WO2013038149A1

WO2013038149A1 - Optical device

Info

Publication number: WO2013038149A1
Application number: PCT/GB2012/052185
Authority: WO
Inventors: William Alden Crossland; Anthony Bernard Davey; John Richard Moore; Daping Chu
Original assignee: Cambridge Enterprise Limited
Priority date: 2011-09-14
Filing date: 2012-09-06
Publication date: 2013-03-21
Also published as: GB201115898D0; TW201312238A

Abstract

In a method of operating a liquid crystal device having a liquid crystal composition with smectic-A properties, a first waveform is applied to optically clear the device so that it is substantially transparent to visible Light and a second waveform is applied to disorder the material of the liquid crystal composition to afford a strongly light-scattering state. The first waveform has a higher frequency than the second waveform, and the method comprises applying a modified waveform to partially scatter at least a portion of the device from the fully-cleared state.

Description

Optical device

The present invention is in the field of photonics. Embodiments relate to an optical device using a Hquid crystal materia? having Sraeetk~A properties, for example display, a panel for affecting the transmission of light or an amplitude spatial light modulator, Embodiments relate to a method of operating such an optical device. An embodiment relates to optical devices in which a disordered state is produced by the process of Sm A dynamic scattering and a clear, uniform state is induced by dielectric re-orientation. Such optical devices can be used to provide variable amounts of light transmission- either locally, for example hi "pixels" or across the whole device without the need for optical polarisers.

Liquid crystals have molecules which tend to self order without freezing and thus gain crystalline attributes even though they still flo and may ill container. The phases of liquid crystals are broadly a generalised sequence of states that such a molecular fluid may pass through on the way from being an isotropic liquid until it feezes as a solid, In general such molecules will be typified by strong anisotropy. The form that this anisotropy takes can be considered where the molecule is typified by a high aspect ratio (much longer than wide, thus "rod" or "lath" like), and may have dipole character, and anisotropic po!arisabiliry. in these eases the average direction of molecular orientation is referred to as the "director".

Nematic liquid crystals typify the commonest liquid crystalline materials and are commonly used in liquid crystal flat screen devices and flat-panel displays. Extending the length of nematic mesogens, or other structural changes, ver often causes them to show further phases upon cooling below the nematic phase, and before solidification, and at lower temperatures the typical character may be of a 'layered fluid". Such layered liquid crystals are called "smectic" liquid crystals, Herein we will only consider the materials normally referred to as "sracctic A", abbreviated to "SmA", liquid crystals. For example the prototypical "5CB" (4'-pentyl-4-biphenylcarbon le)j "50CB" (is the ether linked pentyl, 4^,-{pea.t loxy)-4-bipheny carbofl.itr.U€)_> is nematie, the "BCB'* (4^?-octy.l- 4-biphe«ylcarbominle) and "80CB" (4'~(octyIox )-4-biplienyiearbonitrile), each exhibit a SmA phase beneat the higher temperature nematie phase, where in the abbreviation "mCB" and "mOCB":- m stands for an integer and refers to the number of carbon atoms in the alky! or alkoxyl chain in 4~eyano~4'~n- alkylhiphenyl and 4-eyano-4'-n-aikoxybiphenyL respectively; for example:

8CB =^» 4-cyano-4'-ocrylbiphenyl and

4-cyauo-4 ' -octyloxybipheny 1

The molecules forming SmA phases have similar properties to those forming nematie phases. They are rod-like and usually have a positive dielectric anisotrop . The introduction of a strong transverse dtpole in order to induce a negative dielectric anisotropy tends to destabilise the SmA phase and may lead to increased chemical instability. Smectic liquid crystals exhibit hysteresis in their switching so that dielectric reorientation (or other disturbances of the smectic structure) does not relax when an applied electric field is removed. Unlike most nematie liquid crystal structures, dieleelrieally re-oriented SmA liquid crystals rest in the driven state until farther forces arc applied.

A panel may b formed by taking planar sheets, for example of glass, and applying to these a transparent conducting layer, typically made of indium tin oxide, the conducting layers being connected to conductors so mat a variable field may be applied. These two sheets may be formed into a panel for example separated by beads of uniform diameter (typically, say, 5-15 micrometers, dependent on. desired cell thickness). This panel is then edge sealed with glue allowing one or more apertures for filling with the liquid crystal material.

