Properties of three colors

Properties of three colors:
The quantitative representation of color perception difference is based on the difference of three color attributes: lightness, hue and chroma. The lightness difference represents the difference in depth, the hue difference represents the difference in hue (i.e., red or blue), and the chromaticity difference represents the difference in brightness. Color difference assessment is very important in industry and commerce. It is mainly used for color matching in production and color quality control of products.
Definition and quantification of color.

Color space: RGB
As early as 1704, Newton proposed that the essence of color is light; In 1854, Glassman summarized Glassman’s law of color mixing. Then in 1855, Maxwell put forward the idea of mixing red, green and blue light to produce various colors.
So can red, green and blue match all colors?
This question depends on “whether all colors can be divided into red, yellow and blue combinations”. That is, can the combination of red, yellow and blue light produce all the colors that human beings can perceive?

Then the scientists began various experiments, and finally found that the answer was: yes, but not.

Through the test, after the spectrum of a specific color is obtained, the intensity value of each wavelength of the color is known, and the intensity value of the equivalent trichromatic light (RGB) is also known.

In this way, we have the matching ratio of RGB trichromatic light equivalent to this specific spectrum.
This ratio can be used to roughly quantify the color.
The ratio after normalization is the so-called chromaticity coordinate.

In this way, all colors in nature can be distinguished by human eyes. As long as you can distinguish the color, you can use this method to quantify. Although color can also be described (uniquely) by spectral curve! From 780 nm to 380 nm, the number is huge and very not intuitive.

Such naming is unrealistic. Therefore, not all colors can be divided into red, yellow and blue combinations.
However, if the RGB ratio value is used for naming, only three data are needed (after normalization, only two data are needed).

It took three hundred years for mankind to reach this point.
On this basis, another mathematical coordinate transformation is carried out (coordinate is changed from rgb to xyz)

In 1931, CIE established a series of color space standards representing the visible spectrum. Since any color can be mixed by RGB three primary colors, CIE-RGB primary color system is defined. However, this system has an obvious disadvantage. When calculating the tristimulus value of color, there will be negative values, which brings inconvenience to a large number of calculations. Since any primary color system can be converted from one system to another, people can choose any desired primary color system to avoid negative values and use it easily. Based on this, CIE also recommended the CIE-XYZ system, which uses the imaginary X, Y and Z primary colors, which do not correspond to the visible colors.

The stimulus values X, Y and Z of the three primary colors of CIE-XYZ are very useful for defining colors, but their disadvantages are complex and not intuitive. For a given color, if its brightness is increased, the luminous flux of each primary color also needs to be increased proportionally, and the chromaticity value is only related to the wavelength (hue) and purity, and has nothing to do with the total radiant energy. Therefore, when calculating the chromaticity of a color, normalize the X, Y and Z values relative to the total radiant energy=(X+Y+Z), and the color matching equation can be normalized to x+y+z=1. According to the color coordinates (x, y), z can be determined, but the three primary color stimulus values X, Y and Z cannot be derived only from x and y, and the Y value with brightness information needs to be used, which is consistent with the Y stimulus value in XYZ. Thus, CIE-xyY color space is defined.

Of course, in this way, the “brightness” information contained in the tristimulus value is completely lost, leaving only the information of relative ratio. So CIE 1931 XYZ chromaticity diagram can only see the information of chromaticity (hue, saturation), but not brightness.

Color space: Munsell
In 1905, the American painter Munsell summarized the experience and research results of color scientists for two centuries and put forward the Munsell color system.

Munsell color classification method belongs to pure psychological color classification method. Its three-dimensional space represents three basic visual parameters of color, namely brightness, hue and saturation.

As the real color samples of Munsell color system, Munsell atlas has been widely used in various color-related industrial production and color scientific research such as textile, dye, paint, ink, medicine, chemistry, photography, color television, etc.

Color space: CIE Lab and CIE Luv
In order to measure and evaluate color differences more objectively and accurately, CIE formally proposed two improved uniform color spaces in 1976, namely CIE1976L * u * v color space and CIE1976L * a * b color space. The two values can be converted to each other. In particular, CIE1976L * a * b color space was the color space with good effect at that time, and was widely used.

L indicates light and dark,+indicates light, – indicates dark; A is red and green,+is red and – is green; B is yellow and blue,+is yellow and – is blue.

The distance between the XYZ system and the two colors represented on its chromaticity diagram is inconsistent with the change perceived by the color observer. This problem is called the perceptual uniformity problem. In order to solve the problem of perceptual consistency in color space, experts have carried out nonlinear transformation on CIE-XYZ system and formulated CIE-L * a * b * color space. The color space coordinate CIE-L * a * b * The color space uses L value to represent the brightness of the color, a value to represent the green-red value of the color, and b value to represent the blue-yellow value of the color. The values of L, a and b can be calculated by XYZ, and the calculation formula is as follows:

Where XiYiZi is the tristimulus value of the reference sample or the tested sample, and XnYnZn is the tristimulus value of the standard illuminator.

CIE-LCH color space is converted from CIE-L * a * b * color space, which uses L to represent the brightness value; C represents the saturation value and H represents the cylindrical coordinates of the hue angle value. The color space coordinates are shown in Figure 2. In daily life, people describe the three attributes of color as lightness L, hue H and saturation C. So using CIE-LCH color space to describe color is more in line with people’s habits of color description in daily life.

If we judge a color by a set of L * a * b * or Lch values alone, it is not of great practical significance, but when we compare two colors, we can judge the difference between them by the parameter difference of the two colors. We can easily know the color status of the current product by comparing the parameter values of the product and the standard color sample. Through two sets of L * a * b * values, we can calculate the color difference between the two colors. The color difference is calibrated with △ Eab *, △ L *, △ a * and △ b *. CIELAB color difference formula is as follows:

△ E * The size of the total color difference, △ L * large indicates white, △ L * small indicates black, △ a * large indicates red, △ a * small indicates green, △ b * large indicates yellow, and △ b * small indicates blue.

In the L *, a *, b * rectangular coordinate system, the cylindrical polar coordinate system of L *, c *, h * can be derived.

Color space: LCh, CMYK, etc
Now there are still many color spaces in different industries. For example: LCh, CMYK, Hunterlab, etc. L is the brightness value, C is the color saturation, and h is the hue angle.

The color meter of LISUN can meet the above color space to define and quantify colors, thus achieving the purpose of color management.

LISUN lauched Portable Colorimeter/Chroma Meter is an innovation color measuring tool with powerful configuration to make color measurement easier and more professional. It support Bluetooth to connect with Android and ISO devices. Portable Colorimeter/Chroma Meter will take you into a new world of color management. It can be widely used to measure color value, color difference value and find similar color from color cards for printing industry, paint industry, textile industry, etc.

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