Analysis of Micro LED large-screen display technology
red, yellow, green, white purple (bright purple)
cyan (greenish blue, aqua blue) blue (purplish blue, ultramarine)
In the information society, display products, as the main interface between people and information, have a significant impact on people’s lives. Given the rapid increase of information — in particular the replacement of traditional single text video information by the multimedia digital audio —higher demand of display quality and resolutuion is proposed for the display device.
The color gamut as a critical factor that aﬀects the color expression and color reproduction has become the focus of the display. Consequently, diﬀerent color gamut standards and the relationship between the color gamut and color reproduction of the display will be analyzed based on the principles of colorimetry in this article.
Principles of colorimetry
In a colorful world, humans can perceive diﬀerent colors due to the visual response generated after human eyes receive light signals of diﬀerent wavelengths.
There are two types of photoreceptor cells, rod cells and cone cells, on the retina of the human eyes. Rod cells have no color recognition function and mainly function in dark light conditions; while cone cells function in bright conditions. Cone cells can be further divided into red‐sensitive cone cells, green‐sensitive cone cells, and blue‐ sensitive cone cells, which are most sensitive to red, green, and blue, respectively.
The visual responses of these three cones on the retina to red, green and blue light can be combined to form diﬀerent color perceptions according to the principle of color mixing, and thus almost all colors in nature can also be synthesized from red, green and blue. Based on the above principles, red, green, and blue are usually used as the three primary colors in modern display technology, and most colors in nature can be reproduced by mixing the three primary colors at diﬀerent ratios, as shown in Figure 1.
The color mixing principle can be quantitatively described using a color equation. For color light C that needs to be displayed, the color equation may be expressed as C[C] = R[R] + G[G] + B[B], where C, R, G, and B represent the relative intensities of matched color light, red light, green light, and blue light, respectively.
This color equation indicates that R units of red primary color, G units of green primary color, and B units of blue primary color can be mixed to work out C units of color C, where R, G, and B are called tristimulus values.
CIE1931 standard chromaticity system
For comparison and unification, in 1931, the International Commission on Illumination (CIE) proposed a Standard Observer and Coordinate System, using 700nm, 546.1nm, 435.8nm as the wavelengths of the three primary colors of R, G, and B, and normalized the color equation into [C] = R/(R + G + B)[R] + G/(R + G + B) [G] + B/(R + G + B) [B]. The chromaticity coordinates r, g, b are defined, where r = R/(R + G + B), g = G/(R + G + B), and b = B/(R + G + B), and then the color equation can be expressed as [C] = r[R] + g[G] + b[B].
Since r + b + g = 1, the matched color C can be quantitatively characterized by any two of the tristimulus values.
Consequently, the CIE 1931 RGB chromaticity diagram characterizes colors quantitatively by using the (r, g) two‐dimensional coordinates, as shown in FIG. 2.
Due to the problem of color matching experiment, negative values exist in the CIE RGB chromaticity diagram, which is not conducive to calculation and understanding. Therefore, the CIE further proposed the CIE 1931 XYZ chromaticity system in which the three primary colors of the RGB system are replaced with three imaginary primary colors XYZ and the tristimulus values R, G, and B are mathematically transformed to tristimulus values X, Y, Z, which are all positive.
Correspondingly, the color equation is expressed as C[C] = X[X] + Y[Y] + Z[Z], which can be normalized into [C] = X/(X + Y + Z)[X] + Y/(X + Y + Z)[Y] + Z/(X + Y + Z)[Z]. The normalized equation can be further expressed as [C] = x[X] + y[Y] + z [Z], in which x=X/(X + Y + Z), y=Y/(X + Y + Z), and z=Z/(X + Y + Z). Since x + y + z = 1, the matched color can be determined by the use of just x and y. Taking x and y as the horizontal and vertical coordinates in a two‐dimensional plane, the CIE 1931 XYZ chromaticity diagram as shown in Figure 3 is obtained and colors can be determined by the coordinate of (x,y).
In the CIE 1931 XYZ chromaticity diagram, a horseshoe‐shaped curve can be obtained by connecting the visible light spectrum from 380 nm to 780 nm.
The numbers annotated next to the horseshoe‐shaped curve are the spectral wavelength values. The colors located on the horseshoe curve is monochromatic light and has the maximum saturation.
