U.S. patent application number 10/512457 was filed with the patent office on 2005-08-11 for display device having a material with at least two stable configurations.
This patent application is currently assigned to ZBD DISPLAYS LIMITED. Invention is credited to Jones, John C.
Application Number | 20050174340 10/512457 |
Document ID | / |
Family ID | 29711933 |
Filed Date | 2005-08-11 |
United States Patent
Application |
20050174340 |
Kind Code |
A1 |
Jones, John C |
August 11, 2005 |
Display device having a material with at least two stable
configurations
Abstract
A device is described that comprises a layer of material (254)
disposed between first and second cell walls (250, 252) and is
capable of adopting, and being electronically latched between, at
least two stable configurations. The layer of material (254)
comprises one or more separate electrically addressable regions
(270, 272, 274, 276) and addressing means are provided to write to
each of said electrically addressable regions using voltage pulses
to selectively latch said layer of material as required. The
addressing means is arranged to write to each of said one or more
separate electrically addressable region using at least first and
second latching scans. The first latching scan being arranged to
selectively latch material having a latching threshold within a
first range and said second latching scan being arranged to
selectively latch material having a latching threshold within a
second range, 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 having a latching threshold
within said first range. A method for addressing a device is also
disclosed.
Inventors: |
Jones, John C; (Malvern,
GB) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
ZBD DISPLAYS LIMITED
Malvern Hills Science Park, Geraldine Road
Malvern, Worcestershire
GB
WR14 3SZ
|
Family ID: |
29711933 |
Appl. No.: |
10/512457 |
Filed: |
October 26, 2004 |
PCT Filed: |
May 29, 2003 |
PCT NO: |
PCT/GB03/02354 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60383610 |
May 29, 2002 |
|
|
|
Current U.S.
Class: |
345/204 |
Current CPC
Class: |
G02F 1/133761 20210101;
G02F 1/133776 20210101; G09G 2310/06 20130101; G09G 2310/02
20130101; G02F 1/1395 20130101; G09G 3/2077 20130101; G02F 1/1391
20130101; G09G 2310/061 20130101; G02F 2201/30 20130101; G09G
3/3637 20130101; G09G 3/3614 20130101; G02F 1/133753 20130101; G09G
3/364 20130101; G09G 3/3629 20130101 |
Class at
Publication: |
345/204 |
International
Class: |
G09G 005/00 |
Claims
1. A device comprising first and second cell walls and a layer of
material disposed between said first and second cell walls, said
layer of material being capable of adopting and being electrically
latched between at least two stable configurations, said layer of
material comprising one or more separate electrically addressable
regions and said device further comprising addressing means to
write to each of said electrically addressable regions using
voltage pulses to selectively latch said layer of material as
required, wherein the addressing means is arranged to write to each
of said one or more separate electrically addressable regions using
at least two latching scans, the first latching scan being arranged
to selectively latch material having a latching threshold within a
first range and the second latching scan being arranged to
selectively latch material having a latching threshold within a
second range, wherein said first latching scan additionally
comprises a blanking waveform to latch all of said material into
one of said at least two stable configurations prior to the
selective latching of material having a latching threshold within
the first range, 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 having a latching
threshold within said first range.
2. A device according to claim 1 wherein said first latching scan
indiscriminately latches (i.e. blanks) material having a latching
threshold within said second range.
3. A device according to claim 1 in which the addressing means
applies one or more further latching scans after application of
said second latching scan, wherein each further latching scan is
arranged to selectively latch material having a latching threshold
within a given range but is insufficient to latch material having a
threshold within the threshold range of any preceding scan.
4. A device according to claim 3 wherein each further latching scan
indiscriminately latches (i.e. blinks) any material having a
latching threshold within the given range of any subsequent
latching scan.
5. A device according to claim 1 wherein the time-voltage product
of the voltage pulse of the first latching scan is greater than the
time-voltage product of the voltage pulse of the second latching
scan.
6. A device according to claim 1 wherein latching of said material
is polarity dependent.
7. A device according to claim 6 wherein said first latching scan
latches material using a latching pulse of a first polarity and
said second scan latches material using a latching pulse of an
opposite polarity to the latching pulse of first polarity.
8. A device according to claim 1 wherein said layer of material
comprises a first region arranged to have a latching threshold
within said first range and a second region arranged to have a
latching threshold within said second range.
9. A device according to claim 8 wherein said layer of material
comprises one or more further regions, each of said further regions
having a latching threshold within the given range of a further
latching scan.
10. A device according to claim 1 wherein the layer of material
comprises a plurality of separate electrically addressable
regions.
11. A device according to claim 10 wherein each of the plurality of
separate electrically addressable regions comprise two or more
regions of different latching threshold.
12. A device according to claim 11 wherein the proportion of said
layer of material having regions of different latching threshold is
weighted within each separate electrically addressable region.
13. A device according to claim 1 wherein 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.
14. A device according to claim 13 wherein said at least first and
second latching scans are applied by the addressing means to each
separate electrically addressable area by application of strobe
voltage pulses to said row electrodes and data voltage pulses to
said column electrodes, said strobe and data voltage pulses being
arranged to produce the required resultant voltage pulse at each
separate electrically addressable region.
15. A device according to claim 14 wherein the addressing means
supplies a select or non-select data pulse to latch or not latch
respectively.
16. A device according to claim 14 wherein each row is addressed in
turn with both said first latching scan and said second latching
scan.
17. A device according to claim 14 wherein each row is addressed in
turn with said first latching scan and subsequently each row is
addressed in turn with said second latching scan.
18. A device as claimed in claim 14 wherein the applied data and
strobe waveforms are substantially dc balanced
19. A device according to claim 1 wherein, for each separate
electrically addressable region, the addressing means is arranged
to latch material having a latching threshold within the second
range into the same configuration as material having a latching
threshold with the first range.
20. A device according to claim 1 wherein, for each separate
electrically addressable region, the addressing means is arranged
so as to be capable of selectively latching material having a
latching threshold with the second range into a different
configuration to material having a latching threshold with the
first range.
21. A device according to claim 1 wherein selective latching during
said first and/or said second scan is arranged to partially latch
material having a threshold within said first range or said second
range respectively.
22. A device according to claim 21 wherein a plurality of data
pulses are used to provide said partial latching.
23. A device according to claim 1 wherein the device comprises a
photosensitive layer such that the latching threshold of said layer
of material is variable in response to optical illumination.
24. A device according to claim 1 and further comprising one or
more colour filter elements.
25. A device according to claim 1 that is capable of adopting, and
being electrically latched between, two stable configurations.
26. A device according to claim 1 wherein the layer of material
comprises liquid crystal.
27. A device according to claim 26 wherein the liquid crystal
comprises nematic liquid crystal material.
28. A device according to claim 26 wherein the transition between
said two stable configurations is mediated by an alignment
transition at said first cell wall.
29. A device according to claim 28 wherein the surface of the first
cell wall that is in contact with said layer of liquid crystal
material is profiled so as to provide at least two stable surface
alignment configurations of the liquid crystal material in the
vicinity of said first cell wall.
30. A device according claim 29 wherein the profiled surface of
said first cell wall comprises a bistable surface alignment grating
structure.
31. A device according to claim 29 wherein the surface of the
second cell wall that is in contact with said layer of nematic
liquid crystal material is profiled so as to provide at least two
stable surface alignment configurations of the liquid crystal
material in the vicinity of said second cell wall.
32. A device according to claim 31 wherein the profiled surface of
said second cell wall comprises a bistable surface alignment
grating structure.
33. A device according to claim 31 wherein the latching threshold
between the at least two stable surface alignment configurations of
the liquid crystal material at said first cell wall are greater
than the latching threshold between the at least two stable surface
alignment configurations of the liquid crystal material at said
second cell wall.
