Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-10-19
2003-05-20
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S101000, C349S102000
Reexamination Certificate
active
06567141
ABSTRACT:
TECHNICAL FIELD
This invention relates to the construction of liquid crystal display devices. In particular, the invention is concerned with a reflection-type liquid crystal display device and a transflective liquid crystal display device employing a single polarizing film method, for effecting bright display in black and white or in color by means of a reflector or a transflective reflector, and a sheet of polarizing film provided in a liquid crystal element of the device.
BACKGROUND TECHNOLOGY
For a reflection-type liquid crystal display device, there has been mainly adopted a construction wherein a TN (twisted nematic) liquid crystal element or an STN (supertwisted nematic) liquid crystal element is disposed between a pair of polarizing films, and a reflector is installed on the outer side of one of the polarizing films.
With such a reflection-type liquid crystal display device, however, external light passes through each of two sheets of the polarizing films twice from the time when the external light enters from the visible side of the device until it goes out towards the visible side after reflected by a reflector, so that reduction in light quantity is increased, thereby lowering brightness of images in display. Moreover, since the reflector is installed on the outer side of a glass substrate of the liquid crystal element, there has arisen a problem that shadows appear on display.
To cope with the problem, a single polarizing film type liquid crystal display device, capable of effecting display with just one sheet of polarizing film, has since been proposed. With such a liquid crystal display device having only one sheet of polarizing film, reduction in light quantity can be decreased in comparison with the case of a conventional reflection-type liquid crystal display device employing two sheets of polarizing films, thereby improving brightness of images in display.
Further, with the a single polarizing film type liquid crystal display device, it is possible to solve the problem of the shadows appearing on display by forming a reflector inside a liquid crystal element.
Such a single polarizing film type liquid crystal display device is comprised of one sheet of polarizing film, one sheet of retardation film, and a liquid crystal element incorporating a reflector, as disclosed in, for example, Japanese Patent Laid-open No. H 4-97121 (JP, 04-97121, A).
With such a conventional single polarizing film type liquid crystal display device as described above, however, a problem has been encountered that excellent black display can not be effected, and contrast becomes low.
In order to effect excellent black display, a low reflectance (a ratio of an outgoing light quantity to an incident light quantity as seen from the visible side) needs to be achieved in black display parts at all wavelengths in the visible light region.
However, with the liquid crystal display device described above, explanation is given on its operation in the normally white mode wherein white display is effected in the “off” state when no voltage is applied to the liquid crystal element, and black display is effected in the “on” state when a voltage is applied to the liquid crystal element, However, in the normally white mode, it is difficult to obtain excellent black display, so that nothing but display in low contrast is effected.
Accordingly, in order to obtain excellent black display, a reflection-type liquid crystal display device of the normally black mode was developed, wherein black display is effected in the “off” state when no voltage is applied to the liquid crystal element, and white display is effected in the “on” state when a voltage is applied to the liquid crystal element, as disclosed in, for example, Japanese Patent Laid-open No. H 7-84252 (JP, 07-84252, A).
Even with such a liquid crystal display device as described above, a low reflectance for light rays over all wavelengths can not realized, because an optimization of &Dgr;nd value indicating a birefringent tendency of the liquid crystal element, placement angles as well as retardation values of retardation films, and placement angles of polarizing films are not sufficient, so that contrast is insufficient.
Furthermore, with the single polarizing film type conventional liquid crystal display device described in the foregoing, it is not possible to install a backlight because the reflector does not allow light rays to pass therethrough, so that display can not be seen at places where external light is weak or at night.
Accordingly, there has been developed a transflective liquid crystal display device, employing a half-mirror made of a thin film of aluminum formed by the vapor deposition method or the sputtering method, or having a reflector provided with an opening for every pixel, so that display is effected by light rays emitted from a backlight at places where external light is weak or at night.
In the case of the single polarizing film type liquid crystal display device, at the time of reflective display using external light when incident light passes through a liquid crystal element back and forth, the liquid crystal element and optical elements such as a retardation film, and so forth, need to be designed such that display in excellent black and white can be obtained by controlling outgoing of reflected light with a sheet of the polarizing film.
On the other hand, at the time of transmissive display using a backlight, since light emitted from the backlight passes through the liquid crystal element only once, the liquid crystal element and the optical elements need to be designed such that display in excellent black and white can be obtained in such a condition as described above by controlling outgoing of reflected light with one sheet of the polarizing film. For these reasons, it has been difficult to obtain high contrast in both reflective display and transmissive display.
A liquid crystal display device having a reflector provided with an opening for every pixel has been disclosed in, for example, Japanese Patent Laid-open No. H 10-282488 (JP, 10-282488, A), however, no description on the conditions concerning a liquid crystal element and optical elements has been given therein at all, and no description on how to achieve good contrast at the time in both reflective display and transmissive display has been given therein at all.
The invention has been developed in view of the technical background described above, and an object of the invention is to provide a single polarizing film type liquid crystal display device of to realize bright display in high contrast by obtaining excellent black display at low reflectance for light rays over all wavelengths.
Further, it is another object of the invention to provide a single polarizing film type liquid crystal display device, capable of effecting reflective display by use of external light and transmissive display by lighting up a backlight, and achieving high contrast at the time in both reflective display and transmissive display.
DISCLOSURE OF THE INVENTION
In order to achieve the objects described above, the liquid crystal display device according to the invention is a reflection-type liquid crystal display device of single polarizing film type which comprises: an STN liquid crystal cell comprised of a 200° to 260° twist-aligned nematic liquid crystal sandwitched between a first substrate having a reflector and first electrodes and a second substrate having second electrodes; a retardation film disposed on the outer side (a side opposite from the side facing the nematic liquid crystal) of the second substrate of the STN liquid crystal element; and further a polarizing film disposed on the outer side of the retardation film, wherein a &Dgr;nd value indicating a birefringent tendency of the STN liquid crystal element is in a range of 0.7 to 0.8 &mgr;m, a retardation value R indicating a birefringent tendency of the retardation film is in a range of 0.35 to 0.40 &mgr;m, and an intersection angle formed by a phase delay axis of the retardation film and an absorption axis or a transmi
Arai Makoto
Kaneko Yasushi
Armstrong Westerman & Hattori, LLP
Citizen Watch Co. Ltd.
Dudek James
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