Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal...
Reexamination Certificate
1996-10-09
2001-04-24
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
C349S177000, C349S183000, C349S191000, C349S128000
Reexamination Certificate
active
06222605
ABSTRACT:
The present invention relates to a liquid crystal device and to a method of making a liquid crystal device. Such devices may be used to provide fast switching liquid crystal displays having applications in television, computer displays, and three dimensional displays. Such devices may also be used in optical systems requiring fast polarisation switches, spatial light modulators, and optical shutters for instance for optical communications.
The term “surface mode liquid crystal device” as used herein means a liquid crystal device in which optical change caused by varying the field across the liquid crystal occurs primarily in the surface layers of the liquid crystal. One example of such a device is known as a pi-cell, in which the liquid crystal is disposed between alignment layers which create parallel alignment. This may be achieved by providing polyimide alignment layers which are rubbed in the same direction. Another example of such a device is known as a Fredericks cell, in which the liquid crystal is disposed between alignment layers which create antiparallel alignment. For instance, the display may have polyimide alignment layers rubbed in opposite directions. Other types of surface mode liquid crystal devices are possible, for instance employing parallel alignment but operating in a splay state as described hereinafter.
Surface mode liquid crystal devices (LCDs) are disclosed in Mol Cryst. Liq. Cryst., 1972, 19, 123-131 “Deformation-of Nematic Liquid Crystals in an Electric Field”, Sov. J. Quant. Electron., 1973, 3, 78-9, “Electro-Optic Switching in Oriented Liquid Crystal Films”, and U.S. Pat. No. 4,385,806. LCDs of the pi-cell type are disclosed in Mol Cryst. Liq. Cryst., 1984, 113, 329-339, “The Pi-cell: A Fast Liquid Crystal Optical Switching Device.”, U.S. Pat. No. 4,635,051 and GB 2 276 730. A typical known pi-cell structure comprises a liquid crystal layer disposed between parallel-rubbed polyimide alignment layers and provided with suitable addressing electrodes. A retarder whose optic axis is perpendicular to the alignment direction of the liquid crystal layer may be disposed adjacent the layer so as to compensate for the retardation of the layer and thus lower the required operating voltage range by allowing zero retardation to be achieved at a finite voltage. This assembly is sandwiched between crossed linear polarisers whose polarisation vectors are at 45 degrees to the optic axes of the liquid crystal layer and the retarder. A display using this structure provides fast switching times between on and off states, for instance of the order of a millisecond or less.
In the absence of an electric field across the liquid crystal layer, the liquid crystal is in a splay mode, as described in more detail hereinafter. For the display to operate in the pi-cell mode, the liquid crystal has to be transformed in to the V state, as also described hereinafter in more detail, by the application of a suitable electric field. However, when a suitable field is initially applied, it takes several seconds for the display to change to the V state. Transformation from the splay state to the V state occurs when the voltage across the cell electrodes exceeds a critical voltage and the drive voltage for the cell must therefore always exceed this critical voltage during operation of the display. If the drive voltage falls below a critical value, the liquid crystal relaxes to a twisted state, in which there is a 180 degree twist of the liquid crystal between the alignment surfaces.
The Conference record of the International Display Research Conference, 1991, 49-52 “Cholesteric Liquid Crystal/Polymer Gel Dispersion Bistable at Zero Field” discloses the use of a polymer to stabilise cholesteric features of a LCD. The display is formed by adding a small quantity of monomer to a cholesteric liquid crystal. Polymerisation of the monomer alters the switching properties of the display such that two quasi-stable states are formed. These states can exist in the absence of an applied field across the liquid crystal and can be addressed by appropriate voltage pulses across the liquid crystal. This paper alleges that the liquid crystal/polymer gel dispersion is responsible for creating these quasi-stable states.