34. A device according to claim 33 wherein the latching threshold
of the liquid crystal material at said first cell wall falls within
said first range and the latching threshold of the liquid crystal
material at said second cell wall falls within said second
range.
35. A device according to claim 1 wherein said layer of material
comprises electrophoretic components.
36. A device according to claim 1 wherein said layer of material
comprises droplets of bistable material in a carrier matrix.
37. A device according to claim 36 wherein said droplets are
coloured.
38. A device according to claim 36 wherein said bistable material
comprises droplets of cholesteric material.
39. A device according to claim 36 wherein said bistable material
comprises particulates.
40. A device having a first layer of material according to any
preceding claim and additionally comprising one or more further
layers of material, each of said further layers of material being
disposed between a pair of cell walls and comprising one or more
separate electrically addressable regions, wherein each of the one
or more separate electrically addressable regions of each of said
further layers of material are electrically connected to said
addressing means in parallel with one of the electrically
addressable regions of said layer of material.
41. A device according to claim 40 wherein the first layer of
material and said one or more further layers of material are
arranged in an optical stack.
42. A method of addressing a display device that comprises a
constrained layer of material capable of adopting, and being
electrically latched between, at least two stable configurations,
said layer of material having one or more separate electrically
addressable regions; said method comprising the steps of, (a)
addressing each separate electrically addressable region of said
display device with a first latching scan to selectively latch
material having a latching threshold within a first range, and (b)
subsequently addressing each separate electrically addressable
region of said display device with a second latching scan to
selectively latch material having a latching threshold within a
second range, wherein said first latching scan indiscriminately
latches (i.e. blanks) material having a latching threshold within
said second range into one of said at least two stable
configurations and said second latching scan is insufficient to
latch material having a threshold within said first range.
43. A method according to claim 42 and additionally comprising the
step, after the steps of addressing said device with said first and
second latching scans, of addressing said display with one or more
further latching scans, each further latching scan being arranged
to selectively latch material having a latching threshold within a
given energy range wherein the upper energy of said given energy
range is lower than the upper energy of the energy range of the
preceding latching scan.
44. A method according to claim 42 in which the device comprises a
plurality of separate electrically addressable areas.
45. A method of writing a frame of information to a display device
having two or more stable configurations and comprising a matrix of
separate electrically addressable regions, wherein said method
comprises the step of multiplexing said device using three or more
addressing fields.
46. A method according to claim 45 wherein the first field blanks
said display device.
47. A method according to claim 46 wherein the second field
selectively latches material having a latching threshold within a
first range.
48. A method according to claim 47 wherein the third field
selectively latches material having a latching threshold within a
second range.
Description
[0001] This invention relates to display devices having at least
two stable states and more particularly to matrix addressed devices
and a method of addressing such devices.
[0002] A variety of display devices exist that exhibit two or more
states that are stable in the absence of an applied electrical
field. The most common devices of this type employ liquid crystal
materials. However other devices, such as electrophoretic,
electrochromic, micro-electromechanical systems (MEMS), particulate
displays, are also known.
[0003] Examples of bistable liquid crystal displays include surface
stabilised ferroelectric liquid crystal (SSFLC) devices as
described by N A Clark and S T Lagerwall, Appl. Phys. Lett., 36,
11, 899 (1980). It has also been shown by Berreman and Heffner in
Appl. Phys. Lett., Vol 37, pg109, 1980 and in patent Applications
WO 91/11747 ("Bistable electrochirally controlled liquid crystal
optical device") and WO 92/00546 ("Nematic liquid crystal display
with surface bistability controlled by a flexoelectric effect")
that a nematic liquid crystal material can be switched between two
stable states via the use of chiral ions or flexoelectric coupling.
A bistable nematic display is also described by Dozov (2003) Proc.
SID, pp 946-948. Electrophoretic displays are described by Liang,
R. C in (2003) Proc. SID, pp 838-841, and electrochromic displays
are also known. Powder based displays of the type described by
Hattori et al (2003) Proc. SID, pp 846-849 are also known.
[0004] WO 97/14990 teaches how a zenithally bistable device (ZBD)
may be constructed using a surface alignment grating of a given
design such that nematic liquid crystal molecules can adopt two
stable pretilt angles in the same azimuthal plane. One of these
states is a high pretilt state, whilst the other is a low pretilt
state and a device is described which can adopt, and can be readily
switched between, either of the two stable liquid crystal
configurations. The two zenithally stable liquid crystal
configurations of WO 97/14990 persist after the driving electrical
signals have been removed and the device has been shown to be
highly resistant to mechanical shock, provide microsecond latching
times at low driving voltages (<20V) and allow a high degree of
multiplexability; see E. L. Wood et al. "Zenithal bistable device
(ZBD) suitable for portable applications", Proceedings of SID,
2000, v31, 11.2, p124-127 (2000). Recently, it has also been shown
by Jones et al, (2003) Proc. SID, pp 954-959 that the latching
threshold for such a ZBD device can alter in response to previously
applied voltage pulses (e.g. a d.c. balance or blanking pulse).
[0005] WO99/34251 teaches another ZBD device having a negative
dielectric anisotropy material in a twisted nematic configuration.
Alternative bistable devices are also described in WO 01/40853,
EP1139151A1, EP1139152A1 and EP1139150A1. Patent application WO
02/08825 describes a zenithally stable device exhibiting
multi-stability with more than two stable states.
[0006] Ferroelectric and ZBD devices of the type described above
operate using polar latching. In other words, a pulse of a first
polarity (say a positive pulse) causes latching into the first
stable state whilst a pulse of a second polarity (e.g. a negative
pulse) causes latching into the second stable state. Bistable
cholesteric and bistable twisted nematic display operate using flow
effects; in such devices the pulse shape, rather than polarity, can
be selected to switch to any one of the stable states.
[0007] To provide displays having a number of separately
addressable elements it is common to construct a liquid crystal
device with a series of row electrodes on one cell wall and a
series of column electrodes on the other cell wall. In this way a
matrix of separately addressable elements is formed and a given
voltage can be applied to each individual element of the device by
applying certain voltages to given rows and columns. The technique
of applying appropriate column and row voltage waveforms to
individually latch each element of the display in turn is commonly
termed multiplexing.
[0008] It should be noted that herein the terms row and columns are
not intended to restrict the waveforms to application to a
particular set of electrodes. Rather the terms are used simply to
distinguish the two sets of electrodes and could be consistently
interchanged throughout. Also, other electrodes are possible, from
alphanumeric characters, to axial and radial circular electrodes.
Arrangements also exist for in-plane electrodes used either on
their own or together with out of plane electric fields. Bistable
liquid crystal devices conventionally use the so-called `line at a
time` multiplexing scheme. Data is continuously applied to one set
of electrodes (e.g. columns) during the time taken to write an
entire frame and the other set of electrodes (e.g. rows) are
sequentially addressed with a strobe voltage. The resultant
waveform at a pixel (i.e. the combination of the strobe and data
waveforms) either causes latching of the pixel into the opposite
state (i.e. when a select data pulse is applied), or leaves it
unchanged (i.e. when a non-select data pulse is applied).
[0009] Two general types of line at a time addressing schemes are
known; two field addressing and blanking. With two-field operation
the frame is divided into two fields for addressing the black
pixels and white pixels separately; known waveforms for
implementing such a scheme are shown in FIG. 1. With blanked
operation, the addressing pulse is preceded some time earlier by a
blanking pulse that indiscriminately selects one state regardless
of the applied data. Known waveforms to implement a blanked scheme
are illustrated in FIG. 2. The blank may be applied a line or lines
ahead, or may be applied for a whole frame or a given number of
lines.
