Twisted nematic devices

Equation (4.1.15) implies that at any point in the medium there are two linear vibrations polarized along the local principal axes. The polarization directions of these two vibrations rotate with the principal axes as they travel along the axis of twist and the phase difference between them is the same as that in the untwisted medium. This result was first derived by Mauguin and is sometimes referred to as the adiabatic approximation. It is this property that is made use of in the twisted nematic device discussed in 3.4.2. 

The electrode surfaces that are in contact with the liquid crystal matrix are treated by a layer in order to align the liquid crystal molecules in a particular direction. This alignment layer consists of a polymer that is brushed in a certain direction. There are several principles in how to do the alignment. In a twisted nematic device, the surface alignment directions at the two electrodes are perpendicular to each other. For this reason, the... 

Twisted nematic Device effect in which the director configuration in the unpowered condition is a 90° twist with the helical axis perpendicular to the plane of the cell. 

FIGURE 4 Variation with rms voltage of transmission with crossed polarizers for three angles of incidence (solid lines) and cell capacitance (broken line) for a twisted nematic device. 

A shutter is by definition an unmultiplexed single-pixel device to which only two (possibly rms) drive voltages, Von and Voff, are applied, one of which may be zero if required. The twisted nematic device itself is a shutter, but there exist applications for which its natural turnoff time is too slow. 

J. Kelly, S. Jamal, and M. Cui, Simulation of the dynamics of twisted nematic devices including flow, J. Appl. Phys., 86, 4091 (1999). 

In our own work on solutions of polysiloxane cyanobiphenyl benzoate ester side-chain copolymers in cyanobiphenyls, we have studied how Ae, An, V,h and yi alter as a function of polymer concentration up to approximately 40% by weight. Typical data for A , and Yx are shown in Figs 19 and 20. The important observations are that kii decreased by different amounts with increasing polymer concentration whilst Yi increased strongly. As discussed in detail elsewhere, the changes in the static properties, i.e. 33, kn, Ae/cx and Vth, lead to improved multiplexing capabUity in twisted nematic devices with a decreased temperature dependence. The 38% by weight... 

Fig. 21. Electro-optic response times for a twisted nematic device as a function of polymer concentration. The polymer was a polysiloxane backbone system (as in Figs 19 and 20) dissolved in a nematic cyanobiphenyl compound. Ihe applied voltage was 5Vn, , the cell thickness was 12jum and data are...

It should be mentioned that selective polarizers or retarding layers can also be applied to twisted nematic devices. Kabayashi [95] has shown that with twisted nematic cells in combination with different additional selective polarizers multicolor field effect devices with over 20 different colors are available. 

Twisted nematic devices offer evolutionary improvements over dynamic scattering displays in the two critical areas of display appearance and multiplex capability. 

Electro-optical liquid crystal display devices were now well established, and the twisted nematic device was obviously the superior one, based as it was upon a field effect in a pure nematic of positive dielectric anisotropy rather than upon the conductivity anisotropy, generated by ionic dopants in nematics of negative dielectric anisotropy, as in dynamic scattering displays. 

The widespread use of the twisted nematic device as the display mode above an active matrix array has highlighted its shortcomings. Such displays, which command a significant price premium in the marketplace, are subject to increasing scrutiny over the quality of the image. The narrow and non-uniform viewing cone of the normal twisted nematic display is increasingly viewed as an unacceptable aspect of performance, and various approaches have been described that seek to provide a better solution [34]. 

In Fig. 62 this linear electrooptic effect is compared with the quadratic effect controlling the state of a twisted nematic device. 

Dielectric anisotropy is an additive molar property. Thus, a small amount of PEBAB [Ae 10] (about 10-15 mol %) dissolved in MBBA [Ac —0.2] will provide a material suitable for twisted nematic devices. The threshold will, of course, be higher for this mixture than for a pure positive one such as 4-pentyl-4 -cyanobiphenyl, where the dielectric anisotropy is much higher. There are other influences on the threshold voltage for liquid crystal cells, principally the materials elastic constants and, in the case of dynamic scattering, material viscosity. The response times also are dependent upon elastic constants, viscosity, and dielectric anisotropy. These factors are discussed at length in a review by Goodman." ... 

The typical cell structure used in the twisted nematic device is shown in Fig. 3. The molecules in each surface layer of the liquid crystal are uniformly aligned in one direction, but with a twist angle of 90° between the preferred direction for the two surfaces. With no... 

The formula presented in Eq. 3 indicates the threshold voltage at which the director starts to reorient. Gerritsma, DeJeu, and Van Zan-ten 4 have measured the magnetic threshold by both capacitive and optical techniques and found that the capacitive threshold is lower than the optical one. Van Doorn has shown that this difference is to be expected, since the fluid starts to reorient by the tilting of the director toward the applied magnetic field before the twist has appreciably changed. Consequently the capacitive threshold, which occurs when the director starts to tilt toward the applied field, is lower than the optical threshold, which occurs when the twist becomes sufficiently nonuniform that the optical vector of the light does not Tol-low the twist. A similar difference has been observed in twisted nematic devices excited with electric fields. Berreman s explanation of the static characteristics of electric-field-excited devices is similar to that of Van Doorn. ... 

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