Contrast guest-host effect

A combination of two dyes (with positive and negative dichroism) has been used [177] for color switching in the guest-host effect. The degree of dye order was 5dye = 0.677 and 0.754 for anthraquinone D /D > 1) and tetrazine (T) /D ) dyes, respectively. High values of color contrast were thus achieved on electrooptical switching. 

White and Taylor have described a new device that combines both the guest-host effect and the cholesteric-to-nematic transition. The pleochroic dye is added to a cholesteric material and the cell transmission changes when the cholesteric undergoes the cholesteric-to-nematic transition. Because of the rotational symmetry of the long axes of the dye molecules about the cholesteric helical axes, contrast ratios of greater than 4 to 1 were obtained without the use of a polarizer. 

Analysis of the electro-optical characteristics of GH-LCDs showed that for all types of guest-host chiral nematic phase change effects the contrast is greatest when... 

FIGURE 5 The upper panel shows homogeneously aligned, negative contrast, and the lower panel shows homeotropically aligned, positive-contrast, single-polarizer guest-host device effects. 

A color difference ratio allows us not only to evaluate the usual luminance contrast (4.59), but also to evaluate a color purity and the color hue that makes the human eye very sensitive to the variation of CDR. Reference [170] demonstrates the increase by a factor of three of the number of distinguished rows of the display when evaluating by the colorimetry methods. This effect may be understood by the fact that the human eye is more sensitive to the color contrast than to the luminance one, as the hue and the chroma both contribute to the former. Color differences between on and off display states could be optimized by a special choice of polarizers [170-172], concentration of a dichroic dye (guest) in a liquid crystal matrix (host) [166, 173], using the reflective screens [173], etc. Colorimetric evaluations are very useful in the quality control of both the guest host and twist displays [172, 174]. 

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. 

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