Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-11-02
2004-06-22
Ngo, Huyen (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S113000, C349S086000, C430S020000
Reexamination Certificate
active
06753937
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns a reflective liquid crystal display device in particular with improved display contrast allowing the displayed characters to contrast clearly with the background on which they are displayed.
There exist different types of reflective liquid crystal display devices. One may, in particular, cite transmissive display devices including a metal or dielectric reflector placed behind the display cell.
A transmissive display device conventionally includes an absorbent polariser placed in front of the cell, and a second absorbent polariser placed behind the cell and whose direction of polarisation is perpendicular to that of the first absorbent polariser placed in front of the cell. Such is the case, in particular, of twist nematic or TN liquid crystal cells. A first linear component of the light is absorbed by the absorbent polariser placed in front of the cell, i.e. on the side receiving the incident light, while the second light component, perpendicular to the first, is transmitted to the TN cell. In the TN cell, the direction of polarisation of the transmitted light is rotated through 90°, so that this light is transmitted without modification by the second absorbent polariser, and is visible to an observer placed behind the cell. In the zones where a control voltage is applied in order to modify the optical conditions of the twist nematic liquid crystal, the light transmitted by the absorbent polariser placed in front of the cell passes through said cell without being modified by the TN liquid crystal and is absorbed by the second absorbent polariser placed behind the cell. The observer, placed behind the cell, thus sees characters displaced in black on a white background.
In order to transform a transmissive cell into a reflective cell, as mentioned above, one need only arrange a metal or dielectric reflector behind said transmissive cell. Thus, in the event chat the display cell is a TN liquid crystal cell, the linear component of the light transmitted to the TN cell by the absorbent polariser placed in front of said cell is rotated through 90° by the liquid crystal, then transmitted without modification by the second absorbent polariser placed behind the TN cell whose direction of polarisation is perpendicular to that of the first absorbent polariser, then finally reflected by the reflector. The light then follows the same path, but in the opposite direction, through the cell and the absorbent polarisers, and is visible to an observer placed in front of said cell, i.e. on the side through which the incident light penetrates.
There also exist reflective liquid crystal cells without any polarisers. This is the case, for example, of polymer stabilised cholesteric texture or “PSCT ” liquid crystal cells.
It will be recalled that the characteristic of certain cholesteric type liquid crystals is having a periodic twisted structure which reflects, by Bragg reflection, light which has circular polarisation in the same rotational direction as that of the liquid crystal. Thus, a cholesteric liquid crystal having a right handed helix will reflect light circularly polarised to the right, whereas it will allow light circularly polarised to the left pass without modification. Conversely, a cholesteric crystal having a left handed helix will reflect light circularly polarised to the left.
Display cells without any polarisers which use reflective liquid crystals such as cholesteric liquid crystals require the use of an absorbent black layer conventionally deposited on the back face of the cell. When a suitable control voltage is applied-to selected electrodes to modify the optical conditions of the liquid crystal located at the crossing point of the electrodes in question, this control voltage switches the liquid crystal into a state in which it is reflective or into a state in which it is transparent. In the state in which it is reflective, the cholesteric liquid crystal has a reflection coefficient substantially equal to 0.5 when the light of a polarisation is reflected, and when the light from the other polarisation passes and is absorbed by the absorbent layer. When the control voltage is applied, the liquid crystal switches into the state in which it is transparent and allows practically all the light which is absorbed by the black layer located on the back face of the cell to pass. The observer situated on the front side of the cell, i.e. on the side through which the incident light penetrates the cell, thus sees a dark display on a light background.
The deposition of the absorbent black layer on the back face of the reflective cells poses problems during the manufacturing of such cells. Indeed, as is known, liquid crystal cells are usually manufactured collectively in batches. These batches include two plates common to all the cells and a network of sealing material connecting the plates and forming the walls of said cells, these walls delimiting cavities intended to be filled with liquid crystal. Each batch is divided into rectilinear strips by glass scratching and breaking techniques, or by sawing along parallel straight lines. As each cell has a filling opening along one edge of strip, the cells are then filled and their openings are sealed, then the strip is divided into individual rectangular cells along straight lines perpendicular to the preceding lines. At this stage, if the contour of the cells has parts deviating from the rectangular shape, such parts are shaped by grinding.
The absorbent black layer is applied over one of the two plates common to all the cells before the batches of cells are divided into individual cells. This absorbent black layer may nonetheless not cover the entire surface of the corresponding plate, since it would be damaged and would disturb the cutting and grinding operations. This manufacturing step could also be effected individually on each cell, but that would make manufacturing more complex and expensive than if it could be performed on a batch of cells. It is for these reasons that the technique currently used consists in selectively depositing absorbent black layers on one of the common plates following, for each cell, the final contour thereof. This solution, which is simpler than if the absorbent layer had to be individually deposited on each cell, remains however complex and requires a high level of precision to execute, which inevitably makes the final cost price of the cells more expensive.
Another recurrent problem encountered with reflective cells concerns the optical losses due to the multiple reflections of the incident light at each interface between two materials of different optical indices. This problem will be better understood upon examining
FIG. 1
annexed to the present Patent Application which is a cross-section of a reflective display device according to the prior art.
Designated as a whole by the general reference numeral
1
, the aforementioned reflective display device includes a first transparent substrate
2
arranged on a front side, i.e. on the side which receives incident light, and a second substrate
4
arranged on a back side, so that substrate
4
extends facing front substrate
2
and parallel thereto.
Substrates
2
and
4
may be made of glass or a plastic material. They are connected via a sealing frame
6
in order to limit between them a cavity for containing a film or layer
8
of reflective liquid crystals, for example of the cholesteric type.
The surface of substrate
2
turned towards substrate
4
carries a first group of electrodes
10
. The surface of substrate
4
turned towards substrate
2
includes a second group of electrodes
12
arranged, for example, in the form of strips perpendicular to electrodes
10
. Each of electrodes
10
and
12
may be formed by a conductive strip deposited and structured by conventional techniques and made, for example, of indium-tin oxide (ITO). The display device is connected to a control circuit for electrodes
10
and
12
, this circuit supplying control voltages to the electrodes to switch the liquid crys
Asulab S.A.
Ngo Huyen
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