U.S. patent application number 10/512195 was filed with the patent office on 2005-09-15 for liquid crystal device with bi- or multistable alignment gratings.
Invention is credited to Jones, John C.
Application Number | 20050200785 10/512195 |
Document ID | / |
Family ID | 29711933 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050200785 |
Kind Code |
A1 |
Jones, John C |
September 15, 2005 |
Liquid crystal device with bi- or multistable alignment
gratings
Abstract
A liquid crystal device is described that comprises a layer of
liquid crystal material contained between a first cell wall and a
second cell wall. The layer of liquid crystal material being
switchable between at least a first state and a second state, said
first state and said second state having sufficiently low splay to
enable rapid electrical switching therebetween. The internal
surface of said first cell wall is arranged to provide two or more
surface alignment configurations of different pretilt to said layer
of liquid crystal material. Said states may persist in the absence
of an applied electric field. In particular, the invention relates
to pi-cell devices that allow for rapid switching from the initial
splay state to the bend states, and also to bistable or multistable
pi-cell devices.
Inventors: |
Jones, John C; (Malvern,
GB) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Family ID: |
29711933 |
Appl. No.: |
10/512195 |
Filed: |
October 22, 2004 |
PCT Filed: |
May 29, 2003 |
PCT NO: |
PCT/GB03/02317 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60383610 |
May 29, 2002 |
|
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|
Current U.S.
Class: |
349/134 |
Current CPC
Class: |
G09G 3/3637 20130101;
G09G 3/364 20130101; G02F 1/1391 20130101; G09G 3/2077 20130101;
G09G 3/3629 20130101; G02F 1/1395 20130101; G09G 2310/06 20130101;
G09G 2310/061 20130101; G02F 1/133753 20130101; G02F 2201/30
20130101; G09G 3/3614 20130101; G02F 1/133761 20210101; G09G
2310/02 20130101; G02F 1/133776 20210101 |
Class at
Publication: |
349/134 |
International
Class: |
G02F 001/141 |
Claims
1. A liquid crystal device comprising a layer of liquid crystal
material contained between a first cell wall and a second cell
wall, the layer of liquid crystal material being switchable between
at least a first state and a second state, said first state and
said second state having sufficiently low splay to enable rapid
electrical switching therebetween, characterised in that the
internal surface of said first cell wall is arranged to provide two
or more surface alignment configurations of different pretilt to
said layer of liquid crystal material.
2. A device according to claim 1 wherein said first state is a bend
state in which the tilt of the liquid crystal material at a point
in the bulk of the cell is greater than the pretilt of the liquid
crystal material at said first cell wall and said second cell
wall.
3. A device according to claim 1 wherein, when switched into said
first state, the liquid crystal molecules in the vicinity of the
cell mid-point are orientated in a direction that is substantially
perpendicular to the first and second cell walls.
4. A device according to claim 1 wherein the internal surface of
said second cell wall is configured to provide two or more surface
alignment configurations of different surface pretilt to said layer
of liquid crystal material.
5. A device according to claim 4 wherein the said second state is a
substantially homeotropic state.
6. A device according to claim 4 wherein the latching threshold
between the two or more surface alignment configurations provided
by the internal surface of said first cell wail is greater than the
latching threshold between the two or more surface alignment
configurations provided by the internal surface of said second cell
wall.
7. A device according to claim 6 wherein the surface alignment
configuration of lowest pretilt at said second cell wall has a
pretilt less than the pretilt of any of the two or more stable
alignment configurations provided at said second cell wall.
8. A device according to claim 6 and further comprising addressing
means to latch between said first state and said second state,
wherein the addressing means uses at least first and second
latching scans, said first latching scan being arranged to
selectively latch material at said first cell wall and said second
latching scan being arranged to selectively latch material at said
second cell wall, wherein said first latching scan is applied prior
to application of said second latching scan and said second
latching scan is insufficient to latch material at said first cell
wall.
9. A device according to claim 1 wherein the internal surface of
said second cell wall is monostable and arranged to provide a
single alignment configuration that imparts a pretilt to said
liquid crystal material of less than 900.
10. A device according to claim 9 wherein the pretilt of each of
the two or more surface alignment configurations at said first cell
wall is greater than the pretilt provided at said second cell
wall.
11. A device according to claim 1 wherein the tilt at the cell
mid-point is greater than 5.degree..
12. A device according to claim 1 wherein any one or more of said
at least first state and second state is twisted.
13. A device according to claim 1 wherein the first cell wall and
the second cell wall carry electrodes to define a plurality of
separate electrically addressable regions.
14. A device according to claim 1 wherein said second state is the
most energetically favourable state that the liquid crystal
material can adopt.
15. A device according to claim 1 wherein the layer of liquid
crystal material is nematic liquid crystal material.
16. A device according to claim 1 wherein the first cell wall is
arranged to provide only two surface alignment configurations of
different pretilt.
17. A device according to claim 1 wherein said second state
persists in the absence of an applied electric field.
18. A device according to any claim 1 wherein said first state
persists in the absence of an applied electric field.
19. A pi-cell device comprising a layer of liquid crystal material
disposed between a pair of cell walls, one or both of said cell
walls being arranged to provide two or more stable alignment
configurations to said layer of liquid crystal material, said two
or more stable alignment configurations comprising a continuous
state and one or more defect states said device being switchable
between said continuous state and any one of said one or more
defect states, wherein one of said one or more defect states is a
bend state in which the magnitude of the tilt of the liquid crystal
material at a point in the bulk of the cell is greater than the
pretilt of the liquid crystal material at either cell wall.
20. A device according to claim 19 wherein, when said device is in
said bend state, the liquid crystal molecules at the midpoint of
the cell lie perpendicular to said cell walls.
21. A pi-cell liquid crystal device in which each of the switched
states persist in the absence of an applied electric field.
22. A pi-cell device comprising a layer of liquid crystal material
located between a pair of cell walls and comprising a plurality of
pixels separated by inter-pixel gaps, wherein the internal surface
of at least one of said pair of cells walls is arranged, in both
said pixel and inter-pixel gaps, to provide two or more surface
alignment configurations of different pre-tilt, wherein the
material is arranged to adopt a substantially non-splayed state in
the absence of an electric field such that th said substantially
non-splayed state persists in said inter-pixel gap.
23. A pi-cell device comprising a layer of liquid crystal material
disposed between a pair of cell walls, said layer of liquid crystal
material being rapidly electrically switchable between at least two
substantially non-splayed states, said device also being
switchable, prior to use, from a splayed state to either of said
non-splayed states wherein the internal surface of at least one of
said cell walls is arranged to impart two or more different pretilt
angles in the same azimuthal plane.
24. A device according to claim 23 wherein the splayed state can be
switched to a non-splayed state in less than 1 second.
Description
[0001] This invention relates to a liquid crystal device, and more
particularly to a bistable liquid crystal device arranged to give a
surface mediated transition to a so-called bend state (also known
as the Pi-cell (.pi.-cell) or optically controlled birefringence
(OCB) configuration).
[0002] Liquid crystal devices (LCDs) typically comprise a thin
layer of liquid crystal material contained between a pair of cell
walls. The internal surface of the cell walls are usually coated
with a certain material, or are suitably adapted in some way, to
impart a degree of surface alignment to the liquid crystal. The
bulk of the liquid crystal then adopts a configuration that depends
on the surface alignment properties of the cell walls and on
various other factors, such as the type of liquid crystal material
and the thickness of the liquid crystal layer. Optically
transparent electrode structures on one or both of the cell walls
allow an electric field to be applied to the liquid crystal
layer.
[0003] A typical liquid crystal display device is designed such
that two, or more, liquid crystal configurations can be selected by
the application of suitable electric fields. The different liquid
crystal configurations are designed to be optically distinguishable
so that optical contrast can be attained from the liquid crystal
device. For example, a liquid crystal device suitably arranged
between a pair of polarisers may have one configuration that will
allow transmission of light through the system and a second
configuration that will prevent it. Optical properties such as the
chromaticity, viewing angle and diffusivity may be modified using
additional passive components, such as retarders, reflective or
scattering layers, micro-lenses and the like.
[0004] Monostable liquid crystal devices, in which the liquid
crystal molecules can only adopt one stable configuration, are
known. Application of an electric field can distort the
configuration of the liquid crystal molecules, but once the
electric field is removed the liquid crystal will relax back to its
single stable configuration after some characteristic time
(typically tens of milliseconds to a few seconds).
[0005] Twisted nematic (TN) and super-twisted nematic (STN) LCDs
are examples of monostable devices. The TN and STN devices may be
switched to an "on" state by application of a suitable voltage, and
will relax back to an "off" state when the applied voltage falls
below a certain threshold level. It should be noted that the terms
"on" and "off" relate to application of high (i.e. switching)
voltage and low (i.e. non-switching) voltage respectively not
necessarily the observed optical transmission of a display. As
these devices are monostable, loss of power leads to loss of the
image.
[0006] Another known monostable device is the pi-cell described in
U.S. Pat. No. 4,566,758. Referring to FIG. 1, the principle of
operation of a prior art pi-cell is illustrated. The pi-cell device
comprises a layer of liquid crystal material 2 contained between a
pair of cell walls 4. The walls comprise electrode structures and
each wall is pre-treated to align the liquid crystal in contact
with the wall in a single and particular direction.
[0007] In the absence of an applied voltage, the liquid crystal
material 2 adopts a splayed configuration shown in FIG. 1a in which
the liquid crystal molecules in the centre of the device lie
substantially parallel to the cell walls 4. The centre of the
device is means a plane parallel to the cell walls, and
approximately equidistant between them. Application of a voltage
greater than a certain value allows the liquid crystal material to
adopt a first bend (or non-splayed) state as shown in FIG. 1b after
a certain time.
[0008] In the first bend state, the liquid crystal molecules in the
centre of the liquid crystal layer are substantially perpendicular
to the cell walls 4. The first bend state is retained after removal
of the applied field and may last for periods of a second or
longer. Application of a higher voltage causes a second bend (or
non-splayed) state to be formed as shown in FIG. 1c, due to the
electric field coupling to the positive dielectric anisotropy of
the liquid crystal material and reorienting the director normal to
the surfaces. The liquid crystal director remains substantially
perpendicular to the cell walls 4 at the cell mid-point in the
second bend state, and liquid crystal material throughout the
remainder of the cell, apart from regions near each surface that
are dominated by the anchoring effect of the surfaces, is also
forced to lie substantially perpendicular to the cell walls. The
surfaces of the pi-cell are designed to give a pre-tilt of the
liquid crystal director that is typically between 5.degree. and
30.degree.. The surface alignment directions are often arranged to
be substantially in opposite directions. However, it is possible to
result in a desired bend state using parallel or near parallel
surface directions using a liquid crystal mixture with a suitable
spontaneous twist (i.e. with a certain pitch) and device cell
gap.
[0009] The pi-ell device provides optical contrast when switched
between the first (low voltage) bend state shown in FIG. 1b and the
second (high voltage) bend state shown in FIG. 1c. Furthermore,
very fast (around 1-2 milliseconds at 25.degree. C. in typical cell
gaps of about 4 .mu.m) switching between the first and second bend
states can be achieved However, removal of the applied voltage for
a prolonged period of time will cause the liquid crystal material
to relax back to the more energetically favourable splayed
configuration of FIG. 1a. Switching from the splayed state to the
non-splayed (bend) state is much slower than switching between bend
states, taking typically 30 seconds or longer
[0010] A particular disadvantage of known pi-cell configurations is
nucleating and stabilising the first bend state for subsequent
operation. It has been found that high voltages may be required to
initially switch from the splay state to the bend (i.e.
non-splayed) state. In certain devices, for example devices in
which each pixel is driven by a thin film transistor (TFT), the
voltage required to switch from the splay state to the bend state
may be difficult to produce, and adds extra cost.