Using such a cell a SmA liquid crystal layer may be formed by filling the panel (typically at an elevated temperature above the isotropic transition for the material). In the SmA devices discussed here, no alignment layers are required unlike nematic devices where uniform alignment of the ceil is essential. On filling and thermally cycling such a SmA panel from room temperature to above the isotropic transition and back again, the liquid crystal will exhibit textures that are typical for the phases. Whilst the nematic, with no surface alignment, ma appear in the well-known Schlieren texture where line defects or threads' scatter light, in the SmA a 'focal conic^* texture is formed as a consequence of the layered structure of the SmA material. There is a sharp spatial variation in the refractive index which results in light, scattering. The appearance of these textures results from the anisoiropy of the refractive index, which is highest when light is travelling orthogonal to the more polarisable axis of the average molecular direction. The variation in refractive index causes light scattering. When viewed (under a microscope) between crossed poiarisers, contrast can also be observed betwee regions of different molecular orientations ,

To electrically address a SmA liquid crystal panel an alternating (AC) field is normally applied, m non-doped materials, positive dielectric anisoiropy of the LC will cause the re-arrangement of initially randomly aligned poly-domains, to align the mesogen with the field direction (normal to the glass surface). The panel will appear clear, as the average orientation of the anisotropic molecules is normal to the glass layer. For most non-doped SmA materials this situation is only reversible by heating the cell to destroy the SmA alignment. If a suitable ionic dopant is dissolved n the SmA liquid crystal host, then under the influence of DC or low frequency (e.g. <200 Hz) electric fields, two orthogonal fortes attempt to orient the suieetic A director:- i) Dielectric re-orientation as described above attempts to align the SmA direc tor (indicating the average direction of the long molecular axis) in the field direction,

ii) Simultaneously, the movement of ions through the SmA electrolyte attempts to align the smectic A director in the direction in which ions find it easer to travel. m SmA materials this is within the layers i.e. orthogonal to the field direction (i.e. the materials have positive dielectric anisotropy and negative conductivity anisotropy). The two competing forces give rise to an electro-hydrodynamic instability in the liquid crystal fluid that is referred to as 'dynamic scattering'. In smeetic-A materials the dynamic scattering state strongly scatters light and (in contrast to the similar state in nematic materials) the disruption of the SmA structure that it produces remains after the electrical pulse causing it has terminated. The reversibility between the clear, uniformly oriented, state and the ion-transit induced, poly-domain, scattering state, depends upon the different frequency domains in whic these processes operate. Dynamic scattering requires the field driven passage of ions through the liquid crystal fluid, it therefore occurs only with DC or low frequency AC drive.

Higher frequencies cause dielectric re-orientation (the ions cannot "move" at these frequencies) thus re-establishing a uniform orientation of the molecules.

Thus the combination of dielectric re-orientation (into a clear transparent state) and dynamic scattering (into a strongly light scattering state) in a suitably doped SmA phase (possessing positive dielectric anisotropy and negative conductivity anisotropy) can form the basis of an electrically addressed display. High frequencies (variable, typically 1000 Hz) drive the SmA layer into an optically clear state and low frequencies (variable, typically < 200 Bz) drive it Into the light scattering state. A key feature of sucb a displa is that both these optical states are set u using short electrical addressing periods, and both persist indefinitely, or until the are re-addressed electrically. This is not true of the related phenomena in nemaiic liquid crystals. It is this property of electro-optic testability (o more accurately multi-stability) that allows SmA dynamic scattering displays to be matrix addressed without pixel circuitry and which results in their extremely low power consumption in page-oriented displays or in smart windows.

There is a need with optical devices using SmA compositions to provide transmission properties intermediate the ''fully scattered" and the "fully cleared*' states.

In one aspect there is disclosed a method of operating a liquid crystal device having a smeetic-A liquid crystal composition, in which a first waveform is applied to optically clear the device so that it is substantially transparent to visible light and a second waveform is applied to disorder the material of the liquid crystal composition, to afford a strongly light~scattermg state, wherein the first waveform has a higher frequency than the second waveform, the method comprising applying a third waveform to partially scatter at least a portion of the device from the cleared state.

