Nematic scattering display

Among the various LCDs, the three most widely used are (i) dynamic scattering display, (ii) twisted nematic display and (iii) guest-host display (Shanks, 1982). 

Dynamic scattering displays, which are the first generation LCDs, make use of a nematic LC with a negative dielectric anisotropy (As = — <0) doped with an... 

Finally, we remember that the Kapustin-Williams domains take place due to the effect of the positive conductive anisotropy of the nematic liquid crystal cr /a and disappear in the region or /a < 0 (Fig. 5.7(a)). The considerable decrease in the threshold voltage of the Kapustin-Williams domains for the large conductive anisotropy c7 /(jj proved to be a useful tool for developing liquid crystal mixtures for a dynamic scattering display with low controlling voltages [56]. 

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 dynamic scattering display [54] exploited scattering of light by a nematic liquid crystal induced by turbulence associated with electrical conductivity [55, 56] in the liquid crystal. The effect provided the first commercially exploited liquid crystal displays, using Schiff s base liquid crystals of negative dielectric anisotropy, which in-... 

Unlike a twisted nematic (TN) display, scattering in a DSM LCD is symmetrical in a cone normal to the display. However, it decreases with increasing cone angles. Both... 

W. H. DeJeu and T. W. Lathouwers, "Dielectric Properties of Some Nematic Liquid Crystals for Dynamic Scattering Displays. Chem. Phys. Lett., Vol. 28. p. 239 (1974) and Ref. 5 therein. 

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 It superseded the nematic djmamic scattering display used in early displays.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... 

Many different kinds of LCD displays can be constructed, but only twisted nematic (TN), dynamic scattering, and guest-host (GH) displays are presently of commerical importance. 

---