[0011] Koma et al (1999) Proceedings of the SID, p28-31 found that
the bend state is nucleated at certain "start points" within each
pixel, usually associated with random irregularities, such as
surface or electrode roughness. Devices without such nucleation
sites did not readily form the desired bend state. It has also been
attempted to use a high voltage for sufficient duration to provide
a bend state, and to then stabilise the bend state using a polymer
stabilisation network. This requires that a UV curable monomer is
added to the liquid crystal and which is cross-linked after the
formation of the required bend state upon application of the
nucleating signal. However, this has been found to lead to ionic
contamination of the liquid crystal material and adds significant
fabrication and yield costs
[0012] A particular problem has also been found to arise when using
pixellated pi-cell devices. In such devices, it is not possible to
apply sufficient voltages to the inter-pixel gap regions, in
particular to the mid-point between adjacent pixels. Hence, the
liquid crystal material in the inter-pixel gap remains in the splay
state. The presence of the splay state in the inter-pixel gap
region tends to promote nucleation of the splay state in any pixels
that have been switched into the bend state. U.S. Pat. No.
6,512,569 describes how a patterned inter-pixel gap can be used
that promotes the formation of the bend state in the inter-pixel
gap region. Just as the bend state does not form when the device
has no nuclueation sites, then a device that is switched into the
bend state will remain in that state if there are no nucleation
sites for the lower energy splay state. Changing the alignment
properties in the interpixel gap requires accurate alignment of the
patterned alignment regions with the inter-pixel gaps of the
electrode structures.
[0013] Although monostable devices are commonplace, it is also
known to provide liquid crystal configurations in which the liquid
crystal material can adopt two or more different configurations
that are stable in the absence of an applied electric field.
Research into bistable LCDs has been prompted mainly by their
inherent ability to store images and the high degree of
multiplex-ability. This negates the need for devices that have
expensive active matrix back-planes and permits line at a time
passive addressing. Since bistable devices do not need constant
refreshing to maintain an image, they offer a low power alternative
for display applications.
[0014] A particular type of bistable device is the zenithally
bistable device (ZBD) described in WO 97/14990. Such a prior art
ZBD device is shown in FIG. 2; liquid crystal material 2 is
sandwiched between a first cell wall 6 and a second cell wall 8.
The device is constructed with a bistable surface alignment grating
on the first cell wall such that nematic liquid crystal molecules
in the vicinity thereof can adopt either one of two stable pretilt
angles in the same azimuthal plane. A homeotropic surface alignment
treatment is applied to the second cell wall 8. Application of
suitable voltage pulses allows switching between the high pretilt
state of FIG. 2a and the low pretilt state of FIG. 2b. The high
pretilt state of FIG. 2a is also termed the continuous state,
whilst the low pretilt state of FIG. 2b is also termed the defect
state because attaining the lower pretilt angle at the first cell
wall results in the formation of liquid crystal defects or
disclination lines at the grating walls. Recently, it has also been
demonstrated in WO 99/34251 that a device can be formed from a
surface that exhibits a plurality of stable states wherein the
multi-stable surface can be attained by appropriate design of the
surface profile.
[0015] The two stable liquid crystal configurations of ZBD persist
after driving electrical signals have been removed, and (see Wood
et. al. SID Digest 2000) the device is highly resistant to
mechanical shock, provides 10s of microsecond latching times at low
driving voltages (<20V) and allows a high degree of
multiplexibility. However, the magnitude of latching speeds
obtained with such devices are currently insufficient for high
update rate applications such as colour high definition television
(HDTV).
[0016] Even with relatively low cell gaps of about 3.5 .mu.m,
typical relaxation times for a ZBD device are similar to those of a
conventional STN display, typically being 20 ms or more at
25.degree. C. At lower temperatures, the optical response time
becomes slower still, being typically looms at 0.degree. C. This is
not satisfactory for many applications where moving images are
required. Even backlit TV applications, where the heat from the
backlight may mean that operating temperatures are about 35.degree.
C. or warmer, and the liquid crystal optical response is
correspondingly faster, the response is still too slow to allow
temporal colour to be displayed (where each frame is divided into
R, G and B sub-frames). For a true colour display a response time
of about 5 ms is required: this is difficult to achieve using
conventional monostable or bistable displays. Ferroelectric liquid
crystals readily allow such speeds to be attained, but they suffer
from a requirement for a low cell gap and uniformity of smectic
layer alignment.
[0017] It is thus an object of the present invention to mitigate at
least some of the aforementioned disadvantages and to thereby
provide an improved liquid crystal device.
[0018] According to a first aspect of the present invention, a
liquid crystal device comprises a layer of liquid crystal material
contained between a first cell wall and a second cell wall, the
layer of liquid crystal material being switchable between at least
a first state and a second state, said first state and said second
state having sufficiently low splay to enable rapid electrical
switching therebetween, characterised in that the internal surface
of said first cell wall is arranged to provide two or more surface
alignment configurations of different pretilt to said layer of
liquid crystal material.
[0019] In other words, the first and second states are non-splayed
states that can be rapidly switched between. The internal surface
of the first cell wall may comprise a surface profile that provides
two or more alignment configurations to give the two stable surface
alignment configurations. For example, the internal surface may
comprises a surface alignment grating embossed in a layer of
material carried on the internal surface of the first cell wall.
The device may advantageously be arranged so that the first state
and/or the second state persist in the absence of an applied
electric field.
[0020] The present invention thus provides a liquid crystal device
that has advantages over known pi-cells. For example, the stability
of the substantially non-splayed states in the absence of an
applied electric field means that images written to a device will
persist when addressing voltages are removed. This enables the fast
switching speed of the pi-cell configuration to be coupled with the
ability to store images in the absence of an applied electric
field. The inherent stability of the device thus allows areas of
devices to be addressed only when image update is required, thus
enabling the power consumption of a device to be reduced when
static or slowly updated images are displayed. This allows, for
example, e-books and laptops to be formed that are capable of
displaying high-resolution TV video rate images when required, but
can use a reduced update rate to conserve battery power when a
lower frequency of update, or partial update, is used.
[0021] The present invention also removes -the need for an initial
(slow) addressing step to switch material from a splayed state to
the non-splayed state or the use of polymer stabilisation matrices
to stabilise a particular non-splayed state. As described below,
even is a splayed state is formed, the surface transition increases
the speed with which the non-splayed state can be selected.
[0022] The terms bend, splay and twist arise from consideration of
the elastic deformations of a nematic liquid crystal material and
is described in more detail in chapter 3 of "The physics of liquid
crystals" by De Gennes and Prost, 1993 (second edition), Oxford
University Press(ISBN 0198520247). In brief, any deformation of a
nematic liquid crystal material may be described in terms of splay,
bend and twist deformation components. FIG. 3 illustrates layers of
nematic liquid crystal material 30 arranged such that the liquid
crystal deformation is pure bend (FIG. 3c); pure splay (FIG. 3a)
and pure twist (FIG. 3b). In a device, any configuration adopted by
liquid crystal material can be described using the three
deformation components (i.e. splay, bend and twist).
[0023] Most alignment states will include two or more elastic
deformations. This is particularly true for parallel-walled cells,
where uniform change in tilt from one surface to another includes
both splay and bend deformations. Moreover, in the vicinity of
grating aligned surfaces the director may undergo substantial
elastic deformation and also include both splay and bend. In such
instances at some distance away from the surface profile (typically
within one pitch distance of the repeating profile into the bulk of
the cell) the director variations in two-dimensions will diminish,
and the surface is said to provide a uniform pre-tilt. Further into
the bulk of the cell the director variation within a particular
state is uni-dimensional, varying in the direction parallel to the
device plane normal according to the applied field and the elastic
deformation associated with the interaction of the two surfaces.
Note, the term pre-tilt is taken to mean this uniform alignment of
the director in close proximity to the surface and induced by the
structure of said surface. The tilt of the director represents the
local orientation of the director field that may vary under the
action of an alignment or electric field.
[0024] The term non-splayed states is used herein to mean a liquid
crystal configuration in which the splay component is small; for
example, a state in which the dominant deformation component is
bend. It should be noted that a homeotropic state (i.e. a
configuration as shown in FIG. 24b in which the liquid crystal
molecules are perpendicular to the cell walls throughout the
thickness of the device) has zero splay and thus falls within the
definition of a substantially non-splayed state.
[0025] A particularly important example of a non-splayed state is
the bend state. In a bend state the tilt of the director in the
bulk of the cell is equal to or greater than the pretilt of both
alignment walls. In particular the bend state will usually have a
point within the bulk of the cell where the director is aligned
perpendicular to the cell-plane For this reason, as described at
line 56 of column 1 of U.S. Pat. No. 6,512,569, such a non-splayed,
bend state may sometimes be termed a vertical, or "V-state".
Furthermore, in such a bend state, the bend deformation either side
of vertically aligned director is in opposing directions. As
described in more detail below, the twist component is determined
by any in-plane rotation of the liquid crystal director through the
thickness of the cell (e.g. from the first cell wall to the second
cell wall) and may be selected as desired to tailor the optical
response. In other words, splayed and substantially non-splayed
states can both be provided in either twisted or untwisted
form.
[0026] Advantageously, the first state is a bend state in which the
tilt of the liquid crystal material at a point in the bulk of the
cell is greater than the pretilt of the liquid crystal material at
said first cell wall and said second cell wall. This may be a ZBD
defect state.
[0027] As described above, zenithal bistable or multistable devices
exhibit one or more defect states (i.e. a state in which a liquid
crystal defect provides the surface alignment configuration at one
surface) and a continuous (non-defect) states. It should be noted
that prior art ZBD devices exhibit hybrid aligned nematic defect
states (e.g. as shown in FIG. 2b), planar homogeneous defect states
or twisted homogeneous defect states rather than a defect state in
which the liquid crystal director (i.e the average direction of the
long molecular axes) in the bulk of the cell is orientated in a
direction that is substantially perpendicular to the cell walls.
The advantage of providing a substantially non-splayed (e.g. bend)
defect state of this type is the ability to rapidly switch to the
second substantially non-splayed state as described above.
[0028] Herein, the cell mid-point is taken as a plane within the
liquid crystal material that lies parallel to said first and second
cell walls and is located substantially halfway between the plane
defining the first cell wall and the plane defining the second cell
wall. For a device with one or more grating surfaces, the half-way
point is taken to within one grating pitch of the average distance
from one surface to the other, where the average is taken over at
least the area of one pixel within the device. A point which is
substantially half way may be anywhere from 1/4 of the distance
between the walls to 34 that distance.
[0029] Conveniently, when switched into said first state, the
liquid crystal molecules in the vicinity of the cell mid-point are
orientated in a direction that is substantially perpendicular to
the first and second cell walls. In other words, the tilt of the
liquid crystal material at said point in the bulk of the cell is
substantially 90.degree.. This may be the so-called ZBD continuous
state.
[0030] Electrical addressing signals are applied to the device to
latch into one of the two states, all of which are non-splayed
states and one of which is preferably a bend state. The electrical
addressing means is arranged so as to ensure that the zenithal
bistable surface within the area of at least one pixel is latched
into a continuous state during at least part of the addressing
signal. Conveniently, this addressing means is provided at the
outset of each pixel-switching event, since it ensures that the
director is in a non-splayed state and not in an undesired splayed
state. This initial non-splayed state is preferably a HAN state,
since this ensures that the change of director field to the
subsequent states is rapid.
[0031] Preferably, the internal surface of said second cell wall is
configured to provide two or more surface alignment configurations
of different surface pretilt to said layer of liquid crystal
material. In other words, a "double ZBD" device is provided in
which both surface can impart two or more different surface pretilt
angles to the liquid crystal material.
[0032] Conveniently, the second state is a substantially
homeotropic (continuous) state. In other words, the liquid crystal
molecules lie in a direction perpendicular to the cell wall
throughout the thickness of cell in the second substantially
non-splayed state.
[0033] Advantageously, in a double ZBD device of the invention, the
latching threshold between the two or more surface alignment
configurations provided by the internal surface of said first cell
wall is greater than the latching thresholds between the two or
more stable surface alignment configurations provided by the
internal surface of said second cell wall. In such a case, it is
also preferred that the surface alignment configuration of lowest
pretilt at said second cell wall has a pretilt less than the
pretilt of any of the two or more stable alignment configurations
provided at said second cell wall; i.e. the pretilt of the ZBD
defect state on the surface of higher threshold is higher than the
pretilt of the ZBD defect state on the surface of lower
threshold.
