Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1995-02-08
2002-05-21
Mengistu, Amare (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
Reexamination Certificate
active
06392624
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of driving a liquid crystal device comprising a liquid crystal material sandwiched between a pair of bases and, more particularly, to a method of driving a liquid crystal device comprising a pair of substrates on each of which a transparent electrode layer and an orientation films are laminated in this order. The substrates are placed opposite to each other with a given spacing between them. A ferroelectric liquid crystal material is injected into the gap.
Twisted-nematic liquid crystal devices presently available as commercial products use TFTs (thin-film transistors) forming an active matrix construction to produce a given gray scale. However, the production yield of the prior art process for fabricating these TFTs is not sufficiently high. Also, the cost is still high. For these reasons, there is a demand for a display device of larger area.
On the other hand, a display device utilizing a ferro-electric liquid crystal of surface stabilized bistable type does not need an active matrix construction consisting of TFTs or the like. Therefore, there is the possibility that an inexpensive, large-area display device is fabricated from this ferroelectric liquid crystal.
In an attempt to use ferroelectric liquid crystals as display devices, researches and developments have been vigorously made in these ten years. Generally, ferroelectric liquid crystals have the following excellent features:
(1) High-speed response. The response is 1000 times as high as the response of the prior art nematic liquid crystal display.
(2) They depend less to the viewing angle.
(3) They exhibit a memory effect.
A known technique for displaying an image on such a ferroelectric liquid crystal display is described by Clark et al. in U.S. Pat. No. 4,367,924. Specifically, the cell gap of the display panels is controlled within 2 &mgr;m. The liquid crystal molecules are oriented by a restricting force which orients the molecules at the interface of the panels. The surface of this ferroelectric liquid crystal assumes only two stable energy states. Because of the response is on the order of microseconds and because of the image memory effect, researches and developments have been earnestly conducted.
In this kind of bistable mode-ferroelectric liquid crystal display, the memory effect is produced and, therefore, flicker which is a problem with a CRT can be prevented. Even in a simple XY matrix construction, the display can be driven with more than 1000 scanning lines. That is, it is not necessary to drive TFTs. Nematic liquid crystals which dominate presently have the disadvantage that the viewing angle is narrow. In contrast with this, the ferroelectric liquid crystal has a wide viewing angle because the molecular orientation is uniform and because the gap between the panels is less than half of the nematic liquid crystal panel.
Such a ferroelectric liquid crystal is constructed as schematically shown in FIG.
16
. In particular, a transparent substrate
1
a
is made of glass. A transparent electrode layer
2
a
and a SiO
2
oblique deposition layer
3
a
are successively deposited on the substrate
1
a
, thus forming a laminate A. The electrode layer
2
a
is made of ITO (indium tin oxide) that is a conductive oxide prepared by doping indium with zinc. The deposition layer
3
a
acts as a liquid crystal orientation film. Similarly, a transparent electrode layer
2
b
and a SiO
2
oblique deposition layer
3
b
are successively deposited on a substrate
1
b
, thus forming a laminate B. The SiO
2
oblique deposition layers
3
a
and
3
b
which are orientation films are placed in an opposite relation to each other. Spacers
4
are inserted to secure a given cell gap. In this way, a liquid crystal cell is fabricated. A ferroelectric liquid crystal material
5
is injected into the cell gap.
Although this ferroelectric liquid crystal has the excellent advantages as described above, it is difficult to realize a gray scale. In particular, the prior art ferroelectric liquid crystal utilizing bistable mode is stable only in two states and so this liquid crystal has been regarded as unsuited for creation of gray scale as required in a video tape recorder.
More specifically, when an external electric field E is applied to the prior art ferroelectric liquid crystal such as an interface stable type ferroelectric liquid crystal, the direction of orientation of molecules M is switched between state 1 and state 2, as shown in FIG.
17
. The molecular orientation variations cause variations in the transmittivity if a liquid crystal display is installed between two mutually perpendicular polarizers. As a result, as shown in
FIG. 18
, the transmittance or transmittivity varies rapidly from 0% to 100% at a threshold voltage of V
th
in the presence of the applied electric field. Generally, this voltage range in which this transmittivity makes a transition as described above is less than 1 V. Furthermore, the threshold value V
th
is affected by minute variations in the cell gap. Therefore, in the prior art liquid crystal display, it is difficult to give a stable voltage range to the transmittivity-applied voltage characteristic curve. Hence, it is difficult or impossible to produce desired gray levels by controlling the voltage.
Accordingly, various methods for overcoming these difficulties have been proposed. In one method, subpixels are formed, and the pixel area is adjusted. Alternatively, pixel electrodes are divided to realize various gray levels (referred to as area gray scale method). Various gray levels are accomplished by repeating switching or line addressing in one field, by making use of high-speed switching of a ferroelectric liquid crystal (referred to as time integration gray scale method). However, these methods do not yet provide satisfactory gray scale.
In particular, in the area gray scale method, as the number of gray levels is increased, the required subpixels are increased. It is obvious that the cost performance is poor in terms of fabrication of devices and also in terms of method of driving the devices. Furthermore, where the time integration gray scale method is used alone, the practicability is low. Moreover, where the time integration gray scale method is employed in combination with the area gray scale method, the practicability is also low.
Accordingly, further methods for representing an analog gray scale in each pixel have been proposed. The distance between opposite electrodes is varied within one pixel, or the thickness of a dielectric layer formed between opposite electrodes is varied, so that a local electric field strength gradient is produced. Alternatively, the material of the opposite electrodes is varied to produce a voltage gradient.
However, complicated manufacturing steps are necessary to fabricate a liquid crystal display having analog gray level display characteristics which are at a practical level. In addition, it is very difficult to control the manufacturing conditions. Further, the manufacturing cost is high.
A still other ferroelectric liquid crystal is proposed in Japanese Patent Laid-Open No. 276126/1991. In particular, fine particles of alumina of 0.3 to 2 &mgr;m are sprayed on an orientation film. The ferroelectric liquid crystal is inverted between a portion in which the fine particles exist and a portion in which the fine particles do not exist. The inversion is controlled by application of a voltage. In this way, various gray levels are produced.
Where this known technique is utilized, it is quite difficult in practice to produce desired gray levels because the fine particles are too large and because the amount of the sprayed particles is not stipulated definitely.
For example, if fine particles having grain diameter of 0.3 to 2 &mgr;m is simply dispersed in a cell gap of 2 &mgr;m, then it is quite difficult in practice to subtly vary inversion of the liquid crystal within one pixel. Furthermore, the ferroelectric liquid crystal produces a visible image when the liquid crystal is in a birefringent
Nito Keiichi
Yang Ying Bao
Yasuda Akio
Mengistu Amare
Sonnenschein Nath & Rosenthal
Sony Corporation
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