Twisted nematic effect

The twisted nematic field effect is probably the most important of the field effects because of its combined properties of very low voltage threshold, low resistive power dissipation, and relatively wide viewing angle in the reflective mode. 

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 optical properties of the twisted nematic field effect are par- 

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.  

4—Curve (a) shows rotation angle of linearly polarized light versus voltage for a nematic liquid crystal at room temperature and 1 kHz curve (b) is transmission versus voltage with parallel polarizers (Ref. [16]). 

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In the case of those LC materials which exhibit negative dielectric anisotropy, cells can be constracted which align vertically and twist on applying a field, exactly the converse to the twisted nematic effect from positive anisotropic LCs. Cells of this type are of interest because they can form a superior black state. 

Most of the electrooptic effects are based on light scattering or on light absorption by polarizers or by dissolved dyes. Electrooptic effects which belong to the second group are tunable birefringence, the twisted nematic effect and the guest-host effect. 

Emphasis has been placed on nematic materials because of the predominance of the twisted nematic effect in display devices. However, its disadvantages have caused considerable interest in the cholesteric-nematic phase change effect and the dye-phase change device in particular.61>62... 

Twisted-Nematic Effect (TNE) The effect in which, in a turned-on LCD, light travels through the display changing from being polarized in one... 

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... 

Liquid-crystal electro-optic phenomena can be divided into two categories—those caused only by dielectric forces and those induced by the combination of dielectric and conduction forces. The two conduction-induced phenomena discussed later are dynamic scattering and the storage effect. Four of the dielectric phenomena, or field effects as they are sometimes known, are discussed first (1) induced birefringence, (2) twisted nematic effect, (3) guest-host interaction, and (4) cholesteric-nematic transition. 

Fig. 3—Side view of twisted nematic effect for (A) V = 0 and (B) V > Vth- The 1 arrows represent the orientation of the nematic moiecuies.

The two-frequency approach can also be used with field-effect materials. Bucher, Klingbiel, and Van Meter have shown that the low-frequency threshold voltage for the twisted nematic effect is increased by the superposition of a signal whose drive frequency is greater than the critical frequency where the dielectric anisotropy becomes negative. They claim that... 

Invention of the Twisted Nematic Effect and Subsequent Field Effects... 

The publication of the twisted nematic effect by Schadt and Helfrich in the February 1971 issue of Applied Physics Letters [14] spurred a strong interest of the scientific community in the new effect and in the development of positive... 

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