[0034] Conveniently, the device comprising addressing means to
latch between said first substantially non-splayed state and said
second substantially non-splayed state, wherein the addressing
means uses at least first and second latching scans, said first
latching scan being arranged to selectively latch material at said
first cell wall and said second latching scan being arranged to
selectively latch material at said second cell wall, wherein said
first latching scan is applied prior to application of said second
latching scan and said second latching scan is insufficient to
latch material at said first cell wall. As described in more detail
below, various implementations of this multiscan addressing
technique are possible.
[0035] Preferably, the internal surface of said second cell wall is
monostable and arranged to provide a single alignment configuration
that imparts a pretilt to said liquid crystal material of less than
90.degree..
[0036] Advantageously, the pretilt of each of the two or more
surface alignment configurations at said first cell wall is greater
than the pretilt provided at said second cell wall.
[0037] Conveniently, the tilt at the cell mid-point is greater than
5.degree..
[0038] Advantageously, any one or more of said at least first state
and second state is twisted. In other words, a twisted pi-cell
structure may be formed. The twist may advantageously be up to
180.degree..
[0039] The first cell wall and the second cell wall preferably
carry electrodes to define a plurality of separate electrically
addressable regions. For example, row electrodes are provided on
said first cell wall and column electrodes are provided on said
second cell wall thereby providing a matrix of separately
addressable regions. Some or all of the pixels may include
non-linear elements, such as back-to-back diodes, thin-film
transistor or silicon logic circuit. Alternatively, the device may
be a single pixel fast optical shutter.
[0040] It is advantageous, for said second state is the most
energetically favourable state that the liquid crystal material can
adopt. For example said second state may be a continuous high-tilt
state with the device arranged such that said second substantially
non-splayed state is the most energetically favourable state that
the liquid crystal material can adopt. In this way, the device will
tend to form the second substantially non-splayed state (i.e. the
continuous state) when constructed. Hence, the liquid crystal
material in the interpixel gaps will form the continuous state
which will ensure the first substantially non-splayed state (rather
than a splayed state) is always formed within each of the
pixels.
[0041] For example, if the zenithal bistable surface is arranged to
form the high tilt continuous state spontaneously on first cooling
then at least part of the inter-pixel gap will remain in a
non-splayed state after switching. For example, the grating may be
made relatively shallow so that it is still bistable (i.e there is
an energy barrier between the high tilt and low tilt states) but
the high tilt state is a lower energy than the low tilt defect
state. Hence, the inter-pixel gap does not act to nucleate a splay
state, but advantageously nucleates the non-splayed states. Unlike
previous methods for introducing non-splay state nucleation sites
into the inter-pixel gaps (preferably surrounding each pixel) this
method can be done at no extra-cost of fabrication, being inherent
to the design of the surface. More information on the design of
surfaces to control pretilt can be found in the prior art described
above.
[0042] Advantageously, the layer of liquid crystal material is
nematic liquid crystal material. Herein nematic liquid crystal
material includes long pitch cholesteric. A chiral dopant may also
be mixed to provide any twist that is required. The liquid crystal
material advantageously has a positive dielectric anisotropy.
[0043] Conveniently, the first cell wall is arranged to provide two
surface alignment configurations of different pretilt. In other
words, the first cell wall has a bistable surface structure; for
example a surface alignment grating. Alternatively, more than two
surface alignment configurations may be provided as described in WO
99/34251.
[0044] According to a second aspect of the invention, a pi-cell
device comprises a layer of liquid crystal material disposed
between a pair of cell walls, one or both of said cell walls being
arranged to provide two or more stable alignment configurations to
said layer of liquid crystal material, said two or more stable
alignment configurations comprising a continuous state and one or
more defect states, said device being switchable between said
continuous state and any one of said one or more defect states,
wherein one of said one or more defect states is a bend state in
which the magnitude of the tilt of the liquid crystal material at a
point in the bulk of the cell is greater than the pretilt of the
liquid crystal material at either cell wall.
[0045] Preferably, when said device is in said bend state, the
liquid crystal molecules at the midpoint of the cell lie
perpendicular to said cell walls
[0046] According to a third aspect of the invention a pi-cell
liquid crystal device is provided in which each of the switched
states persist in the absence of an applied electric field.
[0047] According to a fourth aspect of the invention, a pi-cell
device comprises a layer of liquid crystal material located between
a pair of cell walls and comprising a plurality of pixels separated
by inter-pixel gaps, wherein the internal surface of at least one
of said pair of cells walls is arranged, in both said pixel and
inter-pixel gaps, to provide two or more surface alignment
configurations of different pre-tilt, wherein the material is
arranged to adopt a substantially non-splayed state in the absence
of an electric field such that the said substantially non-splayed
state persists in said inter-pixel gap.
[0048] According to a fifth aspect of the invention, pi-cell device
comprises a layer of liquid crystal material disposed between a
pair of cell walls, said layer of liquid crystal material being
rapidly electrically switchable between at least two substantially
non-splayed states, said device also being switchable, prior to
use, from a splayed state to either of said non-splayed states
wherein the internal surface of at least one of said cell walls is
arranged to impart two or more different pretilt angles in the same
azimuthal plane. Advantageously, the splayed state can be switched
to a non-splayed state in less than 1 second.
[0049] The invention will now be described, by way of example only,
with reference to the following drawings in which:
[0050] FIG. 1 shows the operation of a prior art pi-cell
device,
[0051] FIG. 2 shows the operation of a prior art ZBD device,
[0052] FIG. 3 shows the so-called splay, bend and twist
deformations of a liquid crystal material,
[0053] FIG. 4 illustrates a multi-scan technique,
[0054] FIG. 5 shows a panel constructed in accordance with the
present invention,
[0055] FIG. 6 shows a cross-section of the panel of FIG. 5 along
the line II-II,
[0056] FIG. 7 illustrates the use of multiple scans to address a
device having a continuum of transitions,
[0057] FIG. 8 shows a five stage multi-scan technique,
[0058] FIG. 9 illustrates how variation of strobe pulse width may
be used in the multi-scan technique,
[0059] FIG. 10 shows typical ZBD latching curves with no variation
in asymmetry,
[0060] FIG. 11 shows ZBD latching curves in which asymmetry is not
retained,
[0061] FIG. 12 provides an expanded plot of four switching regions
of a ZBD device,
[0062] FIG. 13 gives and expanded view of the display of FIG. 5
when addressed in accordance with the present invention,
[0063] FIG. 14 provides examples of row and data signals that can
be used to implement the present invention,
[0064] FIG. 15 shows strobe and data signals that can provide three
latching scans in accordance with the present invention,
[0065] FIG. 16 shows how each scan of FIG. 15 can be applied to the
whole display in turn,
[0066] FIG. 17 shows how each of the three latching scans of FIG.
15 can be applied to each line in turn
[0067] FIG. 18 shows the measured latching response of a ZBD
cell,
[0068] FIG. 19 shows measured defect-to-continuous and
continuous-to-defect transitions for a cell comprising regions of
different grating pitch,
[0069] FIG. 20 is a series of photomicrographs showing latching,
using multiple scans, of a cell comprising regions of different
grating pitch,
[0070] FIG. 21 plots experimental data of the defect-to-continuous
and continuous-to-defect transitions for two regions of a ZBD cell
of 0.6 .mu.m and 0.8 .mu.m pitch,
[0071] FIG. 22 shows photomicrographs of two ZBD cell areas of 0.6
.mu.m and 0.8 .mu.n pitch addressed using the present
invention;
[0072] FIG. 23 shows the electro-optic response of a double ZBD
device,
[0073] FIG. 24 shows how a double ZBD device can be addressed using
multiple scans from a first blanked state,
[0074] FIG. 25 shows how a double ZBD device can be addressed using
multiple scans from a second blanked state, and
[0075] FIG. 26 shows an exploded view of a double ZBD device
fabricated in accordance with the present invention,
[0076] FIG. 27 illustrates the operation of a single ZBD surface
pi-cell of the present invention,
[0077] FIG. 28 illustrates the operation of another ZBD surface
pi-cell of the present invention,
[0078] FIG. 29 shows in more detail the prior art transition from a
splayed state to a bend state,
[0079] FIG. 30 shows a counter example of a prior art ZBD device in
which surface switching does not occur when forming a bend
state,
[0080] FIG. 31 illustrates a double ZBD pi-cell ZBD device in
accordance with the present invention,
[0081] FIG. 32 shows examples of substantially non-splayed
states,
[0082] FIG. 33 show various splayed states,
[0083] FIG. 34 shows the energies of defect and continuous states
in a ZBD device, and
[0084] FIG. 35 shows rms operation of a device of the present
invention.
[0085] Referring to FIG. 4, the principle of the multiscan
technique is illustrated. The pulse duration versus voltage plot of
FIG. 4 shows the latching properties of a first region 60 and a
second region 62 of a bistable device. The first and second regions
have different latching energies. For each area, a solid line
represents the 90% latched point (i.e. .tau.V.sub.90%) and a dashed
line represents the 10% latched point (i.e. .tau.V.sub.10%). This
nomenclature is well known in the art and is described above with
reference to FIG. 3.
[0086] The first region 60 and the second region 62 are addressed
in separate scans. The selection of the data voltages for the two
distinct areas of behaviour are shown in
[0087] FIG. 6. A first scan using a strobe voltage pulse of
V.sub.s1 is used in combination with a select (+V.sub.d) or a
non-select (-V.sub.d) data voltage pulse to provide selective
switching of the first region 60. A second scan using a strobe
voltage V.sub.s2 is used in combination with the select (+V.sub.d)
or non-select (-V.sub.d) data voltage pulse to provide selective
switching of the second region 62.
[0088] The use of two scans (i.e. the first scan and the second
scan) enables the number of electronic drivers needed to address
the entire panel to be reduced, and/or allows the use of lower data
voltages (albeit with longer line-address-times). The invention
thus allows a bistable panel to be latched into the required state
with a low data voltage possible and/or with a reduced number of
scan electrodes and/or drivers. The approach may be used to
compensate variations of the latching response using multiple scans
of the display.
[0089] The use of two scans (i.e. the first scan and the second
scan) in which the first scan comprises an initial blanking pulse
may be also be described as a three field multiplexing scheme. In
other words, field one is the blanking pulse, field two applies
pulses to address regions having a latching threshold within a
first range and field three applies pulses to address regions
having a latching threshold within a second range. A frame (i.e.
the pattern of information written to the display) is thus written
by the three fields.
[0090] It can thus be seen that the present invention permits
discrimination for the two regions using data voltages that are
slightly greater than the partial latch width, namely: 1 V d ( V
100 % - V 0 % 2 ) Area1 ( V 100 % - V 0 % 2 ) Area2 ( 3 )
[0091] This has the potential to significantly reduce the data
voltage towards a minimum of the partial latch width. This lower
data voltage reduces power consumption during update and decreased
optical transients and growback effects.
[0092] If the variations occur on the same scan electrode (whether
by design, or because the variations are random), the same data is
required to address both areas 1 and 2. This is done by ensuring
the higher voltage area (area 1) is addressed first. The signal
used to address area 1 into the desired states (say, black for
Vs1-Vd and white for Vs1+Vd) is also applied to area 2 on the same
row. The parts of the addressed row with the lower threshold (area
2) are latched by either resultant latch, which therefore appear
(in this example) white, regardless of the data. In a subsequent
scan of the line, however, the strobe voltage is reduced to Vs2,
thereby allowing these areas to be addressed. In this subsequent
scan, neither resultant (Vs2-Vd nor Vs2+Vd) has sufficient energy
to latch area 1, and so the entire row is addressed with the
desired image.
[0093] FIG. 5 shows a panel designed to exhibit three separate
thresholds on each row electrode. The panel has four row electrodes
70a-70d (collectively referred to as row electrodes 70) and eight
column electrodes 72a-72h (collectively referred to as column
electrodes 72). Row driver electronic 74 and column driver
electronics 76 are also provided. The row and column electrodes
overlap to provide thirty-two regions which can be separately
addressed by application of a voltage to an appropriate row and
column. Each row electrode 70 comprises three areas with distinct
latching thresholds; a first area 80, a second area 82 and a third
area 84.