As noted above, the frequency of the first waveform (used to clear) is relatively higher than that of the second waveform, which is typically at around mains frequency.

It is proposed in a co-pending application to obtain greyscaie by first of all applying the second waveform to disorder the device so that it is in a light- scattering state and then t partly clear it by using a modified version of the clearing waveform . Since the frequency of the first waveform is relatively high compared to the second waveform, this method of obtaining greyseaie is likely to be relatively fester than the method described in. this specification. Speed may be desirable in some situations. However it has been found, surprisingly, that starting from the clear state and selectively scattering portions of the device to achieve greyseaie has

advantages. These include the following;

1. The relationship between the stimulus (e.g. voltage, pulse width) used to partially scatter, and the amount of scattering is relatively linear compared to the relationship between a stimulus used to partly clear,

2. The result of partial scattering is more stable with time than the result of partial clearing. The amount of partial clearing takes time to settle by comparison with the amount of time taken to partly scatter.

3. Partial clearing shows a high, degree of "overshoot", so application of a partial clearing waveform produces a response in which the amount of clearing far exceeds the steady state. Partial scattering has much less of an overshoot.

The third waveform ma be a modified version of the second waveform

The second waveform ma be a repetitive de balanced waveform consisting of a quasi-continuous series of cycles, the number of cycles being at least equal to a predetermined number, wherein the predetermined number fully scatters the device, and the step of applying a third waveform ma comprise applying a number of cycles of the second waveform fewer than the predetermined number. The method may comprise determining a riiimber of cycles of the second waveform sufficient fully to scatter the device, and applying a fewer riiimber of cycles, as said modified waveform, to partially scatter the device. The first waveform may he dc balanced and have constant amplitude.

The second waveform may be dc balanced and have constant amplitude.

The second waveform and the third waveform may be at least substantially sinusoidal Tlie step of applying a third waveform may comprise applying one or more cycles of a waveform ha ving the frequency of the second waveform and an amplitude less than the amplitude of the second waveform. The third waveform may have at least substantially the same waveshape as the second waveform,

The method may comprise providing a third waveform having varying amplitude.

The second waveform may ha ve a predetermined pulse width, and the step of applying a third waveform may comprise at least one cycle having the

frequency of the second waveform and having a pulse width different to the predetermined pulse width.

In another aspect a liquid crystal device has a smectic-A liquid crystal composition, wherein which a first waveform is applied to optically clear the device so that it is substantially transparent to visible light and a second waveform is applied to disorder the material of the liquid crystal composition to afford a strongly light-scattering state, wherein the first waveform has a higher frequency than the second waveform, th device furthe comprising a control circuit for applying a modified waveform to partially scatter at least one portion of the device from the c lear state In the device, the control circuit is configured to respond to a control input for selecting a desired degree of scattering of the at least one portion of the device. The control circuit .may be configured to take into account the temperature of the liquid crystal composition.

The smectic A Mquid crystal composition may be a composition as described in FCT/US 10/27328, claiming priority from US patent application. 61/314039, incorporated herein by reference

The liquid crystal composition may comprise, in weight %:

(a) 75% in total of at least one sUoxaae of the general formula 1:

wherein

p ^~ 1 to 10, e.g. 1 to 3,

q - 1 to 12, e.g. 6 to 10,

t^∞0 or 1,

k = 2 or 3,

A is a phenyl or cyc!ohexyl ring which may be the same or different and are bonded together in para positions,

R^~ a C_w alfcyl group, e.g. methyl, which may be the same or different,

X ~ a C 2 alkyl group, and

Z - 1% CI, Br, I, CN, N¾, N0₂, NMe_2? NCS, C¾, or OC¾, CF_¾ OCF₃, CH₂F, CHF₂ especially CN; 0,001 ~ 1% in total of at least one quaternary ammonium salt of the general formul II:

wherein:

T^∞ a methyl group or a siJyl or siloxaiie group and

v ~ 1 to 30, for example v~ to 19, e.g. myristyl (v^lS,

T-metJtyi) or cet l (v^«15 and T~methyl)_s

Rl, R2 and R3_f which may be the same or different, are Cm alkyl, e.g. methyl or ethyl,

