Dichroic contrast ratio

Fig. 6. Computed dependence of perceived contrast ratio on cell thickness for a reflective display with three different guest-host dyes having the indicated dichroic ratios R. Dye concentration adjusted to maintain an on-state brightness of 50%. Concentration of chiral dopant adjusted to maintain 3 turns of the cholesteric helix in the cell.

Fig. 7. Reflected display brightness versus perceived contrast ratio for three guest-host dyes having the indicated dichroic ratios. Increasing the concentration of the dye generally decreases the display brightness and increases the contrast ratio. Values used for this computation ng=1.76, nQ=1.50, d/p 3.0 and d=10.0 ym.

Values of f are summarized in Table 1 for a number of commercially available nematic mixtures. The substance with the highest f value has the lowest operating voltage for a given contrast ratio. The index does not include the effect of the host material on the dichroic ratio, which can be large. 

In the abovementioned two absorption-type transflective LCDs, only one polarizer is employed instead of two. Therefore, the overall image in both T- and R-modes is relatively bright. However, due to the hmited dichroic ratio of the employed dye molecules (DR 15 1), a typical contrast ratio of the guest-host LCD is around 5 1, which is inadequate for high-end full color LCD applications [29]. Thus, the absorption-type transflective LCDs only occupy a small portion of the handheld LCD market. 

In the case of dichroic (especially monochrome) displays, the contrast ratio is sometimes measured at A ,ax which shows the maximum capability of the dye but does not correspond to the photopic response. The contrast ratios calculated using the radio-metric equipment (or in arbitrary units of radiance) also differ from those evaluated using photometric equipment. Bloom and Priestly [94] therefore defined the perceived contrast ratio (PCR) ... 

As predicted from theory, the order parameter has a dramatic impact on the contrast ratio (Fig. 11). The order parameter of the dye is the only parameter that increases both the contrast ratio and the transmission simultaneously. Figure 11 shows that for a good Heilmeier display the dichroic order parameter should be as high as possible (preferably >0.75). 

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... 

Dichroic displays presently used in avionics have a reflective contrast ratio of >25 1 and a transmissive contrast of about 6 1 with a brightness of over 18% at a 45° viewing angle. These displays operate from -30° to +85 °C. The low-temperature operation is assisted by a heater. 

The contrast ratio of the phase change dichroic display is found to decrease with an increase in temperature (see Fig. 22). This decrease is basically caused by the reduced absorption in the quiescent state that results from the decrease in the order parameter and pitch of the dichroic mixture at higher temperatures. The cells show faster switching speed and lower memory at higher temperature. The switching becomes slow at lower temperature due to an increase in the viscosity of the mixture, and a heater is usually required for low temperature operation. 

Figure 22. Temperature variation of the contrast ratio of a double cell phase change dichroic LCD.

Further, two thick rigid polarizing films spoil device flexibility to be unnecessary. Therefore, guest-host twisted liquid crystal devices with dichroic dyes [7] were fabricated to absorb the all-polarization-angle incident light. In this device, a nematic liquid crystal with low birefringence is introduced, and optical rotation effect is suppressed in the twisted liquid crystal layer. The contrast ratio of the display is inferior, but the guest-host nematic liquid crystal of twist orientation is suitable for simple text display. 

Kumar P, Raina KK (2007) Morphological and electro-optical responses of dichroic polymta-dispersed liquid crystal films. Curr Appl Phys 7 636-642 Lee SH, Lim TK, Shin ST, Park KS (2002) A method for improving contrast ratio of polymta-dispersed liquid crystal film using the oriented azo-dye molecules in polymer matrix. Jpn J Appl Phys 41 208-210... 

---