[0094] A cross-sectional view along the line II-II of the panel
shown in FIG. 5 is given in FIG. 6. Referring to FIG. 6, an
alignment grating forming the first area 80, second area 82 and
third area 84 is shown. Also shown is the column electrode 72h, row
electrodes 70a and 70b, a lower (homeotropic, mono-stable)
alignment layer 86 and optical components 88. The optical
components 88 may include polarisers, compensation plates,
diffusers and/or reflectors used in any of a number of
configurations familiar to those skilled in the art. It should be
noted that one or both of the optical components 88 indicated may
not be required to implement a certain device configuration.
[0095] It should be noted that the areas 80, 82 and 84 may be
formed from other methods to alter the thresholds. Such methods
include providing holes in the electrodes, alignment variation
(e.g. photo-alignment), differences in pretilt, changes of grating
shape or anchoring properties. The change may be on the bistable
surface, or on the opposed monostable surface.
[0096] The dielectric material of the alignment grating forming the
first, second and third areas 80, 82 and 84 is selected to be a
different thickness in each of the three areas. This changes the
cell gap and the voltage applied across the modulating medium (due
to voltage drop across the dielectric layer), leading to different
latching thresholds in the first, second and third areas 80, 82 and
84. It is assumed that the third area 84 has the highest latching
threshold, because the dielectric mis-match of the alignment layer
is more significant than the change in cell gap. However, it would
be appreciated that it would also be possible to design the cell so
that the first area 80 has the highest threshold.
[0097] In operation, each of the four rows 70 is sequentially
blanked and then scanned three times, with appropriate data
synchronously applied to the columns 72. Alternatively, all rows
may be blanked initially and simultaneously and subsequently each
scanned sequentially, either in turn or in some predetermined
sequence. For the first scan of a particular row, the voltage (Vs1)
is sufficiently high to indiscriminately latch the two lower
threshold areas (i.e. the first and second areas 80 and 82) into
one state regardless of the data applied to the column. The data
signal, however, combines with Vs1 to either latch the third area
84 into the required state or to leave it unchanged. In the second
scan of the row, the applied voltage is reduced to Vs2 chosen so
that it latches the first area 80 indiscriminately of the data,
whilst leaving the third area 84 unchanged; the second area 82 is
discriminately latched according to the data +Vd. The addressing of
the row is completed on the third scan, where Vs3 leaves both the
second and third areas 82 and 84 unchanged, but discriminately
latches the first area according to the data.
[0098] In this fashion, it is possible to reduce the number of
electronic drivers needed to address the entire panel. It can thus
be seen that three times the number of pixels can be addressed
without the cost associated with the additional driver electronics.
In the panel described with reference to FIGS. 5 and 6, the image
is twelve-by-eight (i.e. ninety-six) pixels, despite only four row
electrodes 70 being used. Other advantages include reduction of the
interpixel gap (i.e. fewer inter-electrode gaps) and hence improved
contrast and reflectivity (i.e. increased aperture ratio for the
pixels).
[0099] It is should be re-emphasised that the present invention is
quite distinct to the various prior art techniques employed to
achieve analogue greyscale. The present invention allows the
electro-optic response of the device to be varied within a single
electrically addressable area (e.g. the area of a overlap of a row
and column electrode) by multiple addressing scans. In contrast,
with analogue greyscale each data signal is modulated to latch the
required proportion of a pixel area. The present invention thus
provides a strobe voltage (which is usually a much higher voltage
that the data voltage) that is modulated over successive scans.
This strobe pulse modulation combined with multiple scans keeps the
data voltage relatively low which, as described above, provides a
number of benefits. Of course, the present invention may be
combined with analogue greyscale techniques to provide a greyscale
device with a reduced number of electronic drivers.
[0100] The method may also be used to reduce the number of drivers
required to produce greyscale by means of spatial dither. In such
an arrangement the areas (e.g. ares 80, 82, and 84 of FIG. 7) may
be arranged to have different areas within each pixel. For example,
the first area 80 may be four times greater in area than the third
area 84, whilst the second area 82 may be half that of the third
area 84. Such a digital weighting is well known to those skilled in
the art of producing linear greyscale with the least number of
separately addressable areas. If analogue greyscale levels are also
included then a different weighting of areas may be used. For
example, if three analogue levels are possible, then a total of
twenty-seven greys can be achieved with 1:3:9 weighted areas. This
occurs for a single row and column (i.e. a separate electrically
addressable area) using the present invention.
[0101] In addition to employing the invention with panels that are
designed to exhibit multiple-thresholds, multiple modulated scans
may be used to compensate for random variations across a panel.
This works in an analogous manner to the previous example, except
the same data is used for each of the multiple scans. In other
words, each scan writes the same data pattern but each scan only
selectively latches material with a defined threshold range. In
this way, the data is written to all regions of the display with
material having a latching threshold within one of the scan
ranges.
[0102] In practice, the latching curves for random variations
across a panel are likely to vary in a continuous fashion, rather
than forming two distinct operating areas. However, even in such
cases, the display can still advantageously be addressed totally in
two scans.
[0103] FIG. 7 shows data and strobe voltages suitable for
addressing a panel in which there is a continuum of latching
transitions. The device can be considered as having a lowest
threshold area (curves 90) and a highest threshold area (curves
92). Data pulses (+V.sub.d and -V.sub.d) and strobe pulses
(V.sub.s1 and V.sub.s2) are selected such that the whole display
can be addressed by two scans; the first with V.sub.s1 combined
with the required data and the second at V.sub.s2 with the required
data, where V.sub.s1>V.sub.s2.
[0104] The result of using two such scans is that the data voltage
is (almost) halved, albeit at the expense of a doubled update rate.
As described below, some overlap of the resultant voltages may be
preferable (e.g. approximately (.delta.V)/2) to ensure that areas
of the cell with switching energies close to the cross over are
latched into the desired state.
[0105] In the case of a device having the properties shown in FIG.
7, the data voltage required to ensure all areas of the panel are
addressed properly is given by: 2 V d V 100 % ( Max ) - V 0 % ( Min
) 4 ( 4 )
[0106] This is half the power needed by typical prior art schemes
as given by equation (2)
[0107] In practice, it is preferable to use V.sub.d slightly higher
than the equality of equation (4) to ensure that the whole display
is in the desired state. Further reductions of V.sub.d are possible
by increasing the number of scans of successively decreasing strobe
voltage. In general, for n scans the data voltage is
correspondingly reduced by a factor of n: 3 V d V 100 % ( Max ) - V
0 % ( Min ) 2 n ( 5 )
[0108] The maximum number of scans that is considered worthwhile
is: 4 n = V 100 % ( Max ) - V 0 % ( Min ) 2 ( V 100 % - V 0 % ) ( 6
)
[0109] where V.sub.100%-V.sub.0% is the inherent partial latch
width of a microscopic region.
[0110] With n-line scanning the energy per update is then in the
range: 5 1 2 nf C ( V d n ) 2 E nf C ( V d n ) 2 ( 7 )
[0111] where f is the number of frame updates (e.g. the frequency
for a constantly updated device) and C is capacitance. The use of a
n times multi-scan approach results in an n times reduction in the
energy required to update the display compared to conventional
update techniques.
[0112] FIG. 8 shows how, for the continuum of transitions shown in
FIG. 7, each line can be scanned five times (i.e. with voltages
V.sub.s1, V.sub.s2, V.sub.s3, V.sub.s4, V.sub.s5) enabling the data
voltage to be reduced by almost a factor of five. It should be
noted that the highest remaining voltage must be used with each
successive scan.
[0113] Referring to FIG. 9, it is illustrated how the slot width of
the strobe pulse could be changed instead of modulating the strobe
voltage Vs between successive scans. In this instance, the longest
duration slot is used first, and subsequent scans are successively
shorter. To ensure wide operating ranges, a combination of both
pulse width (.tau.) and pulse voltage (V) modulation may be
preferred. In addition to alteration of pulse width and duration,
changes to the resultant pulse shape and/or altering the delay
between pulses may be used to provide the required
discrimination.
[0114] Referring to FIG. 10, the switching curves of a ZBD device
comprising first, second and third areas having different latching
properties is shown. A first curve 121, a second curve 122 and a
third curve 123 illustrate the voltage and time slot required to
latch the device into the continuous state from the defect state in
the first, second and third areas respectively. A first curve 121',
second curve 122' and third curve 123' illustrate how a negative
voltage pulse of a given time slot can latch the device from the
continuous state from the defect state. The three different
latching areas may be engineered, or may arise from
non-uniformities across the device.
[0115] Symmetric devices are so-called when the same magnitude
(i.e. .vertline..tau.V.vertline.) of voltage pulse latches both
continuous-to-defect and defect-to-continuous, or where the
difference in thresholds remains constant from one transition to
another. A three area symmetric ZBD device having the properties
shown in FIG. 10 may be latched into the defect state by the
following procedure:
[0116] (i) Blanking the whole device into the defect state by
applying a blanking pulse 124 of negative polarity.
[0117] (ii) Applying a 1.sup.st scan of positive polarity (i.e. to
switch from the defect state to the continuous state) with
non-select data. This provides a first resultant pulse 126 which
leaves the first area (i.e. the area having the first curve 121)
unchanged, in the defect state. The second area (i.e. the area
having the second curve 122) is partially blanked into the
continuous state, and the third area (i.e. the area having the
third curve 123) is fully blanked into the continuous state.
[0118] (iii) Applying a 2.sup.nd scan is of negative polarity (i.e.
to switch from the continuous state to the defect state) with
select data. This provides a second resultant pulse 128 which
leaves the first area unchanged, in the defect state. The second
and third areas are now fully selected into the defect state.
[0119] Thus the device gives the desired final state even though
area two was only partially latched to the defect state during the
first scan. As an aside, if the data waveform is reversed a
blanking pulse 124' would be used to switch the three areas into
the continuous state. The first scan would then contain select data
to provide a resultant pulse 130 that switches all areas into the
defect state, and the second scan would have non-select data
providing a resultant pulse 132 that does not switch any of the
three areas.
[0120] However the above addressing methods assume that any
asymmetry between the two transitions (i.e. the
continuous-to-defect and defect-to-continuous) remains constant.
Variations of offset, cell gap, or the pitch of the grating will
result in little or no change to the amount of asymmetry of the
device response. However, certain variations (e.g. in the mark to
space ratio or shape of the grating) may result in a change to the
amount of observed asymmetry.
[0121] The effect of asymmetry in latching response on the
multi-scan technique of the invention is illustrated in FIG. 11. A
first curve 131, a second curve 132 and a third curve 133
illustrate the voltage and time slot required to latch the device
into the continuous state from the defect state in first, second
and third areas respectively. A first curve 131', second curve 132'
and third curve 133' illustrate how a negative voltage pulse of a
given time slot can latch the device from the continuous state to
the defect state.
[0122] The device of FIG. 11 thus has three sample areas that
exhibit latching properties with constant asymmetry and with
switching voltages equidistant apart. If the strobe and data
voltages are selected so that both scans overlap by the partial
latching width of the second area (i.e. the curves 132 and 132'),
clean switching is observed over the two scans.
[0123] FIG. 12 shows an expanded view of the first, second and
third curves 131, 131', 132, 132', 133 and 133'. Switching curves
132A and 132A' of a fourth area are now also shown. The fourth area
(i.e. curves 132A and 132A') has similar latching properties to the
second area (i.e. curves 132 and 132'), but with a variation in the
asymmetry of the switching.
[0124] It can be seen that if the device is blanked into the defect
state by a blanking pulse 134, then a non-select pulse applied
during the first scan (i.e. a resultant pulse 136) will partially
latch the fourth area into the continuous state. Furthermore, a
select pulse applied during the second scan (i.e. resultant pulse
138) will only partially latch the fourth area back into the defect
state. If the second area is already partially latched before the
second scan is applied, the lower than full-switching voltage may
be sufficient to switch the partial state into the defect state;
however this will not apply for a large variations in asymmetry,
this will not be possible. However, it can be seen that widening
the overlap of adjacent scans would resolve this issue.
