Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-10-01
2004-08-31
Thomas, Tom (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S179000, C349S180000, C349S186000
Reexamination Certificate
active
06784955
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the design of a permanently bistable twisted nematic liquid crystal display, and in particular to such a display has two stable twist states with no applied voltage. The display can be switched between these two stable states by the application of special voltage pulses. Due to this bistability, no power is needed to maintain the display, once it is formed.
BACKGROUND OF THE INVENTION
There are many types of bistable liquid crystal displays (LCD). Among them, the bistable twisted nematic (BTN) LCD has one of the best optical properties with good contrast and brightness. This display relies on adjusting judiciously the cell thickness to liquid crystal pitch (d/p) ratio to produce the metastable twist states.
Bistable twisted nematic (BTN) liquid crystal displays (LCD) have been around since the early discovery by Berreman et al (D. W. Berreman and W. R. Heffner,
Appl. Phys. Lett.
37, 109 (1980)). In the early prior art, the bistable twist states are 0 and 360° twist respectively. Recently, Kwok et al (full reference) extended these BTN to the general case of &phgr; and &phgr;+2&pgr;, where &phgr; can be optimized to yield better optical properties such as contrast and on-state brightness.
In this latter BTN, &phgr; can be optimized to produce excellent optical properties. However, the major problem of this BTN is that the &phgr; and &phgr;+2&pgr; twist states are metastable with short lifetimes. Indeed, the intermediate &phgr;+&pgr; twist state is more stable and both the &phgr; and &phgr;+2&pgr; states will decay to it in a matter of seconds.
There are several variations of the basic BTN. Hoke et al (C. D. Hoke and P. J. Bos,
Soc. Info. Disp. Symp. Digest,
29, 854 (1998)) use a polymer wall trying to stabilize this BTN. The results are not satisfactory as the two twist states are still short-lived. Bryan-Brown et al (GB9521106.6) demonstrated a grating aligned bistable nematic display that can be switched by sub-millisecond pulses and has infinite time memory. The display has been produced and disclosed. Its practicality is in doubt because of the need of a special surface structure in the construction of the display. Switching from one state to the other is also not easy.
Dozov et al (I. Dozov, M. Nobili and G. Durand,
Appl. Phys. Lett.
70, 1179 (1997)) investigated a surface-controlled bistable nematic display using simple planar monostable anchoring. A high pretilt angle and a small cell gap are necessary to produce the bistability and switching between the 0 and &pgr; twist states. This display is permanently stable in that the 0 and &pgr; states do not decay to any intermediate states. It is similar to the &phgr; and &phgr;+2&pgr; BTN.
SUMMARY OF THE INVENTION
According to the present invention there are disclosed designs of permanently bistable twisted nematic liquid crystal displays with optimized optical properties. The stable twist states differ by an angle of &pgr;, or 180°. They are denoted &phgr; and &phgr;+&pgr;. The aligment of the &phgr; and &phgr;+&pgr; twists are given by the rubbing condition that favors the &phgr; twist. The d/p ratio is adjusted to favor the &phgr;+&pgr;/2 twist. This enables both the &phgr; and &phgr;+&pgr; twist states to be stable. The d/p ratio favored &phgr;+&pgr;/2 twist state is not stable under the given rubbing conditions. Hence the lifetimes of the &phgr; and &phgr;+&pgr; states are infinite.
The switching between the &phgr; and &phgr;+&pgr; twist states are effected through many techniques, such as having a small cell gap, or having a three electrode structure. One possible example of switching uses a triode structure. However, it is not essential to the present invention and other schemes are possible.
The new BTN can either be transmittive requiring two polarizers, or reflective requiring just one polarizer. In general, the optical properties depend on &agr; and &ggr;, which are the input and output polarizer angles, with respect to the input director of the liquid crystal cell, and the twist angles &phgr;
1
and &phgr;
2
of the two bistable twist states. The optical properties also depend on d, the liquid crystal cell thickness and &Dgr;n is the birefringence of the liquid crystal material used. With all these values defined, the optical properties and construction of the liquid crystal display is uniquely defined. A perfect optimization procedure is given to give the exact values of the construction of the BTN to give the best optical properties.
It is also possible to construct the BTN in the reflective mode with just one top polarizer. In this case, there is no output polarizer and the display is easier to construct and more economical to make. We also will give the construction parameters for such reflective BTN to obtain the best optical properties. An important aspect of the invention is that the bistable twist states differ by an angle of 180°. rather than 360° as in the previous cases.
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Guo Jian-Xin
Kwok Hoi-Sing
Brock II Paul E
Burns Doane Swecker & Mathis L.L.P.
The Hong Kong University of Science and Technology
Thomas Tom
LandOfFree
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