Function-oriented Knowledge Base \ Electronic paper \ Improve image quality of paper-like display
An initializing alternating drive voltage prevents
flexible display particle coagulation
Use Play, Pause, Rewind
to control the animation of the image.
These controls work for Internet Explorer only. If you use another
web browser, right-click the image and select the Play
option.
Introduction Paper-like displays have small weight, high
contrast, and brightness in natural light, and are capable
of changing an image with only a small energy consumption.
In a display medium with particles in a layer of a contrasting
liquid, the liquid penetrates into gaps between the coloring
particles attracted to the display surface and reduces the
color density provided by the coloring particles. The reduced
color density results in insufficient contrast. Dry solid
electrophoretic displays are free of the problems related
to liquid. A completely solid display medium contains solid
conductive coloring toner (e.g., black) particles and insulating
coloring toner (e.g., white) particles in an atmosphere of
an indifferent gas. However, after several changes of the
image, substantial adhesion and coagulation occur in the display
medium that encloses the particles in the space between the
two electrodes, being repeatedly displayed on the image in
the form of dot-like defects. In addition, if the medium is
comprised of an element that maintains a gap between the electrode
substrates and separates the space between the electrodes
into a plurality of cells, the particles gradually adhere
to the gap element. As a result, the defects in the image
lower the display contrast. In order to obtain a higher display
contrast, it is necessary to prevent particle coagulation
in the flexible display.
Description To prevent particle coagulation in a flexible
display, applying an initializing, alternating drive voltage
to the electrodes prior to the switching voltage is proposed.
During the operation of a display, a coloring particle can
vary the electrification amount or size due to frictional
electrification. Such variations influence the mobility of
the particles under the effect of an electric field between
the electrodes. In addition, the mobilities of the particles
differ for one and the same electric field, depending upon
the state of adhesion of the particles to a substrate or the
contact state between adjacent particles. Correspondingly,
if an electric field is applied between the electrodes, the
mobile particles move, while those that cannot move adhere
to the substrate or an adjacent particle. The particles that
cannot easily move form coagulation with repeating changes
in the display. The initializing, alternating drive voltage
is applied before or after the display drive voltage. When
the initializing, alternating drive voltage is applied between
the electrodes, an electric field with a rapidly changing
direction is formed. Under the influence of the alternating
field, the mobile particles move reciprocally between the
electrodes. While moving, the mobile particles collide with
those that cannot easily move. The latter overcome their adhesion,
dissociate from the electrode or adjacent particles, and become
capable of moving. As a result, particle coagulation is prevented.
In addition, even after particle coagulation is formed, the
particles that do not coagulate are mutually mobile and collide
repeatedly against the coagulation, thereby dissociating the
coagulation. Therefore, an initializing, alternating drive
voltage prevents particle coagulation in a flexible display.
Additional information The frequency of the initializing voltage ranges
from 20 Hz to 20 kHz. Frequencies below 20Hz correspond to
the voltage rapidly switching the image color and even facilitate
coagulation accumulation. At frequencies over 20 kHz, the
movement of the particles lags behind the changes in the field
and the influence of collisions on the coagulation dissociation
is eliminated. The time the initializing alternating voltage
is applied ranges from several periods to 0.5 second. The
magnitude of the initializing alternating voltage is 150-350
V. The resulting contrast is determined by the ratio between
the densities of black (1.6) and white (0.3), which are sustained
for 30-40 thousands of display changes. In addition to the
contrast, the speed of operation of the display is increased.