[0010] As described above, the combination of the strobe voltage
pulse with a select data pulse causes latching whilst the
combination of the strobe voltage pulse with a non-select data
pulse does not cause latching. The voltage and duration of the
select and non-select data waveforms are conventionally chosen to
ensure that, when combined with the strobe pulse, a pixel is either
latched or not as required; i.e. each pixel is selectively latched.
The data and strobe pulses are thus chosen in light of the latching
response of the particular display being addressed.
[0011] The latching response of a typical prior art bistable liquid
crystal device is illustrated in FIG. 3. In this example,
electrical pulses of the correct polarity and sufficient energy
begin to nucleate domains of one state causing (say) a change in
the reflectivity. These small domains remain after the pulse with
no further change in reflectivity. Applying pulses that are more
energetic causes further domains to be created, and the area of the
new state to increase. The reflectivity increases from (say) 0% to
10% at (.tau.V).sub.10 and 90% at (.tau.V).sub.90 and eventually
the pixel is fully latched into the new state. The width of the
latching transition is termed partial latching. By convention, this
is taken as the width of the transition measured on an area of the
display that cannot be resolved by unaided eye.
[0012] Furthermore, as shown schematically in FIG. 4, two or more
areas of a display panel 2 may have different latching
characteristics. For example, a first area 4 may have a different
latching threshold to a second area 6. The latching curve
differences across the cell may arise from unwanted variations such
as random variations in the alignment and surface energy, or longer
range variations of cell gap, voltage (e.g. through resistive
losses along the electrodes, or through differences between driver
circuits), and temperature. The response for the first area 4 and
the second area 6 are shown in FIG. 5 as curves 8 and 10
respectively. Again, the dotted lines represent the 10% latching
point and the solid lines the 90% latching point.
[0013] An example of how the strobe voltage (V.sub.s) and data
voltage (V.sub.d) would conventionally be selected in this instance
is indicated by the dotted lines 12 in FIG. 5. Here, the magnitude
of V.sub.s and V.sub.d are chosen so that the resultant
V.sub.s+V.sub.d causes the desired change of state for both the
first and second areas, whereas V.sub.s-V.sub.d is insufficient to
latch either area. Clearly, this means that V.sub.d is required to
be high, to counter variations across the cell. If there were no
variation, V.sub.d could approach the partial latch width of the
device. The magnitude of the required V.sub.d is given by: 1 V d V
100 % ( Area1 ) - V 0 % ( Area2 ) 2 ( 1 )
[0014] which can typically lie between 4V and 8V (depending on the
size of the display).
[0015] A consequence of the relatively high data voltage (e.g. 4V
to 8V) is a high power consumption of the display panel. For
example, assuming a polar response of the type described above the
data waveforms are usually bipolar pulses (+/- and -/+) to ensure
do balance. With a large number of rows, the energy A) dissipated
by a panel of cell gap f, capacitance C is dominated by the data
waveform voltage (V.sub.d) and lies within the range of:
1/2f.CV.sub.d.sup.2.ltoreq.E.ltoreq.f.CV.sub.d.sup.2 (2)
[0016] depending on the image displayed. It can thus be seen that a
data voltage has a significant effect on the power consumption of
the panel.
[0017] Furthermore, applying a high data voltage to each column can
also have a detrimental effect on the optical properties of rows
other than the line being addressed at any one particular time. As
described above, bistable cholesteric and bistable twisted nematic
displays are caused to latch by flow effects, whilst ferroelectric
and zenithal bistable device are caused to latch using polar
switching. However, most bistable liquid crystal devices will also
respond to the RMS signal of the applied field in addition to any
polar latching response. For a positive dielectric anisotropy
material the director orients parallel to the applied field,
whereas for a negative material, the liquid crystal orients
perpendicular to the field. The amount of liquid crystal
reorientation to an RMS signal is related directly to the elastic
constants of the liquid crystal and inversely to the dielectric
anisotropy.
[0018] In such devices, the re-orientation of the director caused
by the RMS effect can cause an undesirable change in orientation
(and hence optical transient) when addressing the display. For
example, areas of the display that are dark before and after the
addressing signal may change to light during the addressing signal,
causing a distracting "flash" of the image. To help reduce this
effect, materials are chosen so that the Fredericksz transition is
at a high voltage and the gradient of the transition is low.
However, this can severely limit the choice of material for a
device.
[0019] A further problem associated with the use of high data
voltages, and present in zenithal bistable devices, is the effect
of any RMS voltage leading to unwanted latching of one (for
positive dielectric anistropy mixtures used in a ZBD device, the
continuous) state. This effect is termed "growback". Similar
effects also occur in other devices, for example the bistable
twisted nematic device.
[0020] Although bistable displays only have two stable states,
greyscale can be provided in a number of ways. For example,
greyscale can be achieved using temporal and/or spatial dither
where the perception of grey-levels is provided by switching each
pixel "on" and "off" at a rate faster than the viewer can perceive
or by dividing each pixel into two or more weighted and separately
addressable sub-pixel regions.
[0021] Employing spatial and/or temporal dither techniques in
bistable display devices is at the expense of increased complexity,
and hence unit cost. For example spatial dither increases the
number of row and column drivers, requires thinner tracks thereby
increasing track resistance and resistive powers losses in the
panel and also requires more accurate etching to ensure linearity
of the greyscale response. Increasing the number of separately
addressable regions also increases the proportion of the display in
which inter-pixel gaps are present; this reduces the aperture ratio
of the device. It is for these reasons, that passively addressed
bistable devices known to those skilled in the art are, for the
present at least, somewhat limited in there ability to produce high
numbers of grey-levels and moving video images.
[0022] Analogue (or domain) greyscale is also known. This is where
partial (i.e. incomplete) switching of domains within a separately
addressable pixel area is used so that different grey-levels can be
formed from varying the number and/or size of domains in the pixel.
This has previously been used in ferroelectric liquid crystals and
bistable cholesterics. For example, see GB 2315876 where the
addition of balls to provide nucleation sites for analogue grey
scale is described.
[0023] The principal disadvantage associated with the use of domain
grey-scale is that there is no operating window for the addressing
waveform; that is, each grey-level is achieved with a specific
addressing waveform. Ensuring the desired waveform is applied to a
particular pixel is problematical because changes to the waveform
applied to the rows and/or columns may arise due to losses along
the resistive electrodes, variations caused by the temperature of
the driving circuitry (which will depend on use and therefore will
vary across the panel) or batch differences for the driving
circuits. Changes in the response of the liquid crystal to the same
field may also occur across the device arising, for example, from
variations of cell gap, thickness of alignment layer, cell
temperature, alignment of the liquid crystal, and possibly image
history. Any such deviations cause a change in the electro-optic
response, and hence an error in the observed analogue grey
level.
[0024] It is also known to provide devices having multiple
thresholds in order to attain analogue greyscale. In such devices,
each pixel is sub-divided into areas which respond differently to
applied electric fields; for example by forming holes in the
electrodes, including passive dielectric layers or inducing
alignment variations etc. One example of inducing multiple
threshold is provided in Bryan-Brown et al, (1998) proceedings of
Asia Display, p1051-1052, where it is demonstrated that grey-scale
may be achieved in a zenithally bistable device using gratings of
different pitch and shape that allow partial switching of an area
of a pixel. A similar analogue greyscale technique is described for
SSFLCs in Bonnett, Towler, Kishimoto, Tagawa and Uchida (1997)
"imitations and performance of MTM Greyscale for FLCs", Proceedings
of the 18th International Displays Research Conference,
LA6-LA7.
