Color basics – Lab, Lch color stereoscopic space

1.1. Definition of “L, a, b“:

During the 1950s-1960s, Hunter Lab established color spaces, represented in three-dimensional coordinates of L a b
“L” represents the brightness of the object: 0-100 means from black to white
“a” represents the reddish-green color of the object: positive values indicate red, negative values indicate green
“b” represents the yellow-blue color of the object: positive values indicate yellow, negative values indicate blue
1.2. Establish the meaning of the “L a b” color space
The “L a b” color space forms a unified color, color description language, and operating specifications. The “L a b” color space is one of the current universal spaces for measuring the color of objects, which is widely used in most fields. L a b can be used to represent the reflected or transmitted color of the physical sample. Unlike the RGB and CMYK color spaces, Lab Color is designed to approximate human vision, it is committed to perceptual uniformity, and its L component closely matches human perception of luminance.

Chromatic aberration is the concept of three-dimensional space using uniform color space, and the perceptual color difference between two colors can be calculated by formula, that is, the distance between two color points in the color space. Chromatic aberration is of great industrial significance. For example, in the printing and dyeing industry, in order to quantitatively evaluate the color quality of products, people hope to be able to measure color difference with a simple quantity, because the characteristics of color determine that it must be evaluated comprehensively with three attributes of color. The establishment of uniform color space makes the measurement of color and visual color difference closely related with instruments.
In 1976, CIE recommended two chromatic aberration formulas that were comparable in terms of the perception of chromatic aberration: the CIE LAB chromatic aberration formula and the CIE LUV chromatic aberration formula.

CIE LAB chromatic aberration formula:
CIE 1976 (L* a* b*) chromatic aberration formula, or CIE LAB chromatic aberration formula:

Where, the brightness difference:
Chromaticity difference:

where , are the brightness and chromaticity indices of samples 1 and 2, respectively, which can be determined by:

The calculation is obtained, where X, Y, Z are the triple stimulus values of the sample color; is the triple stimulation value obtained by the human eye after the surface of a completely diffuse object is passed through the CIE standard illuminator, wherein.

2.1 Definition of “L” “C ” “h”:

In the figure, “HUE” is the hue, represented by “h”, “CHROMA” is the chromaticity, and “C” is denoted by “C”. “L” stands for brightness, “C” is chromaticity, which is the degree of color saturation, and “h” is hue, which is the overall tendency of color. “L” , “C” and “h” are the three elements of color.
2.2. Establish the meaning of “L”, “C”, “h”
Brightness is the eye’s perception of the light source and the brightness of the surface of the object, and the brightness determines not only the degree of illumination of the object, but also the reflection coefficient of the surface of the object.
Saturation (chromination, freshness) usually refers to the vibrancy of a color. From a scientific point of view, the vibrancy of a color depends on the degree to which the hue emits a single light.
The characteristics of the hue are determined by the spectral composition of the light source and the perception of the human eye by the ratio of radiation of each wavelength reflected on the surface of the colored object. It is the qualitative appearance that color can present, and it is the standard for distinguishing various colors accurately. In many cases, we generally use (L a b) color space to describe color, but in some cases, such as brightness, saturation, but hue changes, we generally use L c h to describe color. For example, the classification of raisin color, measuring the change in the color of honey, and so on.

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