US20050200785A1 - Liquid crystal device with bi- or multistable alignment gratings - Google Patents

Liquid crystal device with bi- or multistable alignment gratings Download PDF

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Publication number
US20050200785A1
US20050200785A1 US10/512,195 US51219504A US2005200785A1 US 20050200785 A1 US20050200785 A1 US 20050200785A1 US 51219504 A US51219504 A US 51219504A US 2005200785 A1 US2005200785 A1 US 2005200785A1
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state
liquid crystal
cell
cell wall
crystal material
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US10/512,195
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English (en)
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John Jones
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ZBD Displays Ltd
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ZBD Displays Ltd
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Priority to US10/512,195 priority Critical patent/US20050200785A1/en
Assigned to ZBD DISPLAYS LIMITED reassignment ZBD DISPLAYS LIMITED ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: JONES, JOHN CLIFFORD
Publication of US20050200785A1 publication Critical patent/US20050200785A1/en
Priority to US11/826,701 priority patent/US7956980B2/en
Abandoned legal-status Critical Current

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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
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    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
    • G09G3/3611—Control of matrices with row and column drivers
    • G09G3/3622—Control of matrices with row and column drivers using a passive matrix
    • G09G3/3629—Control of matrices with row and column drivers using a passive matrix using liquid crystals having memory effects, e.g. ferroelectric liquid crystals
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    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
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    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/137—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
    • G02F1/139—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent
    • G02F1/1393—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent the birefringence of the liquid crystal being electrically controlled, e.g. ECB-, DAP-, HAN-, PI-LC cells
    • G02F1/1395—Optically compensated birefringence [OCB]- cells or PI- cells
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    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
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    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
    • G09G3/3611—Control of matrices with row and column drivers
    • G09G3/3614—Control of polarity reversal in general
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    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1337—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers
    • G02F1/133753—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers with different alignment orientations or pretilt angles on a same surface, e.g. for grey scale or improved viewing angle
    • G02F1/133761—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers with different alignment orientations or pretilt angles on a same surface, e.g. for grey scale or improved viewing angle with different pretilt angles
    • G—PHYSICS
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    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1337—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers
    • G02F1/133776—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers having structures locally influencing the alignment, e.g. unevenness
    • G—PHYSICS
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    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/2007—Display of intermediate tones
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    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
    • G09G3/3611—Control of matrices with row and column drivers
    • G09G3/3622—Control of matrices with row and column drivers using a passive matrix
    • G09G3/3629—Control of matrices with row and column drivers using a passive matrix using liquid crystals having memory effects, e.g. ferroelectric liquid crystals
    • G09G3/3637—Control of matrices with row and column drivers using a passive matrix using liquid crystals having memory effects, e.g. ferroelectric liquid crystals with intermediate tones displayed by domain size control
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    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
    • G09G3/3611—Control of matrices with row and column drivers
    • G09G3/3622—Control of matrices with row and column drivers using a passive matrix
    • G09G3/3629—Control of matrices with row and column drivers using a passive matrix using liquid crystals having memory effects, e.g. ferroelectric liquid crystals
    • G09G3/364—Control of matrices with row and column drivers using a passive matrix using liquid crystals having memory effects, e.g. ferroelectric liquid crystals with use of subpixels

Definitions

  • This invention relates to a liquid crystal device, and more particularly to a bistable liquid crystal device arranged to give a surface mediated transition to a so-called bend state (also known as the Pi-cell ( ⁇ -cell) or optically controlled birefringence (OCB) configuration).
  • a so-called bend state also known as the Pi-cell ( ⁇ -cell) or optically controlled birefringence (OCB) configuration.
  • Liquid crystal devices typically comprise a thin layer of liquid crystal material contained between a pair of cell walls.
  • the internal surface of the cell walls are usually coated with a certain material, or are suitably adapted in some way, to impart a degree of surface alignment to the liquid crystal.
  • the bulk of the liquid crystal then adopts a configuration that depends on the surface alignment properties of the cell walls and on various other factors, such as the type of liquid crystal material and the thickness of the liquid crystal layer.
  • Optically transparent electrode structures on one or both of the cell walls allow an electric field to be applied to the liquid crystal layer.
  • a typical liquid crystal display device is designed such that two, or more, liquid crystal configurations can be selected by the application of suitable electric fields.
  • the different liquid crystal configurations are designed to be optically distinguishable so that optical contrast can be attained from the liquid crystal device.
  • a liquid crystal device suitably arranged between a pair of polarisers may have one configuration that will allow transmission of light through the system and a second configuration that will prevent it.
  • Optical properties such as the chromaticity, viewing angle and diffusivity may be modified using additional passive components, such as retarders, reflective or scattering layers, micro-lenses and the like.
  • Monostable liquid crystal devices in which the liquid crystal molecules can only adopt one stable configuration, are known.
  • Application of an electric field can distort the configuration of the liquid crystal molecules, but once the electric field is removed the liquid crystal will relax back to its single stable configuration after some characteristic time (typically tens of milliseconds to a few seconds).
  • Twisted nematic (TN) and super-twisted nematic (STN) LCDs are examples of monostable devices.
  • the TN and STN devices may be switched to an “on” state by application of a suitable voltage, and will relax back to an “off” state when the applied voltage falls below a certain threshold level.
  • the terms “on” and “off” relate to application of high (i.e. switching) voltage and low (i.e. non-switching) voltage respectively not necessarily the observed optical transmission of a display.
