May.2020 29
Views: 1413
Discussion On LCD Soft Screen And Hard Screen Technology
In the past 10 years, wide-view LCD technology has been completely divided into two types: vertical-oriented soft screen and horizontal-oriented hard screen. PSVA, HVA (CSOT) and UV2A (Sharp) all belong to the soft screen VA technology, while the horizont
In the past 10 years, wide-view LCD technology has been completely divided into two types: vertical-oriented soft screen and horizontal-oriented hard screen. PSVA, HVA (CSOT) and UV2A (Sharp) all belong to the soft screen VA technology, while the horizontal alignment hard screen technology is represented by FFS, ADS (BOE), IPS (LG), PLS (Samsung) and AHVA (AUO).

With the exception of traditional hard-screen LCD manufacturers such as BOE and LG, all other soft-screen LCD manufacturers are developing and producing hard-screen LCD products. Hard screen technology has become the absolute mainstream of the entire LCD market. 

In the market share of the display panel, why is the horizontal hard screen technology of FFS/ADS class more powerful than the soft screen technology of vertical orientation of VA type? It also starts with the technical pros and cons of FFS/ADS and VA classes. The following comparison is carried out from the aspects of manufacturing cost, field of view angle, response time, picture quality, color uniformity, refresh rate and so on.

1. In terms of manufacturing cost. 

PSVA is a variation of traditional VA technology in liquid crystal alignment. Compared with FFS horizontal alignment technology which can achieve alignment only by friction or polarized UV irradiation, PSVA has a considerable increase in both liquid crystal material cost and process cost. 

2. Visual angle. 

VA vertical alignment technology uses 4-domain pixel electrode to realize self-redemption of visual field angle by flipping liquid crystal molecules in four directions. the attenuation of side view brightness and contrast is better than that of common view LCD technology such as TN, but the color offset is still insufficient, and the use of compensation film is still not optimistic. The 8-domain design increases the dispersion degree of liquid crystal molecules by applying different voltages to two 4-domain partitions in one pixel, which can improve the color deviation, but there is still a considerable gap compared with FFS horizontal alignment LCD without compensation film. The gene of FFS/ADS horizontal alignment technology determines that it is inherently excellent in visual angle, and the side view display effect of VA can be much higher than that of VA without using compensation film. 

3. Response time. 

PSVA adopts the method of "polymer stabilization" to realize alignment, and the inclination of the initial arrangement direction of liquid crystal molecules relative to the electric field direction is very small, and most liquid crystal molecules need to be rotated nearly 90 degrees in the driving process. However, the tilt angle of the liquid crystal molecules of the FFS/ADS horizontal alignment technology is generally more than 5 degrees compared with the electric field direction, and it only needs to rotate more than 80 degrees in the driving process, and most of the liquid crystal molecules do not need to rotate to the limit angle in the driving process. The response time of FFS horizontal alignment LCD is significantly better than that of PSVA LCD. 

4. Picture quality. 

From the pixel structure, we can see that another big disadvantage of VA is that Vcom is located on the CF side, which is too far away from the pixel electrode, resulting in insufficient storage capacitance, so it is necessary to set some additional Vcom wiring on the TFT to increase the storage capacitance, and then occupy the valuable pixel opening area, and finally affect the transmittance. On high-resolution 8K products, the pixel opening area is already insufficient, and the additional storage capacitance that must be set further reduces the transmittance. The deficiency of storage capacitance is also very prone to the problem of picture flicker. 

5. Color uniformity. 

VA technology due to its driving state of liquid crystal molecular dispersion is insufficient, red, green and blue under different gray levels of light efficiency change range is quite different, red, green and blue gray scale curve (Gamma) can only be modulated separately, in addition to increase the difficulty and cost in the design and manufacturing process, but also enlarge the backlight brightness, Gap and other reasons caused by color differences, and then affect the brightness and color uniformity. 

6. Refresh rate. 

The main threshold of high refresh rate is adequate storage capacitance and sufficient charging. The storage capacitance of VA technology is already too small, and the high resolution will greatly reduce the charging time. For VA technology, there is a constant need to weigh all kinds of Trade Off, when making high refresh rate products, and it is easy to fall into a dead cycle. A comprehensive display of various indicators: to put it simply, it is 90 points in the front of the VA category, but fails in the side; there is no dead angle in FFS/ADS 360 degrees. 

In a word, FFS horizontal alignment hard screen has become the absolute mainstream display mode of TFT-LCD because of its low manufacturing cost, wide field of view, faster response time, extreme picture quality, uniform color, high refresh rate and so on. The hard screen technology with multiple advantages has also become the ultimate choice of LCD technology.
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