% neutral gray, and the lower the blue saturation.
Colorimeter Lab color difference value
Color difference refers to the difference between two colors, that is, the difference in hue, saturation and brightness. A colorimeter is an instrument that can automatically compare the color difference between a sample and the product being tested. It measures and outputs three sets of data, L, a, and b, and the color difference data after colorimetry, such as ΔE, ΔL, Δa, and Δb.
In 1931, CIE (International Commission on Illumination) established a series of color space standards for representing the visible spectrum. The basic CIE color space standard is CIE-XYZ, which is based on the visual ability of a standard observer, that is, it reflects the range of colors visible to the standard human eye. Based on CIE-XYZ, there are also standard color spaces such as CIE-xyY, CIE-Lab, and CIE-Lch.
A colorimeter is an optical testing instrument designed and developed in accordance with CIE standards. It simulates the human eye's sensitivity to red, green, and blue light, and then measures and calculates the three stimulus values X, Y, and Z through photoelectric integration or spectrophotometry, which are then converted into chromaticity parameters such as L, a, and b using relevant formulas. Since different types of colorimeter use different color measurement methods, the accuracy of their color measurement results will also vary. Therefore, it does not make much practical sense to simply use a set of Lab values to judge a color. However, when we compare two colors, we can use the Lab difference between the two colors to determine the difference between them. Among them:
"L" represents the brightness of the object: 0-100 represents from black to white
"a" represents the red and green color of the object: positive values represent red, negative values represent green
"b" represents the yellow-blue color of the object: positive values represent yellow, negative values represent blue
All colors can be perceived and measured through the Lab color space. These data can also be used to express the color difference between the standard sample and the test sample, and are usually expressed as △Eab (total color difference) △L, △a, △b. The calculation formula is as follows:
△L=L sample-L standard (brightness difference), △L+ means white, △L- means dark
△a=a sample-a standard (red/green difference), △a+ means reddish, △a- means greenish
△b=b sample-b standard (yellow/blue difference), △b+ means yellowish, △b- means bluish
Among them, △L*, △a*, △b* are the differences in lightness L* and chromaticity index a*, b* between the reference sample and the sample being tested. The color difference value is expressed as ΔE*, which is the comprehensive total color difference. The specific calculation method is related to the brightness difference △L, the red-green difference △a, and the yellow-blue difference △b.
