Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in series
Reexamination Certificate
1998-08-19
2001-10-02
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in series
C349S096000, C349S112000, C349S117000, C349S181000
Reexamination Certificate
active
06297863
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a reflective liquid crystal display device.
BACKGROUND OF THE INVENTION
A liquid crystal display device is thin and light, and therefore, has been widely used as a display for a personal digital assistant. A passive liquid crystal device which does not emit a light itself and expresses a display by changing the transmissivity and can be driven with several volts of effective voltage. Accordingly, when the liquid crystal display device is used as a reflective one which comprises a reflector provided at the lower side of the liquid crystal display device and expresses a display by reflecting the outside light, a display device with an extremely low consumption of electricity can be provided.
A conventional reflective color liquid crystal display device comprises basically a liquid crystal cell having a color filter and a pair of polarization films by which the liquid crystal cell is sandwiched. In this case, the color filter is provided on one of substrates of the liquid crystal cell, and a transparent electrode is formed on the color filter provided on the substrate of the liquid crystal cell. A voltage is applied to the liquid crystal cell to change the alignment of the liquid crystal molecules. Accordingly, the transmissivity of each color filter is changed so as to express a color display.
The transmissivity of one polarization film is 45% at most, as a whole. In this case, the transmissivity of a polarization component parallel to an adsorption axis of the polarization film is about 0% and the transmissivity of a polarization component perpendicular to the adsorption axis of the polarization film is about 90%. Accordingly, in a reflective liquid crystal display device comprising two polarization films, when the outside light is incident and reflected by a reflector and goes outside, the light passes through the polarization film four times. As a result, when adsorption by the color filter is not taken into account, the reflectance of the outside light is obtained by the following formula;
0.9
4
×50%=32.8%
Accordingly, when the color filter of the conventional reflective color liquid crystal display device is removed and the device is used as a black and white panel, in the same way, the reflectance reaches 33% at most.
In order to brighten a light display, several reflective liquid crystal display devices comprising only one polarization film provided on the upper side of a liquid crystal cell that is sandwiched by the polarization film and a reflector have already been proposed (refer to Japanese Laid-Open Patent Publication No. 7-146469 and No. 7-84252). In these proposed reflective liquid crystal display devices, the light passes through the polarization film only twice. When adsorption by a color filter is not taken into account, the reflectance of the outside light is obtained by the following formula.
0.9
2
×50%=40.5%
Therefore, these proposed reflective liquid crystal display devices can be expected to increase the reflectance by at most about 23.5% in comparison with that of the liquid crystal display device comprising two polarization films.
A reflective color liquid crystal display device in which a color display is expressed by using the birefringence of a twist orientation nematic liquid crystal layer and a polarization film, without using a color filter, has been disclosed in Japanese Laid-Open Patent Publication No.6-308481. A color liquid crystal display device in which the birefringence of liquid crystal and phase difference film is used has been disclosed in Japanese Laid-Open Patent Publication No.6-175125 and No.6-301006.
However, a reflective liquid crystal device comprising two polarization films can not have sufficient reflectance to obtain sufficient brightness when a color display is expressed by using the additional color filter.
Further, a reflective liquid crystal display device comprising one polarization film expresses a color display using a color filter and the brightness is maintained by increasing the reflectance. In the above-mentioned conventional constitution, it is difficult to express a black and white display in achromatic color, and it is especially difficult to express a black display in achromatic color with low reflectance.
In the reflective color liquid crystal display device in which a colored display is expressed by using the birefringence of twist alignment nematic liquid crystal layer and a polarization film, without using a color filter, or the color liquid crystal display device in which the birefringence of liquid crystal and phase difference film is used, since a color filter is not provided, the efficient reflectance to obtain the practical brightness can be ensured even if two polarization films are used. However, in the above-mentioned color liquid crystal display devices, since a color display is expressed by using the birefringence, on the principle, it is difficult to express a multi-grade and multi-color display such as a 16 grade 4096 color display or a 64 grade full-color display. Further, the above-mentioned color liquid crystal display devices have other problems such as the range of color purity and color reproduction is also being narrow.
In a reflective liquid crystal display device with black and white mode comprising two polarization films, a white display with high reflectance can't be obtained, either.
SUMMARY OF THE INVENTION
In order to solve the above-mentioned problems in the prior art, it is an object of the present invention to provide a reflective liquid crystal display device that can express a black and white display in achromatic color having a bright white display and a high contrast display.
To accomplish the above-mentioned object, one embodiment of a reflective liquid crystal display device according to the present invention comprises a liquid crystal cell including a first substrate, a second substrate and a nematic liquid crystal layer, the nematic liquid crystal layer being sealed between the first substrate and the second substrate, a polarization film provided at a first substrate side of the liquid crystal cell, a polymer film provided between the polarization film and the liquid crystal cell, and a light reflector provided at a second substrate side of the liquid crystal cell. The twist angle of nematic liquid crystals in the nematic liquid crystal layer between a pair of the first and second substrates is in the range of 0° to 90°. The product of the birefringence of the nematic liquid crystal layer &Dgr;n
LC
and the thickness of the liquid crystal layer d
LC
, &Dgr;n
LC
·d
Lc
, is in the range of 0.35 &mgr;m to 0.50 &mgr;m.
The birefringence difference &Dgr;R is in the range of −0.20 &mgr;m to 0.00 &mgr;g m, wherein &Dgr;R=R
film
−&Dgr;n
LC
·d
LC
and R
Film
is the retardation of the polymer film. When the reflective liquid crystal display device is seen from the first substrate side, the direction of the liquid crystal which is twisted from the first substrate side to the second substrate side is designated as the positive direction, &phgr;
LC
represents an angle between a base line which is set in the in-plane direction of the substrate and the longer axis direction of a liquid crystal molecule which is positioned most closely to the first substrate, &phgr;
F
represents an angle between the base line and the retardation axis direction of the polymer film, &phgr;
P
represents an angle between the base line and the adsorption axis direction of the polarization film, &phgr;
F
−&phgr;
LC
is in the range of 70° to 110° and &phgr;
P
−&phgr;
F
is in the range of −90° to −60°.
According to the above-mentioned liquid crystal device, a normally white reflective liquid crystal display device that can express the change of black and white in achromatic color, and a bright display, can be provided.
To accomplish the above-mentioned object, another embodiment of a reflective liquid crystal display device according to the present invention comprises a liquid c
Iwai Yoshio
Ogawa Tetsu
Sekime Tomoaki
Yamaguchi Hisanori
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P,C,
Ngo Julie
Sikes William L.
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