Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-12-21
2001-10-30
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
Reexamination Certificate
active
06310595
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a high-definition liquid crystal display element having a large screen, in which a voltage drop in electrodes is suppressed, and to a driving method thereof.
BACKGROUND OF THE INVENTION
A liquid crystal display device has such advantageous features as a flat display element that it is thin and light-weight, and has low power consumption and low driving voltage. For this reason, the liquid crystal display device is indispensable as a display of information devices, such as lap-top personal computers, word processors, portable terminals, and portable televisions.
At present, the liquid crystal display devices that are most commonly used are the ones adopting the STN (Super-Twisted Nematic) system, and the TN (Twisted Nematic) system employing TFTs (Thin Film Transistors) as the driving elements. However, these liquid crystal display devices all adopt nematic liquid crystal, and this presents many problems to be solved to realize high-definition and large capacity displaying.
The liquid crystal display device adopting the STN system has a relatively simple structure, in which a pair of transparent substrates, each having stripe transparent electrodes, are simply faced each other, and the device is driven by a driving system a so-called simple matrix driving system. This liquid crystal display device is desirable in terms of easy manufacturing and the manufacturing costs, yet due to the limitation of such characteristics as response speed of the liquid crystal and the applied voltage versus transmittance characteristic, the development of the device has come to the level where, practically, the number of scanning lines cannot be increased any further.
Meanwhile, the liquid crystal display device of an active-matrix system adopting TFTs offers full-color dynamic images, which are compatible with that produced by cathode-ray tubes. However, in this liquid crystal display device, it is required to provide the switching elements (TFTs) without causing a pixel failure in each pixel, and manufacturing becomes harder and harder as the display capacity is increased. This liquid crystal display device also has a problem that the transmittance is lowered by the arrangement wherein the switching element is provided on each pixel.
In order to solve these problems, liquid crystal display devices adopting ferroelectric liquid crystal have been getting an attention in recent years. As disclosed in Appl. Phys. Lett. 36(1980) pp. 899-901 (N. A. Clark and S. T. Lagerwall), the ferroelectric liquid crystal has desirable characteristics such as memory effect, high response speed, and wide viewing angle. For this reason, the liquid crystal display device of a simple-matrix system adopting ferroelectric liquid crystal is more suitable for high-definition displaying with large capacity pixels.
A conventional liquid crystal display device adopting ferroelectric liquid crystal is provided with a pair of electrode substrates facing each other and a liquid crystal layer of ferroelectric liquid crystal formed therebetween. One of the electrode substrates has an arrangement wherein a plurality of transparent signal electrodes are placed parallel to one another on a surface of a glass substrate, and a transparent insulating film and a transparent alignment film are formed thereon. The other electrode substrate has an arrangement wherein a plurality of transparent scanning electrodes are provided on a surface of a glass substrate so that the scanning electrodes are orthogonal to the signal electrodes, and a transparent insulating film and a transparent alignment film are deposited thereon. The alignment films are subjected to a uniaxial aligning process such as rubbing.
The ferroelectric liquid crystal is filled in a spacing formed between the glass substrates combined with each other by a sealing material. The glass substrates are sandwiched by a pair of polarizing plates which are so positioned that their polarization axes are orthogonal to each other. Between the alignment films are provided spacers as required.
As shown in
FIG. 7
, a molecule
51
of the ferroelectric liquid crystal has spontaneous polarization
52
in a direction orthogonal to its long axis direction. Also, the molecule
51
is subjected to a force proportional to the vector product of (i) an electric field generated by the driving voltage applied between the signal electrodes and the scanning electrodes and (ii) the spontaneous polarization
52
, and is moved along the surface of cone trajectory
53
.
Thus, to the observer, as shown in
FIG. 8
, the molecule
51
is perceived as though it is switching between positions P
a
and P
b
on the axes of liquid crystal trajectory. For example, when the pair of polarizing plates are positioned so that their polarization axes coincide with the direction of the arrow A-A′ and the direction of the arrow B-B′ of
FIG. 8
, respectively, a dark display is obtained when the molecule
51
is in the position P
a
and a light display produced by birefringence is obtained when the molecule
51
is in the position P
b
.
The aligning states of the molecule
51
in the positions P
a
and P
b
are equivalent in terms of elastic energy, and for this reason the aligning states, that is, the optical states, are maintained even after the electric field is removed. This is called the memory effect, and is the unique property of the ferroelectric liquid crystal, which is not found in the nematic liquid crystal.
Therefore, in the liquid crystal display device in which the ferroelectric liquid crystal having such memory effect and having a high response speed as given by the spontaneous polarization is adopted to the simple matrix system, it is possible to carry out displaying with higher definition and larger capacity.
Incidentally, as disclosed in Tomita et al., Euro Display 96:, 251, in the active-drive liquid crystal display device adopting TFTs as the driving elements, conventionally, the delay time of the driving signal is defined by an RC equivalent circuit. In the TFT liquid crystal display, it is required that a pixel frequency F
p
and a time constant RC per line satisfy the relationship F
p
·RC≦1. F
p
is in the order of several ten MHz, and the corresponding RC is in the order of nano seconds. This allows the TFT liquid crystal display to display dynamic images.
Meanwhile, the passive-drive liquid crystal display has not been examined thoroughly to realize dynamic image displaying, for the reason that the time constant RC is larger than that of the TFT liquid crystal display, and that there are only a few liquid crystal materials available which exhibit high speed response. Thus, even in the ferroelectric liquid crystal display having a potential to realize dynamic image displaying, no conditions for displaying dynamic images, namely, no relationship between the delay time (time constant RC) and the width &tgr; Of the driving pulse had been obtained. RC and &tgr; are both in the order of micro seconds. Therefore, the dynamic image display characteristics of the TFT liquid crystal display cannot be applied to the passive-drive liquid crystal display.
Japanese Unexamined Patent publication No. 77946/1995 (Tokukaihei 7-77946) discloses a method for changing the time constant in a passive-drive liquid crystal display device. In this method, a desired time constant is given per pixel by changing the resistance value of an electrode leading to the pixel in a static-drive liquid crystal display device. Because the effective value of the driving signal is changed in accordance with the time constant, the display color can be changed without using color filters.
When driving the liquid crystal display device by application of a voltage, the capacity formed between the electrodes (signal electrodes and scanning electrodes) and the substrates are charged and discharged repeatedly. Here, a charge is stored in and released from the capacity, and as a result a current flows into the liquid crystal cell via the electrodes. Also, the voltage applied t
Hjerpe Richard
Laneau Ronald
Renner, Otto, Boisselle & Sklar LL
Sharp Kabushiki Kaisha
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