[0025] Analogue addressing of multiple threshold bistable LCDs such
as those described above, still relies on line-at-a-time addressing
as described above for a two-state bistable device: a strobe signal
is applied to a first electrode (say, the rows) and appropriate
data signal applied to the set of electrodes on the opposite
internal surface of the display (say, the columns). Selection
between the different states is achieved by modulating the data
signal so that it combines with the strobe to latch one or more of
the sub-pixel regions, leading to the desired intermediate
state.
[0026] Addressing multiple thresholds exacerbates the problems
described above that are associated with the use of a high data
voltage. For example, assume the lowest data voltage required to
ensure discrimination between two data signals (i.e. .+-.Vd=latch
and not-latch) across an entire panel due to random variations is
given by .vertline.Vd.sub.min.ve- rtline.. For m grays to be
discriminated (ie non-overlapping addressing windows) the data
voltage must be (m-1)Vd.sub.min. Take, for example, the case where
4V data is needed to display a black and white image across a whole
panel. If the each pixel is subdivided into three areas to provide
four separately distinguishable and addressable analogue levels,
the voltage swing required to ensure that each grey is addressed at
all parts of the panel is then increased to 12V. This adds expense
to the cost of the drivers required, the power dissipated during
update is high, and the contrast of the panel is severely reduced
by unwanted "flash" whilst addressing.
[0027] According to a first aspect of the invention, a device
comprises a layer of material disposed between first and second
cell walls and is capable of adopting, and being electrically
latched between, at least two stable configurations, said layer of
material comprising one or more separate electrically addressable
regions and said device further comprising addressing means to
write to each of said electrically addressable regions using
voltage pulses to selectively latch said layer of material as
required, wherein the addressing means writes to each of said one
or more separate electrically addressable region using at least
first and second latching scans, said first latching scan being
arranged to selectively latch material having a latching threshold
within a first range and said second latching scan being arranged
to selectively latch material having a latching threshold within a
second range, 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 having a latching threshold
within said first range.
[0028] In this manner, the first latching scan will selectively
latch material within each electrically addressable region that has
a threshold within the first range. In other words, the required
stable configuration of material having a threshold within the
first range will be selected by the addressing means during the
first scan; this may require application of a voltage pulse
sufficient to provide latching (i.e. to alter the material from one
stable configuration to another), or a voltage pulse insufficient
to latch the material, depending on the initial configuration of
the material and the final desired configuration. A certain
required pattern (e.g. an image to be displayed) is thus written
across the device to any regions thereof having a latching
threshold within the first range. Preferably, the first latching
scan will also indiscriminately latch (i.e. blank) material having
a latching threshold within said second range.
[0029] After application of the first latching scan, the second
latching scan is applied. This second latching scan is arranged to
selectively latch material having a threshold within the second
range into the required state. In other words, the required stable
configuration of material having a threshold within the second
range will be selected by the addressing means during the second
scan; this may require application of a voltage pulse sufficient to
provide latching or to prevent latching depending on the initial
configuration of the material. The second latching scan is arranged
so it does not latch material having a threshold within the first
range. A certain pattern (e.g. an image to be displayed) is thus
written across the device to any regions thereof having a threshold
within the second range.
[0030] Further scans may also be applied, as described below, but
each scan follows the basic rule of latching material within a
selected range but having no latching effect on material within the
threshold ranges of preceding scans. In this manner the entire
layer of material on the display is sequentially addressed by a
series of two or more latching scans that latch regions of reducing
latching threshold (e.g. by applying a voltage pulse of reduced
duration or voltage).
[0031] It should be noted that when a polar bistable device (e.g. a
ZBD device) is used, the latching threshold within the first and
second ranges will each comprise sub-ranges defining the
continuous-to-defect and defect-to-continuous transitions (e.g. see
FIG. 12). One such transition (say, defect-to-continuous) will be
over a sub-range of a first (say, positive) polarity whilst the
second sub-range (say, continuous-to-defect) will be over a
sub-range of a second (say, negative) polarity. However, for a
particular device, defining one such transition (e.g.
continuous-to-defect) inherently defines the second transition
(e.g. defect-to-continuous).
[0032] Selective latching during the second latching scan may have
some overlap with selective latching of the first latching scan. An
overlap can advantageously be used to overcome problems associated
with asymmetry in the latching responses as described below with
reference to FIG. 12 to 14. The term "first range" as used herein
thus means any material with a latching threshold within a range
that is selectively latched by said first scan but is not latched
by said second scan.
[0033] The device may have defined regions in which the material
latches within the first and second (and any subsequent) ranges, or
such threshold changes may arise from manufacturing or other random
variations. The technique may also be used to allow devices to be
addressed over larger temperature ranges. For example, at a first
temperature the whole display may have a threshold within said
first range whereas at a second temperature the whole display may
have a threshold within the second range. As the temperature of the
device varies from the first temperature to the second temperature,
the proportion of the device being selectively latched by the first
latching scan will gradually reduce from one hundred percent to
zero.
[0034] The use of multiple scans in accordance with this invention
to address a device having two or more stable configurations thus
provides a number of advantages. Firstly, in the case of a device
having a number of regions of different latching threshold, the
different latching regions can be addressed sequentially using the
two or more scans. This allows the number of regions that can be
separately addressed to be greater than the number of separate
electrically addressable regions thereby reducing the interpixel
gap and/or providing a device of higher resolution. The two
different latching regions can be deliberately fabricated or may
arise from random variations. If the regions are deliberately
fabricated, the ratio of the different latching regions within each
separate electrically addressable region can be controlled or
"weighted" to provide greyscale as described in more detail
below.
[0035] In the case of a multiplexed device, the use of two or more
scans in accordance with the invention will reduce the data voltage
required for each scan. As illustrated by equation (2) above,
reducing the data voltage will significantly reduce the power
consumption of the device even though the number of scans is
increased. In fact, the present invention allows the minimum data
voltage that is required to address the entire device to be reduced
by a factor approaching the number of scans. Hence, the use of a
first latching scan and a second latching scan would almost halve
the power consumption of a device compared to a prior art single
scan. A number of further advantages associated with a reduced data
voltage include a reduction in optical transients or so-called
"flash" and less "growback" into one state from another.
[0036] Furthermore, the present invention can be used to increase
the range of latching thresholds over which the device can be
operated. This means, for example, an image can be written to the
device across a wider temperature range (allowing for instance
displays to be operated without temperature sensors), or used in
combination with a manufacturing processes that has wider
batch-to-batch/panel-to-panel variations than acceptable using
prior art devices. It would be appreciated by the skilled person
that reducing power consumption and increasing the range of
thresholds within which the device can operate are complementary.
Increasing the range of device operation is at the expense of an
increased number of scans or an increased data voltage per latching
scan.
[0037] Advantageously, said first latching scan comprises an
initial blanking waveform to latch all of said material into one of
said at least two stable configurations. In other words, a blanked
addressing scheme may be used. The blanking scan may comprise a
mono-polar, a bi-polar pulse, or a pulse of any required shape. A
variety of such blanking pulses, which are preferably dc balanced,
are known and used in prior art blanked addressing schemes. A
further blanking waveform may also be applied between the first and
second latching scans. The further blanking is arranged to
indiscriminately latch material having a threshold within the
second range, but to have no effect on the material selectively
latched during the first scan (i.e. material having a threshold
within the first range). Alternatively, the blanking effect of the
first latching scan on material having a latching threshold within
the second range may negate the need for the further blanking
pulse.
[0038] Conveniently, the addressing means applies one or more
further latching scans after application of said second latching
scan, wherein each further latching scan is arranged to selectively
latch material having a latching threshold within a given range but
is insufficient to latch material having a threshold within the
threshold range of any preceding scan. It is also preferred that
each scan indiscriminately latches (i.e. blanks) any material
having a latching threshold within the given range of all
subsequent latching scan.
