INDEX threshold voltage

Fig. 17. Polymer dispersed Hquid crystal display (PDLC). (a) U < clear state, where U) is the threshold voltage of the ceU. and rij represent the indexes of refraction for light polarized parallel and perpendicular to the director of the Hquid crystal represents the index of refraction of the isotropic...

Table 3.18 Transition temperatures (°C), elastic constants (ku, k22, kjj, 10 N), dielectric anisotropy ( e), dielectric constant measured perpendicular to the director (b2.)> birefringence ( n), refractive index measured parallel and perpendicular to the director ( ng and xto), viscosity (yi, mm s ), threshold voltage (V), rate of change of voltage with temperature (%°C ) and twist angle (°) measured at 20° C unless otherwise stated for three typical nematic mixtures for STN-LCDs ...

Scheffer and Nehring proposed a useful index/, for selecting a host material to give lower operating voltage by combining the linear relationship of versus An for a fixed contrast and the threshold voltage relationship with material parameters. 

Fig. 4.2 The structure and operation of the H-PDLC transmission gratings. Liquid crystal droplet planes are perpendicular to the glass plates. The grating spacing A is typically in the range of 0.5 — 5 pm. In the absence of external voltage the droplet directors are preferentially aligned perpendicular to the droplet planes, while above some threshold voltage they reorient along the electric field, i.e. perpendicular to the glass walls. This causes a decrease of the refractive index mismatch between the liquid crystal domains and the polymer matrix and hence the diffraction efficiency is reduced. The directions of s and p polarisations of incident optical beam are also denoted...

The spectral response of a holographic PDLC to applied electric fields is shown in Figure 11.27, where white incident hght is used [37]. At 0 V, due to the periodic refractive index, the cell has a high narrow reflection peak. When the applied voltage is increased, the liquid crystal is ahgned toward the layer normal direction. The amplitude of the oscillation of the refractive index decreases and the reflection of the cell also decreases. The drive voltage is approximately equal to the product of the field threshold of the Freedericksz transition of the liquid crystal layer and the cell thickness. 

The maximum value of An(V) is the index of refraction anisotropy, An = n II — n . Typically, An is about 0.2 to 0.3 This anisotropy is so large that, for monochromatic radiation, the transmitted light intensity undergoes many maxima and minima as the voltage increases above threshold. With white light, variable colors can be observed as a function of voltage. 

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