Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1996-07-18
2003-05-06
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S165000, C349S117000
Reexamination Certificate
active
06559916
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a reflective guest-host liquid-crystal display device. More particularly, the present invention relates to technology for improving the efficiency of utilizing incident light by removing a polarizer.
Liquid-crystal display devices operate in various modes. At the present time, the dominant mode is a TN (Twisted Nematic) or STN (Super Twisted Nematic) mode in which a twist-oriented or supertwist-oriented nematic liquid crystal is used. However, in respect of operating principles, these modes require a pair of polarizers. Since light absorption occurs in the polarizers, transmittance is low, and a bright display screen cannot be obtained. In addition to these modes, a guest-host mode which uses a dichroic dye has been developed. A liquid-crystal display device of a guest-host mode makes a display by using the anisotropic properties of the absorption coefficient of the dichroic dye added to the liquid crystal. When the dichroic dye of a bar-shaped structure is used, the dye molecules have the property of being oriented parallel to the liquid crystal molecules. Therefore, when the molecule orientation of the liquid crystal is changed by applying an electric field, the orientation of the dye is also changed. Since whether or not the dye develops a color depends upon the orientation, it is possible to switch the coloring and colorless states of the liquid-crystal display device by applying a voltage.
FIGS. 7A and 7B
show the structure of a Heilmeier-type guest-host liquid-crystal display device.
FIG. 7A
shows a state in which no voltage is applied, and
FIG. 7B
shows a state in which a voltage is applied. This liquid-crystal display device uses a p-type dye and a nematic liquid crystal (N
p
liquid crystal) whose dielectric anisotropy is positive. The p-type dichroic dye has an absorption axis which is substantially parallel to the molecular axis, and strongly absorbs polarized components Lx parallel to the molecular axis and hardly absorbs polarized components Ly perpendicular to the polarized components Lx. In the state shown in
FIG. 7A
in which no voltage is applied, the polarized components Lx contained in the incident light are strongly absorbed by the p-type dye, and the liquid-crystal display device is made colored. As compared with this, in the state shown in
FIG. 7B
in which a voltage is applied, the N
p
liquid crystal having positive dielectric anisotropy is turned on in response to the electric field, and in accordance with this, the p-type dye is also oriented in a perpendicular direction. For this reason, the polarized components Lx are hardly absorbed, and the liquid-crystal display device is made colorless. The other polarized components Ly which are contained in the incident light are not absorbed by the dichroic dye regardless of the voltage applied state or the voltage non-applied state. Therefore, in the Heilmeier-type guest-host liquid-crystal display device, a single polarizer is interposed beforehand to remove the other polarized components Ly.
In the guest-host liquid-crystal display device using a nematic liquid crystal, a dichroic dye which is added as a guest is oriented in the same way as the nematic liquid crystal. Although the polarized components parallel to the orientation of the liquid crystal are absorbed, the polarized components perpendicular to said first polarized components are not absorbed. Therefore, in order to obtain a sufficient contrast, a polarizer is disposed on the incident side of the liquid-crystal display device, and the polarization direction of the incident light is aligned with the orientation of the liquid crystal. However, when this is done, since 50% (actually 40% or so) of the incident light is lost in principle by the polarizer, the display becomes dark as in the TN mode. As a method of reducing this problem, the mere removal of the polarizer causes the on/off ratio of absorbance to be decreased considerably. Therefore, this is not appropriate, and various improvement measures have been proposed. For example, a construction has been proposed in which a polarizer is removed from the incident side, while a &lgr;/4 phase shifter and a reflection plate are mounted on the emergence side. In this method, the polarization directions of two polarized components located perpendicularly to each other are rotated by 90° in the forward path and the backward path, and thus the polarized components are interchanged with each other. Therefore, in the off state (the absorption state), each polarized component will be absorbed in either the incident optical path or the reflection optical path. However, since a &lgr;/4 phase shifter and a reflection plate are provided externally in this structure, the liquid-crystal display device itself must be made to be a transmission type. In particular, in a case in which an active matrix type structure is adopted to achieve a high resolution and to display a moving image, since thin-film transistors for driving pixel electrodes are integrated on a substrate, the pixel aperture ratio is low in the transmission type, and a considerable portion of the incident light is shut out. Therefore, even if the polarizer is removed, the screen of the display device cannot be made to be markedly bright. Meanwhile, as another measure, by making it possible to absorb all of the polarized components contained in the incident light by using a cholesteric liquid crystal, the polarizer can be removed. However, this method has a drawback in that an halftone display cannot be made due to the strong hysteresis of the cholesteric liquid crystal.
A phase shifter will now be described below.
Generally speaking, a phase shifter refers to a birefringence plate (crystal plate) for providing a predetermined optical path difference (therefore a phase difference) between linearly polarized light vibrating at mutually perpendicular directions when said light passes through a plate. When the thickness of the birefringence plate is denoted as d, the refractive index of the linearly polarized light which vibrates along the electrical principal axes perpendicular to each other as n
1
and n
2
, the optical path difference is given as |n
1
−n
2
|. Phase shifters having optical path differences of &lgr;/4, &lgr;/2 and &lgr;/1 (&lgr; is the wavelength of the light used in a vacuum) are called a &lgr;/4 phase shifter, a &lgr;/2 phase shifter, and a &lgr;/1 phase shifter, respectively, and correspond to phase shifters of &pgr;/2, &pgr;, and 2&pgr;, respectively. For example, the &lgr;/4 phase shifter is a birefringence plate whose thickness is so determined as to introduce an optical path difference of a 1/4 wavelength between the linearly polarized light which vibrates perpendicular to each other. A thin film or the like in which a muscovite is cleaved to a proper thickness is used. Or, a synthetic resin plate or the like in which molecules are oriented in one direction is used. When linearly polarized light having a direction of 45° with respect to the principal axis is made to enter this plate, the transmitted light becomes circularly polarized light.
The &lgr;/4 phase shifter has various uses, and in recent years, it is used in a polarization control element of a flat-panel-type display, such as a liquid-crystal display device. However, for a conventional &lgr;/4 phase shifter, a thin film in which a muscovite is cleaved to a proper thickness, a synthetic resin plate in which molecules are oriented in one direction, or others is used. It is difficult to prepare such a &lgr;/4 phase shifter of a large area, and the &lgr;/4 phase shifter cannot be incorporated into a large flat panel display. Further, since the conventional &lgr;/4 phase shifter has a considerable degree of thickness, it cannot be incorporated into the inside of a liquid-crystal cell or the like which forms a flat panel display, and serious structural limitations occur.
SUMMARY OF THE INVENTION
In order to solve the above-described problems of the prior art, a reflective guest-host liqui
Kataoka Hideo
Shigeno Nobuyuki
Urabe Tetsuo
Sonnenschein, Nash & Rosenthal
Sony Corporation
Ton Toan
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