Nematic liquid crystals display modes

F. Di Pasqual, H. Deng, F. A. Fernandez, et al.. Theoretical and experimental study of nematic liquid crystal display cells using the in-plane-switching mode, IEEE Trans. Electron Devices, 46, 661 (1999). 

F. J. Kahn, Reflective mode, 40-character, alphanumeric twisted-nematic liquid crystal displays, SID Tech. 

Many liquid crystal display modes have been devised, but the versatility and balance of properties offered by the twisted nematic (TN) device have proved very difficult to beat. It superseded the nematic djmamic scattering display used in early displays. To improve its performance, the TN device has been developed into new displays, i.e., STN and active matrix-addressed TN. Displays using dichroic dyes find a niche market in large information displays (airport displays), and, recently, devices using liquid crystals in conjunction with polymeric materials have been discovered. 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

Afterwards there appeared what has become the main application liquid crystal displays (LCDs) based on the twisted nematic (TN) mode. These are commonly used for flat panel displays (e.g., desk calculators). Thin film transistor (TFT) LCDs enabled a large number of segments (e.g., 640 x 1024) to be used and they had advantages like... 

The basic modes of construction of the various commercial types LCD described in detail in Chapter 3 share many common elements. They consist of a very thin layer of a nematic liquid crystal mixture enclosed between two transparent parallel glass substrates held apart by solid spacers and glued together around the edges, see Figure 2.11 for a schematic representation of the optical elements, some or all of which can be combined to construct an LCD. The cell gap (d ss 2-10 pm) should be as uniform as possible in order to minimise variations in the optics of the display. Thick cells are turbid, due to... 

Prior to the development of thin film transistors (TFT) and active matrix technology for liquid crystal displays, the maximum number of hnes or rows in any display with an acceptable contrast was severely limited by the shallow voltage-transmission characteristics of, for instance, the twisted nematic mode. This inspired work to develop a liquid crystal device that could remain in either of two states (ideally black and white) after the removal of the electric field used to switch the liquid crystal into the selected state. With a memory in the liquid crystal an unlimited number of hnes can be displayed using a simple passive matrix and the constraints are instead in the refreshment requirements. 

Ferroelectric liquid crystals also show high resolution due to the very thin gaps allowed by their mode of action. In the transmission mode, they are typically less than 2 xm thick, compared to 4-8 xm in nematic LCDs. In reflective ferroelectric liquid crystal displays, this thickness is reduced by half, resulting in a thickness of less than 1 jxm. Thus, pixels as small as 5 xm have been demonstrated. 

S. T. Wu and C. S. Wu, Mixed-mode twisted-nematic cell for transmissive liquid crystal display. Displays 20, 231 (1999). 

S. H. Lee, K. H. Park, J. S. Gwag, et al., A multimode-type transfleetive liquid crystal display using the hybrid-aligned nematic and parallel-mbbed vertically aUgned modes, Jpn. J. Appl. Phys., part 1, 42, 5127 (2003). 

Liquid crystal displays (LCDs) have many advantages over other display types. They are flat and compact, possess extremely low-power consumption (microwatts per square centimeter in the case of the twisted nematic effect), their color and contrast does not fade with an increase in the illumination intensity, they work both in transmissive and reflective modes in a wide operating temperature range and with a long lifetime. Besides that, LCDs are the most economically produced flat display systems. LCDs have... 

Table 4.1.1 Typical display modes of nematic liquid crystals... 

In addition to the TN and IPS modes in nematic displays, himdreds of other displays methods were invented and tested over the last 30 years. Out of them the bistable nematic development based on surface flexoelectric interactions, the bistable cholesterics displays based on switching between planar and focal conic textures and the polymer dispersed liquid crystal displays found some applications. None of them, however, offer better than a few milliseconds switching time. 

The wide view film (WV film) was developed by FUJlblLM Co., Ltd. (see Fig. 4.5) and released in December 1995. It is an optical compensation film for liquid crystal displays (LCDs). LC display modes are classified into several categories, but the most widely used mode for note PCs, monitors, and cell phones was the TN (twisted nematic) LCD because of its low power consumption. 

---