The Conference record of the International Display Research Conference, 1994, 480-483, “Polymer Stabilised SBE Devices” discloses a technique for inducing a bulk pretilt of the liquid crystal in super-twisted birefringent effect (SBE) devices also known as super-twisted nematic (STN) devices. A monomer is added to the liquid crystal and is polymerised while applying a voltage across the liquid crystal cell. The effect of the polymer is to eliminate stripe formation from SBE or STN displays by inducing a bulk pretilt of the liquid crystal.
GB 2 286 896 discloses LCD's in which isolated pockets or regions of liquid crystal are formed by polymerising or cross-linking relatively high concentrations of pre-polymers.
According to a first aspect of the invention, there is provided a method as defined in the appended Claim
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According to a second aspect of the invention, there is provided a device as defined in the appended Claim
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According to a third aspect of the invention, there is provided a device as defined in the appended Claim
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Preferred embodiments of the invention are defined in the other appended claims.
It is thus possible to provide a surface mode liquid crystal device which is stabilised in a predetermined surface mode state. For instance, in the case of a pi-cell stabilised in the V state, the V state is retained in the absence of an applied field across the liquid crystal. The V state is retained in the absence of a field so that no time is required to reform the V state after the device has been switched off. Further, such a device may be used with a drive scheme in which one drive voltage is zero. The wide viewing angle of the pi-cell mode is unaffected by this process. The effect on switching speed or response time of the device is relatively small so that the device retains the advantages of pi-cells while overcoming the disadvantages described hereinbefore.
REFERENCES:
patent: 4385806 (1983-05-01), Fergason
patent: 4635051 (1987-01-01), Bos
patent: 5231522 (1993-07-01), Sumiyoshi
patent: 0616240 (1994-09-01), None
patent: 2276730 (1994-10-01), None
patent: 2286896 (1995-08-01), None
Scheffer et al “A New, Highly Multiplexable Liquid Crystal Display”, Appl. Phys. Letter 45(10) Nov. 1984—pp. 1021-1023.*
D. S. Fredley et al., The Conference Record of the International Display Research Conference, “Polymer Stabilized SBE Devices”, pp. 480-483, 1994. (Previously Submitted).
Heinz J. Deuling, Molecular Crystals and Liquid Crystals, “Deformation of Neumatic Liquid Crystal in an Electric Field”, vol. 19, pp. 123-131, 1972. (Previously Submitted).
Konno et al., Proceedings of the Fifteenth International Display Research Conference, “OCB-Cell Using Polymer Stabilized Bend Alignment”, pp. 581-583, Oct. 16, 1995.
Hasebe et al.,Japanese Journal of Applied Physics, Effect of Polymer Network Made of Liquid Crystalline Diacrylate on Characteristics of Liquid Crystal Display Device:, vol. 33, No. 11,pp. 6245-6248, Nov. 1994.
Bos et al., SID 93 Digest, “Molecular Crystals and Liquid Crystals”, vol. 113, pp. 329-339, 1984.
European Search Report mailed Feb. 19, 1998 for 96307391.1.
Heinz J. Deuling, Molecular Crystals and Liquid Crystals, 1972, vol. 19. “Deformation of Nematic Liquid Crystal in an Electric Field”, pp. 123-131.
P.D. Berezin et al., Sov. J. Quant. Electron., vol. 3, No. 1, Jul.-Aug. 1973, “Electrooptic Switching in Oriented Liquid-Crystal Films”, pp. 78-79.
Philip J. Bos et al., Mol. Cryst. Liq. Cryst., 1984, vol. 113, “The pi-CELL: A Fast Liquid-Crystal Optical-Switching Device”, pp. 329-339.
D. K. Yang et al., The Conference Record of the International Display Research Conference, “Cholesteric Liquid Crystal/Polymer Gel Dispersion Bistable at Zero Field”, pp. 49-52. 1994.
D. S. Fredley et al., The Conference Record of the International Display Research Confe
Raynes Edward Peter
Tillin Martin David
Towler Michael John
Duong Tai V.
Sharp Kabushiki Kaisha
Sikes William L.
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