CIE color space

In daily life, we often describe colors in the following languages: red, red, lemon yellow, medium yellow, brown, sapphire blue, sky blue...

In fact, the transmission of color information in this way is very inaccurate. Everyone has different definitions and opinions on color language. All of them cannot guarantee the accurate tradition of color information. This method is not suitable for color industry production and management.

Another phenomenon is also very common in daily life. It is the same image. Although the RGB or CMYK values ​​of the pixels are the same, the color perception of different monitors or different printers is different. It is often seen that manufacturers have different colors. The brand's mobile phone is compared to prove that the mobile phone camera display is the best. In the face of this situation, I just want to say that you put the photo on the computer display, is the effect so good? Similarly, you can use PS technology to re-image an image and then look good on a monitor. Changing a monitor may not have the effect.... The only truth is that RGB and CMYK are Device-dependent, which means that the color rendering effect is related to the carrier device.

Those who work in the color matching industry must know the same color material, and there are color differences between batches; when the same formula is transferred between different factories, the formula needs to be fine-tuned to compensate for the difference between the color batches. To meet the final color requirements. In the same way, the display screen manufacturer will also control the difference between the stations of each display. A larger advantage for smaller companies in large companies is that they can better control the differences between product batches and maintain better stability.


How can we achieve accurate color transfer? There are basically two ways to do this:

One is to buy color cards as a medium for color transfer, such as Sweden's NCS natural color card, Germany's RAL color card, American Pantone color card, China's CNCS color card, etc.; color transfer only need to buy the same color card , the color card can be transmitted, and the corresponding color card can be found by telling the other party that the color card is labeled. There are several major disadvantages in this way. 1 It is necessary to pay for the color card, and many are not cheap; 2 color cards will fade after a long time, and often see the colorist holding the color card that has been worn for a long time as the target color for color matching; 3 Even if it is a new color card, there are certain differences between different batches; 4 different color card systems are not connected between each other. For example, Pantone's color number does not correspond to the RAL color number. Of course, color cards are also good, one is more intuitive, and the other is to create income for those companies...


The second way is to digitize the color using the CIE color space. Commonly used CIE color spaces are CIE XYZ, CIE xy, CIE uv, CIE u'v', and CIELAB color space. The CIE color space is based on human eye characteristics and is device-independent. The same value means that the colors are the same. For example, the same CIELAB values ​​of the two color samples mean that the two color samples have the same visual perception color. The following describes each color space:


CIEXYZ color space

The following figure shows the CSY1931 and 1964 XYZ standard chromaticity observer spectral tristimulus values, which correspond to the color observation of the 2° field of view and the 10° field of view, respectively, where the Y curve of 1931 is consistent with the visual V(λ). The calculated Y value represents luminance information. Standard Chroma Observer Spectral Tristimulus Value Curve is the basic basis for all color calculation and color measurement instrument design. It shows the response of three photoreceptor cells of the human eye to different wavelengths. It can be seen from the figure that the response range of the human eye is basically At 400-700 nm, light in the range of more than 400-700 nm has minimal effect on human perception of color, which is also mentioned in my first article. The visible range can also be defined as 400-700 nm.

For the object color, the formula for calculating the CIEXYZ value is

S(λ) is the spectral power distribution curve of the light source, R(λ) is the spectral reflectance of the object, and x(λ), y(λ) and z(λ) are the spectral three-stimulus of the CIE1931 or 1964 standard chromatic observer. A value function, where k is the corresponding normalization coefficient such that the Y value of the fully diffuse reflector is 100.

For the color of the light source, the calculation formula is

CIE xy, CIE uv and CIE u'v' color spaces

With the above CIEXYZ value, the CIE chromaticity coordinate xy can be calculated by the following formula. Since the CIE xy chromaticity diagram is non-uniform, that is, the same color distance in different regions on the CIE xy map does not represent the same color difference.

In order to improve the non-uniformity of the CIE xy chromaticity diagram, in 1960 CIE officially recommended the CIE1960 Uniform Color Space (UCS), which is the uv chromaticity diagram. The calculation formula is as follows:

To further improve the uniformity of the chromaticity diagram, CIE released the 1976 UCS diagram, the u'v' diagram, in 1976, calculated as follows:

These color spaces are often used for color descriptions of light sources or display screens. For example, many display manufacturers often use RGB three primary colors to display the color gamut of their display screens in these color space positions. In fact, this is a one-sided evaluation. It is also extremely misleading, and the display performance includes many aspects. Let's talk about it next time.

CIELAB color space

Since the CIE XYZ color space is not uniform and cannot directly express the amount of visual perception, in other words, someone tells you a CIEXYZ value, you can't quickly respond to what color, how bright it is, etc. Therefore, CIE recommended the CIELAB uniform color space in 1976, mainly for the representation and evaluation of object colors, in order to more objectively and accurately measure and evaluate the difference in color. CIELAB color space is a Cartesian coordinate system, as shown in the following figure, L* indicates brightness, and its chromaticity coordinates a* and b* are red-green direction and yellow-blue direction, respectively.


among them


Where X, Y, and Z are the tristimulus values ​​of the color, and Xn, Yn, and Zn are the tristimulus values ​​that the CIE standard illuminator illuminates on the fully diffuse reflector to reflect into the observer's eye, where Yn = 100.

In the CIELAB color space, in addition to L*, a*, b*, chroma C*ab and hue angle hab are often used to characterize colors because they correspond to visual perception. The following formula can be used to calculate chroma and Hue angle:


For hue angle hab, when a*> 0 and b* > 0, 0° 0, 90° 0 and b* < 0, 270°


CIELAB color space


In addition, there is a caveat to the definition of the CIELAB value when defining the source and the standard observer field of view, such as CIELAB (D65, 10 °), which indicates that the same color sample has its CIELAB value under different light sources. Different visual perceptions that also represent their color are different.

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