[0125] The basics of the invention are described above. However, in
a practical device the invention is likely to be implement using a
sequence of addressing pulses to enable the multiple pixels of the
device to achieve the desired state. As also described above, prior
art schemes include both two-field and blanked addressing. Both
types of these types of addressing are possible with the present
invention.
[0126] The following examples show addressing schemes applied
bistable devices capable of being in either state A or state B for
a given point in the device. Two points or areas on the cell are
considered (i.e. AA, AB, BA or BB), the first requiring a higher
threshold to latch than the second (i.e. high, low). It is assumed
that a positive voltage (+Vs and +Vd) tend to latch the pixel into
state A, whereas a negative voltage (-Vs, -Vd) latch a pixel into
state B. In a display device, it will be common for one state to
appear reflective or white (say state A) and the other to be dark
(say state B). Where the pixel differs from the desired state (i.e.
errors), the state is indicated in bold. The aim of the addressing
scheme is both to ensure that there are no errors after the
addressing sequence is complete and that the desired state is
reached in the shortest time (that is, the least number of
steps).
[0127] To highlight the advantages of the invention, several
counter-example addressing sequences will first be considered.
Firstly, take the situation shown in table 1 below. This uses a
blanking pulse to latch both high and low areas into state B (BB).
In the line address period, the first pulse is at amplitude V2 and
has positive polarity to latch into state A. Selective latching of
the low (second) threshold areas occurs according to whether or not
the data is positive; none of the high (first) threshold areas
receive pulses of sufficient energy to cause latching. In the
second period, +V1 is applied, which combines with the data to
latch the high areas into state A or leave them unchanged,
depending on the data. However, all of the low areas are latched
into state A regardless of the data. If the voltage were -V1 in
this period, the low areas would be indiscriminately latched into
state B instead. Neither case leads to the desired image,
irrespective of starting configuration.
1TABLE 1 Blanked low then high/no data inversion Desired Initial
Final Blanked State State into B Data +V2 +V1 BB AA BB + BA AA BB
BB BB - BB BA AA AA BB + BA AA AA BB BB - BB BA BA AA BB + BA AA BA
BB BB - BB BA AB AA BB + BA AA AB BB BB - BB BA
[0128] In table 2 the last period is at -V2 and no part of the
signal is sufficient to latch the high threshold area into the A
state. When combined with the same data, this means that the whole
of the second period has no effect and is redundant. If the data
were to be inverted in the second period, the pixel would be
latched to BB, regardless of the initial condition.
2TABLE 2 Blanked both low/no data inversion Desired Initial Final
Blanked State State into B Data +V2 -V2 BB AA BB + BA BA BB BB BB -
BB BB AA AA BB + BA BA AA BB BB - BB BB BA AA BB + BA BA BA BB BB -
BB BB AB AA BB + BA BA AB BB BB - BB BB
[0129] Tables 3 and 4 show examples of two-field addressing that do
not give the desired results. In table 3, the positive voltages are
both applied in the first field and the negative voltages in the
second. The second period in each field is redundant.
[0130] Inverting the data in the second field (as in scheme of
table 4) does not reduce the errors.
3TABLE 3 Two field high then low/no alternation/no data inversion
Desired Initial Final State State Data +V1 +V2 -V1 -V2 BB AA + AA
AA AB AB BB BB - BA BA BB BB AA AA + AA AA AB AB AA BB - AA AA BB
BB BA AA + AA AA AB AB BA BB - BA BA BB BB AB AA + AA AA AB AB AB
BB - AA AA BB BB
[0131]
4TABLE 4 Two field high then low/no alternation/data inversion
Initial Desired Data Data State Final State Field 1 +V1 +V2 Field 2
-V1 -V2 BB AA + AA AA - BB BB BB BB - BA BA + AB AB AA AA + AA AA -
BB BB AA BB - AA AA + AB AB BA AA + AA AA - BB BB BA BB - BA BA +
AB AB AB AA + AA AA - BB BB AB BB - AA AA + AB BB
[0132] Table 5 shows a scheme that uses the sequence of +V1 and -V2
strobes, but still leads to error where the high threshold area is
required to latch from an initial state A to the desired state
B.
5TABLE 5 Single field high then low/alternating polarities/no data
inversion Desired Initial Final State State Data +V1 -V2 BB AA + AA
AA BB BB - BA BB AA AA + AA AA AA BB - AA AB BA AA + AA AA BA BB -
BA BB AB AA + AA AA AB BB - AA AB
[0133] Tables 6 to 9 provide examples of how to address two areas
with different thresholds according to the present invention. The
examples all use the scheme to lower the data voltage required to
address the panel and the desired states are either AA or BB (never
AB or BA). The same principles apply to cases where the thresholds
are deliberately altered to give individually addressable areas,
but then the data may vary from one period to the next.
[0134] Table 6 shows a simple addressing scheme in that each area
is blanked prior to the appropriate addressing signal being
applied. Initially there is no restriction on the blank, which is
chosen so that the whole panel is in state B, regardless of the
initial state. This blank might be applied to all of the rows
simultaneously, or it might be limited to one or several lines
ahead of the addressing sequence. It may be DC balanced itself, or
it might include parts that compensate for the net DC over the
whole frame. Data can be applied to the columns to ensure blanking
during this period, but the blank pulse will often be applied
simultaneous to scan signals on other rows of the display. In such
cases, the pulse is designed to latch into one particular state
regardless of the data applied to the columns (i.e. the data
associated with the scan signals on the other rows).
[0135] The blank is followed by the high latching pulse (+V1 in
this example) together with the appropriate data on the columns,
thereby latching the high threshold areas selectively, and latching
the low threshold areas into the opposite state indiscriminately.
Once the high threshold areas are addressed, the low threshold
areas only must be blanked back to the first state to prepare them
for the addressing the low threshold states in the following
period. Ideally the blank pulse is selected such that it latches
the low threshold areas completely without affecting the high
threshold areas that have already been addressed.
6TABLE 6 Separate blanks for high then low. Blank High Blank
Initial Desired Final (and low) low only State State into B Data
+V1 into B +V2 BB AA BB + AA AB AA BB BB BB - BA BB BB AA AA BB +
AA AB AA AA BB BB - BA BB BB BA AA BB + AA AB AA BA BB BB - BA BB
BB AB AA BB + AA AB AA AB BB BB - BA BB BB
[0136] An alternative, and potentially more advantageous scheme, is
shown in table 7. This scheme takes advantage of the fact that the
first high switching voltage (+V1) effectively blanks the low
threshold areas whilst selectively addressing the high threshold
areas. Hence, if the following signal is inverted in polarity (as
well as being set to the appropriate amplitude) then it combines
with the data to give the desired state. In this fashion, two slots
are required to ensure that the lower data addresses both
areas.
7TABLE 7 Blanked high then low/alternating polarities Blank High
Initial Desired (and low) State Final State into B Data +V1 -V2 BB
AA BB + AA AA BB BB BB - BA BB AA AA BB + AA AA AA BB BB - BA BB BA
AA BB + AA AA BA BB BB - BA BB AB AA BB + AA AA AB BB BB - BA
BB
[0137] Tables 8 and 9 illustrate similar schemes to that shown in
table 7, but do not use a blanking pulse, instead using three slots
to achieve the desired final states.
8TABLE 8 One and a half field high then low/alternating polarities
Desired Initial Final State State Data +V1 -V1 +V2 BB AA + AA AB AA
BB BB - BA BB BB AA AA + AA AB AA AA BB - AA BB BB BA AA + AA AB AA
BA BB - BA BB BB AB AA + AA AB AA AB BB - AA BB BB
[0138]
9TABLE 9 One and a half field high then low/alternating polarities
Desired Initial Final State State Data +V1 -V2 +V2 BB AA + AA AA AA
BB BB - BA BB BB AA AA + AA AA AA AA BB - AA AB BB BA AA + AA AA AA
BA BB - BA BB BB AB AA + AA AA AA AB BB - AA AB BB
[0139] As described above, the addressing sequence described in
table 7 above can be used to address each row in a higher number of
scans, thereby allowing the data voltage to be reduced further and
significantly. Case 10 extends the scheme of case 7 by dividing the
range of random thresholds into three (i.e. areas of three distinct
thresholds). This illustrates the use of the invention to
compensate for random variations.
10TABLE 10 Addressing three areas using multiple scanning. Desired
Blank Initial State Final State All into B Data +V1 -V2 +V3 BBB AAA
BBB + AAA AAB AAA BBB BBB BBB - BAA BBB BBB AAA AAA BBB + AAA AAB
AAA AAA BBB BBB - BAA BBB BBB BAA AAA BBB + AAA AAB AAA BAA BBB BBB
- BAA BBB BBB BBA AAA BBB + AAA AAB AAA etc
[0140] FIG. 13 shows how the scheme of CASE 10 is used to address 2
rows (scan) divided into three areas with different thresholds, and
8 columns (data), such as that described with reference to FIGS. 7
and 8;
[0141] FIG. 13a shows a two row (i.e. rows 70a and 70b) by four
column (i.e. columns 72a, 72b, 72c, 72d) segment of the display
shown in FIGS. 5 and 6. Row 70a is blanked black by a resultant
blanking pulse produced by application of suitable signals to row
70a and the columns 72a-72d. Row 70b remains unchanged by the data
signal applied to the column 72a-72d, represented by the grey
status.
[0142] FIG. 13b shows the high threshold area (i.e. the first area
80) of the upper row being addressed. A select data waveform is
applied to column 72b, whilst non-select data waveforms are applied
to columns 72a, 72c and 72d. The desired pattern is thus written to
the pixels of the first area 80 of the row electrode 70a The
resultant is sufficient such that the lower threshold areas (i.e.
the second area 82 and the third area 84) are indiscriminately
blanked white.
[0143] In FIG. 13c, the strobe voltage is reduced to Vs2 and
polarity inverted. This blanks the lowest threshold area (i.e. the
third area 84) back to black whilst leaving the highest threshold
area (i.e. area 80) unchanged. Only the middle area (i.e. the
second area 82) combines with the select and non-select data that
are applied to the columns 72a-72d, to give discrimination.
[0144] FIG. 13d shows the third scan in which the voltage is
reduced to Vs3 and polarity inverted. This addresses only the
lowest threshold area (i.e. the third area 84) to the desired
state, whilst leaving both higher threshold areas (i.e. the first
area 80 and the second area 82) unchanged. Row 70a is now
completely addressed.
[0145] FIGS. 13e and 13f shows how the process described above with
reference to FIGS. 15a and 15b is repeated for row 70b. In this
manner, data can be written to each pixel of the display.
[0146] It can thus be seen that despite only requiring drivers to
address two rows, a total of six-by-eight pixels are addressed,
rather than two-by-eight pixels. For simplicity, monopolar strobe
and data signals are shown. It would however be recognised by the
skilled person that, in practice, bipolar data may be preferred.
For example, ZBD devices may operate better using such bipolar
pulses in a frame scan, line scan or section scan arrangement.
[0147] The standard rules associated with prior art addressing
techniques should generally also be followed when implementing the
present invention. For example, the total signal applied to the
rows must be DC balanced over a certain period, usually taken to be
the complete frame. Also, the data signal should be DC balanced for
each line to prevent unwanted latching for certain pixel patterns.
Furthermore, the strobe (sometime also termed scan) pulses may be
taken to be either bipolar or monopolar as long as the net
resultant DC over time is zero. This DC balance prevents breakdown
of the liquid crystal material. In fact, it has recently been found
that ZBD devices operate better using bipolar pulses. This is due
to the poling effect of the leading (non-latching, dc balancing)
pulse lowering the latching threshold for the trailing (latching)
pulse.
[0148] Examples will now be given in which the scan sequence is
taken to be continuous for each line, each following on from the
other until that line is complete and the next line (in whatever
order) is addressed. This combines both the advantages of using
bipolar addressing with an addressing scheme of the present
invention.