  • loss of power leads to loss of the image.
  • 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.
  • the liquid crystal material 2 adopts a splayed configuration shown in FIG. 1 a in which the liquid crystal molecules in the centre of the device lie substantially parallel to the cell walls 4 .
  • the centre of the device is means a plane parallel to the cell walls, and approximately equidistant between them.
  • Application of a voltage greater than a certain value allows the liquid crystal material to adopt a first bend (or non-splayed) state as shown in FIG. 1 b after a certain time.
  • 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. 1 c, due to the electric field coupling to the positive dielectric anisotropy of the liquid crystal material and reorienting the director normal to the surfaces.
  • the liquid crystal director remains substantially perpendicular to the cell walls 4 at the cell mid-point in the second bend state, and liquid crystal material throughout the remainder of the cell, apart from regions near each surface that are dominated by the anchoring effect of the surfaces, is also forced to lie substantially perpendicular to the cell walls.
  • the surfaces of the pi-cell are designed to give a pre-tilt of the liquid crystal director that is typically between 5° and 30°.
  • 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.
  • the pi-ell device provides optical contrast when switched between the first (low voltage) bend state shown in FIG. 1 b and the second (high voltage) bend state shown in FIG. 1 c. Furthermore, very fast (around 1-2 milliseconds at 25° C. in typical cell gaps of about 4 ⁇ 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. 1 a. 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
  • 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.
  • TFT thin film transistor
  • bistable LCDs Although monostable devices are commonplace, it is also known to provide liquid crystal configurations in which the liquid crystal material can adopt two or more different configurations that are stable in the absence of an applied electric field.
  • Research into bistable LCDs has been prompted mainly by their inherent ability to store images and the high degree of multiplex-ability. This negates the need for devices that have expensive active matrix back-planes and permits line at a time passive addressing. Since bistable devices do not need constant refreshing to maintain an image, they offer a low power alternative for display applications.
  • bistable device is the zenithally bistable device (ZBD) described in WO 97/14990.
  • ZBD zenithally bistable device
  • FIG. 2 Such a prior art ZBD device is shown in FIG. 2 ; liquid crystal material 2 is sandwiched between a first cell wall 6 and a second cell wall 8 .
  • the device is constructed with a bistable surface alignment grating on the first cell wall such that nematic liquid crystal molecules in the vicinity thereof can adopt either one of two stable pretilt angles in the same azimuthal plane.
  • a homeotropic surface alignment treatment is applied to the second cell wall 8 .
  • Application of suitable voltage pulses allows switching between the high pretilt state of FIG. 2 a and the low pretilt state of FIG. 2 b.
  • a device can be formed from a surface that exhibits a plurality of stable states wherein the multi-stable surface can be attained by appropriate design of the surface profile.
  • the two stable liquid crystal configurations of ZBD persist after driving electrical signals have been removed, and (see Wood et. al. SID Digest 2000) the device is highly resistant to mechanical shock, provides 10s of microsecond latching times at low driving voltages ( ⁇ 20V) and allows a high degree of multiplexibility.
  • ⁇ 20V driving voltages
  • HDTV colour high definition television
  • a liquid crystal device comprises a layer of liquid crystal material contained between a first cell wall and a second cell wall, the layer of liquid crystal material being switchable between at least a first state and a second state, said first state and said second state having sufficiently low splay to enable rapid electrical switching therebetween, characterised in that the internal surface of said first cell wall is arranged to provide two or more surface alignment configurations of different pretilt to said layer of liquid crystal material.
  • 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.
  • the internal surface may comprises a surface alignment grating embossed in a layer of material carried on the internal surface of the first cell wall.
  • the device may advantageously be arranged so that the first state and/or the second state persist in the absence of an applied electric field.
  • the present invention thus provides a liquid crystal device that has advantages over known pi-cells.
  • 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.
  • the present invention also removes -the need for an initial (slow) addressing step to switch material from a splayed state to the non-splayed state or the use of polymer stabilisation matrices to stabilise a particular non-splayed state. As described below, even is a splayed state is formed, the surface transition increases the speed with which the non-splayed state can be selected.
  • FIG. 3 illustrates layers of nematic liquid crystal material 30 arranged such that the liquid crystal deformation is pure bend ( FIG. 3 c ); pure splay ( FIG. 3 a ) and pure twist ( FIG. 3 b ).
  • any configuration adopted by liquid crystal material can be described using the three deformation components (i.e. splay, bend and twist).
  • 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.
  • non-splayed states is used herein to mean a liquid crystal configuration in which the splay component is small; for example, a state in which the dominant deformation component is bend. It should be noted that a homeotropic state (i.e. a configuration as shown in FIG. 24 b in which the liquid crystal molecules are perpendicular to the cell walls throughout the thickness of the device) has zero splay and thus falls within the definition of a substantially non-splayed state.
  • a particularly important example of a non-splayed state is the bend state.
  • a 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.
  • 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”.
  • V-state the bend deformation either side of vertically aligned director is in opposing directions.
  • 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.
  • splayed and substantially non-splayed states can both be provided in either twisted or untwisted form.
  • the first state is a bend state in which the tilt of the liquid crystal material at a point in the bulk of the cell is greater than the pretilt of the liquid crystal material at said first cell wall and said second cell wall.
  • This may be a ZBD defect state.
  • 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.
