Contrast ratio and viewing angle

The optical anisotropy (i.e. the birefringence) of a nematic liquid crystal is a very important property for the contrast ratio and viewing angle in liquid crystal displays. The optical anisotropy of the nematic is caused by the anisotropy of the molecular polarisability of the constituent anisometric molecules (rod-like molecules) and their long range orientational ord. ... 

LCD monitors are emerging as a promising new application. For this application, high contrast ratio, wide viewing angle and fast switching are required. 

Figure 5. Contrast ratio versus viewing angle for Heilmeier, dye-doped TN (DDTN), first-minimum TN (FMTN), and second minimum TN (SMTN) LCDs.

Several MTN modes with twist angle varying from 45° to 90° have been used for direct-view and projection displays, depending on the desired contrast ratio and optical efficiency. In transflective LCDs, the 75° and 90° MTN cells are frequentiy used. Therefore, we only discuss these two modes here. 

In a transflective LCD, the reflective part usually has a lower contrast ratio and narrower viewing angle than the transmissive part. Why ... 

H. L. Ong, Low voltage, high contrast ratio, and wide viewing angle TN LCDs for microdisplay, mobile phone and PDA mobile video applications, SID Tech. Digest, 37, 748 (2006). 

Table 2. Contrast ratio and percent transmission of Heilmeier, dye doped TN, and TN displays versus viewing angle [16].

Dye-doped TN has the same cell geometry as TN, except for the fact that the liquid crystal fluid is doped with a small amount of dye. The addition of dye increases both the contrast ratio and the viewing angle. Only a few studies on dye-doped TN LCDs have been reported [15, 16, 25-27, 102]. Scheffer and Nehring [27] have given a detailed theoretical analysis of dye-doped TN LCDs. 

The variation with temperature of the different properties of the materials must be taken into account during device design so that the driving signal can always induce the electro-optic effect over the desired temperature range with acceptable speed, wide viewing angle contrast ratio, and low power consumption. 

---