[0149] It should be noted that rather than addressing each line in
turn, the display may be scanned from top to bottom at the first
strobe voltage, followed by subsequent scans of the whole display
at a reducing strobe voltages. This arrangement is likely to be
advantageous as it allows all of the rows to be connected to a
single driver chip and to be scanned at one voltage first, before
the total voltage level from that driver is reduced for the
following scan, and so on. This enables low cost four level (STN)
drivers to be used. In such cases, it may be preferable to ensure
that both blank and scan signals are bipolar.
[0150] An example of a scheme used to address a single row using
the method described in table 7 is shown in FIG. 14. This shows a
four slot scheme (-1, -1, +1, -2)Vs_(.+-.1, .+-.1, 1, 1)Vd wherein
the first two slots provide the blanking, and the latter two slots
give discriminate latching (1>2). Four slots are required to
allow the data signal to be DC balanced. Although selective
latching occurs in the last two slots only, the first two slots are
used to good effect, providing blanking immediately prior to
selection.
[0151] The row waveform in this instance is not DC balanced within
the line. This can be done using extra pulses either before or
after the signal. If timed immediately before the scan signal as
shown, the DC balancing pulses act to improve blanking.
[0152] Alternatively (for example, due to limitations in the
waveforms possible from the driver circuit) the whole waveform
might be incorporated into a six slot line: (+2, +1, -1, -1, +1,
-2)Vs_(+1, -1, .+-.1, .+-.1, 1, 1)Vd.
[0153] Referring to FIG. 15, a three scan multiplexing scheme of
the present invention is shown. A blanking pulse is followed by
first, second and third strobe pulses synchronised with appropriate
select or non-select data. The duration of each strobe pulse is
reduced from scan to scan and inverted in polarity from the
previous strobe pulse.
[0154] FIG. 16 shows how the first scan (i.e. the use of the first
strobe pulse) can be applied to each line which is then followed by
application of the second scan to each line, followed by the
application of the third scan to each line. Hence, the entire
display receives the first scan, then the second scan and finally
the third scan. FIG. 17 show an alternative arrangement in which
each line is latched using the three scans before the three scans
are applied to the next line.
[0155] It should be noted that a combination of the schemes shown
in FIGS. 16 and 17 is also possible. For example, consider a ten
line display. Lines one to five (say) could be addressed in turn by
the first, second and third scans. Subsequently, lines six to ten
(say) could then be addressed by the first, second and third scans
in turn. Various other combinations could be employed as required,
so long as in each frame each separate electrically addressable
region receives the first scan, second scan and third scan in the
correct order.
[0156] The bipolar pulse latching response of a 3.5 .mu.m ZBD cell
at 25.degree. C., measured using bipolar pulses, is shown in FIG.
18. This shows the asymmetric latching thresholds, which may
require overlapping addressing regions as described above. Negative
fields, with respect to the grating, latch to the continuous state
B at lower voltages than positive fields latching to the defect
state A. Also indicated are the thresholds for the reverse
transition caused by ionic contaminants to the liquid crystal. The
voltages may also be variable in order to compensate for global
variations, such as those of temperature. Voltages may also be
selected to take into account any panel to panel variations.
[0157] In order to demonstrate the present invention a test cell
has been built. The cell used in this investigation is denoted as
cell number Z641, which is a ZBD greyscale cell having a number of
areas fabricated using alignment gratings with different pitch and
mark to space ratios. However, to illustrate the present invention
the areas having fixed mark to space ratio and varied pitch will be
considered as these areas have substantially constant asymmetry in
the two switching thresholds they exhibit.
[0158] The pitch of the discrete areas in the cell is varied
between 0.6 .mu.m and 1.0 .mu.m in 0.1 .mu.m increments, and the
resulting latching transitions from all these areas at a
temperature of 25.degree. C. are shown in FIG. 19. The dashed and
solid lines in the figure show 10% and 90% levels of switching
respectively. In particular FIG. 19a shows the various
continuous-to-defect latching transitions whilst FIG. 19b shows the
various defect-to-continuous transitions.
[0159] It can be noted that the width of the bistability window is
insufficient for the whole range of grating pitches. This results
in growback for the 0.6 .mu.m pitch area, and little or no shift in
the transitions on increasing the pitch from 0.9 .mu.m to 1.0
.mu.m. FIG. 19 shows that the typical partial switch widths vary
from 0.4V to 1.1V for the C to D transitions, and 0.7V to 2.1V for
the D to C transitions.
[0160] The cell is firstly used to demonstrate how multiple scans
in accordance with the invention can be used to reduce the data
voltage while correcting for non-uniformities in the device
switching. Note that the following is carried out on the lightbox,
in order to observe the whole of the device at any one time. This
means that the temperature cannot be controlled, and will be
greater than 25.degree. C., therefore resulting in lower switching
voltages across all the areas. However the transitions of each
grating pitch area will still be shifted in voltage.
[0161] If a time slot of 100 .mu.s is selected, then the C to D
transition was found to require a data voltage of 2.25V for the
transition from C to D, and 2.75V for the transition from D to C,
in order to fully switch all areas under the application of a
single bipolar pulse. If two scans are applied, with an overlap of
1.0V selected therebetween (note the effect of partial switch
widths is discussed above), the first scan switching D to C with
voltages V.sub.s=19.6V, V.sub.d=1.6V, the second switching C to D
with voltages V.sub.s=19.9V, V.sub.d=1.4V, then all of the regions
of cell Z641 with fixed mark to space ratio, are addressed fully
either into the continuous state or the defect state.
[0162] Referring now to FIG. 20, the effect of the multi-scan
technique on the test cell is shown. FIG. 20d shows the pitch (in
.mu.m) of the grating in the different areas of the test cell.
[0163] To illustrate the multi-scan technique, the device was
initially blanked in the defect state, and then two scans were
applied, the first scan with polarity to switch into the continuous
state, the second scan with polarity to switch into the defect
state. When switching the device to the defect state, the first
scan contains non-select data, and the areas with higher threshold
voltages remain in the defect state after the first scan as the
non-select resultant is insufficient to switch into the continuous
state. Other areas however are switching into the continuous state,
as their threshold voltages are lower. This is shown in FIG. 20a as
areas of shorter pitch (therefore lower threshold voltage) are
switched into the continuous (black) state.
[0164] It can be seen, e.g. as there is not a clear distinction
between the 0.8 .mu.m and 0.9 .mu.m pitch areas, that many
non-uniformities are present in the test cell. In addition, a large
amount of growback to the defect state is present in the 0.6 .mu.m
area, therefore resulting in a greater proportion of the area being
in the defect state than should otherwise be the case. A skilled
person would appreciate that the number of defects and the level of
non-uniformities as present in the test cell would be significantly
reduced in any production display.
[0165] FIG. 20(b) shows the device fully switched after the second
scan on switching to the defect state, which incorporates select
data in addition to the strobe with polarity to switch -defect.
This voltage is sufficient to switch the areas into the defect
state that were switched continuous in the first scan, and is
insufficient to switch the areas defect that were not switched
continuous.
[0166] On switching the device into the continuous state, the first
scan now incorporates select data, which switches all areas into
the continuous state, and the second scan incorporates a non-select
data, which leaves all areas unchanged in the continuous state. The
final state is shown in FIG. 20(c), although the device is
unchanged by the second scan.
[0167] It has thus been demonstrated that all the areas of the
greyscale cell Z641 of fixed mark-to-space ratio can be addressed
using two scans of opposite polarity using data voltage of 1.6V and
1.4V in the first and second scans respectively. This compares with
a data voltage of 2.25V that would be required to switch the same
areas using a single scan. A reduction in the data voltage of 33%
has thus been demonstrated.
[0168] It is noted that further reduction in the applied data
voltage will be required in order to reduce the data voltage to a
level below the Fredericksz transition which is in the region of 1V
for this device. However in typical cells (i.e. where the grating
is fabricated with a fixed pitch and mark-to-space ratio) the local
partial switch width is often 0.5V which is much narrower than the
1-2V different in many cases in the greyscale cell used here. Given
such a narrow local partial switch region, data voltages of less
than 1V may be used, which is lower than the Fredericksz
transition. As described above, reducing the data voltage below the
Fredericksz transition prevents display "flash" during
addressing.
[0169] The areas with grating pitch 0.6 .mu.m and 0.8 .mu.m are
positioned adjacent to one another on the greyscale device, and
therefore allow investigation of the second application of using
the present invention to reduce the number of drivers by
fabricating areas of different threshold voltage. Now the cell is
placed in the temperature stage, and set at a temperature of
25.degree. C. The switching curves of the two areas being
considered are shown in FIG. 23. FIG. 21a shows the
defect-to-continuous transition for the two areas of the greyscale
cell Z641 with 0.6 .mu.m and 0.8 .mu.m grating pitch, whilst FIG.
21b shows the continuous-to-defect transition for the same areas.
Dashed and solid lines show 10% and 90% levels of switching
respectively. The first scan is defined by the first arrow 200
(FIG. 21a) and the second scan by the second arrow 202 (FIG.
21b).
[0170] It can be seen from FIG. 21 that the largest difference in
switching voltages for the two areas occurs at a time slot of
between 50 .mu.s and 100 .mu.s (with 100 .mu.s being selected as
the time slot in this demonstration). If we therefore use the D to
C transition as the first scan, with a strobe voltage of 24.5V, and
the C to D transition as the second scan with a strobe voltage of
24V, then using a data voltage for both scans of 1V, depending on
the combination of select or non-select data waveforms for the two
scans we can select 4 separate states.
[0171] FIGS. 22a-22d are photomicrographs of the 0.6 .mu.m and 0.8
.mu.m regions described with reference to FIG. 21 above. FIG. 22e
illustrates the position of the two different regions in the
photomicrographs.
[0172] The cell is blanked into the defect state to latch both
areas white. The defect-to-continuous transition is used as the
first scan, with a strobe voltage of -24.5V. The
continuous-to-defect transition is the second scan with a strobe
voltage of 24V. The first and second scans use a data voltage of
1V. Depending on the combination of select or non-select data
waveforms for the two scans, four separate states can be selected.
FIG. 22a shows 0.6 .mu.m/OFF, 0.8 .mu.m/ON; FIG. 22b shows 0.6
.mu.m/ON, 0.8 .mu.m/ON; FIG. 22c shows 0.6 .mu.m//FF, 0.8 .mu.m/OFF
and FIG. 22d shows 0.6 .mu.m/ON, 0.8 .mu.m/OFF. The definition of
the labels shown in FIG. 22 are given as the polarity of the data
in the 1.sup.st/2.sup.nd scans, where + data has the same polarity
as the corresponding strobe, and - data has the opposite polarity
as the corresponding strobe.
[0173] Two areas with grating pitch 0.6 .mu.m and 0.8 .mu.m can
thus be addressed selectively, using two scans and a 1V data pulse.
Depending on the combination of select or non-select waveforms for
both scans, four separate states can be selected. This allows the
number of drivers to be reduced, for use in either greyscale, or a
standard black and white device. This is achieved by fabricating
areas of different grating pitch.
[0174] The multiscan technique can also be used to ensure operation
across a wide temperature range with the need for a temperature
sensor. The first scan is arranged to latch material where the
threshold is high (e.g. low temperature) and subsequent scans latch
material with a threshold in decreasing ranges (i.e. higher
temperatures). This removes the requirement temperature sensing
circuits and thus reduces costs. The temperature variations may be
local or global.
[0175] It has thus been demonstrated that a display can be
addressed using Multiscan in all cases, provided that the overlap
of adjacent alternating scans is sufficient. Given no change in
asymmetry of the two transitions across the cell, then this overlap
corresponds to the local partial switch width. However if the
asymmetry of the two transitions changes, then a larger overlap is
required which may reduce the data voltage reduction benefits of
the technique.
[0176] As outlined above, patent application WO97/14990 describes a
zenithally bistable device (ZBD) having an alignment grating on at
least one surface. Moreover, WO97/14990 describes the use of a
zenithally bistable alignment grating on both surfaces of a device;
herein such a device shall be termed a double ZBD device.
[0177] Firstly, it has been found that an electric field of a
certain polarity applied across a double ZBD cell results in an
electric field oriented into one surface, and away from the other.
Hence, the field acts to latch one surface from state A to B (say
from low tilt, defect state to high tilt, continuous state) whereas
the same field tends to latch the opposite surface from B to A.
