ChLCD
Kent Displays, Inc. (KDI) was founded in 1993 by Dr. William Doane and Mr. William Manning to commercialize ChLCDs which were studied at the Liquid Crystal Institute at Kent State University. At first, glass-based ChLCDs were developed for information displays for various signage and consumer applications. This transitioned to using plastic substrates for displays to reduce weight and increase ruggedness. In 2009, the Boogie Board® product was introduced using the eWriter LC film.
Cholesteric liquid crystal (ChLC) material is a natural reflector as the inherent chiral structure of ChLC leads to a Bragg-type reflection of the incident light.i Reflective ChLCDs have selective reflection properties. For normal incidence, the reflection of light occurs within a certain bandwidth 
, where 
are the components of refractive index locally parallel and perpendicular to the liquid crystal director, and p is the cholesteric pitch. An example of typical reflection spectra for the Boogie Board is shown in Figure 2 (Left). The pitch is defined through the concentration c of a chiral dopant and helical twisting power (HTP) of the chiral dopant as follows:
.
ChLCDs are bistable and can be switched between two stable states: planar, the reflective state, and focal conic, the slightly scattering transmissive state. Being bistable, ChLCDs require power only when being switched from one image to another. The eWriter uses written pressure to write lines to the planar texture and a small voltage pulse to switch or erase to the focal conic texture. A schematic showing the transitions between the different textures of the eWriter is shown in Figure 2 (Right), where the planar texture is created by pressure or flow of the ChLC and the focal conic texture is created by applying a small voltage pulse to the eWriter LC film.
Fig. 2. (Left) Reflection spectra for a typical Boogie Board eWriter LC film. The written planar texture has a peak bandwidth in the yellow green, approximately 570nm. The focal conic texture appears as the color of the back absorbing layer, in this case the back absorbing layer is black. (Right) A schematic showing the texture transitions for the Boogie Board eWriter LC film.
When the written image is erased, the written planar image is electrically switched to the focal conic texture and during the high voltage pulse the cholesteric helix is untwisted to the metastable homeotropic texture that relaxes to a metastable transient planar texture and then to a stable planar texture. During the following low voltage pulse, the switched planar texture is electrically switched to the focal conic texture.ii
i P. G. de Gennes and J. Prost, The Physics of Liquid Crystals, Oxford University Press: New York (1993).
ii J. W. Doane, A. Khan, Flexible Flat Panel Displays, G. Crawford (Ed.) , John Wiley & Sons, Chapter 17 (2005).