Q is an oxidative!}* stable ion, especially a CIO₄ ion,

(c) 20-65% in total of at least one polarisa te linear molecule having an alkyl chain, the molecule having the general fonnula HI:

D— A —Y (III)

wherein:

D stands for a Cj.js straight chained alkyl or alkoxy group optionally containing one or more double bonds;

k. - 2 or 3,

A^* is a phenyl, cyclohexy!, pynmidme, ,3-dtoxaxie, or

L4-bieyelo[2,2,2]oetyl ring, wherein each A may be the same or different arid are bonded together in para positions, the terminal ring attached to Y optionally being a phenyl and

Y is located in the para position of the terminal ring of the group A and is selected from. Z (as defined above In connection with Fonnula I), straight chained alkyl, C ₆ straight chained alkoxy, OCHF_2} M e₂, CR OCOCH_3i and COO¾; and (d) 2 ~ 20%, optionally 5 - 15, in total of at bast one side chain liquid crystal polysiloxane of the general formula IV:

(IV)

wherein:

a, b and c each independently have a value of 0 to 100 and are such that a+b+e has an average value in the range 3 to 200, e.g. 5 to 20; and a is such that the chain units of the fomiula Y~R₂SiO- [SiRj-OJa represents 0 to 25 mole percentage of the compound of the general formula IV, and c is such that the units of the formula chain

0 to 1 mole percentage of the compound of the general fomiula IV,

m ^~ 3 to 20, e.g. 4 to 12;

t = 0 or 1 ,

k^™ 2 or 3

A is a phenyl or cyclohexyl ring which may be the same or different and the rings are bonded together in para positions, R ^~ a Cj.3 alky! group, e.g. methyl, each of which may be the same or different, and

Y ~ a Ci 2 alkyl group, a chromophore or a calamitic liquid crystal group and each of which may he the same or different, and Z is as defined above in connection with Formula L and wherein the amounts and nature of the components are selected such that the composition possesses smeetic A layering, as detected by X-ray diffraction.

Such a composition has a relatively high and well-defined switching threshold, In other words the voltage gradient between electrodes of a cell containing the composition must reach a we!i-defmed level before the composition is affected.

The siloxane oligomeoe moiety (a) ma be a compound of the formula la:

where X, R, p, q and t are defined above in connection with Formula I and g and h each independently stand for 0, 1 or 2 and j stands for 1, 2 or 3, subject to the requirement that g+h+j is 2 or 3.

The side chain siloxane liquid crystal, component (d), which may be polymer, copolymer or terpolymer, may be a compound of the general formula IVa

(IVa) where a_? b c, m and t are as defined to connection with Formula IV , g ^« 0, I or 2, h~ 0, 1 or 2, j ~ 1, 2, or 3. subject to the .requirement that is 2 or 3; each R may be the same or different and is an alkyJ group, e.g. methyl; and Y a C;.₈ alkyl group, a chromophore or a calamitic liquid crystal group.

The ionic anion (b) of formula II may be a compound of the formula (Ila);

where v, Rl_s R2, R3 and Q are as defined in claim I. in connection with Formula IL

The ionic anion, of formula II may be a compound of the formula lib:

wherein v, Rl_s R2, R3 and Q are as defined in claim 1 or claim 4 in connection with Formula II and T is a silyl or siloxane group.

Component (c) may comprise an organic calamitic mesogen which exhibits either a nematic or a Smectic A liquid crystal, phase.

The at least one polarisable linear molecule, component (e), may compound of the formula Ilia and/or a compound of the formula Mb.

where a - t to 15 and b ^« 1. to 13; f - 0 or 1, j ^« 1,2 or 3; g Ο,Ϊ,ΟΓ 2. h 0, Lor 2 , subject to the requirement that g+h+j does not exceed 3.

The composition may further include:

(e) up to 10% by weight in total of at least one positive or negative dichroic dye, optionally a cyan, yellow, magenta, red, green or blue dye or an emissive dye, e.g. a fluorescent or phosphorescent dye, the dye being aligned with neighbouring mesogenic components of the composition.

The composition may include:

(f) up to 10% of one or more viscosity-reducing solvents or diluents.

The compositions may further include:

(g) u to 1.0 wt% of at least one molecule e.g. a lath-shaped molecule, that is not a liquid crystal, but which can he incorporated into the formulation., without degrading the smectic A layer quality of the composition.