[0039] Preferably, latching of said material is polarity dependent.
The desired state in polarity dependent materials can be selected
using a voltage pulse of the appropriate magnitude, duration and
polarity. Example of such devices include the ZBD and SSFLC devices
described above in which a pulse of (say) positive polarity latches
into a first state whilst a pulse of negative polarity latches into
a second state. It should also be recognised that the invention is
equally applicable to non-polarity dependent devices such as
bistable cholesteric and bistable twisted nematic devices where
latching between states is controlled by pulse shape. It is
advantageous, when using polarity dependent materials, for said
first latching scan to latch material using a latching pulse of a
first polarity and said second scan to latch material using a
latching pulse of an opposite polarity to the latching pulse of
first polarity.
[0040] It should be noted that each latching pulse may be combined
with one or more further pulses of opposite polarity to ensure dc
balance is maintained. For example, each latching pulse (i.e. the
pulse that causes the selective latching) may be preceded or
followed by a pulse of opposite polarity that is shaped so as not
to cause latching. Alternatively, the first latching scan and the
second latching scan may comprise latching pulses of the same
polarity (e.g. positive) and a blanking pulse of the opposite
polarity (e.g. negative) is applied between said first and second
scans to blank (only) material having a threshold within the second
range. DC balancing may also be performed over several scans.
[0041] Conveniently, said layer of material comprises a first
region arranged to have a latching threshold within said first
range and a second region arranged to have a latching threshold
within said second range. A number of techniques may be used to
provide different threshold regions within a device, as described
above. In addition to altering the properties of the material, for
example by controlling alignment of material at the surface, the
electrode properties may be altered. The provision of separate
latching regions of different and known thresholds enables driving
from the same (scan line) driving circuit, thereby reducing the
number of electronic drivers required and hence the cost of the
panel.
[0042] Advantageously, when further (i.e. third and subsequent)
latching scans are applied, said layer of material comprises one or
more further regions, each of said further regions having a
latching threshold within the given range of a further latching
scan.
[0043] Conveniently, the layer of material comprises a plurality of
separate electrically addressable regions. Advantageously, each of
the plurality of separate electrically addressable regions comprise
two or more regions of different latching threshold. The proportion
of said layer of material having regions of different latching
threshold may advantageously be weighted within each separate
electrically addressable region. This allow spatial dither to be
implemented.
[0044] It should be appreciated that the provision of regions
having different latching threshold within a device is known.
However, prior art devices exploit such threshold differences using
analogue greyscale techniques. Analogue greyscale applies a
latching voltage within a predetermined range in a single scan; a
proportion of the separate electrically addressable region is
latched depending on the voltage applied. Prior art analogue
greyscale thus provides no ability to selectively latch material of
different thresholds and within a single separate electrically
addressable area to different state. It should be noted that the
present invention may thus be combined with analogue greyscale
techniques. For example, the first latching scan could be arranged
to cause partial latching of material having a threshold within the
first range. Similarly, the second and any further latching scans
could be arranged to cause partial latching.
[0045] It is preferred that 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. In such an arrangement, said at least first and second
latching scans are advantageously applied by the addressing means
to each separate electrically addressable area by application of
strobe voltage pulses to said row electrodes and data voltage
pulses to said column electrodes, said strobe and data voltage
pulses being arranged to produce the required resultant voltage
pulse at each separate addressable region. This allows the
implementation of so-called line at a time addressing.
[0046] Conveniently, the energy or time-voltage product of the
voltage pulse of the first latching scan is greater than the energy
of the voltage pulse of the second latching scan. The energy may be
varied from scan to scan by reducing the pulse voltage, the pulse
width or both the voltage and pulse width. Different properties may
also be varied between subsequent scans; for example, the voltage
of the strobe pulse could be reduced from the first scan to the
second scan whilst the pulse duration is reduced (or the pulse
shape changed etc) from the second scan to the third scan. It would
be appreciate that, for certain devices the energy of the pulse may
be invariant and the pulse shape or timing (e.g. delay) changed to
alter the latching effect of the pulse.
[0047] Conveniently, the addressing means is arranged to supply
select or non-select data pulses to latch or not latch
respectively. In other words, line at a time addressing can be
implemented using select and non-select data pulses in combination
with a strobe pulse to provide select and non-select resultant
pulses.
[0048] Advantageously, each row is addressed in turn with both said
first latching scan and said second latching scan (i.e.
sequentially addressed). Alternatively, it may prove advantageous
in certain circumstances with said first latching scar and
subsequently for every row to be addressed with said second
latching scan (i.e. subsequently addressed).
[0049] Furthermore, a combination of subsequent and sequential
addressing may be used so that a proportion of the display is
latched by the first and second latching scans, and then a second
portion of the display is latched by the first and second latching
scans. For example, the top half of the display me be latched using
the first and second latching scans, followed by latching of the
bottom half of the display by the first and second latching
scans.
[0050] In order to prevent charge build up over time, it is
preferred that the applied data and strobe waveforms are
substantially dc balanced.
[0051] A skilled person would recognise that the present invention
can be implemented using any one of a number of different
addressing schemes. In fact, the majority of prior art blanked and
two field schemes could be adapted in accordance with the present
invention.
[0052] Advantageously the device is configured such that, for each
separate electrically addressable region, the addressing means is
arranged to latch material having a latching threshold within the
second range into the same configuration as material having a
latching threshold with the first range. In this manner the
required pattern can be written to each separately electrically
addressable region of the device irrespective of any variations in
the latching threshold across the device. In the case of a
multiplexed device this reduces the data voltage required, and the
power consumption of the device can thus be reduced as described
below.
[0053] Alternatively, the device may be advantageously configured
such that, for each separate electrically addressable region, the
addressing means is arranged so as to be capable of selectively
latching material having a latching threshold with the second range
into a different configuration to material having a latching
threshold with the first range. Hence, any material within a
separate electrically addressable regions and having a threshold
within the first range can be selectively latched into a stable
configuration different to that adopted by material within that
region having a second threshold. This has numerous advantages,
especially when each separate addressable region is fabricated with
well defined areas of different latching threshold. In particular,
such an arrangement allows the amount of scan electronics and
electrodes to be reduced in a device whilst maintaining the same
number of elements that can be latched into the state required.
[0054] Conveniently, selective latching during said first and/or
said second scan is arranged to partially latch material having a
threshold within said first range or said second range
respectively. In other words, known analogue greyscale techniques
can be used in conjunction with the present invention.
[0055] The device may advantageously comprise a photosensitive
layer such that the latching threshold of said layer of material is
variable in response to optical illumination. For example, a
photo-conductive layer may be included in the device to enable the
latching threshold of the layer of material to be alter. This would
allows the threshold to be altered between frames, and could be
used to control the amount of said layer of material that forms the
first region having a latching threshold within said first range
and/or, the amount of said layer of material that forms the second
region having a latching threshold within said second range.
[0056] Colour filter elements may also be advantageously
provided.
[0057] Advantageously, the layer of material comprises liquid
crystal such as a nematic liquid crystal material. Herein, the term
nematic shall include long pitch cholesteric material. Chiral
dopants may also be included in the nematic liquid crystal to
impart any required twist.
[0058] Advantageously, the transition between said two stable
configurations is mediated by an alignment transition at said first
cell wall.
[0059] Conveniently, the surface of the first cell wall that is in
contact with said layer of nematic liquid crystal material is
profiled so as to provide at least two stable surface alignment
configurations of the liquid crystal material in the vicinity of
said first cell wall. A number of known techniques may be used to
provide the profiled surface, such as photolithography or the
embossing of a deformable material.
