Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-11-08
2004-09-21
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
06795146
ABSTRACT:
The present invention relates to the field of liquid crystal display devices.
STATE OF THE ART
Liquid crystals are commonly used in display devices. In nematic displays, which constitute the preferred application of the present invention, a nematic liquid crystal is used which is achiral or chiralized, e.g. by adding a chiral dopant. The orientation and the anchoring of the liquid crystal close to the surfaces are defined by alignment layers or treatments applied to the substrates. In the absence of a field, this serves to impose a uniform or slightly twisted nematic texture.
Most devices that have been proposed and implemented in the past are monostable. In the absence of an electric field, the device implements only one texture. This texture corresponds to an absolute minimum for the total energy of the cell. Under a 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 returns to its single monostable texture.
Another class of the nematic display 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 implemented in the cell, using the same anchorings on the surfaces. The terms “bistable” or “metastable” are generally used to designate two states having the same energy or energies that are very close, and that are capable of lasting substantially indefinitely in the absence of an external command. In contrast, the term “metastable” is used for states having energy levels that are slightly different and that are liable to switch after a long relaxation time. Switching between the two states is implemented by applying suitable electrical signals. Once a state has been written, it remains stored in the absence of a field because of the bistable (or metastable) nature of the crystal. This memory of bistable displays is most attractive for numerous applications. Firstly it enables images to be refreshed at a slow rate which is very favorable 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 video to be displayed in high resolution.
A typical example of a known bistable display [document 1] is shown diagrammatically in FIG.
1
. In that case, one of the bistable textures (T
0
) is uniform (or, generally, lightly twisted), whereas the other (T
360
) presents an additional twist of ±360°. The spontaneous cholesteric pitch p
0
of the material is selected so that p
0
≈2·d (where d is the thickness of the liquid crystal layer) so as to equalize the energies of the two topologically equivalent states T
0
and T
360
. A third texture T
180
, which is topologically different from the textures T
0
and T
360
is also possible using the same anchorings, and its energy is lower since it is better adapted to the spontaneous twisting of the material. However, in the absence of a field, T
0
and T
360
remain stable and do not transform into T
180
because of topological constraints. Under a strong electric field, a fourth texture is achieved that is almost homeotropic, with the molecules being perpendicular to the substrates almost throughout, except in the vicinity of the plates. It is this texture that makes it possible to switch between the metastable textures T
0
and T
360
. The particular final texture is selected under hydrodynamic control launched at the end of the control signal (backflow effect).
Another example of a known bistable display [document 2] is shown diagrammatically in FIG.
2
. The two bistable textures T
0
(uniform or lightly twisted) and T
180
which differ by twist of ±180° are topologically incompatible. The spontaneous pitch p
0
of the nematic is selected to be close to four times the thickness d of the cell, i.e. p
0
≈4·d so as to make the energies of T
0
and T
180
substantially equal. Without a field there does not exist any other state of lower energy: T
0
and T
180
are genuinely bistable. Under a strong field, an almost homeotropic texture (H) is obtained, with at least one of the anchorings on the substrates being broken: the molecules are normal to the plate in the vicinity of this surface. At the end of the control pulse, the cell is guided towards one or other of the bistable states depending on whether coupling between the movement of the molecules close to the two surfaces is elastic or hydrodynamic: elastic coupling returns towards the T
0
state while hydrodynamic coupling returns towards the T
180
state.
In order to enable the information displayed on the device to appear, it is necessary for the textures it implements to have different optical properties. Most devices operate with polarized light and use additional optical elements: polarizers, filters, compensating plates, etc. These elements and their orientations relative to the anchorings on the two surfaces are selected as a function of the configuration of the display so as to optimize pertinent optical performance: contrast, brightness, color, viewing angle, etc.
For monostable displays, optimization must apply to an entire continuum of states implemented under fields of various strengths, because these states are displayed throughout the duration of an image. A very large number of optical geometries have been proposed and implemented for various devices, taking account of the particular features of each of said displays. For each display, the configurations of additional elements are also adapted depending on whether they are used in transmission or in reflection.
The optics of the two above-mentioned types of bistable display are very different from that of monostable devices. Firstly, throughout most of the time an image lasts, only two textures exist in the cells of the display: textures corresponding to the two bistable states. The optimum configuration must enable maximum contrast between these two states, while minimizing transient optical effects during switching, due to passing quickly through intermediate states under a field. Furthermore, the main difference between the two bistable textures, an additional twist of 180° or 360° 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 a strong electric field (close to 10 volts per micrometer (V/&mgr;m)). The liquid crystal layer must therefore be very fine (d≈2 &mgr;m to 3 &mgr;m) in order to enable control to be performed using reasonable voltages so optical optimization must take these requirements into account.
Until now, bistable devices have been considered above all in transmission mode, which is the mode in which they were originally proposed.
However, bistable memory is very useful in reflection mode: a bistable display operating in reflection can retain and display an image for a very long time without consuming any energy, neither for its own operation (it is bistable), nor for lighting purposes (it does not require an internal light source).
Recently, certain particular reflective configurations have been proposed for bistable devices having a twist difference of 360° [documents 3, 4, and 5]. They use a single polarizer parallel to the nematic director on the front substrate. The state with little twist T
0
has twist of 63.6° [document 3] and of −36° [document 4]. The contrast specified in those two cases is less than 10 in white light.
The configurations that have been proposed in the past for reflective bistable displays operate in “normal” contrast, i.e. with a black state that is uniform or of small twist (T
0
) and with a white state that is highly twisted (T
180
or T
360
). That configuration which is relatively easy to implement can theoretically achieve contrast of about 60 in white light. Unfortunately it is very sensitive to variations in the thickness d and i
Dozov Ivan N.
Martinot-Lagarde Philippe R.
Stoenescu Daniel N.
Blakely & Sokoloff, Taylor & Zafman
Chowdhury Tarifur R.
NEMOPTIC
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