Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-11-24
2002-08-13
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06433847
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a reflection-type liquid crystal display device, and in particular to a reflection-type liquid crystal display device which carries out display without a polarizing plate.
BACKGROUND OF THE INVENTION
Currently, liquid crystal display devices have been in wide use as a color display with thin and light weight features, for example. In particular, a reflection-type liquid crystal display device, since it does not require a backlight, has a feature which allows the power required for a light source and also a space and weight of the backlight to be reduced. Thus, the reflection-type liquid crystal display device is suitable for devices which are desired to be made light and thin.
As for contrast characteristics of the display, a large decrease in contrast ratio is observed in CRT, etc., in outside day light. Also, as with CRT, in the case where the intensity of surrounding light is much greater than the display light, for example, as in the case of under direct sun light, a large reduction in contrast ratio is inevitable even in a transmissive liquid crystal display device which has been subjected to a low reflection treatment. In contrast, the reflection-type liquid crystal display device can have a display light which is proportional to the quantity of the surrounding light, and therefore is suitable particularly as a portable information terminal device, digital camera, and portable video camera, etc., when they are used outside.
As described, the reflection-type liquid crystal display device has very promising applications in many fields, yet, to this date, no reflection-type liquid crystal display device which can withstand practical use has been developed for the reasons of insufficient performance, for example, in contrast ratio, reflectance, full-color display, and a response to high-definition display and motion images.
The following describes the reflection-type liquid crystal display device in more detail.
A conventional liquid crystal display device using a twist nematic (TN mode) has the arrangement in which a pair of polarizing plates are used, and while it excels in characteristics of contrast ratio and viewing angle dependence, the reflectance is intrinsically low. Also, because a liquid crystal modulation layer and light reflecting layer are separated by the thickness of a substrate, etc., there occurs parallax due to a shift in optical paths of the incident illumination light and outgoing reflected light.
A liquid crystal display device employing a birefringence mode which carries out display by controlling the birefringence of the liquid crystal layer by an electric field (“ECB mode” hereinafter), or a mixed mode in combination of the TN mode and ECB mode allows an arrangement including a single polarizing plate, in which reflectance can be improved while maintaining the advantage of a high contrast ratio of the liquid crystal display device employing only the TN mode. However, such a liquid crystal display device has the problem of poor viewing angle characteristic and coloration in a white display which is caused by wavelength dependence. Further, since a polarizing plate is still used, the utilizing efficiency of the light is inevitably reduced in half or less.
In view of these drawbacks, there has been developed a guest-host liquid crystal element in which the liquid crystal is pigmented in the system with no polarizing plate (“GH” hereinafter). However, this device has the problems that it is less reliable by the pigment added and that a high contrast ratio cannot be achieved due to a low dichromatic ratio of the pigment. In particular, a lack of contrast greatly reduces the color purity in a color display using a color filter, which requires the use of a color filter having high color purity. However, the use of such a color filter having high color purity reduces brightness by the color filter provided, posing the problem that the advantage of high brightness of this system as realized by the omission of the polarizing plate is lost.
On these background, there has been developed a liquid crystal display device in the system employing a polymer-dispersed type liquid crystal, for which high brightness and high contrast display can be expected.
This system utilizes the switching characteristic of optical transmissive state and scattering state of the polymer dispersed-type liquid crystal, which states are switched by controlling a voltage applied to the polymer dispersed-type liquid crystal. This system does not use the polarizing plate and the utilizing efficiency of light can be increased. Further, in this system, in evaluation from a view of color quality, a desirable white display can be expected because of relatively small wavelength dependence as compared with that of the ECM mode and because it is free from the problem of absorption profile of the polarizing plate itself (i.e., characteristic of the polarizing plate which absorbs blue light and the incident light becomes yellowish).
For example, Japanese Unexamined Patent Publication No. 104250/1995 (Tokukaihei 7-104250) (Published Date:Apr. 21, 1995) discloses a liquid crystal display device employing such a system, in which polymer dispersed-type liquid crystal is arranged on a black substrate. In this liquid crystal display device, the polymer dispersed-type liquid crystal will be in the scattering state when no voltage is applied, by which the liquid crystal becomes “nebular” and a white display is realized, and the polymer dispersed-type liquid crystal will be in the transmissive state when a voltage is applied, by which the black substrate provided underneath becomes visible and a black display is realized, thereby realizing a monochromatic display.
Japanese Unexamined Patent Publication No. 10528/1998 (Tokukaihei 10-10528) (Published Date:Jan. 16, 1998) discloses a reflection-type liquid crystal display device in which a polymer dispersed-type liquid crystal is provided on a reflecting plate whose reflectance is non-uniform and periodic.
Japanese Unexamined Patent Publication No. 90352/1997 (Tokukaihei 9-90352) (Published Date:Apr. 4, 1997) discloses a liquid crystal display element having an arrangement in which a reflecting medium with a tilt angle with respect to a substrate surface is periodically provided on a light absorbing medium, on which is further provided a polymer dispersed-type liquid crystal adjacent to the reflecting medium. In this system too, a white display is realized by utilizing the scattering state of the polymer dispersed-type liquid crystal when no voltage is applied, and a. black display is realized by utilizing the transmissive state of the polymer dispersed-type liquid crystal when a voltage is applied and by absorption of light into the light absorbing medium.
The above publication (Tokukaihei 9-90352) uses the reflecting medium with a shape which is not flat but with a tilt angle with respect to the substrate surface. Japanese Unexamined Patent Publication No. 20290/1998 (Tokukaihei 10-20290) (Published Date:Jan. 23, 1998) discloses a liquid crystal display device including a reflecting medium which is characterized by its pyramid shape or cone shape.
Further, Japanese Unexamined Patent Publication No. 134266/1993 (Tokukaihei 5-134266) (Published Date:May 28, 1993) discloses a display element in which a liquid crystal layer and a polymer material layer are deposited, and which utilizes reflection by interference based on a difference in refractive indices of the two layers and the distance of each layer.
Japanese Unexamined Patent Publication No. 281477/1997 (Tokukaihei 9-281477) (Published Date:Oct. 31, 1997) discloses a reflection-type direct viewing color display device using a hologram which splits incident white light with a predetermined angle by dispersion.
Further, Japanese Unexamined Patent Publication No. 320480/1996 (Tokukaihei 8-320480) (Published Date:Dec. 3, 1996) discloses a color display device with the feature of a light absorbing color filter and a light reflecting color filter deposited.
J
Schechter Andrew
Sharp Kabushiki Kaisha
Ton Toan
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