  • prior art ZBD devices exhibit hybrid aligned nematic defect states (e.g. as shown in FIG. 2 b ), 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.
  • 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.
  • the half-way point is taken to within one grating pitch of the average distance from one surface to the other, where the average is taken over at least the area of one pixel within the device.
  • a point which is substantially half way may be anywhere from 1 ⁇ 4 of the distance between the walls to 34 that distance.
  • 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.
  • the tilt of the liquid crystal material at said point in the bulk of the cell is substantially 90°. This may be the so-called ZBD continuous state.
  • Electrical addressing signals are applied to the device to latch into one of the two states, all of which are non-splayed states and one of which is preferably a bend state.
  • the electrical addressing means is arranged so as to ensure that the zenithal bistable surface within the area of at least one pixel is latched into a continuous state during at least part of the addressing signal.
  • 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.
  • 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.
  • a “double ZBD” device is provided in which both surface can impart two or more different surface pretilt angles to the liquid crystal material.
  • the second state is a substantially homeotropic (continuous) state.
  • 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.
  • 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.
  • 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.
  • the device comprising addressing means to latch between said first substantially non-splayed state and said second substantially non-splayed state, wherein the addressing means uses at least first and second latching scans, said first latching scan being arranged to selectively latch material at said first cell wall and said second latching scan being arranged to selectively latch material at said second cell wall, wherein said first latching scan is applied prior to application of said second latching scan and said second latching scan is insufficient to latch material at said first cell wall.
  • this multiscan addressing technique are possible.
  • 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°.
  • 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.
  • the tilt at the cell mid-point is greater than 5°.
  • any one or more of said at least first state and second state is twisted.
  • a twisted pi-cell structure may be formed.
  • the twist may advantageously be up to 180°.
  • the first cell wall and the second cell wall preferably carry electrodes to define a plurality of separate electrically addressable regions.
  • 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.
  • the device may be a single pixel fast optical shutter.
  • said second state is the most energetically favourable state that the liquid crystal material can adopt.
  • 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.
  • the device will tend to form the second substantially non-splayed state (i.e. the continuous state) when constructed.
  • 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.
  • 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.
  • 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.
  • the inter-pixel gap does not act to nucleate a splay state, but advantageously nucleates the non-splayed states.
  • 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.
  • the layer of liquid crystal material is nematic liquid crystal material.
  • 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.
  • the first cell wall is arranged to provide two surface alignment configurations of different pretilt.
  • the first cell wall has a bistable surface structure; for example a surface alignment grating.
  • more than two surface alignment configurations may be provided as described in WO 99/34251.
  • a pi-cell device comprises a layer of liquid crystal material disposed between a pair of cell walls, one or both of said cell walls being arranged to provide two or more stable alignment configurations to said layer of liquid crystal material, said two or more stable alignment configurations comprising a continuous state and one or more defect states, said device being switchable between said continuous state and any one of said one or more defect states, wherein one of said one or more defect states is a bend state in which the magnitude of the tilt of the liquid crystal material at a point in the bulk of the cell is greater than the pretilt of the liquid crystal material at either cell wall.
  • the liquid crystal molecules at the midpoint of the cell lie perpendicular to said cell walls
  • a pi-cell liquid crystal device in which each of the switched states persist in the absence of an applied electric field.
  • a pi-cell device comprises a layer of liquid crystal material located between a pair of cell walls and comprising a plurality of pixels separated by inter-pixel gaps, wherein the internal surface of at least one of said pair of cells walls is arranged, in both said pixel and inter-pixel gaps, to provide two or more surface alignment configurations of different pre-tilt, wherein the material is arranged to adopt a substantially non-splayed state in the absence of an electric field such that the said substantially non-splayed state persists in said inter-pixel gap.
  • pi-cell device comprises a layer of liquid crystal material disposed between a pair of cell walls, said layer of liquid crystal material being rapidly electrically switchable between at least two substantially non-splayed states, said device also being switchable, prior to use, from a splayed state to either of said non-splayed states wherein the internal surface of at least one of said cell walls is arranged to impart two or more different pretilt angles in the same azimuthal plane.
  • the splayed state can be switched to a non-splayed state in less than 1 second.
