Ways to improve response time of liquid crystal display (LCD)
Liquid crystal display (LCD) has main potential role in the market. Even though, LCD has some limitations such as color accuracy, viewing angle, response time, motion blur, black details reproduction and video processing delay. According to these, in this paper I am reviewing some basics understanding of liquid crystal display (LCD) technology and the way to improve response time.
In the earlier time, the display technology was identically by tube television, its called cathode ray tube (CRT). Since the CRT invention in 1879 by German physicist Karl Ferdinand Braun, the development of the display technology was take into account. Now, in the daily life we cannot avoid the display technology role. Therefore, recently the display technology becomes one of the most important technologies in the world. Many scientists from different background and institution or company are investigating the display technology using variety methods according to the CRT weakness such as large, heavy and relatively fragile. In the development of display technology the CRT technology displaced by liquid crystal display (LCD), plasma display, field emission display (FED), organic light emitting display (OLED), etc.
2. Liquid crystal display (LCD)
Liquid crystals are materials that are not quite solid, also not liquid. Reinitzer an Austrian botanist discovered them in 1888 . In 1963, a scientist from Radio Corporation of America (RCA) at New Jersey, United State of America (USA) discovered that liquid crystals in their natural state allow light to flow straight through liquid crystal . However, when liquid crystal is given an electrical charge they redirect, or bend the light. That exposure provided the first indication that liquid crystals could used in a new form of display device. Then, there are two things about liquid crystals which interesting. First, they were small, so display devices using them potentially are extremely thin. Secondly, unlike CRTs, displays using liquid crystals would be non-emissive. In other words, they would not generate their own light. Instead, they could utilize external lighting, including, among other things, ambient light. Therefore, they would use small amounts of power.
In the improvement of liquid crystal technology, several additional insights made. First, it was apparent that the rod-shaped (calamitic) liquid crystals had to be fashioned into a screen. The other shape is disk like shape, which is currently as new material for viewing angle enhancement . Next, polarizing filters added to the device. LCD displays utilize two sheets of polarizing material with a liquid crystal solution between them. An electric current passed through the liquid causes the crystals to align so that light cannot pass through them. A polarizing filter works somewhat like a shutter, leasing light in or blocking it out, depending upon the angle of the light.
Colored display in LCD technology is formed by positioning matrices of red, green and blue pixels. Those are very close in distance. Then, to control the pixels, and to form the image required, are used either 'passive' or 'active' matrix driver methods. The active matrix LCD is known as a thin film transistor (TFT) display. The passive matrix LCD has a grid of conductors with pixels located at each intersection in the grid. A current is sent across two conductors on the grid to control the light for any pixel. An active matrix has a transistor located at each pixel intersection, requiring less current to control the luminance of a pixel. For this reason, the current in an active matrix display can be switched on and off more frequently improving the screen refresh time, for example our mouse will appear to move better across the screen.
3. Response time improvement
Response time of a LCD technology is certainly an important specification to consider when we make a choice among various LCD monitor models available in market. Response time means how fast the pixels are able to make transition between two states. In other words, how fast they go from black to white or vice versa. LCD technology uses liquid crystals, which allow and block light provided by backlight device. Response time estimated in milliseconds or ms.
LCD has two types of response time, on and off response times1. For on response time, it mainly depends on rotational viscosity and the LC dynamics due to different electric field dependent dynamics. For off response time, it mainly depends on rotational viscosity, cell gap and elastic constants of the LC.
In this part is presented of response time improvement. To make the LCDs faster, the developments of low viscosity LC, lowering cell gap and control of the liquid crystal (LC) dynamics in a well defined way are important. T. Miyashita et all , showed the offset codebook mode (OCB) can improved response time intrinsically. Figure 1 shows molecular dynamics dependent on applied voltage. The first liquid crystal molecules in splay alignment then transits to twist state. Finally it goes through to bend state as increasing voltage. The black and white states are controlled in bend state. In this device, the same flow direction in switching accelerates response time and it is below 10 ms in all grey levels though the rotational viscosity of nematic LC used is larger than 90 mPa S.
Fig 1. Voltage dependent dynamic of OCB device4 Fig 2. Scheme of fringe-field switching (FFS)
S.H. Hong et all, reported the response time can be improved using four domain vertical alignment (VA) . In the VA mode, the LC molecules are vertically aligned so that the polarization state of linearly polarized light passed through a polarizer does not change while an analyzer blocks propagating through the cell, and thus the light. Four domains VA cell has the concept of in-plane field in vertical and horizontal directions. The liquid crystal director tilts down in four directions. As the result, owing to fully self-compensation effects, the device shows wide viewing angle over 60° and excellent color characteristics, also shows faster response time < 40 ms.
Table 1. Response time dependence of four domain VA cell on the cell gap6
S.H. Lee et all reported that fringe-field switching (FFS) in the nematic liquid crystal used a positive LC with low value of rotational viscosity and optimized light efficiency in a maximum way by optimizing rubbing direction and retardation of the cell, its called ultra-FFS device. Ultra FFS can achieved response time 28 ms, high image quality like cathode ray tube (CRT), best among existing thin film transistors (TFTs)-LCD. The scheme of ultra FFS can be seen in the figure 2.
The response time one of the main key to enhance the image quality of LCD. The response time can be achieved using several method even though has an intrinsic difficulty. At present, every industry is aggressively working on improvement of response time as small as possible.
S.H. Lee et all, Journal of the Korean Physical Society, Vol. 39, pp. S42_S48, 2001
Sun Xiaowei, (Assoc. Prof), Lecture notes on liquid crystal display, NTU, 2007
T. Miyashita and T. Uchida, IEICE TRANS. ELECTRON. E79 C, 1076, 1996
S.H. Hong, J. Appl. Phys., Vol. 87, No. 12, 15 June 2000
S. H. Lee et al, Int. Symp. Liq. Cryst. 20, 2000