Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-01-21
2001-11-20
Hjerpe, Richard (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S090000, C345S094000, C345S208000, C349S086000, C349S092000, C349S093000, C349S096000
Reexamination Certificate
active
06320563
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to liquid crystal displays. More particularly, the present invention relates to cholesteric liquid crystal displays. Specifically, the present invention relates to a dual frequency cholesteric display and method of driving this display.
BACKGROUND ART
Liquid crystal displays take advantage of a liquid crystal's ability to reflect and scatter light. This light reflecting ability is in part due to liquid crystal's tendency to form textures. The term texture describes the molecular orientations within a liquid crystal display cell. Cholesteric liquid crystals exhibit three alignments. These are the planar texture, focal conic texture, and homeotropic texture. Cholesteric crystals exhibit a helical molecular structure. The helical structure is formed by stacked long molecules that are progressively displaced through a small angle. When these liquid crystals are in the focal conic texture, the individual helical domains are in a random arrangement. This random arrangement weakly scatters light. The helical axis is more or less parallel to the supporting surfaces. In the homeotropic texture, the liquid crystal material adopts a completely undeformed director configuration. In this configuration, the director points perpendicular to the supporting surfaces. Finally, in the planar texture, the helical axis is aligned perpendicular to the supporting surfaces. As the liquid crystal material moves from one of these textures to another, its light propagating attributes change.
Cholesteric liquid crystals are used for reflective displays because they exhibit Bragg reflection in the planar texture. In the focal conic texture, cholesteric liquid crystal material scatters light. They are both stable at zero field. For a regular cholesteric liquid crystal with a positive dielectric anisotropy, the transition from the planar texture to the focal conic texture is direct and is achieved by applying a low voltage pulse. However, the transition from the focal conic texture to the planar texture is indirect. The material must be switched from the focal conic texture to a third state, a homeotropic texture, by a high voltage pulse, and then the material relaxes to the planar texture. The need to switch the material to the homeotropic texture is disadvantageous because the voltage required to switch the material to homeotropic texture is high, response time is increased, and it is difficult to make use of cumulative effect with the homeotropic texture. These disadvantages make it impractical to use known cholesteric liquid crystals in video rate displays.
It is known to provide a dual frequency cholesteric liquid crystal material responsive to high and low frequency voltages. However, it is only known to apply a single high or low frequency of varying duration to change the appearance of the material. This results in a slow and unacceptable addressing speed.
Thus, it is desirable to develop a drive scheme for switching directly from the focal conic texture to the planar texture without first switching to a homeotropic texture. It is also desirable to provide a cholesteric display that would be conducive to video rate applications.
DISCLOSURE OF INVENTION
It is, therefore, a primary object of the present invention to provide a dual frequency cholesteric liquid crystal display and drive scheme.
It is another object of the present invention to provide a display and drive scheme, as above, to switch cholesteric liquid crystal material directly from a focal conic texture to a planar texture without first switching to a homeotropic texture.
It is a further object of the present invention to provide a display and drive scheme, as above, that switches a cholesteric liquid crystal by selectively applying multiple electric pulses.
It is still another object of the present invention to provide a drive scheme for a cholesteric liquid crystal display, as above, that simultaneously applies high and low frequency electric pulses.
It is an additional object of the present invention to provide a drive scheme for a cholesteric liquid crystal display, as above, wherein the liquid crystal material is switched cumulatively between the textures by multiple pulses so the amplitude or the duration of the pulses, or both, can be reduced.
The foregoing and other objects of the present invention, which shall become apparent as the detailed description proceeds, are achieved by a method of addressing a dual frequency cholesteric liquid crystal material disposed between opposed substrates, wherein one of the substrates has a first plurality of electrodes facing a second plurality of electrodes on the other substrate, and wherein the intersection of the first and the second plurality of electrodes forms a plurality of pixels, the method comprising the steps of selectively applying high and low frequency voltages to the plurality of pixels, wherein the high frequency voltage causes the material to exhibit one texture and the low frequency voltage causes the material to exhibit another texture, and adjusting a voltage amplitude value for each high and low frequency to obtain a desired reflectance for each pixel.
Other aspects of the present invention are attained by a dual frequency cholesteric display, comprising a pair of opposed substrates, wherein one of the substrates has a first plurality of electrodes facing a second plurality of electrodes on the other substrate, a dual frequency bistable cholesteric liquid crystal material disposed between the substrates, wherein the material and the intersection of the first and second plurality of electrodes forms a plurality of pixels, means for selectively applying high and low frequency voltages to the plurality of pixels, wherein the high frequency voltage causes the material to exhibit one texture and the low frequency voltage causes the material to exhibit another texture, and means for adjusting a voltage amplitude value for each high and low frequency to obtain a desired reflectance for each pixel.
These and other objects of the present invention, as well as the advantages thereof over existing prior art forms, which will become apparent from the description to follow, are accomplished by the improvements hereinafter described and claimed.
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Xu Ming
Yang Deng-Ke
Hjerpe Richard
Kent State University
Renner Kenner Greive Bobak Taylor & Weber
Zamani Ali A.
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