The at least one molecule that is not a liquid crystal may comprise a compound of the formula (V):

The composition ma also include:

(h) up to 50% by weight, e.g. up to 40%, in total of at least one birefringen altering additive, e.g. birefringence increasing additives, for example:

where R - CM₀ alkyl, n -· 0 or 1, L is selected hydrogen, or C'1.3 alkyl and X - CN, F, NCS, C¾ OCF₃ or C_w alkyi or

where R is a Cue alkyl group,.

R ~ a Ci.so lk l group.

where R~ a C_h lk ! group

where R^:::: a C1.10 alky! group

The total amount of the birefringence-altering additive component (h) and the total amount of component (c) ma be in the range of 35 - 73 wt%. e.g. 40 ~ 65 wt% or 45 -- 60 t%. The composition may have a birefringence in the range 0.15 to 0.3. and preferably 0.16 to 0,2, at 20°C and 589am and be opaque in the disordered state and clear in the ordered state,

The composition may include up to 10% by weight in total of at least one positive or negative diehroic dye, optionally a cyan, yellow, magenta, red, green or bine or a black dye, or an emissive dye, e.g. a fluorescent or phosphorescent dye, the dye being aligned with neighbouring raesogenic components of the composition.

The composition may have a birefringence in the range 0,07 to 0.15, and preferably 0.1 to 0,13, at 20°C and 58 rmi_f (ii) is translucent in the disordered state and clear in the ordered state and (iii) includes up to 10% by weight in total of at least one positive or negative dichroic dye. optionally a cyan, yellow, magenta, red, green or blue dye, or a black dye or an emissive dye, e.g. a fluorescent or phosphorescent dye, the dye being aligned with neighbouring mesogenic components of the composition. In the drawings:

Figure I is a plan view of a first example of a liquid crystal panel.

Figure 2 is a cross-section along the line 11-IIA of Figure 1;

Figure 3 is a cross-section similar to that of Figure 2, through a second example of a liquid crystal panel;

Figure 4 shows a schematic diagram of a drive arrangement for a liquid crystal panel:

Figure 5 shows some exemplary waveforms.

Figure 6 shows the eifect of an initial scattering waveform followed by a series of partial clearing pulses applied to a pixel of an SmA device; and Figure 7 shows the effect of an initial clearing waveform followed by a series of partial sea tiering pulses appl ed to the pixel used to provide the results of Figure 6. in the drawings, like reference signs refer to like parts.

Referring to Figs 1 and 2, display panel 400 has first and second substrates 10,420. In this embodimen t both of the substrates are tr ansparent to visible light and are of glass, thus being generally rigid. In other embodiments, transparency and rigidity may not be required, and some embodiments use substrates of relatively flexible material, for example a polymer such as PET. in this embodiment the panel is dimensioned such that a total voltage across a pixel of 100 volts is insufficient to affect the liquid crystal material (eg to cause a state change). The thickness of the liquid crystal material is typically in the range of 2 - 50 microns, e.g. 5-15 microns.

The panel 400 has a first set of electrodes 430 shown in Fig I as extending laterally across the device 400, and these are referred to for convenience herein as row electrodes. The panel 400 has a second set of electrodes 440 extending perpendicular to the row electrodes 430, and these are referred to for

convenience herein as row electrodes. It will be understood of course that the device 400 need not be oriented as shown. The electrodes 430, 440 in this embodiment are transparent to visible light Examples of suitable materials are gold or ΠΌ.

The column electrodes 440 are disposed on the inner surface of the first substrate 410, and the row electrodes on the inner surface of the second substrate 420. The substrates are maintained in spaced relationship by spacers 450, shown here as spheres. The spacing between the substrates forms a chamber which contains a smectic A compositio 460 As previous discussed. the liquid crystal composition is a tbermotropic liquid crystal smectic A composition exhibiting a smectic type A phase made up of multiple layers, wherein under the influence of different electric fields applied between the electrodes, the alignment of the layers of the composition can become more ordered or more disordered, the composition has stable states in which the alignment of the layers of the composition are differently ordered including an ordered state, a disordered state and intermediate states, the composition being such that, once switched to a given state by an electric field, it remains substantially in that state when the field is removed.

