Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-11-03
2003-06-10
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S113000, C349S115000
Reexamination Certificate
active
06577364
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a reflective liquid crystal device. Such devices may be used, for instance, in hand-held and laptop equipment such as computers, diaries and personal organisers.
DESCRIPTION OF THE RELATED ART
Proc. Ind. Acad. Sci., 1955, 41A 130 and 137 disclose circular polarisers and quarter wave plates made of combinations of birefringent plates to provide improved achromaticity. The individual retarders are combined with different azimuthal orientations of their optic axes to achieve the improvement in achromatic performance.
Seki et al, Mol. Cryst. Liq. Cryst., 1995, 263, 499 and Seki et al, Eurodisplay, 1996, 464 disclose a liquid crystal device (LCD) of the reflective electrically controlled birefringence (ECB) type comprising a nematic liquid crystal and a quarter wave plate. The optic axis of the quarter wave plate is crossed with that of the nematic liquid crystal and is at 45° to a polariser disposed on one side of the nematic liquid crystal. The untwisted liquid crystal and quarter wave plate are disposed between the polariser and a mirror and a normally white state is achieved with the liquid crystal providing a retardation of &lgr;/4. A black state is achieved by controlling the liquid crystal so as to provide zero retardation. This black state is effectively provided by the quarter wave plate and is achromatic only to the degree that the quarter wave plate is achromatic. A further retarder of negative birefringence with its optic axis perpendicular to its plane may be included to improve viewing angle performance.
Uchida et al, Asia Display, 1995, 599 discloses a reflective display in which a polariser and a mirror are disposed on opposite sides of a hybrid aligned nematic (HAN) liquid crystal layer and a retarder. The retarder is biaxial having an optic axis out of the plane to improve viewing angle and an in-plane optic axis which co-operates with the retardation of the liquid crystal layer to provide black and white states. The retarder in-plane axis is at 45° to the polarising axis of the polariser and is crossed with the optic axis of the liquid crystal layer. A normally black state is provided when the net retardation of the liquid crystal layer and the retarder is equal to &lgr;/4. A white state is provided when the retardation of the liquid crystal is such that it cancels the retardation of the retarder. Such a display depends for its achromaticity on optimising the dispersion of the optical elements.
Ishinabe et al, Eurodisplay, 1996, 119 discloses a full colour reflective LCD in which a HAN liquid crystal layer and a biaxial retarder are disposed between a linear polariser and a mirror. The optic axis of the HAN layer is crossed with the optic axis of the retarder and is at 45° to the polarising axis of the polariser. A normally black state is achieved by making the difference between the retardations of the liquid crystal layer and the retarder equal to &lgr;/4. Achromaticity is improved by adjusting the dispersion of materials of the liquid crystal layer and the retarder so that the birefringences partially compensate each other.
Kuo et al, Asia Display, 1995, 135 also discloses an LCD in which a HAN liquid crystal layer and a biaxial retarder are disposed between a linear polariser and a mirror. A dark state is provided when the total retardation is equal to an odd number of quarter wavelengths. The retarder has an in-plane optic axis which is crossed with the liquid crystal axis and is at 45° to the polariser axis. The display may be operated in the normally black mode, as described hereinbefore, or in the normally white mode, in which case a larger passive retarder is used. Achromaticity may be improved by optimising the dispersion of the elements or by adjusting the dispersion so that the birefringences compensate each other.
Wu et al, Applied Physics Letters, 1996, 68, 1455 discloses a reflective LCD in which a twisted nematic liquid crystal cell and a retarder are disposed between a polariser and a mirror. The twisted nematic liquid crystal cell has a twist angle of 90°, is relatively thin, and has its input director angled at 20° to the axis of the polariser. The retarder provides a retardation of &lgr;/4 and has an optic axis angled at 45° to the polariser axis. The cell operates in the normally white mode where the retardations of the liquid crystal cell and the retarder cancel each other in the white state and the black state is obtained by reducing the retardation of the liquid crystal cell to zero. Accordingly, the achromaticity depends on the achromaticity of the retarder.
Kuo et al, Eurodisplay, 1996, 387 discloses a similar twisted nematic display which is operated in the normally white mode and again achieves a black state whose achromaticity depends on the achromaticity of the retarder.
Fukuda et al disclose in three papers (IDRC, 1994, 201; SID Journal, 1995, 3, 83; Asia Display 1995, 881) a reflective supertwisted nematic (STN) LCD comprising a single polariser and a single retardation film. The twist of the liquid crystal is between 220 and 260° and the device operates in the normally white mode. STN liquid crystal is used to allow high multiplex ratios i.e. small voltage differences between on and off voltages and hence a large value of d&Dgr;n of the liquid crystal, for instance greater than 0.6 micrometers. Achromaticity is improved by varying the dispersive properties of the liquid crystal and the retarder.
PCT WO 96/31577 discloses a reflective display comprising a linear polariser and a reflector between which are disposed a first retarder adjacent the polariser and a second retarder adjacent the reflector. The first retarder provides a retardation of &lgr;/2 and has an optic axis aligned at 15° to the polarising direction of the polariser. The second retarder comprises a liquid crystal layer providing a retardation of &lgr;/4 with an optic axis which is switchable in the plane of the retarder between angles of 75° and 120° with respect to the polarising direction.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a reflective liquid crystal device comprising a linear polariser, a polarisation preserving reflector, and a retarder arrangement, characterised in that the retarder arrangement comprises at least three retarders, a first of which is disposed between the polariser and the reflector, a second of which is disposed between the first retarder and the reflector, and a third of which is disposed between the second retarder and the reflector, at least one of the first, second and third retarders comprising a liquid crystal layer which is switchable between a non-reflective device state, in which the retardation of the retarder arrangement is equal to (2n+1)&lgr;/4, where n is an integer and &lgr; is a wavelength of visible light, and a reflective device state.
In the non-reflective device state, the first retarder may have a retardation of substantially &lgr;/2, the second retarder may have a retardation of substantially &lgr;/2, and the third retarder may have a retardation of substantially &lgr;/4.
In the non-reflective device state, the optic axis of the first retarder may be substantially at an angle of (&agr;+B.180°) to the polarisation or absorption axis of the polariser, the optic axis of the second retarder may be substantially at an angle of (x.&agr;+C.180°) to the polarisation or absorption axis, and the optic axis of the third retarder may be substantially at an angle of (2(&bgr;−&agr;)+sign(&agr;).45°+D.180°) to the polarisation or absorption axis, where x is a positive real number, B C and D are integers and sign (&agr;) if the sign of &agr;. &agr; may be substantially equal to 6.90. x may be substantially equal to 5.
The first or second retarder may comprise the liquid crystal layer whose optic axis rotates during switching by an angle substantially equal to 22.5° about the normal direction of light passage.
The second retarder may comprise the liquid crystal layer and may have a retardati
Gilmour Sandra
Saynor Kirstin Ann
Tillin Martin David
Towler Michael John
Kim Robert H.
Nguyen Dung
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