Optical Properties of the Activated State

In the following it is always assumed that the liquid crystal is characterized by a positive dielectric anisotropy, Ae. In a nontwist-ed structure with zero pretilt a reorientation from a planar to a homeotropic alignment takes place above the Freedericksz threshold voltage 

In a TN cell with zero pretilt the corresponding threshold voltage for the transition from the twisted to the nontwisted structure is given by [1] 

The reduced threshold voltage Vth defined by the ratio U IUq, is given by 

The influence of different viewing angles on the electro-opical curve for fixed temperature is shown in Fig. 5. Increasing temperature results in a shift of the electro-optical curve to lower reduced voltages. This is shown in Fig. 6 for direct view (azimuthal angle 0=0°) and three different temperatures T=0°C, 20 °C and 40 °C. 

The electro-optical curve is different for a display operated in the 1st or 2nd minimum condition. This is shown in Fig. 7 for azimuthal angles 0=0° and 45°. 

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