1. Field of the Invention
The present invention relates to a new liquid crystal display device, and a liquid crystal display apparatus using such device.
2. Description of the Related Art
In recent years, the liquid crystal display apparatus (hereinafter referred to as LCD) is widely used for a word processor, personal computer, projection TV, small TV, or the like.
In the LCD, an image is indicated by controlling the changes of transmittancy of light. For a control of the kind, there are, among others, a method for controlling light by a combination of the polarization effect of liquid crystal molecules and the polarizer; a method utilizing the phase change of liquid crystal by the application of light scattering and transmission; and a method utilizing the changes of lighting and shading of colors by adding pigments to the liquid crystal, and controlling the absorbing amount of visible light of the pigments.
The LCD using the method wherein the polarization effect and polarizer are combined is a twisted nematic LCD having the molecular arrangement which is twisted 90.degree. for example In principle, it is possible to control the polarized light by use of a thin liquid crystal layer and application of a low voltage. As a result, the TN LCD presents the characteristics of a high contrast ratio with a quick response speed at a low power consumption. This TN LCD is applicable to a watch and a calculator by a simple matrix drive, and also, to a full-color liquid crystal TV by a combination of an active matrix drive provided with switching devices per pixel, and color filters.
However, since these LCDs in which the polarization effect and polarizer are combined should use a polarizer on principle, the amount of transmitted light of the LCD is considerably reduced. That is, the amount of transmitted light is reduced at least to 50% or less because at least one polarizer is used. Also, the represented colors and contrast ratio of these LCDs vary greatly depending on the viewing angles due to the orientation of the liquid crystal molecular arrangement. Therefore, these LCDs have the viewing angle dependence. In addition to this viewing angle dependence, the transmittance indicated by the amount of transmitted light is low against the amount of incident light. Actually, therefore, the TN LCD has not arrived at the stage as yet that its display capability exceeds that of the cold-cathode ray tube completely.
On the other hand, the LCD utilizing the phase change of the liquid crystal, and the LCD controlling the absorbing amount of visible light of pigments are, among others, the PC liquid crystal which uses an electric field to be applied to generating the phase change from the cholesteric phase having the molecular arrangement of a helical structure to the nematic phase having the molecular arrangement of a homeotropic structure, and the LCD using the White Tailor GH liquid crystal formed by adding pigments to liquid crystal molecules. These LCDs do not use any polarizer on principle. Therefore, the transmittance is not reduced. Also, being able to provide wide viewing angles, these LCDs are used for an automobile equipment, projection display and the like.
However, unless the thickness of the liquid crystal layer is made comparatively large or the helical strength of the liquid crystal molecules is enhanced, these LCDs cannot obtain sufficient light scattering. This is because of the fact that the light scattering is caused by various arrangements of liquid crystal molecules. That is, in order to cause light to scatter sufficiently, it is necessary to provide the helical axes in all the alignments with respect to the direction of the incident light in a case of the cholesteric phase having the helical structure, for example. For the provision of such helical axes in many numbers of alignments, the thickness of the liquid crystal layer must be made larger inevitably.
Therefore, there is a problem that not only a high driving voltage is required for these LCDs, but also the response speed becomes extremely slow. As a result, it is difficult to apply them to a display device requiring a large capacity of representation (many numbers of pixels).
Also, with these LCDs, it is difficult to conduct the gradation display because the transmittance changes abruptly as the applied voltage increases.
Further, when the mutual changes are controlled with respect to the conditions of the light scattering and transmission by application of an electric field, hysteresis occurs in the electro-optic characteristics because the molecular arrangement of liquid crystal differs conspicuously depending on whether it is in the state of light scattering or transmission. There are various theories dealing with the causes of this generation of hysteresis, and the causes are yet to be clarified. However, it is known that the hysteresis tends to occur when the liquid crystal molecules form the state of light scattering (that is, the state where the molecular arrangements of liquid crystal become an aggregate of fine domains) when electric field is not applied or the molecular arrangements are extremely different.
As above-mentioned, if hysteresis exists in the applied electric field-transmittance characteristics of LCDs, the multiplexed drive is difficult. Other problems are also encountered practically.
There have been proposed a LCD having means for obtaining light scattering (generally referred to as DS effect) by use of the Nn liquid crystal in which a conductive substance such as an organic electrotype is dissolved into the other LCD utilizing the phase change of crystal liquid, and by application of a high voltage at a low frequency, and a LCD having means for obtaining light scattering by application of thermo-optical effect. However, in these cases, too, the problems are encountered as above-mentioned.
Further, there have been proposed the polymeric scattering LCDs in which light scattering is enhanced as shown in FIG. 36A where many numbers of capsules are formed in polymer 3 held between substrates 1 and 2, and liquid crystal is sealed in them to form a capsule structure, and liquid crystal 6 is diffused among fabric polymers 5 to form a fabric polymer structure as shown in FIG. 36B. However, due to the method of its fabrication and principle, there are limits to the polymeric arrangement, and the mixing ratio of the polymer and liquid crystal layer for these polymeric scattering LCDs. Also, only a part of the applied voltage is charged to the liquid crystal because the voltage supplied from the outside source is divided and applied to the polymer and liquid crystal. As a result, the actuality is that if it is intended to satisfy a required driving characteristic for a high response speed at a sufficiently low power consumption, the light scattering capability cannot be obtained satisfactorily.
In these methods, too, the molecular arrangement of liquid crystal differs greatly depending on whether it is in the state of light scattering or transmission. Consequently, the hysteresis occurs in the electro-optic characteristics as above-mentioned. To countermeasure this, it may be possible to reduce the hysteresis to a certain extent by controlling the liquid crystal arrangement in the state of light scattering (by mixing a hydrophobic substance with polymer so that the liquid crystal arrangement on the inner surface of the capsule cell can be controlled, for example). However, this countermeasure brings about the weakened light scattering at the same time. Therefore, this is not practical. In the polymeric scattering LCD, too, problems are encountered as in the cases of the other LCDs utilizing the phase change of liquid crystal.
As a method for scattering light, it is conceivable that an alignment process is executed per fine region so that the liquid crystal molecules can be configured in various directions on the surfaces of the two substrates having electrodes, and that liquid crystal is held in the space formed by these surfaces inverted to oppose each other as inner surfaces. However, it is practically difficult to arrange the directions of alignment process to be different per fine region (rubbing direction, for example). This method cannot be any means for solving the aforesaid problem with respect to the generation of hysteresis, either.