  • FIG. 1 shows the operation of a prior art pi-cell device
  • FIG. 2 shows the operation of a prior art ZBD device
  • FIG. 3 shows the so-called splay, bend and twist deformations of a liquid crystal material
  • FIG. 4 illustrates a multi-scan technique
  • FIG. 5 shows a panel constructed in accordance with the present invention
  • FIG. 6 shows a cross-section of the panel of FIG. 5 along the line II-II
  • FIG. 7 illustrates the use of multiple scans to address a device having a continuum of transitions
  • FIG. 8 shows a five stage multi-scan technique
  • FIG. 9 illustrates how variation of strobe pulse width may be used in the multi-scan technique
  • FIG. 10 shows typical ZBD latching curves with no variation in asymmetry
  • FIG. 11 shows ZBD latching curves in which asymmetry is not retained
  • FIG. 12 provides an expanded plot of four switching regions of a ZBD device
  • FIG. 13 gives and expanded view of the display of FIG. 5 when addressed in accordance with the present invention
  • FIG. 14 provides examples of row and data signals that can be used to implement the present invention
  • FIG. 15 shows strobe and data signals that can provide three latching scans in accordance with the present invention
  • FIG. 16 shows how each scan of FIG. 15 can be applied to the whole display in turn
  • FIG. 17 shows how each of the three latching scans of FIG. 15 can be applied to each line in turn
  • FIG. 18 shows the measured latching response of a ZBD cell
  • FIG. 19 shows measured defect-to-continuous and continuous-to-defect transitions for a cell comprising regions of different grating pitch
  • FIG. 20 is a series of photomicrographs showing latching, using multiple scans, of a cell comprising regions of different grating pitch
  • FIG. 21 plots experimental data of the defect-to-continuous and continuous-to-defect transitions for two regions of a ZBD cell of 0.6 ⁇ m and 0.8 ⁇ m pitch
  • FIG. 22 shows photomicrographs of two ZBD cell areas of 0.6 ⁇ m and 0.8 ⁇ n pitch addressed using the present invention
  • FIG. 23 shows the electro-optic response of a double ZBD device
  • FIG. 24 shows how a double ZBD device can be addressed using multiple scans from a first blanked state
  • FIG. 25 shows how a double ZBD device can be addressed using multiple scans from a second blanked state
  • FIG. 26 shows an exploded view of a double ZBD device fabricated in accordance with the present invention
  • FIG. 27 illustrates the operation of a single ZBD surface pi-cell of the present invention
  • FIG. 28 illustrates the operation of another ZBD surface pi-cell of the present invention
  • FIG. 29 shows in more detail the prior art transition from a splayed state to a bend state
  • FIG. 30 shows a counter example of a prior art ZBD device in which surface switching does not occur when forming a bend state
  • FIG. 31 illustrates a double ZBD pi-cell ZBD device in accordance with the present invention
  • FIG. 32 shows examples of substantially non-splayed states
  • FIG. 33 show various splayed states
  • FIG. 34 shows the energies of defect and continuous states in a ZBD device
  • FIG. 35 shows rms operation of a device of the present invention.
  • the pulse duration versus voltage plot of FIG. 4 shows the latching properties of a first region 60 and a second region 62 of a bistable device.
  • the first and second regions have different latching energies.
  • a solid line represents the 90 % latched point (i.e. ⁇ V 90% ) and a dashed line represents the 10% latched point (i.e. ⁇ V 10% ). This nomenclature is well known in the art and is described above with reference to FIG. 3 .
  • 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 s1 is used in combination with a select (+V d ) or a non-select ( ⁇ V d ) data voltage pulse to provide selective switching of the first region 60 .
  • a second scan using a strobe voltage V s2 is used in combination with the select (+V d ) or non-select ( ⁇ V d ) data voltage pulse to provide selective switching of the second region 62 .
  • the use of two scans i.e. the first scan and the second scan
  • 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.
  • the use of two scans in which the first scan comprises an initial blanking pulse may be also be described as a three field multiplexing scheme.
  • field one is the blanking pulse
  • field two applies pulses to address regions having a latching threshold within a first range
  • 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.
  • the present invention permits discrimination for the two regions using data voltages that are slightly greater than the partial latch width, namely: V d ⁇ ( ⁇ V 100 ⁇ % ⁇ - ⁇ V 0 ⁇ % ⁇ 2 ) ⁇ Area1 ⁇ ( ⁇ V 100 ⁇ % ⁇ - ⁇ V 0 ⁇ % ⁇ 2 ) ⁇ Area2 ( 3 )
  • 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 Vs 1 ⁇ Vd and white for Vs 1 +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.
  • the strobe voltage is reduced to Vs 2 , thereby allowing these areas to be addressed.
  • neither resultant (Vs 2 ⁇ Vd nor Vs 2 +Vd) has sufficient energy to latch area 1 , and so the entire row is addressed with the desired image.
  • FIG. 5 shows a panel designed to exhibit three separate thresholds on each row electrode.
  • the panel has four row electrodes 70 a - 70 d (collectively referred to as row electrodes 70 ) and eight column electrodes 72 a - 72 h (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 .
  • FIG. 6 A cross-sectional view along the line II-II of the panel shown in FIG. 5 is given in FIG. 6 .
  • an alignment grating forming the first area 80 , second area 82 and third area 84 is shown.
  • the column electrode 72 h Also shown is the column electrode 72 h, row electrodes 70 a and 70 b, 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.
  • 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.
  • 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.
  • each of the four rows 70 is sequentially blanked and then scanned three times, with appropriate data synchronously applied to the columns 72 .
  • all rows may be blanked initially and simultaneously and subsequently each scanned sequentially, either in turn or in some predetermined sequence.
  • the voltage (Vs 1 ) 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 Vs 1 to either latch the third area 84 into the required state or to leave it unchanged.
  • the applied voltage is reduced to Vs 2 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 Vs 3 leaves both the second and third areas 82 and 84 unchanged, but discriminately latches the first area according to the data.
  • 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).
  • 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.
  • 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.
  • the present invention may be combined with analogue greyscale techniques to provide a greyscale device with a reduced number of electronic drivers.
  • the method may also be used to reduce the number of drivers required to produce greyscale by means of spatial dither.
  • the areas e.g. ares 80 , 82 , and 84 of FIG. 7
  • the areas may be arranged to have different areas within each pixel.
  • the first area 80 may be four times greater in area than the third area 84
  • 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.
  • 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.
  • the latching curves for random variations across a panel are likely to vary in a continuous fashion, rather than forming two distinct operating areas.