[0178] If the two surface of a double ZBD device are the same, the
A to B and B to A transitions of both surface will be the same and
hence an applied field will always tend to latch the device into
either of the hybrid states AB or BA. In other words, both surfaces
will switch at the same applied (negative or positive) voltage and
hence only the hybrid states can be selected.
[0179] It has been found that a first improved double ZBD can be
produced by constructing a device with the same grating on both
surfaces, but with each surface arranged so that the transition
from low tilt (e.g. state A) to high tilt state (e.g. state B) has
a higher threshold energy (.tau.V) than the reverse transition (B
to A). In other words, the transition from A to B occurs at a first
magnitude of voltage (but different voltage polarity) for both
surfaces whilst the transition from B to A occurs at a second
magnitude of voltage (but different voltage polarity) for both
surfaces. These so-called asymmetric transitions provide a degree
of independent control over switching at each surface.
[0180] FIG. 23 shows the measured electro-optic response of a
double ZBD device having asymmetric transitions. Curves 221A shows
the transition at the first surface (S1) from the high tilt state
(state B) to the low tilt state (state A), whilst curves 222B show
the transition at the second surface (S2) from the low tilt state
(state A) to the high tilt state (state B). Curves 221B shows the
transition at the first surface from the low tilt state (state A)
to the high tilt state (state B), whilst curves 222A show the
transition at the second surface from the high tilt state (state B)
to the low tilt state (state A). The dotted lines represent the
onset of the transition and full lines are for full latching. The
cell had three different optical transmission states, due to the
equivalence of the hybrid states AB and BA.
[0181] The latching thresholds were measured for bipolar pulses,
with latching defined in each,.case using the trailing pulse. The
switching results shown graphically in FIG. 23 are summarised, for
750 .mu.s pulses with data signals of .+-.3V applied to the
columns, in Table 11.
11TABLE 11 Results for a double ZBD cell. Onset Completion
Transition Voltage Voltage S2 = B to A -15.2 -15.8 S1 = A to B
-12.6 -13.4 S2 = A to B 12.6 13.4 S1 = B to A 15.2 15.8
[0182] As an example, consider addressing the double ZBD device
described with reference to FIG. 20 using an addressing scheme of
the type used in the prior art. A first pulse applied to the
addressed row of +20V ensures that the S1 is latched into state A
and S2 into B (i.e. state AB). Blanking pulses such as this are
often applied one or more lines ahead of the appropriate addressing
signal. The +20V magnitude is sufficiently high to blank into BA,
irrespective of the data applied. This allows data for some
previous line to be applied simultaneously to the blank pulse.
[0183] After blanking, the row of interest is ready to be
addressed. The first pulse of the addressing sequence should be of
the opposite polarity to the blank and centred between the
asymmetric transition energies. In this example, a pulse of -14V
was applied. This latches S1 into the A state and S2 into state B
when the data is +3V since the resultant -17V is above both
transitions, but leaves both surfaces unchanged for negative data
(resultant of -11V).
[0184] In the final pulse of the addressing sequence the polarity
is inverted and the magnitude is reduced, so that the data causes
latching or not of the lower threshold surface, but leaves the
higher threshold surface unaffected. In this example, +11V was
applied. Where the data is +3V, the voltage drop across the cell is
only +8V, and the pixel is unchanged (either AB or BA from the
first pulse). If the data is -3V, the +14V resultant latches S2
into state B and the pixel is either AB or BB. However, if the
pixel is in the state AB from the first pulse, it will remain so
even after the second pulse. The state AA has not been achieved.
This addressing sequence is summarised in Table 12 in which the
first letter corresponds with S1, the second with S2 and bold
letters denote error. It is seen that any attempt to latch S2 into
the required A state, will inevitably also latch S1 into state
B.
12TABLE 12 Example of a prior art addressing sequence applied to
double ZBD. Desired Final Blank State (+) Data 1 -V1 Data 2 +V2 BB
AB + BA - BB AB AB - AB - AB BA AB + BA + BA AA AB - AB + AB
[0185] The multi-scan technique described above can be applied to
double ZBD when it is arranged for the two surface to have
different latching thresholds, irrespective of the resulting tilt
in the low pre-tilt state. It is then possible to address the
device so that the surface with the higher threshold is selectively
latched in a first scan, whilst the surface with the lower
threshold is selectively latched in a second scan.
[0186] The latching energy of a bistable grating surface may be
varied by altering the grating shape (for example, the altering the
pitch to depth ratio, the mark to space ratio, or the degree of
asymmetry) or surface properties (e.g. surface energy). Providing
different top and bottom surfaces leads to a wider addressing
window in which selection of the desired state is possible
independently of variations or changes of condition. In such cases,
the bistable alignment on each surface may be gratings of different
shapes, but different grating materials might be used for the two
surfaces. Differences of dielectric constant for the two surfaces
leads to different electric field profiles at the surface (even for
the same grating shape), thereby resulting in different thresholds.
Alternatively, the gratings might be coated with different
materials, thereby altering the transition thresholds due to
differences in surface energy.
[0187] A double ZBD device can thus be constructed in which the
threshold voltage for a transition on the first surface differs
from the threshold voltages of the analogous transitions on the
second surface. Because of the reversal of field for the top and
bottom surfaces, this may even be achieved using surfaces with
equivalent alignment properties top and bottom. In other words, an
improved operating window results when asymmetric transitions are
used, but the polarities are inverted (i.e. for one surface A to B
is lower than B to A, but vice-versa for the other transition).
[0188] As an example, consider selection of conditions AA and BB
where the first letter represents the higher threshold surface
state, and the second letter the lower threshold surface state. The
use of multi-scan addressing requires that the higher threshold
surface be latched first if required. The first pulse applied to
selectively latch the higher threshold surface will always latch
the lower threshold surface, thereby leading to a transient hybrid
state. This first pulse can followed by a second pulse that may
selectively (i.e. according to the data) latch the lower threshold
surface, without affecting the condition of the higher threshold
surface.
[0189] The use of two scans to separately address the top and
bottom surfaces allows all four states (AA, AB, BA and BB) to be
discriminately selected, as shown in table 13. In this example,
.vertline.V1.vertline.&g- t;.vertline.V2.vertline., a +V.sub.d
data pulse latches towards AB whilst a -V.sub.d pulse latches
towards BA. In each case, the first letter in table 13 denotes the
high threshold surface and the second letter denotes the low
threshold surface. It is possible that the negative thresholds and
positive thresholds may be reversed, but the same basic principles
would still apply.
13TABLE 13 Addressing sequence for Dual ZBD according to the
present invention. Desired Initial Final Blank State State (-) Data
1 +V1 Data 2 -V2 BB Aa BA - AB + AA BB Ba BA + BB + BA BB AB BA -
AB - AB BB BB BA + BB - BB AA Aa BA - AB + AA AA Ba BA + BB + BA AA
AB BA - AB - AB AA BB BA + BB - BB BA Aa BA - AB + AA BA Ba BA + BB
+ BA BA AB BA - AB - AB BA BB BA + BB - BB AB Aa BA - AB + AA AB Ba
BA + BB + BA AB AB BA - AB - AB AB BB BA + BB - BB
[0190] Referring to FIG. 24, the addressing sequence of a dual ZBD
in accordance with the present invention is illustrated.
[0191] FIG. 24a shows a ZBD cell comprising a nematic liquid
crystal layer 230 contained between first and second bounding glass
walls 232 and 234. First and second electrodes 236 and 238 are
applied to the internal surfaces of the first and second bounding
glass walls 232 and 234 respectively. The liquid cell in FIG. 24a
can be in any initial configuration; e.g. the mixture of different
optical states shown.
[0192] A first alignment surface 240 is applied to the first
electrode 236 and a second alignment surface 242 is applied to the
second electrode 238. Each of the alignment surfaces comprise a
surface relief structures (e.g. a grating) that can impart two
stable alignment conditions to the nematic liquid crystal material
in the vicinity thereof. However, the first alignment surface is
arranged to provide latching between the two bistate surface states
at a higher voltage threshold than the second surface.
[0193] FIG. 24b shows the orientation of the ZBD cell after
blanking using a high negative pulse. A hybrid state (i.e. AB) is
thus formed.
[0194] A first scan is then applied using a positive strobe pulse.
If a negative (i.e. select) data pulse is combined with the
positive strobe pulse, the resultant pulse is sufficient to latch
both the high threshold surface and the low threshold surface; the
hybrid state BA shown in FIG. 24c is thus formed. If a positive
(i.e. non-select) data pulse is combined with the positive strobe
pulse, the resultant is insufficient to latch the high threshold
surface but will latch the low threshold surface; the state AA
shown in FIG. 24d is thus formed. The first scan thus
indiscriminately latches the lower threshold surface, and
selectively latches the higher threshold surface.
[0195] Once the first scan is complete, a second scan is applied
using a negative strobe pulse of a lower magnitude or duration than
the positive strobe pulse of the first scan. The second scan is
arranged to selectively latch the lower threshold surface, but has
no effect on the higher threshold surface.
[0196] If the BA state of FIG. 24c was selected during the first
scan, the resultant pulse produced during the second scan will
latch the lower threshold surface to the state shown in FIG. 24e if
a positive (select) data voltage is applied. Application of a
non-select data pulse results in the BA state of FIG. 24c being
retained as shown in FIG. 24f.
[0197] If the AA state of FIG. 24d was selected during the first
scan, the resultant pulse produced during the second scan will
latch the lower threshold surface to the state shown in FIG. 24h if
a positive (select) data voltage is applied. Application of a
non-select data pulse results in the AA state of FIG. 24d being
retained as shown in FIG. 24g.
[0198] In this manner, multiple scans allow the state at the two
surfaces of the device to be readily selected. In other words,
states AA, BB, AB or BA may be chosen as required. It should be
noted that although FIG. 24 shows initial blanking into state AB,
it is also possible to use the technique after the device has been
blanked into state BA. This is illustrated in FIG. 25.
[0199] FIG. 25a shows the liquid crystal material in a mixed
configuration. After application of a positive blanking pulse, the
hybrid state BA of FIG. 25b is formed. The first scan can either
form the BB state of FIG. 25c or the AB state of FIG. 25d. If the
BB state is selected in the first scan, this can be retained (FIG.
25e) or the BA state of FIG. 25f can be selected. If the AB state
is selected in the first scan, this can be retained (FIG. 25h) or
the AA state shown in FIG. 25g can be selected.
[0200] A person skilled in the art would recognise that double ZBD
devices could be used in various optical arrangements known to
those skilled in the art. It should be noted that a good optical
response is obtained when state A for both surfaces has zero tilt,
and state B has 90.degree. tilt (i.e. parallel to the surface
material). For example, a transmissive device could be produced
using two polarisers or a single polariser and a reflector could be
used to provide a reflective device. The optical characteristic
could also be altered using compensation films, colour filters etc.
The double ZBD arrangement gives excellent viewing angle
characteristics for homeotropic and twisted nematic states.
[0201] Referring to FIG. 26, four segments of a double ZBD device
are shown. The device comprises a first cell wall 250 and a second
cell wall 252 that constrain a layer of nematic liquid crystal
material 254. A first row electrode 256 and a second row electrode
258 are provided on the internal surface of the first cell wall
250. A first column electrode 260 and a second column electrode 262
are provided on the internal surface of the second cell wall 252. A
first surface alignment grating 264 is provided to align liquid
crystal material at the first cell wall 250, and a second alignment
grating 266 is provided to align liquid crystal at the second cell
wall 252. The groove directions of the first and second gratings
are orthogonal. A pair of polarisers 268 are also provided; one
polariser placed either side of the cell and arranged such that
their optical axes are orthogonal and lie along the groove
direction of the respective surface grating. A backlight 270 is
also provided.
[0202] The device of FIG. 26 thus contains four separately
electrically addressable areas. The liquid crystal in the first
electrically addressable area 270 (defined by the overlap of the
second row electrode 258 and the second column electrode 262) is
shown latched into the BB state and provides a black state. Liquid
crystal in the second electrically addressable area 272 (defined by
the overlap of the first row electrode 256 and the second column
electrode 262) is shown in the BB state and provides a white
state.
