Threshold characteristics, displays

The simplest way to overcome the scanning limitations encountered in passive matrix liquid crystal display panels is to provide one or more nonlinear circuit elements at the intersection of the gate and data electrodes, thus producing sharper threshold characteristics for each pixel. Presently, diodes and intrinsic nonlinear devices are used for these nonlinear circuit elements. 

Figure 4. Threshold characteristic of Heilmeier display compared to those of a TN and a dye-doped TN display. The transmittance of the Heilmeier, dye-doped TN, and TN displays are 16.0%, 26.7%, and 35.5%, respectively. Operating voltage, 10 V.

The threshold characteristic of Heilmeier displays and that of TN displays having the same polarizer and host liquid crystal are plotted in Fig. 4. The figure shows that the... 

Figure 7. Threshold characteristics of Heilmeier displays with 0° and 90° twisted homogeneous alignments.

Figure 10. Effect of cell gap on the threshold characteristic and contrast of a Heilmeier display. The trans-mittances of cells with 5, 8, and 11 im cell gaps are 23.6%, 20.0%, and 16.0%, respectively. Operating voltage, 10 V.

Heilmeier displays can be multiplexed only for low levels, as the threshold characteristic is not sharp enough for high level multiplexing. The 90° twisted geometry provides... 

The threshold characteristic of the dye-doped TN display is very similar to that of a TN display (see Fig. 4). The contrast ratio of a dye-doped TN LCD is heavily dependent on the polarization efficiency of the polarizer, and thus polarizers with very high polarization efficiency (>99%) should be used. The threshold and operating voltages also increase slightly with an increase in thickness. The contrast ratio shows a similar effect to the Gooch Tarry curve in TN displays. The contrast ratio has been found to maximize at the first and second Gooch Tarry minima [16, 88]. It has been found that the incorporation of dyes broadens the Gooch Tarry minima [16, 27]. The addition of a small amount of dichroic dye decreases the values of the maxima and increases... 

This equation was derived for an infinitely thick cholesteric layer where there is no torque exerted by the boundaries and cholesteric material has its natural pitch. However, it holds reasonably well for practical devices with large d/p ratios, and in particular for White-Taylor displays with homeotropic boundaries having no forced pitch. The threshold characteristic shows a noticeable hysteresis. The winding voltage Vnc is given by (see Fig. 16)... 

Figure 16. Threshold characteristic of a phase change dichroic display ...

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