What is color gamut?

The development of LCD screens has gone through several stages, including the upgrade of backlight from CCFL to LED light strips, the transformation of the body from heavy to thin, the expansion of color gamut from ordinary to high color gamut, and further development to quantum dot technology, from non-dimmable to regional dimming. It has been constantly improving to provide better visual effects.

For users such as designers who have high requirements for color, the color gamut parameters of the display are crucial. Therefore, when selecting a display, the color gamut parameters are a very important consideration.

This article will systematically introduce the definition and standards of the display color gamut, explore various mainstream methods to improve the high color gamut through backlight technology, and look forward to the future prospects of high color gamut display technology.

1. Definition of color gamut

Color gamut is color space, color refers to color, and gamut refers to range, which is the sum of all visible light. There are two ways to represent it in two-dimensional space: 1) using x, y coordinate system (CIE 1931 non-uniform chromaticity space); 2) using u', v' coordinate system (CIE1976 uniform chromaticity space). The position marked with color on the chromaticity space diagram is the visible light color area, which is a horseshoe shape.

So what is a color gamut chromaticity diagram? We all know that red, green, and blue are the three primary colors, and any color that can be recognized by us is a combination of three different color spectra.

In 1931, the CIE International Illumination Association proposed the CIE-XYZ color gamut chromaticity diagram, which is the color specification commonly used in the industry.

The CIE-XYZ color gamut chromaticity diagram shows the range of all colors that the human eye can perceive. The horizontal and vertical coordinates represent the stimulus value, and the color gamut consists of a straight line and a curve. The wavelength of light marked on the curve is in nm.

CIE-1931 color gamut chromaticity diagram

In the figure above, the inverted "U"-shaped area surrounded by dotted lines represents the color range visible to the naked eye. The triangles surrounded by the other three color lines represent the color range that can be restored by each standard.

In fact, the most advanced display technology still cannot fully realize all the colors of CIE-1931, so according to the application in photography, videography, printing and other fields, various industries have formulated corresponding color standards, and selected specific areas in the CIE-1931 color gamut chromaticity diagram as scales to define a variety of color gamut standards.

2. 4 common color gamut standards

At present, there are generally four most common computer monitor screen color gamut standards on the market, namely sRGB, NTSC, Adobe RGB, and DCI-P3. The difference is mainly the breadth of the range of colors covered.

The NTSC color gamut was customized by the National Television Standards Committee of the United States in 1953. The purpose was to customize a set of color standards for the CRT color TV that had just appeared at that time. The NTSC TV standard they launched is a set of radio and television transmission protocols, which are used in the radio and television systems of the United States, Japan and other countries. Of course, this also means that the NTSC color space is more used in the television industry.

The sRGB color space is a color space jointly developed by Microsoft and HP in 1996. Due to the strong user base of Windows, almost all mainstream devices, from PCs and Macs to cameras, scanners, printers, projectors, etc., support sRGB. The color space of most content on the Internet, including text, pictures, and videos, is also based on sRGB.

Adobe RGB is a color space launched by professional software manufacturer Adobe in 1998. The original intention was to include both sRGB (a color space commonly used in computers) and CMYK (a color space commonly used in printing), so that digital photos taken can not only be displayed and edited normally on computers, but also be printed with lossless and correct colors. Adobe RGB covers a wider range of colors than sRGB, and is favored by designers, so it is widely used in professional photography and post-production fields.

DCI-P3 is a color space used in digital cinemas, so it is often promoted as a "movie color space." It is a color gamut standard dominated by human visual experience, which matches the full color gamut that can be displayed in movie scenes as much as possible, and has a wider range of red/green systems. It is currently widely used in Apple products, so if you use MAC, try to choose a monitor with high DCI-P3 color coverage to achieve good results.

Rec. 2020 is a wide color gamut standard suitable for HDTVs and future 4K TVs.

3. How to choose a screen by color gamut?

Adobe RGB is a color gamut standard launched by Adobe. For users in the photo editing, color grading, video editing, printing and publishing industries, and users with high color requirements, you can pay more attention to the color gamut display of Adobe RGB values.

The sRGB color gamut standard is a definition proposed for computer external devices. For ordinary office and web browsing, just buy sRGB color gamut devices.

NTSC, as a TV standard, also has the widest color gamut among the three. So the radio, television and film and television industry practitioners among the monitor users can mainly refer to its values. In the LCD liquid crystal display industry, it is usually benchmarked against the NTSC color gamut standard.

The DCI-P3 color gamut is suitable for film and television practitioners.

Fourth, factors affecting the size of the color gamut

Two direct factors affecting the size of the color gamut: the color filter (CF) used on the LCD glass; backlight design.

It is remixed by R/G/B after the transmittance CF. Different models of OC use different color filters, which requires us to use different LED white light color areas to adjust the white point color coordinates of the LCD display.

