Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-06-30
2002-06-18
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S121000, C349S113000, C349S139000
Reexamination Certificate
active
06407787
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of reflective liquid crystal display devices.
BACKGROUND OF THE INVENTION
Liquid crystal display devices (LCDs) are used in a variety of appliances including display for mobile personal digital assistants, taking the advantage of the thin and light features. An LCD is a light receiving device which does not emit light itself but changes the light transmittance for displaying information. Since the LCD can be driven with a few volts, a reflective LCD, in which a reflector is provided underneath the LCD to display information using reflected external light, realizes an extremely low power consuming display device.
A conventional reflective color LCD includes a liquid crystal cell provided with a color filter and a pair of polarizing films interposing the liquid crystal cell. The color filter is disposed on one of substrates of the liquid crystal cell, and a transparent electrode is formed on the color filter. The voltage applied to this liquid crystal cell changes the ordering direction or orientation of the liquid crystal molecules, and thus changes the light transmittance of the liquid crystal for each color filter to display colored information.
The transmittance of the polarized light parallel to an absorption axis of the polarizing film is almost 0%, and that of the vertically polarized light is almost 90%. Light constituents vertical to the absorption axis in the non-polarized natural light is 50% of the total light. Accordingly, overall reflectance in the reflective LCD using two polarizing films in which the light passes through the polarizing films four times before exiting the reflective LCD is as follows when absorption of the light by the color filter and loss on the reflecting face are not considered:
(0.9)
4
×50%=32.8%.
The reflectance is thus limited to around 33% even for a black and white panel.
In order to achieve brighter display, several prior arts disclose configuration to employ only one polarizing film on an upper side of the liquid crystal cell, and interpose the liquid crystal cell between one polarizing film and a reflector (e.g. Japanese Laid-open Patent Nos. H7-146469 and H7-84252). In this case, the light passes through the polarizing film only twice, and overall reflectance is as follows when absorption of the light by the color filter and loss on the reflecting face are not considered:
(0.9)
2
×50%=40.5%.
The overall reflectance improves by about 23.5% at the maximum (=(40.5/32.8)×100%−100%), compared to the configuration using two polarizing films.
Color LCDs which do not employ the color filter are disclosed in the Japanese Laid-open Patent Nos. H6-308481, H6-175125, and H6-301006. The Japanese Laid-open Patent No. H6-308481 discloses the reflective color LCD which uses birefringence of a twisted nematic liquid crystal layer and a polarizing film for color display. The Japanese Laid-open Patent Nos. H6-175125 and H6-301006 propose the color LCD which uses birefringence of the liquid crystal layer and a phase retardation film for color display.
However, the reflective LCD using two polarizing films may not be able to secure reflectance for achieving sufficient brightness.
The reflective LCD using one polarizing film displays color information by the use of the color filter, and secures sufficient brightness by increasing the reflectance. This configuration, however, makes achromatic display of black and white difficult. In particular, achromatic black color which has low reflectance may not be displayed.
The reflective LCD using birefringence of twisted nematic liquid crystal layer and polarizing film for color display, and the color LCD using birefringence of the liquid crystal layer and a retardation film do not use the color filter. Since these types of color LCDs eliminate the use of the color filter, reflectance for sufficient brightness is securable even if two polarizing films are used. However, since the display is colored by birefringence, multi gray levels and multi-color display such as 4096 colors in 16-step gradation or full color in 64-step gradation may theoretically be difficult. Color purity and color reproducibility range may also be narrow.
The reflective LCD in the black and white mode which uses two polarizing films may not be able to achieve high reflectance for the white mode.
The present invention aims to offer a reflective liquid crystal display device (LCD) which achieves bright white display, high contrast, and achromatic black and white display.
SUMMARY OF THE INVENTION
The reflective LCD of the present invention includes a liquid crystal cell in which a nematic liquid crystal layer is sealed between first and second substrates; a polarizing film disposed on the first substrate side of the liquid crystal cell; two retardation films consisting of a structural component having small chromatic dispersion in refractive index anisotropy disposed between the polarizing film and liquid crystal cell; and optical reflecting means disposed on the second substrate side.
A twisting angle of the nematic liquid crystal layer is from 45° to 90°, and a product of birefringence &Dgr;nLC of the nematic liquid crystal layer and thickness dLC of the liquid crystal layer, &Dgr;nLC−dLC, is from 0.20 to 0.30 Mm. The retardation value RF
1
of the retardation film at the polarizing film side (a product of refractive index anisotropy and thickness of the retardation film) is from 0.23 &mgr;m to 0.28 &mgr;m. The retardation value RF
2
of the retardation film at the liquid crystal cell side is from 0.13 &mgr;m to 0.18 &mgr;m. The direction normal to the film face of the two retardation films is determined as the z axis, and the direction of a slow axis is determined as the x axis in orthogonal coordinates (x, y, z). When a z coefficient Qz defined by Formula 1, using refractive indexes nx, ny, and nz to each axis direction in the above orthogonal coordinates, is from 0.3 to 1.0; a set of Formulae 2 to 4, or a set of Formula 5 to 7 is satisfied:
Qz
=(
nx−nz
)/(
nx−ny
) (1);
75
°≦&phgr;P
≦95° (2);
95
°≦&phgr;P−&phgr;F
1
≦115° (3)
155
°≦&phgr;P−&phgr;F
2
≦175° (4);
−15
°≦&phgr;P
≦105° (5);
−115
°≦&phgr;P−&phgr;F
1
≦−105° (6);
−175
°≦&phgr;P−&phgr;F
2
≦−165° (7);
where
&phgr;P=angle of the absorption axis direction of the polarizing film;
&phgr;F
1
=angle of the slow axis direction of the retardation film on the polarizing film side; and
&phgr;F
2
=angle of the slow axis direction of the retardation film on the liquid crystal cell side.
All angles are measured relative to a reference line which is a bisector of a larger angle between the ordering direction of liquid crystal molecules closest to the first substrate and the ordering direction of liquid crystal molecules closest to the second substrate. A twisting direction of the nematic liquid crystal layer from the first substrate to second substrate is determined as a positive direction.
With this configuration, the reflective LCD of the present invention in the normally white mode achieves bright display and achromatic color change between back and white.
In particular, when the set of Formulae 2 to 4 is satisfied, it is preferable to set the angle &phgr;P of the absorption axis direction of the polarizing film from 90° to 120° or from 155° to 185°. This further achieves better characteristics with high contrast.
When the set of Formulae 5 to 7 is satisfied, it is preferable to set the angle &phgr;P of the absorption axis direction of the polarizing film from 0° to 30° or from 60° to 90°. This also achieves better characteristics with high contrast.
Furthermore, the reflective LCD of the present invention preferably sets the twisting angle of the nematic liquid crystal layer from 60° to 65°.
This further achieves better characteristics.
The z coefficient Qz of the retardation fi
Iwai Yoshio
Sekime Tomoaki
Yamaguchi Hisanori
Matsushita Electric - Industrial Co., Ltd.
Ratner & Prestia
Rude Timothy
Sikes William L.
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