Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-06-04
2003-12-09
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S039000, C349S177000, C345S087000
Reexamination Certificate
active
06661491
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display comprising an OCB-mode (Optically Self-Compensated Birefringence mode) liquid crystal display panel.
BACKGROUND OF THE INVENTION
In recent years, with advance in multimedia technologies, a great deal of image information has been distributed. As a means for displaying such image information, liquid crystal displays have rapidly spread. This is because liquid crystal displays with high contrast and wide viewing angle have been developed and put to practical use with development of liquid crystal technologies. At present, the liquid crystal displays are equal to CRT (Cathode Ray Tube) displays in display performance.
However, current liquid crystal displays are not suitable for use in display of moving images because of a low response speed of liquid crystal. While it is required that the liquid crystal respond within one frame period (16.7 msec) in a current NTSC (National Television Standard Committee) system, the current liquid crystal displays require more than 100 msec to respond between gray scales in multiple gray scale display, thereby causing a displayed moving image to be blurred. In particular, since the response between gray scales in a region where a drive voltage is low is extremely slow, a satisfactory moving image display is not attained.
Accordingly, many attempts have been conventionally made to provide high-speed responsive liquid crystal displays. While various liquid crystal display methods of high-speed response have been summarized by Wu et al. (C. S. Wu and S. T. Wu, SPIE, 1665, 250 (1992)), methods capable of achieving a response characteristic necessary for displaying the moving image are limited.
Currently, liquid crystal displays comprising an OCB-mode liquid crystal display panel, a ferroelectric liquid crystal display panel, or an anti-ferroelectric liquid crystal display panel are believed to be promising as liquid crystal displays having high-speed responsiveness suitable for display of the moving image.
Among these liquid crystal display panels, the ferroelectric liquid crystal display panel and the anti-ferroelectric liquid crystal display panel having a layered structure suffer from many problems associated with their practical uses such as: low shock resistance, limited range of available temperatures, and high temperature dependency of property. In view of these, attention has been focused on the OCB-mode liquid crystal display panels using nematic liquid crystal as liquid crystal displays suitable for display of the moving image.
The high-speed responsiveness of the OCB-mode liquid crystal displays was demonstrated by J. P. Bos in 1983. Since it was thereafter demonstrated that the provision of retardation films brought about displays with wide viewing angle and high-speed responsiveness, the OCB-mode liquid crystal display panels have been studied and developed.
FIG. 36
is a cross-sectional view schematically showing a constitution of the conventional OCB-mode liquid crystal display panel. Referring to
FIG. 36
, the OCB-mode liquid crystal display panel comprises a first glass substrate
81
provided with a transparent counter electrode
82
on a lower surface thereof and a second glass substrate
88
provided with a transparent pixel electrode
87
on an upper surface thereof. A first alignment layer
83
is formed on a lower surface of a counter electrode
82
and a second alignment layer
86
is formed on an upper surface of the pixel electrode
87
. Liquid crystal molecules have been filled into a gap between these alignment layers
83
,
86
to be formed into a liquid crystal layer
84
. The alignment layers
83
,
86
have been subjected to alignment treatment to align the liquid crystal molecules in parallel with one another and in the same direction. The thickness of the liquid crystal layer
84
is defined by a spacer
85
.
A first polarizer
91
is provided on an upper surface of the first glass substrate
81
and a second polarizer
92
is provided on a lower surface of the second glass substrate
88
. These polarizers
91
,
92
are provided in cross nicole, that is, such that their optical axes are orthogonal to each other. A first retardation film
89
is provided between the first polarizer
91
and the first glass substrate
81
and a second retardation film
90
is provided between the second polarizer
92
and the second glass substrate
88
. As the retardation films
89
,
90
, negative retardation films whose main axes are hybrid-arranged are used.
In the OCB-mode liquid crystal display panel so constituted, by application of a voltage, the liquid crystal is caused to transition from spray alignment
84
a
to bend alignment
84
b,
in which state, an image is displayed. Since the response speed of the liquid crystal of the OCB-mode liquid crystal display panel is significantly improved as compared to a TN-mode (Twisted nematic mode) liquid crystal display panel, the liquid crystal display panel suitable for moving imaged is play is realized. In addition, the provision of the retardation films
89
,
90
can achieve wide viewing angle.
As described above, the OCB-mode liquid crystal display panel displays an image when the liquid crystal is in the bend alignment state. Therefore, an initialization process for transitioning from initial spray alignment to bend alignment (hereinafter simply referred to as spray-bend alignment transition) is essential.
FIGS. 37A-37C
are views for explaining the initialization process for performing the spray-bend transition in the conventional liquid crystal display, wherein
FIG. 37A
is a graph showing change in the rate of the spray-bend transition, and
FIGS. 37B
,
37
C are graphs each showing a waveform of a voltage applied to the liquid crystal display panel in the initialization process.
In
FIG. 37A
, a longitudinal axis indicates the rate of transition from initial spray alignment to bend alignment in the liquid crystal layer included in the liquid crystal display panel. In
FIGS. 37B
,
37
C, longitudinal axes respectively indicate potential difference between the source line and the counter electrode and potential difference between the gate line and the source line.
As shown in
FIG. 37B
, in the initialization process, a predetermined voltage is applied intermittently to the source line and the counter electrode so that the potential difference between the source line and the counter electrode becomes 10V or more. Also, as shown in
FIG. 37C
, a predetermined voltage is applied to the gate line and the source line so that the potential difference between the gate line and the source line becomes 10V or more over the whole initialization process. As a result, as shown in
FIG. 37A
, the rate of transition to the bend alignment is increased stepwise and the spray-bend transition is completed when the initialization process is terminated.
By the way, how the spray-bend transition takes place is observed and the observation result shows that a nucleus of the bend alignment is generated from a specific spot and grown. Hereinbelow, this nucleus is named “transition nucleus”.
Publication of Examined Patent Application No. Hei. 10-20284 discloses a liquid crystal display panel in which a convex/concave portion made of a conductive material is formed at a predetermined position on the side of an array substrate for the purpose of generating the transition nucleus. In this constitution, since the electric field strength applied to a region of the liquid crystal layer on the convex/concave portion becomes larger than that around the region, the generation of the transition nucleus is facilitated. Consequently, the spray-bend transition smoothly takes place.
However, in the conventional liquid crystal display, the spray-bend transition sometimes takes place with low reliability because of insufficient strength of the electric field. In this case, the spray-aligned region is locally left and becomes a luminescent spot, which is observed as dot defect.
SUMMA
Kimura Masanori
Nakao Kenji
Shiota Akinori
Suzuki Daiichi
Tanaka Yoshinori
Chung David
Matsushita Electric - Industrial Co., Ltd.
Parker Kenneth
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