Backlight design requires that the spectrum peak of the LED white light RGB is close to the RGB filter peak of CF, and at the same time, the half-wave width of the RGB three colors is as narrow as possible to reduce the cross-effect of RGB, so as to obtain a higher color gamut value.

Five, common methods to improve the color gamut

After the LCD glass is confirmed, the CF is also fixed. The key factor to improve the color gamut of the LCD display is the backlight. In the backlight design, there are two ways to improve the color gamut:

LCD liquid crystal itself does not display images. The reason why images can be seen is that electrical signals must be added to the liquid crystal and a backlight is required. In the structure of the liquid crystal glass, the color gamut is affected by the color filter (Color Filter, abbreviated as CF), which consists of three filters: red, green, and blue. Only light sources with a spectrum close to the filter can pass through the filter. After the LED white light passes through the CF, a new mixed white light is obtained.

1. Use high color gamut LED to improve color gamut

White light LED with ordinary color gamut is composed of blue light chip + Yag powder, and the NTSC color gamut is about 72%. There are many ways to realize high color gamut LED. The following is a comparison of the respective solutions, see the figure below.

Chip + green powder + new red powder solution, the key to realizing high color gamut LED lies in the selection of parameters such as the peak value and half-wave width of the color powder. The color powder spectrum is selected to match the color filter spectrum, and the half-wave width of the emission spectrum is narrow, so as to effectively improve the LED color gamut.

Here we focus on the new red powder KSF. KSF, KGF, and KTF are all fluoride phosphors, of which KSF is a cubic crystal, and KGF and KTF are hexagonal crystals. New red powder (KSF) is potassium fluorosilicate excited by tetravalent manganese, which is widely used in high color gamut LED. KSF Phosphors are hygroscopic and easily oxidized.

At high temperatures, they easily undergo reversible chemical reactions with water, and the color of the split changes from orange to brown. The brightness of fluoride phosphors will decay greatly under high temperature conditions, and it can return to normal after returning to normal temperature. Due to the characteristics of fluoride phosphors, their storage conditions are very strict, and it is necessary to avoid damage to the powder by temperature and humidity; during the application process, materials with good air tightness and heat dissipation are required, so the LED bracket and glue must be selected in a targeted manner.

2. Use quantum dots to improve the color gamut

Quantum dots are semiconductor nanocrystals, and their main components are: zinc, cadmium, selenium and sulfur atoms. Quantum limits the area of electrons and holes, giving quantum dots a discrete energy level structure. Quantum dots emit colored light when stimulated by light or electricity. Different sizes of quantum dots will cause the spectrum of quantum dots to be excited to be in different bands. The size or different components of quantum dots can be adjusted according to needs, so that quantum dots emit a single and symmetrical spectrum.

The main characteristics of quantum dots are as follows: nanocrystals with a particle size of 1 to 10 nm; chemical reactions with water and oxygen will cause failure; they can emit light of a specific frequency under the action of electricity or light, and inorganic luminescent materials are more stable than organic luminescent materials, and have higher luminous efficiency; the luminescent color is single and pure, and the half-wave width is ultra-narrow (≤35 nm); the practical application is highly operable, and different colors of light can be emitted by simply changing the size of the quantum dots.

From an environmental perspective, quantum dots are divided into two types: cadmium quantum dots and cadmium-free quantum dots. At present, cadmium quantum dots are superior to cadmium-free quantum dots in color gamut and light efficiency, and the cost of cadmium-containing quantum dots is relatively low in high color gamut backlight design costs. The content of cadmium in quantum dot components is relatively low and within the scope of environmental protection regulations, so cadmium-containing quantum dots are widely used in the industry; cadmium-free quantum dots are harmless and environmentally friendly, and its breakthrough will be the next development direction of quantum dots.

In the field of display technology, the main applications of quantum dots include two aspects: based on the electroluminescent properties of quantum dots, develop quantum dot light-emitting diode display technology, namely QLED; based on the photoluminescent properties of quantum dots, make quantum dots into quantum films or quantum dot diffusion plates, and apply them to high color gamut backlight technology. When quantum dots are used in LED packaging, the problems of heat dissipation and water and oxygen barrier are difficult to solve. When applied to membranes and diffusion plates, the display effect is better and the reliability is stronger.

Sixth, the prospects of high color gamut backlight technology

Resolution and color gamut are the most intuitive feelings of users about a display device. At present, 4K/8K has met the user's needs for clarity to a certain extent, and the color gamut will be the hot spot that users will pursue next.

The improvement of color gamut allows people to understand the color display capabilities of the device more intuitively, which greatly improves the user's sensory experience. With the development of society and the improvement of material level, users' pursuit of electronic products is also constantly improving. In the next few years, the proportion of high color gamut will continue to increase, and the era of high color gamut display may be ushered in.

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