  • the display can still advantageously be addressed totally in two scans.
  • FIG. 7 shows data and strobe voltages suitable for addressing a panel in which there is a continuum of latching transitions.
  • the device can be considered as having a lowest threshold area (curves 90 ) and a highest threshold area (curves 92 ).
  • Data pulses (+V d and ⁇ V d ) and strobe pulses (V s1 and V s2 ) are selected such that the whole display can be addressed by two scans; the first with V s1 combined with the required data and the second at V s2 with the required data, where V s1 >V s2 .
  • the result of using two such scans is that the data voltage is (almost) halved, albeit at the expense of a doubled update rate.
  • some overlap of the resultant voltages may be preferable (e.g. approximately ( ⁇ V)/2) to ensure that areas of the cell with switching energies close to the cross over are latched into the desired state.
  • V d slightly higher than the equality of equation (4) to ensure that the whole display is in the desired state. Further reductions of V d are possible by increasing the number of scans of successively decreasing strobe voltage. In general, for n scans the data voltage is correspondingly reduced by a factor of n: V d ⁇ ⁇ V 100 ⁇ % ⁇ ( Max ) ⁇ - ⁇ V 0 ⁇ % ⁇ ( Min ) ⁇ 2 ⁇ n ( 5 )
  • n ⁇ V 100 ⁇ % ⁇ ( Max ) ⁇ - ⁇ V 0 ⁇ % ⁇ ( Min ) ⁇ 2 ⁇ ( ⁇ V 100 ⁇ % ⁇ - ⁇ V 0 ⁇ % ⁇ ) ( 6 )
  • V 100% ⁇ V 0% is the inherent partial latch width of a microscopic region.
  • the energy per update is then in the range: 1 2 ⁇ nf ⁇ C ⁇ ( V d n ) 2 ⁇ E ⁇ nf ⁇ C ⁇ ( V d n ) 2 ( 7 )
  • f is the number of frame updates (e.g. the frequency for a constantly updated device)
  • C is capacitance.
  • FIG. 8 shows how, for the continuum of transitions shown in FIG. 7 , each line can be scanned five times (i.e. with voltages V s1 , V s2 , V s3 , V s4 , V 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.
  • the slot width of the strobe pulse could be changed instead of modulating the strobe voltage Vs between successive scans.
  • the longest duration slot is used first, and subsequent scans are successively shorter.
  • a combination of both pulse width ( ⁇ ) and pulse voltage (V) modulation may be preferred.
  • changes to the resultant pulse shape and/or altering the delay between pulses may be used to provide the required discrimination.
  • 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.
  • Symmetric devices are so-called when the same magnitude (i.e.
  • a three area symmetric ZBD device having the properties shown in FIG. 10 may be latched into the defect state by the following procedure:
  • the device gives the desired final state even though area two was only partially latched to the defect state during the first scan.
  • 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.
  • any asymmetry between the two transitions i.e. the continuous-to-defect and defect-to-continuous
  • 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.
  • 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.
  • 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.
  • the device of FIG. 11 thus has three sample areas that exhibit latching properties with constant asymmetry and with switching voltages equidistant apart. If the strobe and data voltages are selected so that both scans overlap by the partial latching width of the second area (i.e. the curves 132 and 132 ′), clean switching is observed over the two scans.
  • FIG. 12 shows an expanded view of the first, second and third curves 131 , 131 ′, 132 , 132 ′, 133 and 133 ′.
  • Switching curves 132 A and 132 A′ of a fourth area are now also shown.
  • the fourth area i.e. curves 132 A and 132 A′
  • has similar latching properties to the second area i.e. curves 132 and 132 ′, but with a variation in the asymmetry of the switching.
  • a non-select pulse applied during the first scan i.e. a resultant pulse 136
  • a select pulse applied during the second scan i.e. resultant pulse 138
  • 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.
  • widening the overlap of adjacent scans would resolve this issue.
  • 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).
  • a positive voltage (+Vs and +Vd) tend to latch the pixel into state A
  • a negative voltage ( ⁇ Vs, ⁇ Vd) latch a pixel into state B.
  • say 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).
  • Tables 3 and 4 show examples of two-field addressing that do not give the desired results.
  • 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.
  • Table 5 shows a scheme that uses the sequence of +V 1 and ⁇ V 2 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.
  • 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).
  • AA or BB ever 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.
  • 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).
  • the blank is followed by the high latching pulse (+V 1 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.
  • 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.
  • 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. TABLE 6 Separate blanks for high then low.
  • 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.
  • TABLE 8 One and a half field high then low/alternating polarities Desired Initial Final State State Data +V1 ⁇ V1 +V2 BB AA + AA A B AA BB BB ⁇ BA BB BB AA AA + AA A B AA AA BB ⁇ AA BB BB BA AA + AA A B AA BA BB ⁇ BA BB BB AB AA + AA A B AA AB BB ⁇ AA BB BB
  • 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. TABLE 10 Addressing three areas using multiple scanning.
  • FIG. 13 shows how the scheme of CASE 10 is used to address 2 rows (scan) divided into three areas with different thresholds, and 8 columns (data), such as that described with reference to FIGS. 7 and 8 ;
  • FIG. 13 a shows a two row (i.e. rows 70 a and 70 b ) by four column (i.e. columns 72 a, 72 b, 72 c, 72 d ) segment of the display shown in FIGS. 5 and 6 .
