Electro-optic scattering type devices with color polariser for s

Details Electro-optic scattering type devices with color polariser for s Electro-optic scattering type devices with color polariser for s

1996-08-20

1998-06-02

Sikes, William L.

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

349 97, G02F 1133

Patent

active

057608601

DESCRIPTION:

BRIEF SUMMARY
This invention relates to novel electro-optic colour devices, more specifically to liquid crystal colour shutters and switches.
Liquid crystal materials are widely used in liquid crystal displays such as watches, calculators etc. Most displays of this type consist of a thin film of a liquid crystalline composition sandwiched in a cell between two substrates, at least one of which is transparent, and having transparent electrodes on their inner surfaces. On applying a potential difference across the electrodes the alignment of the molecules of the liquid crystalline composition is altered, resulting in an electro-optic effect in the material, which is exploited by the display. Most often electro-optic effects in the nematic liquid crystal phase are exploited in such displays. Examples of types of displays include the twisted nematic, the Freedericksz effect device, cholesteric memory mode device, cholesteric to nematic phase change effect device, dynamic scattering effect device, two frequency switching effect device and the `supertwist` effect device. Other types of device include active matrix twisted nematics and pi-cells.
In general the above liquid crystal devices comprise cells made from glass substrates. There are numerous problems associated with the manufacture of glass cells and therefore devices, for example the limitations concerning the angle of view of such devices in practice limits the size of such cells to approximately 50 cm across the diagonal.
A type of liquid crystal display has been developed in which droplets of a suitable liquid crystalline composition are dispersed in a solid light transmissive polymeric matrix. In the art such materials are known as polymer dispersed liquid crystals (PDLC) and/or as nematic-curvilinear aligned phase (NCAP) materials because of their construction and physics respectively. Typically PDLCs and NCAPs consist of a dispersion of nematic liquid crystal material in a continuous polymer matrix. Light is scattered from such materials because of a mismatch between the refractive index of the droplets and the matrix. When no field is applied incident light is efficiently scattered because of the mismatch of the refractive index of the droplets and the matrix. When a field is applied the alignment of the liquid crystal results in the droplets having an effective refractive index closer to the ordinary refractive index of the liquid crystal, n.sub.o, for light incident normal to the material. The matrix has a refractive index, n.sub.p, essentially equal to n.sub.o and therefore the material becomes transparent to incident light.
Problems associated with these type displays include low contrast.
Recent published work has shown that improved contrast ratio may be achieved by exploiting depolarisation properties between crossed polarisers; Yoshida et al Journal of the SID, Jan. 4, 1993 pp 417-22.
Some of the properties which need to be considered for optimising the performance of PDLC and related devices are set out below.
The ordinary refractive index n.sub.o of the composition should match or be similar to the refractive index of the polymer material, so that in the presence of an applied Afield the material becomes as transparent as possible to incident light. Further desirable characteristics of the liquid crystal composition are a high birefringence, .DELTA.n, a high nematic to isotropic transition temperature T(N-I) and a low threshold voltage, V.sub.th, for device operation.
There are various methods by which a PDLC can be constructed. J. W. Doane in Liquid Crystals, Applications and Uses vol I, ed. B. Bahadur pp361-95 1990, and references therein describes some of them. One of the methods is polymerization induced phase separation (PIPS). For the PIPS method it is important that the liquid crystal material must mix with a monomer with which it is soluble. The liquid crystal composition should be sufficiently soluble in the monomer chosen and be sufficiently less soluble in the polymer so that phase separation occurs. Some other techniques include: Thermally

REFERENCES:
patent: 4610507 (1986-09-01), Kamamori et al.
patent: 5099343 (1992-03-01), Margerum et al.
Journal of the Society for Information Display, Dec. 1993, USA, vol. 1, No. 4, ISSN 0734-1768, pp. 417-422, Yoshida H et al `A full-color TFT-LCD with a polymer-dispersed structure` cited in the application see sections 1,3.1,3.2,4.2; figure 2.

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