Function-oriented Knowledge Base \ Electronic paper \ Improve image quality of paper-like display
Multiplicity of two-color ball particles in micro-capsule enhances contrast ratio of display image
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Introduction Between a pair of transparent addressable electrodes,
a single-layer gyricon display contains a multiplicity of
two-color balls arranged in a single layer in a dielectric
liquid. A gap required for the liquid to enter between the
two-color balls must be provided for rotating the two-color
balls, so the latter cannot be closely packed. For this reason,
despite the high contrast between colors of the two hemispherical
portions of a two-color ball in a dichromatic gyricon display,
it is impossible to obtain a sufficiently black color as a
whole, as well as a high contrast ratio. It is necessary to
enhance the contrast ratio.
Description Using a multiplicity of identical two-color
ball particles within an individual microcapsule to enhance
the contrast ratio is proposed. Particles which are composed
of a white hemisphere and a hemisphere of one of the primary
colors are positioned within an individual microcapsule in
a colorless, transparent dielectric liquid. The two-color
ball particles are substantially smaller than the corresponding
particles of a single-layer display. The hemispheres of a
two-color particle possess different zeta-potentials in the
dielectric liquid. Therefore, the hemispheres are charged
with opposite charges and the two-color particle exhibits
a dipole moment. Under this condition, the charge of the colored
hemisphere is higher than that of the white hemisphere and
the two-color ball electrophoretic particle as a whole is
charged into the same polarity as that of the colored hemisphere.
The particle in the dielectric liquid thus has a non-zero
monopole charge. Due to the monopole charge, the two-color
ball particles exposed to an electric field applied between
the electrodes detach from an electrode of the same potential
(to which they were drawn by a previous image switching) and
migrate to the electrode with the potential opposite to the
charge of the particle. During switching, the dipole moment
reverses the particles. The particles cover the entire internal
hemisphere of the microcapsule without any gaps previously
required in gyricon displays in order to rotate a two-color
ball. The dispersion and absorption of light in the dielectric
liquid at the microcapsule edge are considerably reduced.
The purity and density of the obtained microcapsule color
are generally higher. The micro-capsules are substantially
closely packed, thereby improving the purity and density of
the obtained color of the entire display, as well as the sharpness
of switching between regions of different colors, including
contrasting ones. The contrast ratio is thus enhanced.
Additional information The electric field is applied to the display
medium for only the time required for the two-color particles
to migrate from one display screen side to the other display
screen side, and is cut off before the particles reach the
opposite display screen sides. In a color display, a display
pixel is formed of three microcapsules. Each microcapsule
contains a group of ball particles, one hemisphere of which
is colored in one of the primary colors. The dielectric constant
of the dispersion medium of one group differs from that of
another group. This provides different values for the mobility
and threshold voltage reversing the two-color ball particles
of these groups. The color of a trichromatic pixel is controlled
by means of one electrode per pixel due to a change in voltage
only.
