Cholesteric liquid crystal
A cholesteric liquid-crystal display (ChLCD) is a display containing a liquid crystal with a helical structure and which is therefore chiral. Cholesteric liquid crystals are also known as chiral nematic liquid crystals. They organize in layers with no positional ordering within layers, but a director axis which varies with layers. The variation of the director axis tends to be periodic in nature. The period of this variation (the distance over which a full rotation of 360° is completed) is known as the pitch, p. This pitch determines the wavelength of light which is reflected (Bragg Reflection). The technology is characterized by stable states i.e. focal conic state (dark state) and planar state (bright state). Displays based on this technology are called “bistable” and don’t need any power to maintain the information (zero power). Because of the reflective nature of the ChLCD, these displays can be perfectly read under sunlight conditions. Examples of compounds known to form cholesteric phases are hydroxypropyl cellulose and cholesteryl benzoate. Some companies, such as Chiral Photonics, have begun to explore CLCs as the basis for photonic devices. A US company, Kent Displays, has developed "no power" Liquid Crystal Displays using Polymer Stabilized Cholesteric Liquid Crystals: these