[0203] The A state of the second alignment grating is arranged to
give a higher pretilt than the A state of the first alignment
grating. Hence, the third electrically addressable area 274
(defined by the overlap of the second row electrode 258 and the
first column electrode 260) provides a light grey state when in the
AB state. This should be compared to the fourth electrically
addressable area 274 (defined by the overlap of the first row
electrode 256 and the first column electrode 260) that provides a
light grey state when in the BA state.
[0204] This transmission difference between AB and BA thus means
that four transmission levels are possible as described above. If
the optics are chosen carefully, this arrangement may provide a
satisfactory viewing angle; note the zero tilt, three state device
AA, AB=BA, BB has perfect LCD optics.
[0205] It has thus been shown that it is possible to use a zenithal
bistable alignment surface on both internal surfaces of an LCD.
Designing the surfaces to give different switching thresholds for
the two surfaces allows three or four states to be addressed
separately. It is preferred that the device uses zenithal bistable
grating surfaces arranged with the grating axes aligned at
substantially 90.degree. to each other. A second preference is that
the low tilt state of the two surfaces is substantially different
(although both should have below 60.degree. pretilt from the
average surface plane) and the two high tilt states are both in the
pretilt range 88.degree. to 90.degree.. Moreover, it has been
described how electrical signals can be provided that allow (at
least) the device to be latched into both surfaces low tilt, or
both surfaces high tilt independently.
[0206] Although the double ZBD device described is shown with
periodic surface alignment gratings, a surface of the type
described in WO 01/40853 may be used as one or both of the
zenithally bistability alignment layers. In such alignment layers,
the surface alignment of the low tilt state varies significantly
from one point on the surface to another. Examples of such surfaces
include homeotropic bi-grating, grating grids, or other such
gratings, or pseudo-random surface features (pillars or blind
holes) with size, shape and spacing in a range that gives zenithal
bistability.
[0207] It should also be noted that the two scans to switch the two
surfaces can be combined with the multiple scans to address
different areas across the display. In other words, neighbouring
double ZBD region may have different thresholds. This can reduce
the data voltage, or reduce the number of electrodes/drivers as
described above.
[0208] Referring to FIG. 27, a bistable pi-cell device according to
the present invention is shown. The device comprises a layer of
liquid crystal material 500 sandwiched between a first cell wall
502 and a second cell wall 504.
[0209] The internal surface of the first cell wall 502 has a
surface profile (not shown) that imparts two stable alignment
configurations having different pretilts to the liquid crystal
material. The internal surface of the second cell wall carries a
monostable surface treatment (e.g. silicon dioxide, an
appropriately designed surface relief structure or a suitable
prepared polymer surface, such as a rubbed-polymer or a
photo-aligned polymer) that imparts a pretilt of less than
45.degree. to the liquid crystal material in the vicinity thereof.
Preferably the pre-tilt of the monostable surface is less than
30.degree. and more preferably less than 25.degree.. Preferably,
the pre-tilt of the monostable surface is greater than 5.degree.
and more preferably greater than 10.degree..
[0210] In operation, the liquid crystal in the vicinity of the
first cell wall is latched between the low pretilt (defect) state
shown in FIG. 27a and the high pretilt (continuous) state shown in
FIG. 27b. It can be seen that the defect state is a bend state, and
that the continuous state is a hybrid state in which the splay
component is small. Hence, because the device is latching between a
bend state and a second substantially non-splayed states, the
switching speed is high (typically below 5 milliseconds).
[0211] Preferably, the pre-tilt of the low tilt, defect state of
the zenithal bistable surface is higher than the pre-tilt on the
opposing surface as shown in FIG. 28. In this instance, if an
unwanted splayed state is formed, the splay occurs closer to the
zenithal bistable surface. Applying a pulse to latch the material
adjacent the bistable surface into the high tilt continuous state
causes the splay to move closer to the grating surface. The splay
is then dissipated rapidly as the surface latches into the high
tilt state.
[0212] This should be compared to the prior art pi-cell device
shown in FIG. 29 in which both surfaces are monostable and the ZBD
device shown in FIG. 30 in which the pretilt at the upper and lower
surfaces is identical and the transition to a bend state occurs
without a surface transition. In these cases, the transition from
splay to bend takes significantly longer to occur than is found
when surface mediated transitions are used in accordance with this
invention.
[0213] In this fashion, the device is designed to provide a
surface-latching mediated transition to the bend state. Using this
surface transition enables the transition from a splay to a bend
state to occur in a time that is orders of magnitude quicker than
is possible using conventional prior-art pi-cell devices.
[0214] For applications where the optical contrast is required to
be maintained throughout long periods without addressing (ie. Image
storage) the device is designed to eliminate formation of splayed
states. This is done by ensuring that there are no nucleation
points for splayed states and/or that the energy of the bend states
is relatively low (for example, using relatively high pre-tilts on
both surfaces).
[0215] Other applications, such as those that require a fast update
speed with regular updating, the device may be designed to give
other important properties, such as wide viewing angle, high
transmissivity/reflectivity, high contrast and good (saturated)
white state. This may mean that the splay state is significantly
lower energy than the bend state, and the device relax into this
state after a period that is of similar (but longer) duration to
the frame update period. For example, the pre-tilt at both surfaces
may be as low as 10.degree., and may be as low as 5.degree..
[0216] Referring to FIG. 31, a second pi-cell device according to
the present invention is shown. The pi-cell device comprises a
first cell wall 502 and a second cell wall 506 with a layer of
nematic liquid crystal material 500 sandwiched inbetween. The first
cell wall 502 and the second cell wall 506 have surface profiles
that can each impart, and can be latched between, two alignment
states having different pretilt. In other words, a so-called double
ZBD device is formed. However, unlike the double ZBD devices
described above, the defect state (i.e. the state shown in FIG.
31a) is arranged to form a substantially non-splayed (bend)
state.
[0217] To enable latching between the two substantially non-splayed
states, the latching thesholds at the first and second cell walls
are arranged to be different as described above. This allow the
multi-scan technique also described above to be used to latch
between the two configurations. As latching occurs between two
substantially non-splayed states, the switching speed is
significantly increased compared with that obtained when latching
to/from a splayed state.
[0218] Ideally, the device is arranged such that the homeotropic
(continuous) state of FIG. 31b is more energetically favourable
than any defect state. This is achieved by careful selection of the
surface profile of the alignment surfaces. In a pixellated device,
this also means that the inter-pixel gap region will tend to form
the continuous state of FIG. 31b. This helps to ensure that any
liquid crystal material latched from the homeotropic state of FIG.
31b adopts the substantially non-splayed state of FIG. 31a rather
then a splayed state. This should be contrasted to convention
mono-stable pi-cell devices in which the inter-pixel gap regions
relax to the splayed state and thus nucleate growth of the splayed
state in the pixel regions.
[0219] There are a number of features of the zenithal bistable
surface that may be varied to ensure a bistable surface that
spontaneously forms a high tilt state on first cooling. In addition
to using a shallow grating (eg low amplitude and/or long pitch)
such as surface may be provided using rounded features (eg a blazed
sinusoidal grating) or a relatively low anchoring energy.
[0220] It is also possible to arrange for both of the surfaces to
be mono-stable, but one of the surfaces has substantially weaker
zenithal anchoring energy, whilst maintaining a low tilt state when
undeformed by an applied electric field. An electrical blanking
pulse is then used at the outset of each addressing sequence that
causes anchoring breaking, aligning the director vertical at the
said weakly anchored surface. In this fashion, the bend state is
again mediated by a surface transition from a low tilt to a high
tilt state. A disadvantage to this type of device, however, is that
the alignment properties of the cell are required to be carefully
arranged to give the two required states (for example, stable
states with different twists).
[0221] Although a bistable surface is described above, it would be
recognised that a surface comprising three or more states (e.g. a
surface of the type described in WO99/34251) could be used. In such
cases intermediate states would be formed that could, for example,
allow the implementation of greyscale.
[0222] Referring to FIG. 32 a number of substantially non-splayed
states are shown.
[0223] The states include vertically aligned nematic (VAN), in
which both surfaces are substantially vertical homeotropic aligned
(i.e. pre-tilts greater than 70.degree., usually greater than
85.degree.). This is a special case, since the 1D director profile
contains neither splay nor bend. A hybrid aligned nematic (HAN) is
another non-splayed state in which one surface is high tilt
(typically greater than 70.degree.) and one surface is low tilt
(typically between 0.degree. and 45.degree.).
[0224] Bend states are also non-splayed and B1, B2 and BT are Bend
states. Bend state can be defines as a state in which the tilt of
the director at some point in the bulk of the cell (i.e. between
the two surfaces) is greater than the pretilt at both surfaces.
Typically there is a point between the walls where the director is
normal to the plane of the cell and the direction of bend changes
either side of this condition. In B1 the pretilt at both surfaces
are similar, and the tilt is substantially 90.degree. close to the
centre of the device. In B1, there is a significant difference in
pre-tilt between the two surfaces and the tilt in the bulk of the
cell is substantially 90.degree. closer to the higher pre-tilt
surface. In the twisted example BT, the director includes a twist
deformation from one surface to the other with the director at some
point in the bulk of the cell (in this case close to the cell
centre) being perpendicular to the cell walls. Switching from HAN
to B1 will take typically 2 ms.
[0225] Referring to FIG. 33, a number of splayed states are shown.
In each of these cases the director in the bulk of the cell
includes points at which the tilt is equal to or lower than the
higher pre-tilt on one cell wall. Note, S4 is a transient state
that may occur on the application of an applied field to an S1
state. Although the director may be at 90.degree. at a point in the
bulk of the cell (ie the director aligned parallel to the applied
field) either side of this point the director has the same
direction of bend. Moreover, there is a point within the bulk of
the cell where the director is substantially splayed (close to the
bottom surface) and the director is at a lower tilt than either of
the two aligning walls. ST is an example of a splayed twist state,
where the director in the bulk of the cell is equal or lower to the
higher of the two aligning surface pre-tilts.
[0226] Referring to FIG. 34, theoretical energies for the
continuous and defect states for a blazed sinusoidal grating
surface from U.S. Pat. No. 6,249,332. Shaded area shows an example
range of grating shapes that give an energy barrier between the
continuous and defect surface states and the surface remains
bistable. Designing the grating to give spontaneous formation of
the C state on first cooling is possible, for example, by using a
groove depth to pitch to the left of the cross-over point but
within the range for bistability.
[0227] A device of the present invention may also be operated as a
monostable device, wherein a surface transition is used at the
beginning of a sequence of frames to ensure that a Bend state is
achieved. This ameliorates the need for high latching voltages,
nucleation points for the Bend state and/or long transition periods
from a splay to bend.
[0228] The device may be in the splay state initially, when it is
switched into constant update mode. Before each frame is addressed,
perhaps using RMS multiplexing (Alt-Pleshko, MLA, 4-line addressing
etc--standard IN or STN methods) or TFT addressing, a series of
pulses is applied to latch the device into the required initial
substantially non-splayed state. Preferably this initial state is a
Bend state. For example, when in the splayed state and initial DC
pulse to latch the zenithal bistable surface into the C state
induces a HAN state. The director in the middle of the cell is
switched quickly to vertical. Then latching to back to the Defect
state induces the bend state. Having achieved the bend state in a
period of typically about 1 millisecond or quicker, the Bend state
may be modulated by the applied field in a similar fashion to a
standard pi-cell arrangement (i.e. between the states of FIGS. 1b
and 1c).
[0229] Alternatively a symmetric grating with two high tilt but
opposite pre-tilt defect states may be used. The anchoring
transition between these symmetric states enables a direct
transition from the splay to bend states.
[0230] Alternatively, a weakly anchored surface may be switched
vertical, and then the direction of tilt reversed (through suitable
balancing of pitch and pretilt) into the bend state In such cases,
the shape of the trailing part of the addressing pulse is varied to
selectively latch into the required bend state rather than the
splay state. This is shown schematically in FIG. 35 where the
states and addressing pulse 800 are illustrated.
* * * * *