Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-11-12
2004-12-14
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S033000, C349S096000, C349S117000, C349S123000, C349S177000
Reexamination Certificate
active
06831716
ABSTRACT:
The present application is a non-provisional application of International Application No. PCT/FR01/01428, filed May 11, 2001.
The present invention relates to the field of liquid crystal display devices.
STATE OF THE ART
Depending on the physical nature of the liquid crystal used, distinctions are drawn between devices in which the liquid crystals are nematic, cholesteric, smectic, ferrolectric, etc. In nematic displays, which constitute the preferred application of the present invention, a nematic liquid crystal is used that is non-chiral or that is made to be chiral, e.g. by adding a chiral dopant. This produces a texture which is spontaneously uniform or lightly twisted, with a helical pitch that is greater than a few micrometers. The orientation and the anchoring of the liquid crystal close to surfaces are defined by alignment layers or treatments that are applied to the substrates.
Most devices that have been proposed or made in the past are monostable. In the absence of an electric field, only one texture is obtained in the device. This texture corresponds to an absolute minimum in the total energy of the cell. Under an electric field, the texture is deformed continuously and its optical properties vary as a function of the applied voltage. When the field is interrupted, the nematic crystal returns to its single monostable texture. The person skilled in the art will recognize that these systems include the modes of operation that are in the most widespread use in nematic displays: twisted nematic (TN), supertwisted nematic (STN), electrically controlled birefringence (ECB), vertically aligned nematics (VAN), etc.
Another class of nematic displays is that of nematics that are bistable, multistable, or metastable. Under such circumstances, at least two distinct textures that are stable or metastable in the absence of a field can be achieved in a cell, using the same anchoring on the surfaces. The terms “bistable” or “multistable” are generally used to designate at least two states having the same energy or energy which is very similar, and which are likely to endure almost indefinitely in the absence of any external command. In contrast, the term “metastable” is used for states having different energy levels which are likely to switch after a relaxation time that is long. Switching between two states is achieved by applying appropriate electrical signals. Once a state has been written, it remains stored in the absence of an applied field because the crystal is bistable. This memory of bistable displays is very attractive in numerous applications. Firstly, it enables images to be refreshed at a slow rate, which is most advantageous for reducing energy consumption in portable appliances. Secondly, in fast applications (e.g. video), the memory makes a very high multiplexing rate possible, thus enabling high resolution video to be displayed.
Recently, a new bistable display [document
1
] has been proposed, using a liquid crystal that is cholesteric or chiralized nematic. The two bistable textures, U (for uniform or lightly twisted) and T differ from each other by twisting through ±180° and they are topologically incompatible (FIG.
1
). The spontaneous pitch p
0
of the nematic is selected to be close to four times the thickness d of the cell (p
0
≈4d) so that the U and T energies are substantially equal. Without an applied field, there exists no other state of lower energy: U and T are genuinely bistable.
Because of the topological incompatibility of the two bistable textures, it is not possible to transform one into the other without continuous bulk distortion. Switching between U and T thus requires a strong external field to induce anchoring transitions on the surfaces. Above a threshold electric field E
c
(threshold for breaking anchoring), an almost homeotropic texture (referenced H in
FIG. 1
) is obtained, with anchoring on at least one of the substrates being broken: the molecules extend normally to the plate in the vicinity of said surface.
When the field is interrupted, the nematic molecules close to the broken surface are in unstable equilibrium, without any anchoring torque, and they can return either to their initial orientation (thus returning to the same texture as they had prior to the field being applied), or else they can turn through 180° and, after relaxation, give rise to a bulk texture with an additional twist through 180°. At the end of the command pulse, the cell is guided in selecting one or other of its bistable states depending on whether the coupling between movements of molecules close to the two surfaces is elastic or hydrodynamic: elastic coupling causes a return to the U state, hydrodynamic coupling towards the T state.
For the information displayed on the device to appear, it is necessary for the textures that are achieved to have optical properties that are different. Most devices operate in polarized light and use additional optical components: polarizers, filters, compensating plates, etc. These elements and their orientations relative to anchoring on the two surfaces are selected as a function of the configuration of the display in order to optimize the relevant optical performance: contrast, brightness, color, viewing angle, etc.
For monostable displays, optimization needs to apply to an entire continuum of states achieved under fields of greater or lesser strengths because these states are on display throughout the duration of an image. A very large number of optical configurations have been proposed and made for a variety of devices (TN, STN, etc.), taking account of the particular features of each of those displays.
The optics of a bistable display in which anchoring is broken are very different from those of monostable devices. Firstly, throughout the major fraction of the duration of an image, only two textures are present in each cell of the display: those that correspond to the two bistable states. The optimum configuration must enable maximum contrast to be achieved between those two textures, while minimizing transient optical effects during switching due to passing rapidly through intermediate states under a field. Furthermore, the main difference between the two bistable textures, an additional twist of 180°, is not a parameter that is available for optimization: it is imposed by the physical mechanism used for achieving two bistable states. In addition, bistable switching requires an electric field that is strong E>E
c
(close to 10 volts per micrometer (V/&mgr;m)) and thus a control voltage U=E·d that is proportional to the thickness d of the cell. The liquid crystal layer must therefore be very fine (d≈2 &mgr;m to 3 &mgr;m) in order to make it possible to achieve control with voltages that are reasonable, and so optical optimization must take these requirements into account.
SUMMARY OF THE INVENTION
An object of the present invention is now to propose a novel display device based on liquid crystals which present properties that are better than those of previously known devices.
In the context of the present invention, this object is achieved by a reflective bistable device and characterized by the fact that it comprises:
a) a liquid crystal material contained between two parallel substrates, the substrates being provided with electrodes on their facing inside surfaces in order to enable an electric field to be applied to said liquid crystal, at least the front substrate and the front electrode being optically transparent;
b) alignment layers or treatments on the electrodes to orient the liquid crystal and enable at least two alternative distinct textures that are stable or metastable in the absence of a field to be implemented, in which one of the textures is either non-twisted or twisted with a total angle lying in the range −90° to +90°, and the other texture presents additional twisting through an angle close to 180°;
c) the thickness d of the liquid crystal layer being selected in such a manner that the product d·&Dgr;n is close to &lgr;
0
/4, where &lgr;
0
is the center wa
Dozov Ivan N.
Martinot-Lagarde Philippe R.
Stoenescu Daniel N.
Blakely & Sokoloff, Taylor & Zafman
Chowdhury Tarifur R.
NEMOPTIC
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