Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1996-02-28
2001-12-04
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S129000, C349S037000
Reexamination Certificate
active
06327014
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a special structure of electro-optic cell (pixel) in a liquid crystal screen providing an enlarged viewing angle compared with known liquid crystal screens.
DESCRIPTION OF THE PRIOR ART
Liquid crystal screens for projection or direct viewing are generally composed of lines (selection lines) and columns (data lines). At each intersection there is a pixel electrode connected to these lines via transistors. The gates of these transistors form the selection lines and are controlled by peripheral control circuits that scan the lines and make the transistors of each line conducting by using the data lines, connected to other peripheral control circuits, to polarizing the electrodes and modify the optical properties of the liquid crystal located between these electrodes and the counter-electrode (or reference electrode), thereby enabling images to be formed on the screen.
One of the major disadvantages of this type of screen is that the optical properties depend on the angular visibility, i.e. the direction of propagation of the light through the pixels, in particular for images having several levels of gray.
FIG. 1
shows a cell of twisted nematic liquid crystals
1
with two viewing planes, the horizontal plane
2
and the vertical plane
3
. In the case of crossed polarizers, a voltage across the electrode terminals creates an electric field through the liquid crystal that aligns the molecules; the pixel is in its non-conducting state and appears black. The angular distribution of the optical transmittance of the light is symmetric about the horizontal plane
2
and asymmetric in the vertical plane
3
. The result is a dark area S and a light area C on opposite sides of the horizontal plane. The direction of maximum contrast D, corresponding to the minimum transmittance, is located in the vertical plane
3
; its orientation depends on the voltage applied across the electrodes of the pixel, in other words the luminosity. Consequently, the quality of the image displayed on the screen varies with the viewing angle; at very large angles the contrast may even be inverted.
The direction of the various viewing zones depends on the orientation of the liquid crystal molecules.
FIG. 2
shows the alignment of these molecules
4
in the case of a twisted nematic liquid crystal in the presence of a voltage U across the electrodes
5
and
6
. On the face towards the liquid crystal these electrodes are covered with an alignment layer
7
(which can be a polyimide) imposing a special direction on the liquid crystal molecules
4
. The directions of the dark and light zones,
8
and
9
, mentioned above are given by the inclination taken by the molecules of liquid crystal in the presence of the electric field. By treating the polyimide coating in a certain manner, we can make the liquid crystal molecules take a pre-inclined position, which is an angle between the molecules closest to the substrate and the substrate itself, in the absence of the electric field.
In order to overcome these problems and obtain a larger angular field of vision, K. H. Yang, in an article in the “11th International Display Research Conference”, 1991, page 68, proposes a solution that is illustrated in FIG.
3
. This consists in dividing each pixel into two domains each having an opposing inclination when a voltage U is applied to the electrodes. The alignment layers of each of these half-pixels are engraved so as to create opposing tilts. This can be done in two stages, separated by a masking stage, of anti-parallel rubbing of the polyimide alignment layers, or by depositing silicon oxide SiO under vacuum. In this case, the effects are neutralized and we obtain symmetrical vision regardless of the angle of the viewing angle.
The drawback of this solution is that it is technically difficult to implement, since it requires the engraving of two different alignment layers of the liquid crystal molecules, which is very difficult to achieve under mass production conditions.
SUMMARY OF THE INVENTION
The present invention resolves these problems through the use of a screen structure that is easy to fabricate.
The invention relates to an electro-optic cell or pixel formed by two substrate layers, on one of which there is a pixel electrode and on the other a counter-electrode, with a layer of liquid crystal molecules between these electrodes, in which the counter-electrode is divided into two parts by a groove, wherein a non-zero polarization voltage is applied between the electrode and the counter-electrode when the pixel is not addressed.
Said groove in the counter-electrode, dividing it into two parts (or domains), preferably cuts this electrode through the middle.
Said polarization voltage is preferably between 0.8 and 1.3 times the value of the threshold polarization voltage of the liquid crystal in the presence of a homogeneous electric field.
Another characteristic of the invention is that the said pixel has homogeneous alignment layers. The division of each pixel into two domains, in each of which the molecules have different inclinations, is produced only by the effect of the electric field between said pixel electrode and said counter-electrode.
In order to avoid any preference for one of the two domains introduced by the alignment layer, the alignment layer may cause the liquid crystal molecules to have either an alignment strictly parallel to the substrate or, in the case of the presence of an intrinsic pre-inclination of the alignment layer, an anti-parallel alignment.
Another improvement of the invention consists in slightly increasing the thickness of the liquid crystal so as to maintain the optical characteristic in the presence of said polarization voltage when the pixel is not addressed. This thickness should preferably be such that its product with the bifringence, divided by the wavelength of the light is slightly greater than the constant {square root over (3/2)}, which is typically between 1.1 and 1.5 times this constant.
Another characteristic of the invention is that an opaque material can be deposited on one of the substrate layers in order to mask the groove of the counter-electrode; moreover, this opaque material can be used as a storage capacity.
Finally, the invention also relates to a liquid crystal screen that makes use of such electro-optic cells.
REFERENCES:
patent: 4759610 (1988-07-01), Yanagisawa
patent: 5054910 (1991-10-01), Kozaki et al.
patent: 5061045 (1991-10-01), Yoneya et al.
patent: 5084778 (1992-01-01), DeJule et al.
patent: 5136407 (1992-08-01), Clerc
patent: 5229873 (1993-07-01), Hirose et al.
patent: 5309264 (1994-05-01), Lien et al.
patent: 5499123 (1996-03-01), Mikoshiba
patent: 5510916 (1996-04-01), Takahashi
patent: 445777 (1991-09-01), None
patent: 0636917 (1995-02-01), None
SID International Symposium-Digest of Technical Papers, Seattle May 16-21, 1993, vol. 24, Part 1, May 16, 1993 Society for Information Display, pp. 269-272, Lien et al. ‘Two-Domain TN-LCDs Fabricated by Parallel Fringe Field Method’.
SID International Symposim-Digest of Technical Papers, San Jose Jun. 14-16, 1994, vol. 25, Jun. 14, 1994, Society for Information Display pp. 594-596, Lien et al.
SID International Symposium Digest of Papers, Boston May 17-22, 1992 vol. 23, May 17, 1992 Society for Information Display pp. 498-801 Koike Y et al.
Haas Gunther
Mourey Bruno
Mulatier Laurence
Thomson Licensing S.A.
Ton Toan
Tripoli J. S.
Wein F. A.
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