No alignment layer is provided.

In use, voltages applied between row electrodes and column electrodes

influence the liquid cry stal composition between the relevant electrodes. For example, referring again to Fig 1, it will be seen that an exemplary row

electrode is marked 430a and an exemplary column electrode is marked 440a. if a low frequency voltage, less than 200 Hz,- e.g. 50 Hz, 60 Hz,- the voltage being of suitable amplitude for the thickness of liquid crystal composition (for example 150 volts), the material of the composition directly associated with the electrode crossover will become scattered and will block transmission of visible light. If a relatively high frequency, over J 000 Hz- e.g. 2kHz - voltage is appli d, mis will clear the composition at that location and light will be transmitted through the composition at that location. Referring to Fig 3, a panel 500 is shown. This panel is generally similar to the one shown in Figs 1 and 2, except that the electrodes 530. 540 are generally continuous across the whole or major part of th substrates 51.0, 520, In this embodiment the substrates 51 (3,520 are transparent to visible light but are of polymer material Referring to Figure 4, an illustrative drive circuit 600 for SraA panel 500 has first and second waveform generators 400, 41.0 and a switch circuit 420, Each column electrode 501-504 of the panel is connected at one end to a respective output node 401-404 of the first waveform generator, and at the other end to a respective output node 411-414 of the second waveform generator. Each row electrode 5 ! 1-5 4 of the panel is connected to a respective one 421-424 of nodes of the switch circuit 420,

The first waveform generator 400 can provide a dc balanced clearing frequency (e.g. 2kHz) to selected ones or all. of the column electrodes 501 -4 via its output nodes 401 -4 The second waveform generator 410 can provide a balanced scattering frequency (e.g. 50 Hz) to selected ones or all of me column electrodes 501-4 via its output nodes 11- 14. The switch, circuit 420 can selectively connect selected ones or all of the row electrodes 51 1- 14 to ground potential or to a tristate (high frequency, floating) condition.

In use the first waveform generator 400 is controlled to provide the clearing frequency to all the column electrodes 501-504, with the second waveform generator 410 inactive, and the switch circuit 420 connecting all the row electrodes 511. -4 to ground. As a result, all pixels of the panel. 500 are cleared. if it is then desired to set (for example) only the pixel at the intersection of column electrode 504 and row electrode 512 to the scattered state, the second waveform generator 41 provides a scattering frequency at its 4^th output node 414 , while leaving all other output nodes 411- 13 inacti e ( at ground potential). In the meantime the first waveform generator is inactive, and the switch circuit 420 grounds its second output node 422, all other output nodes 421 ,423-4 being instated. A sufficient number of scattering pulses (e.g.

sinusoids) is output from the second waveform generator to cause scattering to occur. It has been found by the inventors that once a pixellated anel is ly cleared, desired pixels can. be partially scattered and will maintain, their partial!y- scattered partially-cieared state. Several passes using bit-plane data from an original image each giving different -partial scatterin of differing areas allows the display of a grey-scale image, which will then be held with no power applied.

The different stages of partial scattering can be produced in several ways. One technique is digital drive and this may be relatively easily implemented since the liming and amplitude can be accurately controlled.

Oreyseal.es can be generated either a linear method (e.g. % or 16 equally weighted bitp!anes are used to progressively scatter the display to give , ¼, S, ½ etc contrast clearing), or a binary method, where the bitplanes are binary weighted to give additive ½. ¼, Vg.

Scattering is a cumulative effect dependent on the amplitude and duration of the scattering .waveform. Variations in the number of scattering cycles, the amplitude, or the timing of the waveform (see Fig 5) may be used to control the scattering.

Fig 5 shows examples of dri ve methods using square wave-type switching to parti lly scatter pixels. A problem occurs if it is needed scatter only very slightly for a small grey step, and a single cycle is too long to provide the desired amount of scattering.

