Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within nematic phase
Reexamination Certificate
2000-05-19
2004-03-16
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within nematic phase
C349S180000
Reexamination Certificate
active
06707527
ABSTRACT:
FIELD OF THE INVENTION
This invention deals with the fabrication parameters for making bistable twisted nematic liquid crystal displays. In particular, the conditions of the twist angles and the retardation of the liquid crystal cell in order to optimize its optical properties such as the contrast ratio, the viewing brightness and the viewing angle are given.
BACKGROUND OF THE INVENTION
Bistable twisted nematic (BTN) displays are displays which possess at least 2 stable twist states under a zero voltage condition. The first such device was reported by Berreman in 1981. A patent on this display was issued in 1985. This BTN display has the advantage that, unlike an ordinary LCD, no holding voltage is needed to display the images. At zero volts, the alignment of the BTN can take on either of two twist states, depending on the choice of the rubbing directions of the alignment layers and the d/p ratio, where d, p are the thickness and pitch of the LC cell respectively.
The physics of the BTN lies in the interaction between the cell boundary conditions and the natural twist of the LC material. If the rubbing of the alignment layers favors a twist angle of &phgr;, and if the natural twist of the nematic LC itself is &phgr;+&pgr;, then the twist states &phgr; and &phgr;+2&pgr; are equally stable (or metastable). Hence, for bistability to occur, the d/p ratio of the LC cell should be ~(0.5+&phgr;/2&pgr;).
The optical properties of the BTN depends strongly on the input polarizer angle &agr;, the output polarizer angle &ggr;, the twist angle &phgr;, and the thickness—birefringence product d&Dgr;n of the LC cell. So far, only a few combinations of (&phgr;, &phgr;+2&pgr;) have been successfully made, including the (−90°, 270°), (0°, 360°) and (90°, 450°) BTNs. However, none of these BTN possesses all the desired qualities of high brightness, high contrast, low color dispersion, and low sensitivity to d&Dgr;n changes (thus ease of fabrication). It is the purpose of this invention to disclose many possible combinations of (&agr;, &ggr;, &phgr;, d&Dgr;n) that can produce BTNs with excellent optical properties and ease of fabrication.
SUMMARY OF THE INVENTION
The present invention gives the fabrication conditions of liquid crystal cells and the input/output polarizer angles in order to achieve excellent optical quality bistable twisted nematic liquid crystal displays. The conditions to be detailed are the twist angle &phgr;, the thickness of the liquid crystal cell d, the liquid crystal birefringence &Dgr;n, the input polarizer angle &agr;, and the output polarizer angle &ggr;.
REFERENCES:
patent: 4505548 (1985-03-01), Berreman et al.
patent: 4529271 (1985-07-01), Berreman et al.
patent: 2001/0005246 (2001-06-01), Takiguchi et al.
Kwok Hoi-Sing
Tang Shu-Tuen
Burns Doane Swecker & Mathis L.L.P.
Kim Robert H.
Rude T L
The Hong Kong University of Science and Technology
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