Research Progress on Improving LCD Grayscale Image Quality a

LCD plays an important role in the field of large-size display by virtue of its mature technology, supporting upstream, middle and downstream industrial chain, high cost performance and long life. In recent years, LCDs have been continuously improving their performance. In the process of competing with other types of display devices, LCD continues to develop and use new technologies, and has shown excellent performance in many aspects, such as obtaining 8K ultra-high resolution, high dynamics with 12-bit or higher grayscale. Range display, equipped with high-brightness and high-saturation backlight to obtain a good dark state and high contrast, with realistic colors and high refresh rate to meet the needs of dynamic display.
 
However, LCD is still facing huge challenges, especially the development trend of LCD is challenged by new display technologies such as organic light-emitting diode displays. New display technologies are in the ascendant, they have better performance in certain performance parameters, so LCD must step up innovation and optimization, especially urgently need to make improvements in its weak areas.
 
At present, LCD still occupies most of the large-size display, and it is developing toward larger and larger sizes. A large size means a large viewing angle, so the gray-scale image quality problem caused by a large viewing angle is an important task that must be faced and solved. There are many performance parameters related to LCD viewing angle, such as contrast, gamma shift, color shift, brightness and color gamut range, etc. These parameters will determine the LCD grayscale image quality.

2 contrast
LCD has the highest contrast and gray-scale contrast. The highest contrast refers to the ratio of the transmittance between the bright state and the dark state, and the gray-scale contrast refers to the ratio of the transmittance under a certain gray level to the transmittance of the dark state. Since the LCD was invented, people have conducted continuous research on its contrast. In general, the contrast of LCD has mainly undergone the following several developments. In the early days, people proposed some display modes with wide viewing angles, which improved the contrast of LCDs under oblique viewing angles, enabling them to be viewed at large viewing angles. During this period, in order to further improve the contrast, people also proposed the use of optical compensation film compensation method, and also developed different compensation methods for different display modes, to obtain a higher contrast. In recent years, in order to make the contrast of the LCD comparable to that of an active light-emitting display, people have proposed the use of high-brightness Mini LED (Light-emitting diode) backlight, local dimming technology, dual-box display technology, etc. These technologies have made the contrast of LCD another qualitative leap, and also helped its development in the field of large-size displays.
 
In the 1970s, the market was dominated by the simpler process of twisted nematic (TN: twisted nematic) LCD. Its front-view contrast was low, and the contrast was strongly dependent on the viewing angle, so it needed to be improved. Its contrast. To solve this problem, people developed some display modes with wide viewing angles at the end of the 20th century, such as: in-plane switching (IPS: in-plane switching) , fringe field switching (FFS: fringe field switching)  And optical compensated bend (OCB: optical compensated bend)  and other display modes. In addition, the liquid crystal molecules in the vertical aligned (VA: vertical aligned) LCD are arranged almost vertically, so under the cross-polarized light, the VALCD has a higher front-view contrast. In 2002, Kikuchi et al. prepared a polymer stable blue phase liquid crystal (BPLC: blue phase liquid crystal) . The BPLC temperature was expanded to a larger range including room temperature, and its stability was guaranteed . Under normal conditions, BPLC is optically isotropic, so BPLCD also has a higher contrast , and also has the advantages of sub-millisecond response speed and no orientation, making it considered as the next generation liquid crystal display mode.
 
After using a suitable optical compensation film in LCD, the dark state light leakage can be improved, and the contrast can also be improved . The optical compensation film is an anisotropic transparent polymer material. Commercially, this layer of polymer material is usually added to the inside of the polarizer to form a wide viewing angle polarizer. The wide viewing angle polarizer can play two roles: one is to correct the extinction ratio of the polarizer under the oblique viewing angle; the other is to compensate the phase change of the liquid crystal in the off-axis direction, reduce the change of the phase retardation, and achieve a better dark state. Figure 1 shows the main components of the LCD, the position of the wide viewing angle optical compensation film, and the display effect before and after adding the optical compensation film . It can be seen that before and after adding the compensation film, the image quality under the large viewing angle has been significantly improved.