  • Row 70 a is blanked black by a resultant blanking pulse produced by application of suitable signals to row 70 a and the columns 72 a - 72 d.
  • Row 70 b remains unchanged by the data signal applied to the column 72 a - 72 d, represented by the grey status.
  • FIG. 13 b 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 72 b, whilst non-select data waveforms are applied to columns 72 a, 72 c and 72 d.
  • the desired pattern is thus written to the pixels of the first area 80 of the row electrode 70 a
  • 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.
  • the strobe voltage is reduced to Vs 2 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 72 a - 72 d, to give discrimination.
  • FIG. 13 d shows the third scan in which the voltage is reduced to Vs 3 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 70 a is now completely addressed.
  • FIGS. 13 e and 13 f shows how the process described above with reference to FIGS. 15 a and 15 b is repeated for row 70 b. In this manner, data can be written to each pixel of the display.
  • the standard rules associated with prior art addressing techniques should generally also be followed when implementing the present invention.
  • the total signal applied to the rows must be DC balanced over a certain period, usually taken to be the complete frame.
  • the data signal should be DC balanced for each line to prevent unwanted latching for certain pixel patterns.
  • 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.
  • 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.
  • 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.
  • FIG. 14 An example of a scheme used to address a single row using the method described in table 7 is shown in FIG. 14 .
  • This shows a four slot scheme ( ⁇ 1, ⁇ 1, +1, ⁇ 2)Vs_( ⁇ 1, ⁇ 1, ⁇ 1, ⁇ 1)Vd wherein the first two slots provide the blanking, and the latter two slots give discriminate latching (1>2).
  • Four slots are required to allow the data signal to be DC balanced. Although selective latching occurs in the last two slots only, the first two slots are used to good effect, providing blanking immediately prior to selection.
  • 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.
  • 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.
  • FIG. 15 a three scan multiplexing scheme of the present invention is shown.
  • a blanking pulse is followed by first, second and third strobe pulses synchronised with appropriate select or non-select data.
  • the duration of each strobe pulse is reduced from scan to scan and inverted in polarity from the previous strobe pulse.
  • FIG. 16 shows how the first scan (i.e. the use of the first strobe pulse) can be applied to each line which is then followed by application of the second scan to each line, followed by the application of the third scan to each line.
  • the entire display receives the first scan, then the second scan and finally the third scan.
  • FIG. 17 show an alternative arrangement in which each line is latched using the three scans before the three scans are applied to the next line.
  • FIGS. 16 and 17 a combination of the schemes shown in FIGS. 16 and 17 is also possible.
  • Lines one to five (say) could be addressed in turn by the first, second and third scans.
  • 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.
  • 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.
  • cell number Z 641 is a ZBD greyscale cell having a number of areas fabricated using alignment gratings with different pitch and mark to space ratios.
  • Z 641 is a ZBD greyscale cell having a number of areas fabricated using alignment gratings with different pitch and mark to space ratios.
  • 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.
  • the pitch of the discrete areas in the cell is varied between 0.6 ⁇ m and 1.0 ⁇ m in 0.1 ⁇ m increments, and the resulting latching transitions from all these areas at a temperature of 25° C. are shown in FIG. 19 .
  • the dashed and solid lines in the figure show 10% and 90% levels of switching respectively.
  • FIG. 19 a shows the various continuous-to-defect latching transitions whilst FIG. 19 b shows the various defect-to-continuous transitions.
  • the width of the bistability window is insufficient for the whole range of grating pitches. This results in growback for the 0.6 ⁇ m pitch area, and little or no shift in the transitions on increasing the pitch from 0.9 ⁇ m to 1.0 ⁇ m.
  • FIG. 19 shows that the typical partial switch widths vary from 0.4V to 1.1V for the C to D transitions, and 0.7V to 2.1V for the D to C transitions.
  • 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° 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.
  • FIG. 20 shows the effect of the multi-scan technique on the test cell.
  • FIG. 20 d shows the pitch (in ⁇ m) of the grating in the different areas of the test cell.
  • 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.
  • 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. 20 a as areas of shorter pitch (therefore lower threshold voltage) are switched into the continuous (black) state.
  • FIG. 20 ( b ) shows the device fully switched after the second scan on switching to the defect state, which incorporates select data in addition to the strobe with polarity to switch -defect. This voltage is sufficient to switch the areas into the defect state that were switched continuous in the first scan, and is insufficient to switch the areas defect that were not switched continuous.
  • the first scan now incorporates select data, which switches all areas into the continuous state
  • the second scan incorporates a non-select data, which leaves all areas unchanged in the continuous state.
  • the final state is shown in FIG. 20 ( c ), although the device is unchanged by the second scan.
  • FIG. 21 a shows the defect-to-continuous transition for the two areas of the greyscale cell Z 641 with 0.6 ⁇ m and 0.8 ⁇ m grating pitch
  • FIG. 21 b 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. 21 a ) and the second scan by the second arrow 202 ( FIG. 21 b ).
  • FIGS. 22 a - 22 d are photomicrographs of the 0.6 ⁇ m and 0.8 ⁇ m regions described with reference to FIG. 21 above.
  • FIG. 22 e illustrates the position of the two different regions in the photomicrographs.
  • 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.
  • four separate states can be selected.
  • FIG. 22 a shows 0.6 ⁇ m/OFF, 0.8 ⁇ m/ON
  • FIG. 22 b shows 0.6 ⁇ m/ON, 0.8 ⁇ m/ON
  • FIG. 22 c shows 0.6 ⁇ m//FF, 0.8 ⁇ m/OFF and FIG.
