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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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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.
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.
US Patent 6486866; Link >>
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