Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-05-18
2004-01-20
Shankar, Vijay (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S084000
Reexamination Certificate
active
06680726
ABSTRACT:
RELATED APPLICATION
This application is related to concurrently filed application titled “Reflective Electrophoretic Display With Stacked Color Cells”(IBM Docket ARC920010075US1).
TECHNICAL FIELD
The present invention relates to electrophoretic cells that form an electrophoretic display. In particular the invention relates to a stacked cell configuration for use in a color electrophoretic display operating in a light-transmissive mode.
BACKGROUND OF THE INVENTION
An electrophoretic cell is a cell comprised of pigment particles suspended in a fluid and uses electrophoresis to switch between the following two states:
Distributed State: Particles are positioned to cover the horizontal area of the cell. This can be accomplished, for example, by dispersing the particles throughout the cell, by forcing the particles to form a layer on the horizontal surfaces of the cell, or by some combination of both.
Collected State: Particles are positioned to minimize their coverage of the horizontal area of the cell, thus allowing light to be transmitted through the cell. This can be accomplished, for example, by compacting the particles in a horizontal area that is much smaller than the horizontal area of the cell, by forcing the particles to form a layer on the vertical surfaces of the cell, or by some combination of both.
The electrophoretic cell can serve as a light valve since the distributed and collected states can be made to have different light absorbing and/or light scattering characteristics. As a result, an electrophoretic cell can be placed in the light path between a light source and a viewer and can be used to regulate the appearance of a picture element or “pixel” in a display. The basic operation of transmissive electrophoretic cells along with the examples of various electrode arrangements are described in IBM's U.S. Pat. Nos. 6,144,361 and 6,184,856.
Transmissive color displays are known that use liquid crystals and crossed-polarizers to control the intensity of light through the color filters in each pixel. The use of linear polarizers limits the transmission of light through the display and, hence the backlight efficiency, brightness and power efficiency of these displays is reduced. These displays also suffer from limited viewing angle.
Liquid crystal displays commonly use a side-by-side arrangement of single color subpixels within each pixel to generate color via spatial color synthesis. The transmission efficiency of such an arrangement is limited by the fact that each subpixel only occupies a fraction of the total pixel area. By arranging the subpixels in a vertical stack, each subpixel can occupy the same lateral area as the pixel itself, and the transmission efficiency can be significantly increased. U.S. Pat. No. 5,625,474 assigned to the Sharp Corporation and IBM's U.S. Pat. No. 5,801,796 describe embodiments of stacked cell arrangements suitable for liquid crystal materials. Since these embodiments rely on liquid crystal materials, however, they remain hindered by the aforementioned limitations of such materials. Electrophoretic displays do not suffer from these limitations and can offer improved transmission characteristics combined with extremely low power requirements.
Electrophoretic color displays have been proposed in the prior art. Japanese Patent JP 1267525 assigned to Toyota Motor Corporation describes an electrophoretic display having colored (blue and yellow) particles with different zeta potentials in a solution of red dye to give a multicolored (yellow, green and red) display. When a certain voltage is applied to the pixels, the yellow particles are pulled to the front transparent electrode and the viewer sees yellow. At a higher voltage, the blue particles are also pulled to the front electrode and the viewer sees green. When the particles are pulled off the transparent electrode, the colors of the particles are hidden by the dye solution and the viewer sees red.
U.S. Pat. No. 3,612,758 assigned to Xerox Corporation describes an electrophoretic display having pigment particles of a single color in a contrasting dye solution. In this scheme, under the influence of an electric field, the particles migrate to a front transparent electrode and the viewer sees the color of the particles. When the field is reversed, the particles migrate away from the front transparent electrode, are hidden in the dye solution, and the viewer sees the color of the dye solution.
WO 94/28202 assigned to Copytele Inc. describes a dispersion for an electrophoretic display comprised of two differently colored particles that are oppositely charged. The polarity of the voltage applied to the cell determines the polarity of the particle attracted to the front transparent electrode, and hence determines the color seen by the viewer.
In the electrophoretic color display references cited above, the use of a backlight is not suggested nor would these embodiments have contrast in a transmissive mode of operation. Transmissive electrophoretic displays based on backlit operation, however, have been proposed in the prior art.
U.S. Pat. No. 4,218,302 assigned to the U.S. Philips Corporation, describes a transmissive electrophoretic display that uses particles to either allow or frustrate the total internal reflection of light at the interface between the interior of the rear window and the suspension fluid. In the regions of this interface where no particles are present, the conditions for total internal reflection are satisfied, and light from the source is reflected back towards the source and the viewer sees no light. In the regions of this interface where the particles are present, the condition for total internal reflection is frustrated, and light from the source passes through the interface and the viewer sees light.
U.S. Pat. No. 4,648,956 assigned to the North American Philips Corporation describes several embodiments of a transmissive electrophoretic display. In these embodiments, the suspension is contained between a transparent, large-area upper electrode and a small-area lower electrode. When the absorbing particles in a selected region of the display are collected on the small-area lower electrode, they cover only a small portion of the horizontal area of the selected region. As a result, most of the light from the source passes through the selected region without being absorbed and the viewer sees light. When the absorbing particles are drawn to the upper electrode, they cover substantially the horizontal area of the selected region so that most of the light from the source is absorbed before it can reach the viewer and the viewer sees dark.
U.S. Pat. No. 5,298,833 assigned to Copytele Inc. describes a transmissive electrophoretic display based on a conductive mesh screen disposed between the backlight and the viewer. The mesh screen covers the viewing area of the display and is immersed in a suspension comprised of black particles in a clear fluid. Particles are either drawn to, or removed form a selected area of the mesh by using patterned transparent electrodes disposed above and below the mesh. When the black particles are drawn to the mesh they sit on the mesh without completely covering its holes. Light from the source is transmitted through the holes and the viewer sees light. When the particles are removed from the mesh, they are drawn to cover the selected transparent electrode. Light from the source is substantially absorbed in the area covered by the electrode and the viewer sees dark.
IBM's U.S. Pat. No. 6,225,971 describes a transmissive electrophoretic display with improved brightness and color gamut, but which relies on spatial color synthesis. As stated above, however, the transmission efficiency of color generation via spatial color synthesis is limited by the fact that each subpixel only occupies a fraction of the total pixel area.
There is a continuing need in the art for a low-power transmissive color electrophoretic display with high brightness, wide-viewing angle, high image contrast, and large color gamut. It would be desirable, therefor
Gordon, ll Joseph G.
Hart Mark W.
Lowe Anthony C.
Swanson Sally A.
Berthold Thomas R.
Johnson Daniel E.
Patel Nitin
Shankar Vijay
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