  • 22 d shows 0.6 ⁇ m/ON, 0.8 ⁇ m/OFF.
  • the definition of the labels shown in FIG. 22 are given as the polarity of the data in the 1 st /2 nd scans, where + data has the same polarity as the corresponding strobe, and ⁇ data has the opposite polarity as the corresponding strobe.
  • Two areas with grating pitch 0.6 ⁇ m and 0.8 ⁇ m can thus be addressed selectively, using two scans and a 1V data pulse.
  • 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.
  • 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.
  • 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.
  • ZBD zenithally bistable device
  • both surfaces will switch at the same applied (negative or positive) voltage and hence only the hybrid states can be selected.
  • 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 ( ⁇ V) than the reverse transition (B to A).
  • ⁇ V threshold energy
  • 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.
  • FIG. 23 shows the measured electro-optic response of a double ZBD device having asymmetric transitions.
  • Curves 221 A shows the transition at the first surface (S 1 ) from the high tilt state (state B) to the low tilt state (state A), whilst curves 222 B show the transition at the second surface (S 2 ) from the low tilt state (state A) to the high tilt state (state B).
  • Curves 221 B shows the transition at the first surface from the low tilt state (state A) to the high tilt state (state B), whilst curves 222 A 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.
  • a first pulse applied to the addressed row of +20V ensures that the S 1 is latched into state A and S 2 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.
  • the first pulse of the addressing sequence should be of the opposite polarity to the blank and centred between the asymmetric transition energies.
  • a pulse of ⁇ 14V was applied. This latches S 1 into the A state and S 2 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).
  • 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.
  • +11V was applied.
  • the data is +3V
  • the voltage drop across the cell is only +8V
  • the pixel is unchanged (either AB or BA from the first pulse).
  • the +14V resultant latches S 2 into state B and the pixel is either AB or BB.
  • 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.
  • 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.
  • 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).
  • 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.
  • the gratings might be coated with different materials, thereby altering the transition thresholds due to differences in surface energy.
  • 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).
  • the first letter represents the higher threshold surface state
  • 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.
  • FIG. 24 the addressing sequence of a dual ZBD in accordance with the present invention is illustrated.
  • FIG. 24 a 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. 24 a can be in any initial configuration; e.g. the mixture of different optical states shown.
  • 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.
  • the first alignment surface is arranged to provide latching between the two bistate surface states at a higher voltage threshold than the second surface.
  • FIG. 24 b shows the orientation of the ZBD cell after blanking using a high negative pulse.
  • a hybrid state i.e. AB
  • 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. 24 c 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. 24 d is thus formed. The first scan thus indiscriminately latches the lower threshold surface, and selectively latches the higher threshold surface.
  • 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.
  • the resultant pulse produced during the second scan will latch the lower threshold surface to the state shown in FIG. 24 e if a positive (select) data voltage is applied.
  • Application of a non-select data pulse results in the BA state of FIG. 24 c being retained as shown in FIG. 24 f.
  • the resultant pulse produced during the second scan will latch the lower threshold surface to the state shown in FIG. 24 h if a positive (select) data voltage is applied.
  • Application of a non-select data pulse results in the AA state of FIG. 24 d being retained as shown in FIG. 24 g.
  • states AA, BB, AB or BA may be chosen as required.
  • FIG. 24 shows initial blanking into state AB, it is also possible to use the technique after the device has been blanked into state BA. This is illustrated in FIG. 25 .
  • FIG. 25 a shows the liquid crystal material in a mixed configuration.
  • the hybrid state BA of FIG. 25 b is formed.
  • the first scan can either form the BB state of FIG. 25 c or the AB state of FIG. 25 d. If the BB state is selected in the first scan, this can be retained ( FIG. 25 e ) or the BA state of FIG. 25 f can be selected. If the AB state is selected in the first scan, this can be retained ( FIG. 25 h ) or the AA state shown in FIG. 25 g can be selected.
  • 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° tilt (i.e. parallel to the surface material).
  • 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.
  • 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
  • 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.
  • the device of FIG. 26 thus contains four separately electrically addressable areas.
  • the liquid crystal in the first electrically addressable area 270 (defined by the overlap of the second row electrode 258 and the second column electrode 262 ) is shown latched into the BB state and provides a black state.
  • Liquid crystal in the second electrically addressable area 272 (defined by the overlap of the first row electrode 256 and the second column electrode 262 ) is shown in the BB state and provides a white state.
  • the A state of the second alignment grating is arranged to give a higher pretilt than the A state of the first alignment grating.
  • 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.
  • 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° 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° pretilt from the average surface plane) and the two high tilt states are both in the pretilt range 88° to 90°. 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.
  • a surface of the type described in WO 01/40853 may be used as one or both of the zenithally bistability 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.
  • the two scans to switch the two surfaces can be combined with the multiple scans to address different areas across the display.
  • neighbouring double ZBD region may have different thresholds. This can reduce the data voltage, or reduce the number of electrodes/drivers as described above.
  • the device comprises a layer of liquid crystal material 500 sandwiched between a first cell wall 502 and a second cell wall 504 .
  • 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° to the liquid crystal material in the vicinity thereof.
  • 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
  • the pre-tilt of the monostable surface is less than 30° and more preferably less than 25°.
