Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-10-09
2002-11-19
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S169000
Reexamination Certificate
active
06483563
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to reflective cholesteric liquid crystal displays and, in particular, to the use and treatment of alignment layer materials to improve performance of such displays.
BACKGROUND OF THE INVENTION
Liquid crystal displays have brought to the world a low-power, flat-screen technology that has enabled many new portable devices. As the demand for more portable devices grows, so does the demand for displays with improved performance. Reflective displays are important not only because they reduce power consumption, but also because they can be clearly seen in sunlight. Displays with bistable memory are attractive for reducing power consumption and increasing battery life compared to displays in which the image needs to be continuously refreshed.
Bistable liquid crystal displays were developed using cholesteric liquid crystalline materials. With a low concentration polymer network, both the focal-conic and planar textures of a cholesteric liquid crystal can be made stable and it is possible to electrically switch between the two states. Reflective cholesteric displays that employ polymer may use cells with rubbed substrates to achieve a homogeneous alignment of the liquid crystal at the surface of the cell substrate. Homogeneous alignment tends to favor the planar texture. The polymer network prevents the focal-conic texture from transforming to the planar texture.
It was discovered that a bistable cholesteric display could be made without a polymer network if inhomogeneous surfaces for the cell substrate were employed. In this case, cell substrates were typically unrubbed. U.S. Pat. No. 5,453,863 describes polymer-free cholesteric display cells which are said to be bistable and can be electrically switched between the two optical textures. The U.S. Pat. No. 5,453,863 discloses that alignment layers of a cell may be rubbed or unrubbed. However, the U.S. Pat. No. 5,453,863 patent does not address the relationship between display performance and variations in the degree of rubbing and choice of alignment layer materials. More importantly, that patent does not disclose how to optimize brightness by increasing the reflectivity of the planar texture or how to optimize contrast by lowering reflectance of the focal conic state. A display with a focal conic state that has high reflectance will have low contrast and is of little commercial value. The U.S. Pat. No. 5,453,863 patent also does not describe how to control the degree of circular polarization of the reflected or transmitted light important in stacked cell configurations.
Cholesteric liquid crystal displays have been popular for portable signs as well as hand held devices where low-power consumption and hence long battery life is important. Being reflective, these displays are readable in bright sunlight as well as room light. A drawback with current cholesteric liquid crystal displays, however, is that their brightness and contrast are not optimized. The brightness is reduced by the defects introduced in the planar texture by the polymer network or by an inhomogeneous surface alignment such as the unrubbed surface in polymer-free displays. Defects also tend to destroy the polarization state of the reflections from the planar texture. Likewise, contrast has been limited by the light scattering nature of the focal-conic texture. Defects in the focal-conic texture affect the light scattering properties of that texture. It is desirable that the focal-conic texture be as transparent as possible in order to show the black or colored back-layer as clearly as possible.
Cholesteric liquid crystal displays may achieve bistability of the focal conic and planar textures through the use of an inhomogeneous aligning surface rather than with polymer present in the bulk of the liquid crystal. An inhomogeneous aligning surface that is typically used in cholesteric liquid crystal displays is in the form of an unrubbed polyimide layer. There are various types of polyimide alignment layer materials characterized by different pretilt angles, or angles by which the liquid crystal director extends from the surface of the substrate. The lack of rubbing results in an azimuthally random distribution of the nematic director at the surface (inhomogeneous alignment). In this case, both the planar and focal-conic textures are stable under zero field conditions. This inhomogeneous alignment layer introduces defects in the uniform planar and focal conic textures, giving them stability, i.e., the ability to maintain set optical textures in the absence of an electric field.
It is conventionally believed that a very high brightness “perfect planar” texture may be induced by homogeneous alignment of the liquid crystal director. However, such an alignment condition alone does not stabilize the focal-conic texture and provide it with desirable low reflectance. It is well known that light reflected from a perfect single domain planar texture is completely circularly polarized. Defects resulting from an inhomogeneous alignment surface or a polymer network substantially reduce the degree of circular polarization of the reflected and transmitted light.
SUMMARY OF THE INVENTION
The present invention is directed to chiral nematic liquid crystal displays which include a “homogeneous” alignment surface on one or both of the substrates (i.e., sides) of a cell. This surface tends to align the liquid crystal director adjacent thereto and provide the display with increased brightness, low focal conic reflectance and/or reflected light that has an increased degree of circular polarization. Aspects of the present invention include a display with one side treated; a display with both sides treated; orientations of a display with the untreated side located nearest to and farthest from a viewer; and a stacked display having a cell with at least one side treated, such as a stacked display in which a second (e.g., lower) cell has both sides treated and a first (e.g., upper) cell has only the side nearest the second cell treated. These different aspects of the invention may be achieved through the use of various alignment techniques such as rubbed polyimide, UV alignment, selection of alignment material such as low or high pretilt, and combinations of the foregoing.
Increased performance of the inventive liquid crystal display is intended to refer to increase compared to a control liquid crystal display device that is identical but has inhomogeneous alignment surfaces such as unrubbed polyimide. With regard to a stacked display, a corresponding cell of the control device should be referred to for purposes of comparison. For example, if a bottom cell of an inventive stacked display has increased brightness, the brightness of a bottom cell of the stacked control display should be considered. Increases of properties are calculated relative to an increase in percentage or degree of the magnitude of the property that is achieved by the control display. For example, an inventive 30% increase in a property, such as brightness, means a brightness that is 1.3 times the brightness of the control display. If not otherwise indicated, it is should be understood that increases or decreases in a property are with regard to the property of the control display.
One embodiment of the present invention is directed to a liquid crystal display having at least one cell with at least one side treated so as to enhance brightness, comprising chiral nematic liquid crystal material having positive dielectric anisotropy. In all embodiments of the present invention, the liquid crystal material is preferably substantially free from polymer. Cell wall structure contains the liquid crystal material. At least one homogeneous alignment surface is effective to substantially homogeneously align the liquid crystal director adjacent thereto. At least one of the cell wall structure and each homogeneous alignment surface cooperates with the liquid crystal material so as to form focal conic and planar textures that are stable in the absence of a field. This homogeneous alignm
Davis Donald J.
Doane J. William
Huang Xiao-Yang
Khan Asad A.
Miceli Gene A.
Dudek James
Kent Displays, Inc.
Watts Hoffmann Fisher & Heinke
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