Full grayscale color gamut

Color is an important attribute for humans to perceive the physical world. In the human visual system, different spectral distributions correspond to the different colors we perceive. Color perception plays an important role in object recognition, classification, and emotional cognition.

In the digital field, extensive and realistic color display technology has always been pursued by display devices, which leads to the concept of color gamut.

 

What is color gamut?

Light is a type of wave, and the wavelength range that the human eye can recognize is 380-750 nanometers, which is called visible light. By mathematically transforming the color and brightness information of visible light and mapping them onto a plane, we can obtain the famous CIE-1931 color gamut map, which represents the entire range of colors that the human eye can recognize.

In the digital field, the color range of a display depends on the mixture of red, green, and blue sub pixels. The maximum color range of a display can be represented by a triangle formed by connecting the extreme points of the red, green, and blue sub pixels. This area represents all the colors that the display device can display, also known as the color gamut of the display.



What is color gamut standard?

At present, the most advanced display technology cannot fully present all colors in the CIE-1931 color gamut, and the color range covered by various displays is also not the same. In view of this, different industries have developed their own corresponding color standards to achieve unified color management. These industries select specific regions as reference scales in the CIE-1931 color gamut map, and then define multiple color gamut standards to ensure consistency of color information in different application scenarios. So, different application scenarios may adopt different color gamut standards.

Common display color gamut standards include sRGB (default color gamut for Windows systems), Adobe RGB (commonly used for printing and photography), and DCI-P3 (commonly used for film production). These three color gamut are much smaller than CIE-1931, and the coverage area in the CIE-1931 color gamut chromaticity diagram is shown in the following figure.

Color gamut percentage refers to the ratio of the triangle area formed by the primary colors of a display and the three primary colors defined by the color gamut standard on the CIE-1931 chromaticity diagram. If the display reaches 100% of the color gamut standard, it means that the color gamut of this display covers the entire color space of the standard. When the proportion is greater than 80%, it can usually be called a wide color gamut.

2 sRGB Adobe RGB DCI-P3.png

Full grayscale color gamut

As a type of microdisplay, silicon-based OLED also possesses the property of color gamut. Full grayscale color gamut, as the name suggests, refers to the ability of a microdisplay to display images with the same range of grayscale color gamut.

Why is it necessary to propose a full grayscale color gamut for silicon-based OLED microdisplays?

This is related to the emission characteristics of silicon-based OLED pixels.

We know that in silicon-based OLED screens, colors are generated by mixing pixel arrays composed of R (red), G (green), and B (blue) sub pixels. Mixing different proportions of red, green, and blue light can result in a variety of colors. For example, a certain proportion of red, green, and blue light mixture will produce white. This process relies on each sub-pixel emitting a specific intensity of light precisely, resulting in accurate color presentation at a macro level.

Schematic diagram of three primary colors. png

However, in silicon-based OLED microdisplays, the current supplied to each pixel is extremely small, especially for low brightness pixels (i.e. low grayscale areas of the image). For low brightness pixels with adjacent brightness levels, the current difference between them is often very subtle. But this subtle difference in current has an amplified effect on color, which may lead to a decrease in color gamut in low grayscale areas.

Analog driving controls the brightness of pixels by changing the current or voltage input to each pixel. In an ideal situation, precisely adjusting the current or voltage of each sub-pixel can accurately control the intensity of the light it emits, thereby mixing accurate colors. However, in practical applications, the slight difference in current between low brightness pixels can lead to inaccurate sub-pixel luminescence intensity, resulting in mixed colors deviating from expectations.

For example, to mix a certain type of blue in the dark area, a specific ratio of red (R), green (G), and blue (B) light is required. However, due to the slight deviation in the current of low brightness pixels, the emission intensity of blue sub pixels is inaccurate, resulting in a deviation in the final mixed color. In other words, when the colors in low grayscale areas become distorted, it means that the range of colors that the screen can accurately display will shrink, that is, the color gamut will decrease.

5-color grayscale image-2.png

Digital driving is the process of adjusting the lighting and extinguishing time ratio of each pixel to achieve brightness control. Specifically, when displaying pixels with lower brightness, digital drivers can set shorter lighting duration and longer extinguishing duration, achieving the desired low brightness presentation effect through precise control of the time ratio. This eliminates the need to rely on distinguishing small differences in current.

For sub pixels, the same principle applies. Its brightness can be precisely controlled based on digital signals, and when performing color mixing, it can strictly mix various colors according to specific proportions of R, G, and B light, ensuring the authenticity of colors and maintaining the same color gamut that can be displayed on the entire screen in the full grayscale range.

4 color gamut contrasts. png

 

In the practical application of silicon-based OLED microdisplays, the full grayscale color gamut plays a profound role.

In terms of XR experience, the full grayscale color gamut can create more realistic visual effects and enhance users' immersion. Once the color gamut changes, the colors in the virtual scene will become unnatural, thereby disrupting immersion.

In the process of industrial integration, the authenticity of colors is of great importance. Taking the field of medical imaging as an example, the full grayscale color gamut helps to accurately present the true color of human tissues, assisting medical staff in making precise judgments on the actual condition of patients; On the contrary, changes in color gamut may cause color deviation, leading doctors to misjudge the type or severity of lesions.

 

Nowadays, wide color gamut has been widely recognized by display manufacturers and users. However, ensuring consistent display performance in different scenarios is also an important feature of high-end products.