TN-cell

Since the walls between heterochiral domains are unacceptable defects in an LC display, enantiomericafly enriched dopants are added to the LC to favor one sign of twist over the other in actual devices, providing a monodomain in the TN cell. It should be noted, however, that the chirality of the structure derives from the interaction of the LC director with the surfaces the molecular chirality serving simply to break the degeneracy between mirror image domains to favor one over the other. 

The T-cells are divided into three classes helper T-cells (Tn-cells), cytotoxic T-cells (T cells) and natnral killer cells (NK). The latter two are involved in killing pathogens, which is discnssed below. The helper T-cells coordinate the whole immnne response. These cells are discnssed first. 

Lehmann postulates 1971 Schadt and Helfrich describe LCs as the 4th state the twisted nematic (TN) cell... 

The passage of light through a twisted nematic (TN) cell type of liquid crystal display (LCD) when the current to the electrodes is off (top) and when it is on (bottom). Liquid crystai dispiay iiiustration by Hans Cassidy. Courtesy of Gale Research. 

Figure 4.6 Working principle of a twisted nematic (TN) cell in the normally white" configuration (left), and the change of transmission with increasing applied voltage (right). In the cell configuration sketched above the threshold voltage (V,, ) for the electrooptical response corresponds to approximately V90 for 90% of maximum transmission.

The first TN-LCD were simple, directly addressed segment displays as still used, e. g., for wrist watches. When attempts were made to increase the information content of the displays by time-sequential addressing in rows and lines (multiplexing) the limits of the TN cell were soon met. At higher multiplex ratios [14] contrast loss occurred, because of ever shorter addressing times. The development of the super-twisted nematic (STN) cell in 1984 [15] pushed the practicable limit to higher multiplex ratios, but it did not lead to a general solution of the problem. 

The switching time r of a TN cell depends mostly on the rotational viscosity which can be influenced by molecular design, and on the elastic splay constant Kp, the correlation of with molecular structure remains quite elusive. 

Figure 4.9 Elastic deformations of calamitic, rod-like liquid crystals in the nematic phase. The corresponding elastic elasticity constants are K, (splay), /<2 (twist), and Kj (bend). K, has the largest influence on the threshold voltage, of TN cells [23f].

The TN cell is schematically shown in Figure 1.19. The liquid crystal sandwiched between two indium-tin-oxide (ITO) coated glass plates is about a few microns in thickness. The liquid crystal molecules on the top and bottom plates are homogeneously aligned and twisted by an angle of 7t/2. The liquid crystal molecules have a positive dielectric anisotropy sa > 0 so that an electric field will rotate the liquid crystal director up. The threshold voltage is given by... 

The experimental set-up used for the Freedericksz cell as well as for the TN-cell is described elsewhere... 

In Table 1 some experimental data for the three compounds investigated are presented. Each time a TN-cell with dual-frequency addressing was used. 

Table 1. Time constants and related values for the compounds investigated using a TN-cell f = 0. 2 kHz < fo< 200 kHz... 

Room temperature nematic liquid crystals have been developed for electro-optical applications [13-15]. In particular, twisted nematic (TN) liquid crystal displays have been widely used for practical display devices [ 13-15,38). In the TN cells, nematic liquid crystals form twisted alignment due to the influence of rubbed aligmnent polymer layers coated on the substrates (Fig. 7a). The TN cells are placed between two crossed polarizers. Without electric fields, the twisted LC aligmnent induces optical rotation of incident polarized... 

Fig. 7 Schematic illustration of TN cells incorporating nematic LC physical gels (a) exhibiting TN alignment without aggregates of hydrogen-bonded molecules (b) exhibiting TN alignment with a randomly dispersed fibrous network (c) exhibiting TN alignment with an oriented fibrons network...

The faster responses described above have been observed for the gels containing randomly dispersed self-assembled fibers (Fig. 7b). Such random dispersion of the fibers in TN cells induces partial light scattering, leading to... 

If an electric field is applied to a TN cell with a twist of 90 and pretilt angle of 0 the deformation starts at the well-known threshold voltage ... 

Results for a variation of An from 0.2-0.1 are given in Fig. 10. In this case the characteristic curves are steeper for smaller An but there is a residual transparency of about 10% in the off state. The curves are calculated for 633 nm laser-light. The optimized refractive index anisotropy for a 10 ym TN cell should therefore have a value between An=0.2 and An=0.1. 

The last part is concerned with a combination of a TN cell with a light collecting plate, the FLAD. A schematic representation is shown in Fig. 13. The TN cells are operated in a transmissive mode. The thickness of the back glass plate should not be larger than 0.7 mm to avoid parallax effects. 

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