Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1996-12-19
2001-12-04
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S099000, C349S102000, C349S180000, C349S181000
Reexamination Certificate
active
06327010
ABSTRACT:
The invention relates to an electrooptical system as defined in the preamble of claim
1
.
Important criteria for assessing the optical properties of electrooptical systems are the values achieved for
contrast
brightness
viewing angle dependence of the contrast and
viewing angle dependence of the colour values.
Initially TN displays (twisted nematic) were operated in the so-called Mauguin region (d·&Dgr;n>>&lgr;), as indicated, for example, in IEEE Transaction and Electron Devices, 25 (1978), 1125-1137. In this region, the polarisation vector of the incident light in the visible spectral region follows the helical structure of the unaddressed cell, irrespective of thickness variations in the cell. However, displays of this type have an extremely high viewing angle dependence of the contrast and thus a greatly limited observation angle range.
A decisive improvement in the viewing angle dependence of the contrast is observed if the system has a value for the product of birefringence &Dgr;n and layer thickness d of the liquid crystal in the range 0.150 &mgr;m≦d·&Dgr;n≦0.600 &mgr;m indicated in DE 30 22 818. This system has the disadvantage that, according to Electronics Letters, 10 (1974), 2-4, a barrier behaviour which is dependent on the cell thickness and wavelength is produced in the sub-Mauguin region, which can result in a certain brightening in the voltage-free state.
U.S. Pat. No. 4,443,065 proposes a double-cell arrangement in which one cell is addressed electrically and used for information display while the other cell serves to compensate the optical path difference d. &Dgr;n of the switched cell. However, arrangements of this type frequently have inadequate values for contrast and brightness as a consequence of the additional liquid-crystal layer.
In electrooptical systems based on ECB (electrically controlled birefringence) or DAP (distortion of aligned phases) effect, the liquid-crystal molecules have a negative dielectric anisotropy &Dgr;&egr;, a homeotropic edge alignment and an untwisted structure as described, for example, in Displays 7 (1986), 3. It has been proposed that the observation angle range can be broadened by using compensation layers based on polymer films (EP 0 239 433 and EP 0 240 379) or liquid-crystal layers (DE 39 11 620) having negative optical anisotropy. The electrooptical properties of such compensated ECB systems are frequently impaired by inadequate values for contrast and brightness.
The object of the present invention was therefore to provide electrooptical systems, based on the TN or ECB effect and containing one or more compensation layers, which are distinguished by improved electrooptical properties and in particular by high values for the contrast and/or brightness and/or viewing angle dependence of the contrast and/or colour values. It has been found that this object can be achieved by the provision of the elctrooptical systems according to the invention.
The invention thus relates to electrooptical systems containing
a tristed nematic liquid-crystal layer between 2 substrates whose insides are provided with electrode coatings and alignment layers thereon, the liquid crystal having a parallel edge alignment and a twist angle of 0≦&bgr;≦100° and in particular 0≦&bgr;≦90° or a homeotropic edge alignment,
one or more layers for compensating the optical path difference of the liquid-crystal layer d·&Dgr;n, and
at least one device for linear polarisation of the light in such an arrangement that the light, before entering and after exiting the liquid-crystal layer, passes through a polarisation device at least once in each case, characterised in that, in order to achieve high contrast and/or high brightness and/or high viewing angle independence of the contrast and/or the colour values, the angle &psgr; which the polarisation device forms on the input side with the preferential direction of the liquid-crystal molecules on the first substrate surface satisfies condition (1) or (2)
&psgr;=(&bgr;+90°)/2±10° (1)
&psgr;=&bgr;/2±10° (2)
if a polarisation device is present on both the input side and the output side, the polariser on the output side being rotated by 90±10° with respect to the polariser on the input side, and it also being possible for the alignment of the polarisers on the input side and the output side to be interchanged,
or satisfies condition (3) or (4)
30°≦&psgr;≦70° for 0≦&bgr;≦45° (3)
35°≦&psgr;≦90° for 45°≦&bgr;≦100° (4)
if a polarisation device is only present on the input side.
The formulation used in equations (1) and (2) is intended to indicate that deviations of up to ±10° from the angle &psgr; given by the equations
&psgr;=(&bgr;+90°)/2
&psgr;=&bgr;/2
are possible. However, the deviations from the optimum angles &psgr;=(&bgr;+90°)/2 or &psgr;=&bgr;/2 are preferably not greater than ±7.5° and in particular less than ±5°.
In arrangements having a polarisation device on both the input side and the output side, the polariser on the output side is rotated by 90°±10° with respect to the polariser on the input side, this formulation again indicating that deviations of up to ±10° from the optimum angle of 90° are possible. However, the deviations from the optimum rotation of the rear polariser are preferably not greater than 7.5° and in particular less than ±5°.
The alignments of the front and rear polarisers may also be interchanged; i.e. if the alignment of the polariser on the input side is given by &psgr; and the alignment of the polariser on the output side is given by &psgr;′, the alignment of the polariser on the input side in another arrangement can be given by &psgr;′ and the orientation for the polariser on the output side can be given by &psgr;. The arrangements described by equations (1) and (2) and the arrangements given by interchanging the polariser alignments are preferred.
The electrooptical systems according to the invention contain an addressable liquid-crystal layer which is arranged between plane-parallel, transparent substrates whose insides are provided with electrode coatings and alignment layers thereon. The electrodes comprise, for example, thin, planar and transparent indium-tin oxide (ITO) or indium oxide coatings. To produce the edge alignment of the liquid crystals, polymer layers, for example polyimide or polyvinyl alcohol layers, which have been given a uniform alignment by rubbing, if desired with simultaneously application of pressure, are generally used. In addition, alignment layers can also be obtained by vapour-deposition of inorganic materials, such as, for example, silicone oxide or magnesium fluoride. A review of the various alignment techniques is given, for example, in Thermotropic Liquid Crystals, G. W. Gray (ed.), pp. 75-77.
If the liquid-crystal layer is operated in accordance with the TN principle, the liquid crystals have a parallel edge alignment, usually with a small pretilt angle in the order of, for example, from 1° to 10°. If, by contrast, the liquid-crystal layer is based on the ECB principle, the liquid-crystal molecules have a homeotropic edge alignment and are usually tilted against the perpendiculars at a small pretilt angle of, for example, 0.5-5°.
In TN liquid-crystal layers, the twist angle, which is between 0° and 100° and in particular between 0° C. and 90°, is usually defined by the alignment of the alignment layers. However, it is also possible for the twist angle &bgr; to be given by a cholesteric pitch of the liquid crystal. Even if the twist angle is not set via the pitch of the liquid crystal, a chiral dope is generally added in a low concentration in order to prevent reverse twist and reverse tilt, as described, for example, in DE 25 07 524.
In conventional ECB liquid-crystal layers, the liquid-crystal molecules are essentially untwisted. By contrast, the ECB liquid-crystal layer in the electrooptical systems according to the in
Baur Gunter
Fehrenbach Waltraud
Scheuble Bernhard
Staudacher Barbara
Duong Tai V.
Merck Patent Gesellschaft mit beschrankter Haftung
Millen White Zelano & Branigan P.C.
Parker Kenneth
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