CIELab color space coordinates The meaning of Lab value measured by colorimeter

Colorimeters rely on the color and optical specifications established by the International Organization for Illumination (CIE). CIELab color space coordinates are the foundation of colorimeter development. Colorimeters calculate color using color formulas based on these coordinates, ultimately determining the three excitation values. Each color corresponds to a unique point in the CIELab color space.

△E total color difference

△L+ means white

△L-indicates darker

△a+ indicates reddish

△a- indicates greenish

△b+ indicates yellowish

△b- indicates bluish

 

scope	Color difference (tolerance)
0 – 0.25 △E	Very small or none; ideal match
0.25 – 0.5 △E	Tiny; acceptable match
0.5 -1.0 △E	Slight to moderate; acceptable in some applications
1.0-2.0△E	Moderate; acceptable in certain applications
2.0 -4.0 △E	There is a gap; acceptable in certain applications
4.0△E or above	Very large; unacceptable in most applications

 

The lightness index L* (brightness axis) represents black and white, 0 is black, 100 is white, and gray is between 0-100.

Chromaticity index a* (red-green axis), positive values ​​are red and negative values ​​are green.

Chromaticity index b* (yellow-blue axis), positive values ​​are yellow and negative values ​​are blue.

All colors can be expressed using the three values ​​of L*a*b*. The difference between the L*a*b* of the samples is expressed as ΔL*Δa*Δb*; ΔE* represents the total color difference.

If ΔL* is positive, it means the sample is shallower than the sample; if it is negative, it means the sample is deeper than the sample.

If Δa* is positive, it means that sample 2 is redder (or less greener) than sample 3; if it is negative, it means that sample 3 is greener (or less redder) than sample 4.

If Δb* is positive, it means that the sample is yellower (or less blue) than the sample; if it is negative, it means that the sample is bluer (or less yellow) than the sample.

A colorimeter, also known as a portable colorimeter, color analyzer, or colorimeter, is a simple instrument for measuring color deviation. It involves fabricating a filter with spectral characteristics similar to those of the human eye and using it to measure the light of a sample. The key is designing the spectral sensitivity of this sensor and being able to measure and display color differences under specific light sources using computer software. It is a commonly used instrument in scientific research and production.

In the field of color detection, colorimeters are used to unify color evaluation standards. A colorimeter can measure an object’s color using L, a, and b chromaticity values. By calculating the difference between the chromaticity values ​​of two color samples, product color differences can be evaluated. This article briefly explains the meaning and range of the Lab chromaticity value measured by a colorimeter.

In 1931, the International Commission on Illumination (CIE) established a series of color space standards for representing the visible spectrum. The most basic CIE color space standard is CIE XYZ, which is based on the visual capabilities of a standard observer and reflects the range of colors visible to the standard human eye. Based on CIE XYZ, other standard color spaces include CIE Lab, CIE xyY, and CIE Lch.

Currently, the CIE Lab color space is the most commonly used in the industry. However, simply using a set of Lab values ​​to represent a color is not very meaningful. However, when comparing two colors, we can use the difference between the two colors’ Lab values ​​to determine the color difference.

Colorimeter Lab color model description:

The Lab color model consists of three elements: lightness (L) and the color-related elements a and b. L* represents lightness, a* represents the range from magenta to green, and b* represents the range from yellow to blue.

The Lab color space is the color space of human vision and is a device-independent color model. It encompasses the full color gamut of RGB and CMYK and can also express colors beyond those colors. The Lab model can express all colors that the human eye can perceive.

Colorimeter Lab value range:

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, among which +127a is magenta, and when it gradually transitions to -128a, it becomes green; by the same principle, +127b is yellow and -128b is blue.

From the Lab mode concept, we know that a*: dark green → 50% gray (neutral gray) → magenta. In this channel’s grayscale image, dark represents green: gray values ​​less than 128 (i.e., 50%) are green. The closer the grayscale value is to 50%, the lower the saturation of green. The farther the grayscale value is from 50%, the higher the saturation of green. Magenta is the brightest color, with values ​​greater than 128 (i.e., 50% gray) representing magenta. The brighter the color, the higher the saturation. Conversely, as values ​​closer to 128 degrees of gray approach neutral gray (the darker the highlight), 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, 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, the lower the blue saturation.

Explanation of the meaning of the Lab value of the 3nh colorimeter:

The precision colorimeter mainly looks at the tested L, a, b and E values.

L: Indicates the brightness of the color. Positive numbers indicate whiter colors, while negative numbers indicate darker colors.

a: represents the red-green value, a positive number indicates a reddish color, and a negative number indicates a greenish color.

b: represents the yellow-blue value, positive numbers indicate yellowish color, negative numbers indicate bluish color.

C: Chroma, the ratio of color components to white components. 0 means no color, and 100 means a very bright color.

h: Hue angle, which uses angles to represent different colors.

E: Indicates the final total color difference value. There is a tolerance for the acceptable color difference.