  • the pre-tilt of the monostable surface is greater than 5° and more preferably greater than 10°.
  • the liquid crystal in the vicinity of the first cell wall is latched between the low pretilt (defect) state shown in FIG. 27 a and the high pretilt (continuous) state shown in FIG. 27 b.
  • the defect state is a bend state
  • the continuous state is a hybrid state in which the splay component is small.
  • the switching speed is high (typically below 5 milliseconds).
  • the pre-tilt of the low tilt, defect state of the zenithal bistable surface is higher than the pre-tilt on the opposing surface as shown in FIG. 28 .
  • 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.
  • the device is designed to provide a surface-latching mediated transition to the bend state.
  • 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.
  • the device 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).
  • 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.
  • the pre-tilt at both surfaces may be as low as 10°, and may be as low as 5°.
  • 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.
  • a so-called double ZBD device is formed.
  • the defect state i.e. the state shown in FIG. 31 a
  • the defect state is arranged to form a substantially non-splayed (bend) state.
  • 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.
  • the device is arranged such that the homeotropic (continuous) state of FIG. 31 b 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. 31 b. This helps to ensure that any liquid crystal material latched from the homeotropic state of FIG. 31 b adopts the substantially non-splayed state of FIG. 31 a 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.
  • zenithal bistable surface may be varied to ensure a bistable surface that spontaneously forms a high tilt state on first cooling.
  • a shallow grating eg low amplitude and/or long pitch
  • rounded features eg a blazed sinusoidal grating
  • relatively low anchoring energy e.g. a relatively low anchoring energy
  • both of the surfaces can 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.
  • 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 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).
  • 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.
  • FIG. 32 a number of substantially non-splayed states are shown.
  • the states include vertically aligned nematic (VAN), in which both surfaces are substantially vertical homeotropic aligned (i.e. pre-tilts greater than 70°, usually greater than 85°). This is a special case, since the 1 D director profile contains neither splay nor bend.
  • VAN vertically aligned nematic
  • HAN hybrid aligned nematic
  • Bend states are also non-splayed and B 1 , B 2 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.
  • the pretilt at both surfaces are similar, and the tilt is substantially 90° close to the centre of the device.
  • there is a significant difference in pre-tilt between the two surfaces and the tilt in the bulk of the cell is substantially 90° closer to the higher pre-tilt surface.
  • 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 B 1 will take typically 2 ms.
  • 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.
  • S 4 is a transient state that may occur on the application of an applied field to an S 1 state.
  • the director may be at 90° 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.
  • 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.
  • 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.
  • the device may be in the splay state initially, when it is switched into constant update mode. Before each frame is addressed, perhaps using RMS multiplexing (Alt-Pleshko, MLA, 4-line addressing etc—standard IN or STN methods) or TFT addressing, a series of pulses is applied to latch the device into the required initial substantially non-splayed state.
  • this initial state is a Bend state.
  • 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.
  • 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. 1 b and 1 c ).
  • 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.
  • 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
  • the shape of the trailing part of the addressing pulse is varied to selectively latch into the required bend state rather than the splay state. This is shown schematically in FIG. 35 where the states and addressing pulse 800 are illustrated.

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  • Nonlinear Science (AREA)
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  • Theoretical Computer Science (AREA)
  • Mathematical Physics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Control Of Indicators Other Than Cathode Ray Tubes (AREA)
  • Liquid Crystal (AREA)
  • Liquid Crystal Display Device Control (AREA)
  • Control Of El Displays (AREA)
  • Electrochromic Elements, Electrophoresis, Or Variable Reflection Or Absorption Elements (AREA)
  • Cookers (AREA)
  • Push-Button Switches (AREA)
  • Devices For Indicating Variable Information By Combining Individual Elements (AREA)
US10/512,195 2002-05-29 2003-05-29 Liquid crystal device with bi- or multistable alignment gratings Abandoned US20050200785A1 (en)

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US10/512,195 US20050200785A1 (en) 2002-05-29 2003-05-29 Liquid crystal device with bi- or multistable alignment gratings
US11/826,701 US7956980B2 (en) 2002-05-29 2007-07-17 Liquid crystal device with bi- or multistable alignment gratings

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US38361002P 2002-05-29 2002-05-29
US10/512,195 US20050200785A1 (en) 2002-05-29 2003-05-29 Liquid crystal device with bi- or multistable alignment gratings
PCT/GB2003/002317 WO2003102683A1 (en) 2002-05-29 2003-05-29 Liquid crystal device with bi- or multistable alignment gratings

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US10/512,195 Abandoned US20050200785A1 (en) 2002-05-29 2003-05-29 Liquid crystal device with bi- or multistable alignment gratings
US11/635,083 Active 2027-04-09 US8130186B2 (en) 2002-05-29 2006-12-07 Display device
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US20050174340A1 (en) 2005-08-11
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EP1512138A2 (en) 2005-03-09
WO2003103013A2 (en) 2003-12-11
WO2003102683A1 (en) 2003-12-11
HK1081710A1 (en) 2006-05-19
US7956980B2 (en) 2011-06-07
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CN1656534B (zh) 2010-12-01
ATE334464T1 (de) 2006-08-15
CN1656534A (zh) 2005-08-17
EP1512138B1 (en) 2006-07-26
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US8130186B2 (en) 2012-03-06
US20070132685A1 (en) 2007-06-14

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