While the saturation gradually decrease with the location of the color go away from the edge to the interior of the horseshoe‐shaped area.
Color gamut of a display screen and color gamut standards
Color gamut can be understood as the range of colors that a display device can display. For the three‐primary‐color display most commonly used in modern display technology, based on the principles of color mixing, the color coordinates of the three primary colors of red, green, and blue used by the display can be positioned and then connected to obtain a color gamut triangle in the CIE 1931 XYZ chromaticity diagram.
The three vertices of the color gamut triangle are the coordinates of the red, green, and blue primary colors of the display device. While the area enclosed by the triangle covers all the colors that the display device can display by mixing the three primary colors. The larger the area of the triangle , the wider the color gamut of the display, and thus the richer the colors that can be displayed.
To facilitate information transmission, a series of color gamut standards are formulated in the display industry, and the most common standards are NTSC, Rec.709, DCI‐P3, and Rec.2020, as shown in FIG. 3.
The NTSC color gamut is a standard released by the National Television Standards Committee of the United States in 1953. This standard was customized for the CRT color TV at that time. It is too old to be applicable for modern display devices. Moreover, fewer modern display content creators use the NTSC color gamut, which means that the display products using NTSC as the color gamut cannot match with the display content.
Rec.709 color gamut standard is the most widely used standard currently, which is released by the International Telecommunication Union in 1990 for high‐definition television (HDTV). With the development of computer technology, sRGB color standard is developed in 1996 by Microsoft together with HP, Mitsubishi, Epson, and other manufacturers.
Anchored to the mass of Microsoft's users, the sRGB standard is supported by most digital image acquisition devices such as digital cameras, digital video cameras, scanners, and monitors.
The supports of both display and image acquisition devices make sRGB standard the most widely used color gamut standard. And thus, Rec.709 color gamut, which is exactly the same as sRGB color gamut, becomes the most prevalent color gamut currently.
DCI‐P3 color gamut is a wide color gamut standard introduced by the American film industry in 2005, and is the most commonly used color standard for digital film devices currently. Compared with the Rec.709 color gamut, the DCI‐P3 color gamut is wider in the green and red regions, resulting in a 25% larger color gamut,as shown in FIG. 3. DCI‐P3 standard become more and more prevalent with the advent of the 4K devices, as the Rec.709/sRGB is not enough to meet the display requirements.
Rec. 2020 is a color gamut standard that was introduced by the International Telecommunication Union in 2012 for ultra‐high‐definition (4K and 8K) devices. As shown in FIG. 3, Rec. 2020 color gamut covers the widest range among all color gamut standards currently.
As shown in FIG. 3, among the above color gamut standards, the Rec.709 color gamut has the smallest area, the DCI‐P3 color gamut can completely cover the Rec.709 color gamut, and the Rec.2020 color gamut can completely cover the DCI‐P3 color gamut.
Although the Rec.709 color gamut area is only 72% of the NTSC color gamut area, it can not be completely covered by NTSC color gamut in the blue area. In fact, the NTSC color gamut can only cover 82% of the Rec.709 color gamut.
When the color gamut of a display device can completely cover the color gamut of the picture to be displayed, the display can fully present the colors in the picture, and restore the creative intention of the video or picture creator. As the creators use Rec.709, DCI‐P3, or Rec. 2020 instead of NTSC as the color gamut to create videos or pictures, display devices use NTSC as the color gamut can not be connected with the videos or pictures directly. Since the NTSC color gamut standard cannot completely cover any of the three color gamut standards of
Rec.709, DCI‐P3 and Rec.2020, using NTSC color gamut as the specification of display devices can not indicate the color gamut coverage of the videos or pictures characterized by Rec.709, DCI‐P3, or Rec. 2020. In other words, the NTSC gamut index of the display device is less valuable for consumers, since it can not show the color reproduction ability of the pictures or videos source data using Rec.709, DCI‐P3, or Rec. 2020 gamut index.
Based on the principle of chromaticity, this paper expounds diﬀerent chromaticity standards and analyzes the relationship between the color gamut index and the color reproduction ability of the display device. As analyzed above, using NTSC color gamut as the index for LED displays can not characterize the color reproduction ability. While using the Rec.709, DCI‐P3, or Rec.2020 color gamut as the color gamut index is a scientific way to characterize the color reproduction ability of LED displays.
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