CBSE Class 12-science Answered

Commonly, liquid crystal material may be modulated to produce a display. Conventional liquid crystal displays commonly use twisted nematic (tn) liquid crystal materials having a pair of states that may differentially pass or reflect incident light. While twisted nematic displays may be reflective or transmissive, cholesteric displays are usually reflective but they may also be transmissive.

In cholesteric displays, the cholesteric material has very high optical activity. Such liquid crystal material switches between a reflective texture called the planar cholesteric texture and the transparent configuration with the focal conic texture. The cholesteric molecules assume a helical configuration with the helical axis perpendicular to the surface of the substrates.

The cholesteric liquid crystal material molecules in response to an electric field align as planar texture with the optical axis, reflecting light of a particular wavelength. Generally, the maximum reflection in the planar cholesteric texture is at a wavelength directly proportional to the material's pitch distance.

Conventionally, an electric field is applied in the direction of the optical axis in order to change the phase and reflectivity of the cholesteric material. However, these changes are generally in the form of either reflecting light of a given wavelength or not reflecting light.

Thus, a given completed cholesteric liquid crystal cell may produce reflected light in a specific color, such as red, green or blue, but not any combination of them. Therefore, the conventional approach is to provide separate cholesteric display elements for each of the three primary colors (e.g., red, green and blue). These separate display elements may be stacked one on top of the other in order to generate the desired tri-color light output. Alternatively, the three elements may be placed side by side each displaying the same color. The three different colors may be achieved using color filter material.