[0060] A mono-stable surface treatment may be applied to the
internal surface of the second cell wall. For example, a
homeotropic surfactant or a planar homogenous layer such as a
rubbed polymer.
[0061] The profiled surface of said first cell wall conveniently
comprises a bistable surface alignment grating structure.
Alternatively, any suitably profiled bistable surface, for example
of the type described in WO 01/40853, EP1139151A1, EP1139152A1 or
EP1139150A1, may be employed.
[0062] Advantageously, the surface of the second cell wall that is
in contact with said layer of nematic liquid crystal material is
profiled so as to provide at least two stable surface alignment
configurations of the liquid crystal material in the vicinity of
said second cell wall. Again, the profiled surface of said second
cell wall advantageously comprises a bistable surface alignment
grating structure.
[0063] Advantageously, the device may be arranged to provide a
.pi.-cell configuration.
[0064] In such a pi-cell configuration, the layer of liquid crystal
material is 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,
wherein 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.
[0065] 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 pi-cell device may advantageously be arranged so that the first
state and/or the second state persist in the absence of an applied
electric field.
[0066] The pi-cell of 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.
[0067] The pi-cell device 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.
[0068] 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. In a device, any
configuration adopted by liquid crystal material can be described
using the three deformation components (i.e. splay, bend and
twist).
[0069] 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.
[0070] 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 has zero splay
and thus falls within the definition of a substantially non-splayed
state.
[0071] 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.
[0072] Advantageously, the first state of the pi-cell 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.
[0073] 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 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.
[0074] 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 3/4 that distance.
[0075] Conveniently, when the pi-cell is 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.
[0076] Electrical addressing signals are applied to the pi-cell
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.
[0077] 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" pi-cell device is provided
in which both surface can impart two or more different surface
pretilt angles to the liquid crystal material.
[0078] 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.
[0079] Advantageously, in a double ZBD pi-cell 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.
[0080] 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.. 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. Conveniently,
the tilt at the cell mid-point is greater than 5.degree..
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..
[0081] 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.
[0082] It is advantageous, for said second state to be 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.
[0083] 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.
[0084] 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.
[0085] 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.
[0086] The pi-cell device may also comprise 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.
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.
[0087] A pi-cell liquid crystal device may also be provided in
which each of the switched states persist in the absence of an
applied electric field.
[0088] Furthermore, a pi-cell device may comprise 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.
[0089] In addition, the pi-cell device may comprise 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.
[0090] Conveniently, the latching threshold between the at least
two stable surface alignment configurations of the liquid crystal
material at said first cell wall are greater than the latching
threshold between the at least two stable surface alignment
configurations of the liquid crystal material at said second cell
wall. Furthermore, it is preferable that the latching threshold of
the liquid crystal material at said first cell wall falls within
said first range and the latching threshold of the liquid crystal
material at said second cell wall falls within said second
range.
[0091] It should be appreciated that two or more of said at least
two stable configurations are preferably optically distinguishable.
A skilled person would recognise how the use of polarisers,
retardation films etc could provide optical contrast between the
various configurations. Preferably, the layer of material used is
bistable. Alternatively, the material may advantageously comprise
three or more stable configurations. Electrophoretic particles (for
example, a charged particle or a charged droplet of liquid) may
also be included in the layer of material.
[0092] The layer of material may advantageously comprise droplets
of bistable material in a carrier matrix. The droplets may be
coloured, and may comprises cholesteric liquid crystal material.
The material may alternatively comprises particulates.
[0093] A device having a first layer of material as described above
may additionally comprising one or more further layers of material,
each of said further layers of material being disposed between a
pair of cell walls and comprising one or more separate electrically
addressable regions, wherein each of the one or more separate
electrically addressable regions of each of said further layers of
material are electrically connected to said addressing means in
parallel with one of the electrically addressable regions of said
layer of material. The first layer of material and said one or more
further layers of material may be advantageously arranged in an
optical stack. In this manner, an optical stack device is provided
of the type described in more detail below.
[0094] According to a second aspect of the invention, a method of
addressing a display device that comprises a constrained layer of
material capable of adopting, and being electrically latched
between, at least two stable configurations, said layer of material
having one or more separate electrically addressable regions is
provided. The method comprises the steps of (a) addressing each
separate electrically addressable region of said display device
with a first latching scan to selectively latch material having a
latching threshold within a first range, and (b) subsequently
addressing each separate electrically addressable region of said
display device with a second latching scan to selectively latch
material having a latching threshold within a second range, wherein
said second latching scan is insufficient to latch material having
a threshold within said first range.
[0095] Conveniently, said first latching scan indiscriminately
latches (i.e. blanks) material having a latching threshold within
said second range into one of said at least two stable
configurations.
[0096] Advantageously, the method additionally comprising the step,
after the steps of addressing said device with said first and
second latching scans, of addressing said display with one or more
further latching scans, each further latching scan being arranged
to selectively latch material having a latching threshold within a
given energy range wherein the upper energy of said given energy
range is lower than the upper energy of the energy range of the
preceding latching scan.
[0097] Conveniently, the device comprises a plurality of separate
electrically addressable areas.
[0098] According to a third aspect of the invention, a method is
provided for writing a frame of information to a display device
having two or more stable configurations and comprising a matrix of
separate electrically addressable regions, wherein said method
comprises the step of multiplexing said device using three or more
addressing fields. The first field advantageously blanks said
display device, and the second field may selectively latches
material having a latching threshold within a first range, whilst
the third field selectively latches material having a latching
threshold within a second range.
[0099] The invention will now be described, by way of example only,
with reference to the following drawings in which;
[0100] FIG. 1 shows a prior art two-field addressing scheme,
[0101] FIG. 2 shows prior art blanked addressing scheme,
[0102] FIG. 3 illustrates the partial latching region of a prior
art bistable liquid crystal device,
[0103] FIG. 4 shows the random variations across a prior art panel
that can provide areas of two different latching ranges,
[0104] FIG. 5 shows the principle of data voltage selection
according to the prior art,
[0105] FIG. 6 illustrates a multi-scan technique according to the
present invention,
[0106] FIG. 7 shows a panel constructed in accordance with the
present invention,
[0107] FIG. 8 shows a cross-section of the panel of FIG. 7 along
the line II-II,
[0108] FIG. 9 illustrates the use of multiple scans to address a
device having a continuum of transitions,
[0109] FIG. 10 shows a five stage multi-scan technique,
[0110] FIG. 11 illustrates how variation of strobe pulse width may
be used in the multi-scan technique,
[0111] FIG. 12 shows typical ZBD latching curves with no variation
in asymmetry,
[0112] FIG. 13 shows ZBD latching curves in which asymmetry is not
retained,
[0113] FIG. 14 provides an expanded plot of four switching-regions
of a ZBD device,
[0114] FIG. 15 gives and expanded view of the display of FIG. 7
when addressed in accordance with the present invention,
[0115] FIG. 16 provides examples of row and data signals that can
be used to implement the present invention,
[0116] FIG. 17 shows strobe and data signals that can provide three
latching scans in accordance with the present invention,
[0117] FIG. 18 shows how each scan of FIG. 17 can be applied to the
whole display in turn,
[0118] FIG. 19 shows how each of the three latching scans of FIG.
