Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Plasma excitation
Reexamination Certificate
1997-10-22
2001-05-15
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Plasma excitation
C349S058000, C349S177000, C349S180000, C345S058000, C345S060000, C340S457100, C252S299100, C428S001400
Reexamination Certificate
active
06233028
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to plasma-addressed liquid crystal display panels commonly referred to as “PALC” display devices. These devices comprise, typically, a sandwich of: a first substrate having deposited on it parallel transparent column electrodes, commonly referred to as “ITO” columns or electrodes since indium-tin oxides are typically used, on which is deposited a colour filter layer; a second substrate comprising parallel sealed plasma channels corresponding to rows of the display crossing all of the ITO columns each of which is filled with a low pressure ionizable gas, such as helium, and containing spaced cathode and anode electrodes along the channel for ionizing the gas to create a plasma, which channels are closed off by a thin transparent dielectric sheet; and a liquid crystal (LC) material located between the substrates. The structure behaves like an active matrix liquid crystal display in which the thin film transistor switches at each pixel are replaced by a plasma channel acting as a row switch and capable of selectively addressing a row of LC pixel elements. In operation, successive lines of data signals representing an image to be displayed are sampled at column positions and the sampled data voltages are respectively applied to the ITO columns. All but one of the row plasma channels are in the de-ionized or non-conducting state. The plasma of the one ionized selected channel is conducting and, in effect, establishes a reference potential on the adjacent side of a row of pixels of the LC layer, causing each LC pixel to charge up to the applied column potential of the data signal. The ionized channel is turned off, isolating the LC pixel charge and storing the data voltage for a frame period. When the next row of data appears on the ITO columns, only the succeeding plasma channel row is ionized to store the data voltages in the succeeding row of LC pixels, and so on. As is well known, the attenuation of each LC pixel to backlight or incident light is a function of the stored voltage across the pixel. A more detailed description is given in Buzak et al., “A 16-Inch Full Colour Plasma Addressed Liquid Crystal Display”, Digest of Tech. Papers, 1993 SID Int. Symp., Soc. for Info. Displ. pp. 883-886.
A problem in the known construction is that too large a column voltage is required for full activation of a LC pixel. Typical driving voltages required with the known construction such as described in the SID Digest 93 publication are in the range of 80-100V.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the invention is an improved PALC display device.
Another object of the invention is a PALC display device requiring lower column driving voltages than were heretofore required.
Another object of the invention is a PALC colour display device requiring reduced column voltages for reliable operation.
To this end a PALC display device is provided comprising a layer of liquid crystal material, data electrodes coupled to the liquid crystal layer and adapted to receive data voltages for activating portions of the liquid crystal layer, a plurality of elongated plasma channels extending generally transverse to the data electrodes for selectively switching on said liquid crystal portions, a dielectric sheet closing off the plasma channels on the side facing the data electrodes, said plasma channels each comprising spaced elongated cathode and anode plasma electrodes and an ionizable gas filling, characterized in that the liquid crystal layer is located between the data electrodes and the plasma channels, the dielectric anisotropy (&Dgr;&egr;) lies between 1 and 9 and the dielectric constant perpendicular to the director (&egr;
⊥
) lies between 1.5 and 5 (1.5≦&egr;⊥≦5; 1≦&Dgr;&egr;≦9).
For PALC devices the required column driving voltage shows a low value for the indicated ranges of &egr;⊥ and &Dgr;&egr;. Furthermore the required column driving voltage shows less variation. At values for &egr;⊥ smaller than 1.5 the LC material is liable to crystallize, which reduces the effectiveness of the LC material.
In a preferred embodiment of the invention the dielectric constant perpendicular to the director &egr;
⊥
is smaller than 3.5. The required column voltage difference becomes smaller as &egr;
⊥
is reduced.
Preferably the dielectric anisotropy &Dgr;&egr; is less than 5 and more than 2. In this range the required column voltage shows a minimum. Furthermore the required column voltage is almost independent of the dielectric anisotropy and shows only a relatively small dependence on the dielectric constant perpendicular to the director.
In a preferred embodiment of the invention the PALC display device has a LC layer having a twist angle between 80 and 120 degrees. Within this range a compromise is achieved between the dynamic range &Dgr;V
appl
on the one hand and the viewing angle on the other hand. An increase of the twist angle reduces the required column driving voltage, however too high an angle reduces the viewing angle. The twist angle is preferably between 85° and 110°, even more preferably between 90 and 100 degrees.
Preferably the liquid crystal material has an elastic ratio k
3
/k
1
smaller than 1.75. Small values of k
3
/k
1
reduce the required column driving voltage further. Preferably the ratio k
3
/k
1
is less than 1.25.
In accordance with the invention an LC layer is selected whose transmission vs. voltage characteristic curve falls steeply between 99% and 1% transmission, i.e. having a low required column voltage (&Dgr;V
appl
), e.g. less than 25 Volts.
PALC display devices constructed in accordance with the invention will increase the fraction of the column voltage effective across the LC pixel elements. For a better understanding of the invention, its operating advantages and specific objects attained by its use, are illustrated and described in the accompanying drawings and descriptive matter, like reference numerals or letters signifying the same or similar components.
REFERENCES:
patent: 4684477 (1987-08-01), Sugimori et al.
patent: 4818428 (1989-04-01), Scheuble et al.
patent: 4896149 (1990-01-01), Buzak et al.
patent: 5221979 (1993-06-01), Kim
patent: 5414440 (1995-05-01), Ilcisin et al.
patent: 5461395 (1995-10-01), Stein
patent: 5747121 (1998-05-01), Okazaki et al.
patent: 5793455 (1998-08-01), Nakamura
patent: 5807499 (1998-09-01), Tarashima et al.
patent: 5853801 (1998-12-01), Suga et al.
patent: 5877831 (1999-03-01), Leenhouse et al.
patent: 6025895 (2000-02-01), Yazaki et al.
“A 16-Inch Full Colour Plasma Addressed Liquid Crystal Display”. Digest of Tech. Papers, May, 1993 SID Int. Symp., Society for Information Display, pp. 883-886.
Bruinink Jacob
Leenhouts Frans
Ngo Julie
Sikes William L.
U.S. Philips Corporation
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