Change in Intensity of the Coloring

The introduction of dye molecules into the liquid crystalline host does not change the majority of the properties of the host, provided that not too much dye is introduced (not more than 1-2%). The N I transition temperature of the liquid crystal, the viscous and elastic properties, the electrical conductivity (provided the dye is not ionic and does not contain ionic impurities), the dielectric permittivities, (provided the dye molecule does not have a large dipole moment), and even the refractive indices all remain the same. The only significant change in the properties of the crystal is the appearance of absorption bands in the visible region of the spectrum and a slight increase in viscosity [151]. 

Since the values of As, Ku, and 71 are practically the same, in an im-piure nematic liquid crystal, as the corresponding parameters for the pure material, there is little point in a specific investigation of the field and the time characteristics of the guest-host effect since the findings would be the same as already discussed. Thus, the threshold of the guest-host effect should be described by (4.9), and the times by (4.30) and (4.31), although 

FIGURE 4.26. The guest-host effect in a nematic Uquid crystal. The orientation of dye molecules and absorption spectra in polarized light (a) no field present (b) the field exceeds the threshold level (P is the polarizer). 

We may distinguish the following characteristics of the guest-host displays (see also Chapter 2)  

Type of contrast. Positive contrast corresponds to the dark symbols on a bright background, negative — to bright symbols on a dark background. Positive contrast is more preferable especially under the conditions of weak illumination [151, 154]. 

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