The twisted nematic device

Consider the general form of n introduced above and now suppose that we apply a magnetic field across the plates of the form 

It is simple to verify that, for any one solution of the equilibrium equations (3.288) and (3.289) which satisfies the boundary conditions (3.285) and (3.286) is indeed the twisted planar solution given by equation (3.283). However, another possibility is to adopt the methods of Section 3.2.2 to search for symmetric solutions satisfying 

Integrating (3.298) and using the boundary conditions (3.285) gives the solution 

Inserting this result into (3.298) and integrating using the boundary conditions (3.286)i gives the solution for the angle as a function of the solution namely, 

The constants 0m and b are obtained from the two relations obtained by the symmetry conditions (3.293) and (3.294) applied to the above solutions. These are 

A noteworthy feature of the twisted nematic (TN) is that the intensity of 

An advance that has extended the application of LCDs to full page computer terminals and other high information content displays without having to resort to TFTs is the supertwisted nematic device. It makes use of the fact that the electrooptic response of the nematic gets progressively steeper as the twist angle is increased, until at a certain 

Modified forms of these displays have been developed - e.g., the dye display which has a pleochroic dye dissolved in the nematic material and requires the use of just one polarizer - but we shall not be discussing them here. Analytical expressions have been derived which simplify the computational effort involved in optimizing the material and device parameters, but one has to rely on numerical modelling to give a complete description of the dynamical characteristics of these devices. Certain unusual dynamical effects observed in the TN device, e.g., the reverse- 

3 The Freedericksz effect in UgUy anisotropic nenuttics periodic 

The Freedericksz transition discussed in 3.4.1 may be called a homogeneous transition since the distortion occurring above the threshold is uniform in the plane of the sample. In low-molecular-weight nematics, which as a rule have relatively small elastic anisotropy k i kjj 2 22), it is the homogeneous transition that is generally observed. Some polymer nematics, however, are known to exhibit high elastic anisotropy - an example is a racemic mixture of poly-y-benzyl-glutamate (PEG) which has k Jk =11.4 and k /k = 13.0 - and in such cases more complex types of field-induced deformations are possible.  

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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. 

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. 

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]. 

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... 

With nonscattering displays, such as the twisted nematic device, this glare can be prevented by the use of a diffuse reflector in place of the mirror reflector. However, there is a loss of brightness due to the depolarization of radiation reflected from the diffuse surface. The compromise is acceptable because the glare-free viewability is usually considered more important than high brightness. 

Experimental observations of nematic Uquid crystals frequently report uniform alignment of the director, corresponding to the above twist solution with r = 0, resulting in a constant solution of the form in (i) above. This type of alignment occurs in samples between two parallel plates when the alignment of the director on both boimdary surfaces is identical. This set-up can be taken one stage further for example, if the director is fixed parallel to both plates and one plate were then to be rotated about its normal, it is anticipated that a twist orientation similar to the above twist solution with r 0 would be induced. This will be discussed further in the context of the twisted nematic device in Section 3.7. 

It should be mentioned that Fraser [92] has carried out a theoretical investigation of the twisted nematic device that incorporates surface pretilt of the director and electric field effects. Much of the analysis is naturally extended from the ideas presented on pretilt in Section 3.4.2 and electric field effects in Section 3.5. 

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