LCD Display
For different display modes, the types of optical compensation films that need to be added are also different Generally, LCDs whose initial directors are arranged horizontally need to add nx>nz>ny type biaxial compensation film or uniaxial compensation film group, such as TN, IPS and FFS display modes. After adding the compensation film, their front-view contrast can be significantly improved, and the contrast under large viewing angles can also be improved. However, the LCD whose initial directors are arranged vertically needs to add a biaxial compensation film or a uniaxial compensation film group of type nx>ny>nz, such as VA display mode. VALCD itself has a high front-view contrast. After adding a compensation film, the contrast under oblique viewing angles can be improved.
There are two ways to achieve local dimming technology: one is to use a high-brightness LED light source and a dimming light valve, which can control the brightness of different areas by controlling whether the light valve transmits or not; the other is to use a dot matrix Mini The LED acts as a backlight source, and the brightness of different areas can be controlled by lighting the Mini LEDs in different positions.

3 Gamma shift
The term "gamma" is derived from the power index of the cathode ray tube grid voltage, which is used to describe the electron flow density of the cathode ray tube, so the gamma index determines the brightness of the pixel. Later, the gamma index has been used until now to describe the relationship between grayscale and brightness in various display devices. Since the liquid crystal material is anisotropic, the LCD will exhibit different electro-optical properties at different viewing angles, and the relationship between gray scale and brightness at each viewing angle will be separated from each other, which is gamma shift.
Figure 4(a) shows the color image of a certain monitor when viewed upright. In this image, the middle 4 pictures are color pictures with different tones, and the black and white grayscale cards with different grays are on the periphery. When viewing the image from an oblique angle of view, the colors of the four color images in the middle have changed, and the grayscale sequence of the grayscale card has also been disordered, and the phenomenon of grayscale conversion to color even appears on the far right of the image, as shown in the figure. As shown in 4(b). This shows that the LCD image will have a gamma shift under oblique viewing angles, and it is more obvious under large viewing angles.

The viewing angles of different positions on the display are different, and the corresponding gamma curves are also different, as shown in Figure 5 . For a large-size LCD, even if the viewer is directly in front of the center of the LCD screen, the images from the four corners still have a larger oblique viewing angle. Although the LCD drive circuit contains a gamma correction module, it can only adjust the overall image of the LCD to an appropriate gamma value, and cannot solve the problem of gamma offset at various viewing angles. Therefore, the LCD must have a gamma offset.

LCD Display

The viewing angles of different positions on the display are different, and the corresponding gamma curves are also different, as shown in Figure 5 . For a large-size LCD, even if the viewer is directly in front of the center of the LCD screen, the images from the four corners still have a larger oblique viewing angle. Although the LCD drive circuit contains a gamma correction module, it can only adjust the overall image of the LCD to an appropriate gamma value, and cannot solve the problem of gamma offset at various viewing angles. Therefore, the LCD must have a gamma offset.

Generally, people quantitatively evaluate the gamma shift of a display device through the average brightness difference between different gray levels. In 2004, Kim et al. proposed the off-axis image distortion index D(θ, ϕ) to evaluate the gamma shift of the LCD at a certain oblique viewing angle, as shown in equation . Among them, θ represents the polar angle, and ϕ represents the azimuth angle. If the polar angle form is used to present the off-axis image distortion index, the azimuth angle corresponding to the maximum off-axis image distortion index needs to be specified to display the specific angle corresponding to the most severe gamma shift. In 2019, Guo et al. proposed to use the azimuth image distortion index DA(θ) to evaluate the gamma shift of the LCD under a certain viewing cone, thereby further improving the evaluation of the gamma shift, as shown in equation (2) As shown in. In the formula, θ represents the polar angle. The calculation needs to first find the gamma curve corresponding to the maximum and minimum gamma offset under a certain viewing cone, and then calculate the gamma offset between the two gamma curves. This index does not need to specify the specific azimuth angle, because it represents the maximum gamma offset within a certain viewing cone. In terms of discrimination conditions, when these two indexes are lower than 0.2, it can be considered that the gamma shift of the LCD is indistinguishable from the human eye.




 


 

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