17 can be applied to each line in turn
[0119] FIG. 20 shows the measured latching response of a ZBD
cell,
[0120] FIG. 21 shows measured defect-to-continuous and
continuous-to-defect transitions for a cell comprising regions of
different grating pitch,
[0121] FIG. 22 is a series of photomicrographs showing latching,
using multiple scans, of a cell comprising regions of different
grating pitch,
[0122] FIG. 23 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,
[0123] FIG. 24 shows photomicrographs of two ZBD cell areas of 0.6
.mu.m and 0.8 .mu.m pitch addressed using the present
invention;
[0124] FIG. 25 shows the electro-optic response of a double ZBD
device, FIG. 26 shows how a double ZBD device can be addressed
using multiple scans from a first blanked state,
[0125] FIG. 27 shows how a double ZBD device can be addressed using
multiple scans from a second blanked state, and
[0126] FIG. 28 shows an exploded view of a double ZBD device
fabricated in accordance with the present invention,
[0127] FIG. 29 shows the operation of a prior art pi-cell
device,
[0128] FIG. 30 illustrates the operation of a ZBD surface pi-cell
of the present invention,
[0129] FIG. 31 illustrates the operation of another ZBD surface
pi-cell of the present invention,
[0130] FIG. 32 shows in more detail the prior art transition from a
splayed state to a bend state,
[0131] FIG. 33 shows a counter example of a prior art ZBD device in
which surface switching does not occur when forming a bend
state,
[0132] FIG. 34 illustrates a pi-cell double ZBD device in
accordance with the present invention,
[0133] FIG. 35 shows examples of substantially non-splayed
states,
[0134] FIG. 36 show various splayed states,
[0135] FIG. 37 shows the energies of defect and continuous states
in a ZBD device, and
[0136] FIG. 38 shows rms operation of a device of the present
invention,
[0137] FIG. 39 shows a cholesteric device operable in accordance
with the present invention, and
[0138] FIG. 40 shows a multi-layer stack device operable in
accordance with the present invention.
[0139] Referring to FIG. 6, the principle of the present invention
is illustrated. The pulse duration versus voltage plot of FIG. 6
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.
[0140] In accordance with the invention, 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 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.
[0141] 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.
[0142] 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.
[0143] 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: 2 V d ( V
100 % - V 0 % 2 ) Area1 ( V 100 % - V 0 % 2 ) Area2 ( 3 )
[0144] 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.
[0145] 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.
[0146] FIG. 7 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.
[0147] A cross-sectional view along the line II-II of the panel
shown in FIG. 7 is given in FIG. 8. Referring to FIG. 8, 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.
[0148] 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.
[0149] 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.
[0150] 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.
[0151] 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. 7 and 8, 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).
[0152] 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.
[0153] 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.
[0154] 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.
[0155] 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.
[0156] FIG. 9 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.
[0157] 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.
[0158] In the case of a device having the properties shown in FIG.
9, the data voltage required to ensure all areas of the panel are
addressed properly is given by: 3 V d V 100 % ( Max ) - V 0 % ( Min
) 4 ( 4 )
[0159] This is half the power needed by typical prior art schemes
as given by equation (2).
[0160] 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 it scans the data voltage is
correspondingly reduced by a factor of n: 4 V d V 100 % ( Max ) - V
0 % ( Min ) 2 n ( 5 )
[0161] The maximum number of scans that is considered worthwhile
is: 5 n = V 100 % ( Max ) - V 0 % ( Min ) 2 ( V 100 % - V 0 % ) ( 6
)
[0162] where V.sub.100%-V.sub.0% is the inherent partial latch
width of a microscopic region.
[0163] With n-line scanning the energy per update is then in the
range: 6 1 2 n f C ( V d n ) 2 E n f C ( V d n ) 2 ( 7 )
[0164] 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.
[0165] FIG. 10 shows how, for the continuum of transitions shown in
FIG. 9, 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.
[0166] Referring to FIG. 11, 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.
[0167] Referring to FIG. 12, 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.
[0168] 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. 12 may be latched into the defect state by the
following procedure:
[0169] (i) Blanking the whole device into the defect state by
applying a blanking pulse 124 of negative polarity.
[0170] (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.
[0171] (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.
[0172] 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.
[0173] 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.
[0174] The effect of asymmetry in latching response on the
multi-scan technique of the invention is illustrated in FIG. 13. 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.
[0175] The device of FIG. 13 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.
[0176] FIG. 14 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.
[0177] 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.
[0178] 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.
[0179] 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).
[0180] 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
[0181] 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
[0182] 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 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
[0183]
4TABLE 4 Two field high then low/no alternation/data inversion
Desired Initial Final Data Data State 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
[0184] 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
[0185] 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.
[0186] 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).
[0187] 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
Desired (and low Initial Final 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
[0188] 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
[0189] 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
[0190]
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
[0191] 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
Initial Final Blank State 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
[0192] FIG. 15 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.
[0193] FIG. 15a 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. 7 and 8. 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.
[0194] FIG. 15b 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.
[0195] In FIG. 15c, 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.
[0196] FIG. 15d show 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.
[0197] FIGS. 15e and 15f 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.
[0198] 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.
[0199] 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.
[0200] 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.
[0201] 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.
[0202] An example of a scheme used to address a single row using
the method described in table 7 is shown in FIG. 16. 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.
[0203] 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.
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.
[0204] Referring to FIG. 17, 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.
[0205] FIG. 18 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. 19 show an alternative arrangement in which
each line is latched using the three scans before the three scans
are applied to the next line.
[0206] It should be noted that a combination of the schemes shown
in FIGS. 18 and 19 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.
[0207] 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.
20. 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.
[0208] 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.
[0209] 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. 21. The dashed and
solid lines in the figure show 10% and 90% levels of switching
respectively. In particular FIG. 21a shows the various
continuous-to-defect latching transitions whilst FIG. 21b shows the
various defect-to-continuous transitions.
[0210] 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 0.61 .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. 21 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.
[0211] 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.
[0212] 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.
[0213] Referring now to FIG. 22, the effect of the multi-scan
technique on the test cell is shown. FIG. 22d shows the pitch (in
.mu.m) of the grating in the different areas of the test cell.
[0214] 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. 22a as
areas of shorter pitch (therefore lower threshold voltage) are
switched into the continuous (black) state.
[0215] 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.
[0216] FIG. 22(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.
[0217] 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. 22(c), although the device is
unchanged by the second scan.
[0218] 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.
[0219] 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.
[0220] The areas with grating pitch 0.61 .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. 23a 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.
23b 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. 23a) and the second scan by the second arrow 202 (FIG.
23b).
[0221] It can be seen from FIG. 23 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.
[0222] FIGS. 24a-24d are photomicrographs of the 0.6 .mu.m and 0.8
.mu.m regions described with reference to FIG. 23 above. FIG. 24e
illustrates the position of the two different regions in the
photomicrographs.
[0223] 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, 4 separate states can be selected.
FIG. 24a shows 0.6 .mu.m/OFF, 0.8 .mu.m/ON; FIG. 24b shows 0.6
.mu.m/ON, 0.8 .mu.m/ON; FIG. 24c shows 0.6 .mu.m/OFF, 0.8 .mu.m/OFF
and FIG. 24d shows 0.6 .mu.m/ON, 0.8 .mu.m/OFF. The definition of
the labels shown in FIG. 24 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.
[0224] 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.
[0225] 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.
[0226] 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.
[0227] 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.
[0228] 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.
[0229] 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.
[0230] 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.
[0231] FIG. 25 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.
[0232] 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. 25 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
[0233] As an example, consider addressing the double ZBD device
described with reference to FIG. 22 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.
[0234] 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).
[0235] 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 AH 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 - BA AB AB - AB - AB BA AB + BA + BA AA AB - AB + AB
[0236] 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.
[0237] 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.
[0238] 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).
[0239] 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.
[0240] 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 +V.1 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
[0241] Referring to FIG. 26, the addressing sequence of a dual ZBD
in accordance with the present invention is illustrated.
[0242] FIG. 26a 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. 26a
can be in any initial configuration; e.g. the mixture of different
optical states shown.
[0243] 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.
[0244] FIG. 26b shows the orientation of the ZBD cell after
blanking using a high negative pulse. A hybrid state (i.e. AB) is
thus formed.
