Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-06-28
2001-12-04
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S097000, C349S133000
Reexamination Certificate
active
06326940
ABSTRACT:
This invention relates to ferroelectric liquid crystal devices.
The surface stabilised ferroelectric liquid crystal device (SSFLCD) possesses the advantage over other liquid crystal devices, such as the twisted nematic liquid crystal device, that it is a bistable device which can be switched between two states by switching pulses of opposite polarity. It is switched into one state by a pulse of one polarity and will remain in that state until a switching pulse of opposite polarity is applied to switch it to the opposite state. By contrast, in operation of a twisted nematic liquid crystal device, a drive signal must be applied continuously to maintain the device in one of its states. SSFLCD's are of particular interest in multiplexed applications as the level to which such devices can be multiplexed is not restricted by the electro-optic characteristic of the device.
As is well known, a conventional SSFLCD typically includes a layer of chiral smectic C* ferroelectric liquid crystal material contained between two parallel transparent substrates provided on their inside surfaces with electrode structures in the form of row and column electrode tracks which cross one another to form an addressable matrix array of pixels. Such pixels are typically addressed on a line-by-line basis by applying data pulses in parallel to the column electrode tracks, each data pulse being either a selection pulse or a non-selection pulse, and by sequentially applying strobe pulses to the row electrode tracks so as to switch selected pixels along each row from one state to the other under the effect of the electric field produced by the voltage difference between the data pulse and the strobe pulse applied to the relevant electrode tracks. In certain addressing schemes, a blanking pulse is applied sequentially to the row electrode tracks of such a voltage and duration as to set all the pixels along each row to one state, irrespective of the data pulses applied to the column electrode tracks. Subsequent application of strobe and data pulses may then be used to switch selected pixels to the other state whilst leaving the remaining pixels in the one state. Addressing schemes of this type are described in Surguy, Ayliffe, Birch, Bone, Coulson, Crossland, Hughes, Ross, Saunders and Towler, “The Joers/Alvey, Ferroelectric Multiplexing Scheme”, Ferroelectrics, 1991, Vol. 122, pp. 63-79.
The interaction between the strobe and data waveforms needs to be carefully controlled to ensure that those pixels which require to be switched are switched successfully and those pixels which are to remain in the same state are not affected by either the strobe pulses or the data pulses applied to them during addressing of the other pixels in the array. This problem is exacerbated by the particular temperature sensitivity of SSFLCD's and by manufacturing inhomogeneities, such as variations in the surface roughness and variations in the cell spacing, which may occur across the width of a device, particularly a large area display. The switching margin, that is the part of the switching characteristic of the device which discriminates between the switched and non-switched states in dependence on the applied switching signal, is reduced by the above factors.
It is an object of the invention to provide a ferroelectric liquid crystal device having a switching response which allows a high degree of discrimination between the switched and non-switched states.
According to the present invention there is provided a ferroelectric liquid crystal device including a cell comprising a layer of ferroelectric liquid crystal material contained between two substrates having at least one alignment layer for determining the surface alignment of the molecules in the liquid crystal material, and addressing means for switching the molecules in the liquid crystal material between a first state and a second state by means of a switching signal, wherein the liquid crystal material and the alignment layer are such that the cell exhibits a &tgr;-V response in the form of a curve having two minima separated by an intermediate maximum (which may be termed the intermediate maximum point), where a ferroelectric liquid crystal device including a cell comprising a layer of ferroelectric liquid crystal material contained between two substrates having at least one alignment layer for determining the surface alignment of the molecules in the liquid crystal material, and addressing means for switching the molecules in the liquid crystal material between a first state and a second state by means of a switching signal, wherein the liquid crystal material and the alignment layer are such that the cell exhibits a &tgr;-V response in the form of a curve having two minima separated by an intermediate maximum, where &tgr; denotes the duration and V denotes the voltage of a switching pulse.
Such a &tgr;-V response, which may be termed a double turnaround response, allows a high degree of discrimination between the switched and non-switched states beyond the turnaround point and offers potential addressing advantages, as will be appreciated more fully from the following description.
REFERENCES:
patent: 5644371 (1997-07-01), Koden et al.
patent: 0809133 (1997-11-01), None
patent: 2315876 (1998-02-01), None
Search Report for GB 9813839.9; Dated Oct. 5, 1998.
Jones John Clifford
Lymer Karl Philip
Hjerpe Richard
Nguyen Francis
Renner , Otto, Boisselle & Sklar, LLP
Sharp Kabushiki Kaisha
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