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United States Patent [w]
Wu et al.
US005889566A [ii] Patent Number: 5,889,566  Date of Patent: *Mar. 30, 1999
 MULTISTABLE CHOLESTERIC LIQUID CRYSTAL DEVICES DRIVEN BY WIDTHDEPENDENT VOLTAGE PULSE
Bao-Gang Wu; Jianmi Gao, both of Richardson; Hongxi Zhou, Dallas; Yao-Dong Ma, Richardson, all of Tex.
Advanced Display Systems, Inc.,
The term of this patent shall not extend beyond the expiration date of Pat. No. 5,625,477.
 Appl. No.: 837,431
 Filed: Apr. 17, 1997
Related U.S. Application Data
 Continuation of Ser. No. 225,836, Apr. 11, 1994, Pat. No. 5,625,477.
 Int. CI.6 G07F 1/1343
 U.S. CI 349/35; 349/175; 349/33;
 Field of Search 349/35, 175, 33;
 References Cited
U.S. PATENT DOCUMENTS
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D.K. Yang and J.W. Doane, "Cholesteric Liquid Crystal/ Polymer Gel Dispersions: Reflective Display Applications," SID 92 Digest, pp. 759-761.
A light modulating apparatus containing cholesteric liquid crystals capable of indefinite zero field retention of optical images, including full color and complete gray scale capability, is described. This invention is predicated on establishment of zero field multistable liquid crystal domains representing a continuous distribution of states ranging from the highly reflective planar to the lightscattering focal-conic structures. The zero field multistable domain structure stability is achieved by controlling solid surface-liquid crystal boundary interactions to provide essential equalization of the total system energy of each of the domain structures. This is the first time this equalization, and thus zero field multistability, has been achieved without the use of a polymer gel addition to create rigid polydomains in the liquid crystal mix. The establishment of a particular domain structure, and thus the degree of light reflection or scattering by individual display pixels, is controlled using either a single relatively long voltage pulse or a number of shorter duration successive voltage pulses. The zero field multistability achieved is remarkably stable against both mechanical shocks and thermal variations. Because these displays eliminate the need for backlighting and image refreshing energy input, they represent ultra low power consumption devices ideal for use with portable display systems. Because these displays also eliminate the need for polarizers, analyzers, retarder and color filters, they are ultra thin and particularly well suited for use in ultra large (i.e., billboard) displays using a building block approach which retains total spatial integrity. These displays can also be fabricated as flexible all-plastic units again utilizing the overall thinness of these devices.
87 Claims, 10 Drawing Sheets
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