[0245] 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. 26c 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. 26d is thus formed. The first scan thus
indiscriminately latches the lower threshold surface, and
selectively latches the higher threshold surface.
[0246] 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.
[0247] If the BA state of FIG. 26c 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. 26e if
a positive (select) data voltage is applied. Application of a
non-select data pulse results in the BA state of FIG. 26c being
retained as shown in FIG. 26f.
[0248] If the AA state of FIG. 26d 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. 26h if
a positive (select) data voltage is applied. Application of a
non-select data pulse results in the AA state of FIG. 26d being
retained as shown in FIG. 26g.
[0249] 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. 26 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. 27.
[0250] FIG. 27a shows the liquid crystal material in a mixed
configuration. After application of a positive blanking pulse, the
hybrid state BA of FIG. 27b is formed. The first scan can either
form the BB state of FIG. 27c or the AB state of FIG. 27d. If the
BB state is selected in the first scan, this can be retained (FIG.
27e) or the BA state of FIG. 27f can be selected. If the AB state
is selected in the first scan, this can be retained (FIG. 27h) or
the AA state shown in FIG. 27g can be selected.
[0251] 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.
[0252] Referring to FIG. 28, 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.
[0253] The device of FIG. 28 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.
[0254] 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.
[0255] 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.
[0256] 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.
[0257] 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.
[0258] 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.
[0259] A pi-cell arrangement could also be provided.
[0260] Referring to FIG. 29, 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.
[0261] 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 equi-distant 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.
[0262] 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.
[0263] 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.
[0264] The pi-cell device provides optical contrast when switched
between the first (low voltage) bend state shown in FIG. 29b and
the second (high voltage) bend state shown in FIG. 29c.
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. 29a. 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.
[0265] 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.
[0266] Koma et al (1999) Proceedings of the S1), 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.
[0267] 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.
[0268] Referring to FIG. 30, 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.
[0269] 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..
[0270] In operation, the liquid crystal in the vicinity of the
first cell wall is latched between the low pretilt (defect) state
shown in FIG. 30a and the high pretilt (continuous) state shown in
FIG. 30b. 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).
[0271] 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. 31. 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.
[0272] This should be compared to the prior art pi-cell device
shown in FIG. 32 in which both surfaces are monostable and the ZBD
device shown in FIG. 33 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.
[0273] 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.
[0274] 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).
[0275] 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..
[0276] Referring to FIG. 34, 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.
34a) is arranged to form a substantially non-splayed (bend)
state.
[0277] 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.
[0278] Ideally, the device is arranged such that the homeotropic
(continuous) state of FIG. 34b 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. 34b. This helps to ensure that any
liquid crystal material latched from the homeotropic state of FIG.
34b adopts the substantially non-splayed state of FIG. 34a 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.
[0279] 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.
[0280] 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).
[0281] 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.
[0282] Referring to FIG. 35 a number of substantially non-splayed
states are shown.
[0283] 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 ID 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.).
[0284] 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.
[0285] Referring to FIG. 36, 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.
[0286] Referring to FIG. 37, 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.
[0287] 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.
[0288] 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 TN 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. 29b
and 29c).
[0289] 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.
[0290] 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. 38.
[0291] Although the examples described herein are primarily
directed to ZBD type devices, it should be appreciated that the
invention can also be applied to any of a number of different
display technologies including all the prior art display
technologies described above. It should also be noted that the term
"display" does not necessarily mean that an image is written that
can be observed by a user. A display may include spatial light
modulators and the like in which amplitude and/or phase-modulation
is imparted to light.
[0292] For example, and with reference to FIG. 39, a droplet based
display 800 is shown. The display comprises a pair of cell walls
802 between which a layer of material 804 is located. The layer of
material 804 comprises a matrix medium 806 in which first droplets
808 and second droplets 810 are contained. The cell walls 802 are
typically glass plates carrying ITO electrodes 812 and having a
black background layer 814 on the lower plate (alternatively the
lower plate may be made from some other black conductive electrode
structure).
[0293] The droplets may be formed from a bistable material that
reflects some wavelengths in one state and transmits those
wavelengths in the second stable state. The device may be arranged
to give two different size droplets 808 and 810 such that the
threshold to switch one droplet will be different to that of the
other droplet. More than two droplet types may be used, and the
droplet size may be the same or different as shown.
[0294] An example of such a device would use a cholesteric liquid
crystal fabricated for example: in the fashion of Yang et al (2003)
Proceedings of SID XXXIV, Book 2, pp959-961: The droplets would be
sufficiently large to give two stable states--either a selective
reflecting substantially planar (Granjean) state or a forward
scattering polydomain sample (ie focal-conic-like), preferably
larger than 15 .mu.m. In the planar state the larger droplet 808
would reflect say red light, but transmit blue and green. In the
other state the larger droplet 808 would transmit all wavelengths.
The smaller droplet 810 would have a lower threshold than the
larger droplet 808 and reflect say green wavelengths in the planar
state.
[0295] The device may contain a further droplets C (not shown) with
a lower wavelength reflected in the planar state. The droplet types
are readily formed in an emulsion separately, and then mixed
together with a matrix material (in contrast to prior art methods,
such as that of Yang et al ibid).
[0296] To provide a colour device, an approximately equal mix of
three droplet types is used, but colour balancing and off-axis
reflectivity may be enhanced using other mixing ratios. Preferably,
the matrix is made from a photopolymerisable or similarly cross
linkable material, so that the layer forms a rigid plastic layer.
One method to achieve different thresholds for the different
droplets is to ensure that the cholesteric compound or mixture
within each droplet have different dielectric anisotropies. Other
methods include using different sized droplets, using aspherical
drops with different eccentricities or alignments, or by varying
the interaction between the liquid crystal and the droplet wall
using different surfactants or wall materials for each of the
droplets.
[0297] In accordance with the present invention, three scans may be
applied to latch droplet types A, B and C of different threshold.
On the first scan, the highest threshold droplets A are either
latched into the reflective planar state or into the scattering
focal conic state. Since droplets B and C do not reflect red light
then if in the scattering state, then when droplets A are in the
scattering texture the red light is eventually absorbed by the
black at the rear electrode. In the second scan, droplets B are
selected by the data into either reflect green or scatter green,
leaving the red and blue light un-reflected (and preferably with
little scattering). The third scan would select droplets C into
either a reflect blue or scatter blue. In this fashion, a
full-colour display can be fabricated without the poor reflectivity
associated with a spatially divided colour panel or the costs
associated with a multiple stack.
[0298] Alternative bistable media might include charged particles
in which the optical properties differ depending on particle
orientation (see, for example, Hattori et al (2003) Proc. SID, pp
846-849.). In such systems, the charge density of the droplets may
be controlled to give different thresholds, as well as droplet
size, shape and interfacial properties. Such particles will usually
cause absorption of light, but will allow different colours to be
mixed.
[0299] Referring to FIG. 40, the present invention may also be
applied to a device that comprises a stack of two or more panels. A
stacked device 900 comprises a first panel 902, a second panel 904
and a third panel 906. Each panel is used to modulate light within
a certain wavelength range.
[0300] The row and column electrodes of each panel are connected to
a single set of drivers. In other words, each column driver 908 is
electrically connected to a column of each panel and each row
driver 910 is electrically connected to a row of each panel. The
three panels are arranged to have different threshold of latching.
For field effect devices a simple method of altering the latching
properties is to vary the cell gap for the different panels. The
application of three scans in accordance with the teachings
described above thus enables each panel to be latched into the
desired state using the single set of driver circuitry. In this
manner, a colour display can be constructed from the three panel
stack with reduced driver cost.
* * * * *