Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-06-24
2004-11-23
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C345S107000, C430S032000, C204S600000
Reexamination Certificate
active
06822783
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrophoretic display unit which utilizes migration of charged electrophoretic particles for displaying an image, and to a method for driving the display unit.
2. Related Background Art
In recent years, with remarkable progress of digital techniques, the amount of information which individual persons can deal with is dramatically increasing. With this technical progress, display units are being developed for less power consumption and a smaller thickness of the unit. Of the display units, liquid crystal display units can meet the above needs, and are developed energetically and have been commercialized. However, conventional liquid crystal display units have problems such that the displayed letters may be not readily readable depending on the viewing angle or light reflection, and heavy visual load due to the flickering or insufficient brightness of a light source. Such problems are not solved yet. Therefore, reflection type display units are expected to be promising.
One example is an electrophoretic display unit disclosed by Harold D. Lees et al. (U.S. Pat. No. 3,612,758).
FIG. 27A
illustrates an example of the structure of the electrophoretic display unit. This kind of electrophoretic unit has a pair of substrates
1
a
,
1
b
counterposed with a prescribed interspace, an insulating liquid filled between the substrates
1
a
,
1
b
, many colored charged electrophoretic particles
3
dispersed in the insulating liquid
2
, and display electrodes
15
a
,
15
b
placed respectively on the substrates
1
a
,
1
b
of the respective pixels. Partitioning walls
7
are provided between the pixels to prevent migration of colored charged electrophoretic particles
3
to other pixels to enable uniform display. In this display unit, colored charged electrophoretic particles
3
, which are charged positively or negatively, are adsorbed onto the display electrode
15
a
or
15
b
depending on the polarity of the voltage applied to display electrodes
15
a
,
15
b
. Insulating liquid
2
and colored charged electrophoretic particles
3
have different colors. With colored electrophoretic particles
3
adsorbed onto display electrode
15
a
at the observer side, the color of particles
3
is visually recognizable (FIG.
27
B), whereas, with colored electrophoretic particles
3
adsorbed onto display electrode
15
b
at the other side, the color of insulating liquid
2
is visually recognizable (FIG.
27
A). Therefore, various images can be displayed by controlling the polarity of the applied voltage for each of the pixels. This type of display unit is classified herein as a “vertical migration type”.
However, in the vertical migration type electrophoretic device, insulating liquid
2
should contain a colorant such as a dye or an ionic substance. The colorant contained is liable to cause instability in the electrophoresis operation by additional charge transfer, which may lower the performance, life, or stability of the display unit.
To solve such problems, electrophoretic display units shown in
FIGS. 28A and 28B
are disclosed in Japanese Patent Application Laid-Open Nos. 49-5598, 49-024695, and 11-202804 (hereinafter referred to as “horizontal migration type electrophoretic display units”). Such a horizontal migration type electrophoretic display unit has also a pair of substrates
1
a
,
1
b
, insulating liquid
2
filled between the substrates
1
a
,
1
b
, many colored charged electrophoretic particles
3
dispersed in the insulating liquid
2
, and a pair of display electrodes
25
a
,
25
b
for each of the pixels. However, the pair of display electrodes
25
a
,
25
b
are not placed to hold insulating liquid
2
therebetween unlike the aforementioned type, but are placed on one substrate
1
b
. In such a horizontal migration type electrophoretic display unit, insulating liquid
2
should be transparent but need not contain a colorant, thereby avoiding the aforementioned problems. In this display unit, one display electrode
25
a
is coated with a colored layer having the same color as charged electrophoretic particles
3
(e.g., black), and the other display electrode
25
b
is coated with another color (e.g., white). The colored electrophoretic particles
3
migrate horizontally (in the direction parallel to the substrate) in accordance with the polarity applied to display electrodes
25
a
,
25
b
, and are adsorbed by display electrode
25
a
or
25
b
. With charged electrophoretic particles
3
adsorbed by display electrode
25
a
, the color of display electrode
25
b
is visually recognizable more readily (FIG.
28
A), whereas with charged electrophoretic particles
3
adsorbed by display electrode
25
b
, the entire pixel is visually recognizable to have the color of electrophoretic particles
3
(FIG.
28
B). Accordingly, various images can be displayed by controlling the polarity of the applied voltage for each of the pixels.
The systems for electrical addressing the display unit having the pixels arranged in a matrix are roughly classified into two systems: active matrix systems and simple matrix systems.
In the active matrix system, a switching element such as a thin film transistor (TFT) is formed in each of the pixels, and the voltage applied to the pixel is controlled independently for each of the pixels. With this system, the horizontal migration type electrophoretic display unit can be driven with a high display contrast. However, this active matrix system has disadvantages of a high cost for processing, and difficulty in formation of thin film transistor on a polymer substrate owing to the high process temperature. These disadvantages are serious especially in formation of a paper-like display for low-cost flexible display. For offsetting such disadvantages, are proposed a process for thin film transistor by use of a polymer material applicable to a printing process, and a process of TFT transfer system which does not need heating of the substrate. However, the possibility of commercialization is not sure.
On the other hand, in the simple matrix system, the necessary constitutional elements for addressing are X-Y electrode lines only. Therefore, this system is of low cost and can be formed on a polymer substrate. For application of a writing voltage to a selected pixel, the voltage for writing is applied to the X electrode line and the Y electrode line crossing at the selected pixels. However, if the electrophoretic display unit is driven by the simple matrix system, the writing can be made also in a part of neighboring pixels around the selected pixels (so-called crosstalk phenomenon) to lower the display contrast significantly. This is caused necessarily, because the electrophoretic display unit does not have a precise threshold to the writing voltage.
To solve this problem in the electrophoretic display having no threshold voltage in principle, a method is disclosed in which the simple matrix is driven by employing a control electrode in addition to the pair of electrodes as a three-electrode system.
Most of the disclosures on the three-electrode system relate to vertical migration type electrophoretic display unit such as the one disclosed in Japanese Patent Publication No. 61-016074 (U.S. Pat. No. 4,203,106).
The three-electrode system for the horizontal migration type electrophoretic display unit is disclosed only in Japanese Patent Gazette No. 02,740,048 (U.S. Pat. No. 5,345,251, Japanese Patent Application International Publication No. 8-507154). In the disclosed system, according to the above Patent Gazette, insulating liquid
2
employed is considered to be not transparent, but to be colored. Therefore, this system is different from the ones of the horizontal migration type electrophoretic display unit disclosed in the aforementioned Japanese Patent Laid-Open Nos. 49-5598 and 11-202804, and the display unit of the present invention in which the insulating liquid is transparent.
Japanese Patent Gazette No. 02,740,048 discloses two constitutions (a first constitution
Matsuda Yojiro
Ukigaya Nobutaka
Sugarman Scott J.
Tra Tuyen
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