In Figures 5c and 5 d possible solutions to this are shown - in Figure 5c the final steps have a decrease in the waveform amplitude, whereas Figure 5d the mark/space ratio of the waveform is reduced, but the amplitude remains the same. Figure 5a shows a dc balanced sinusoidal waveform having amplitude approx -fci OO volts and a .frequenc of 50 Hz, i.e. a normal scattering waveform. The number of cycles shown represents the number of cycles necessary for complete scattering

Figure 5 b shows a sequence of fixed amplitude fixed waveform partial scattering cycles, where each individual waveform is a dc balanced series of complete cycles. The lei andmost waveform is four full cycles and is to partially scatter (as required on. selected individual pixels f a pixellated display) The next waveform has 3 complete cycles and acts to scatter a small amount more, and so on for the next (2 cycles) and the last ( a single cycle). Depending on the requirements, a sequence of 4 stages could either give four levels or grey, or could be used to give a binary format of 16 levels,

In Figure 5c an initial 4 cycles and the next 3 cycle waveforms are at full amplitude but the 2 cycle waveform and the single cycle have decreases in the waveform amplitude . In Figure 5d the mark space ratio of the waveform reduces, whereas the amplitude remains the same

Combinations of methods of Figure 5 c and Figure 5d can also be used. Similar results can be achieved when modifying other waveshapes- for example square waves, triangle waves, sawtooth waves and composite waves.

As previously noted, the scattering effect is generally linear with voltage, with number of cycles etc. The relationship depends on the precise nature of the sraectic-A composition, on cell dimensions, temperature etc. An example control circuit to determine the amount of stimulus, for example number of scattering cycles, amplitude of scattering cycles etc needed to provide a desired amount of clearing uses look-up table conversions, including cell temperature, to achieve the desired accuracy and reproduceability of results. Referring again to Figure 4, the second waveform generator 410 is controlled at a control input 450 to apply partial scattering waveforms to field a desired greyscale level of scattering to a desired pixel In one embodiment the control input.450 causes a single number of scattering cycles to be applied, and the desired greyscale level of a pixel is achieved by sequentially and successively scattering that pixel by addressing it and then subjecting it to that single number of scattering cycles one or more times.. For example, one two or three- scattering sinusoids migh be selected as the single number

In another embodiment, the control input 450 may cause the second waveform generator 410 to output scattering waveforms modified to have different fixed amplitudes, different varying amplitudes and different mark -space ratios, similar to those shown in Figure 5.

The time taken to scatter a pixel is dependent on both eel! thickness and frequency- for example higher cell thickness is likely to take longer to

clear/scatter. As the scatter frequency falls towards dc, the necessary voltage may reduce or the scattering time may fall .

Referring to Figure 6. the lower graph shows a scattering (second) waveform 6 1 applied to a pixel. In this case, a total number of 32 cycles would provide full scattering of the pixel of concern. The u pper plot shows the response in arbitrary units, with zero being fully scattered (i.e. dark) and 75 being fully clear (i.e. transparent); and thus shows the resultant response 604 by the LC composition. The scattering waveform- here a mains frequency waveform is followed by a series of sixteen partial clearing (first) waveforms 602, all of which are the same. Each partial clearing waveform, for this example, has 2 full cycles of the ί kHz waveform that is applied for clearing purposes, so giving rise to 16 grey levels.

The effect of this stimulus will be seers to be highly varied- so for example the respo se 605 to the fourth partial clearing waveform 602 has an end result of a relatively small "jump* in clearing by comparison to the response 606 to the sixth waveform 602. A later partial clearing waveform provides a very small response 607, Moreover the first discussed response 605 has a remarkable overshoot iollovved by a peri od of settling. The overshoo results in a temporary high degree of clearing, which drops back to a steady state that is constant,

The second discussed response 606 also has an overshoot, which is however. less pronounced. Follo wing the overshoot, the degree of clearing falls back but then- instead o f being constant- rises to a higher level to achieve its stead state,

The third-discussed response 607 starts by overshooting to fully clear then falls to a less degree of clarity before rising virtually to the fully clear state.

These plots are for a constant temperature.

Turning now to Figure 7, this shows a clearing pulse 701, which produces a transition 705 from fully-scattered to Mly-ordered in the SmA composition. This clearing pulse is followed by 16 identical partial scattering waveforms 702. Each of these, in this embodiment, consists of 2 cycles at mains (50Hz) frequency. Inspection of the upper tr ace shows that the amount of scattering per waveform application is substantially uniform... In other words the responses 707 are close to identical and the change of degree of scattering is very similar for each application, of a partial clearing pulse, giving rise to a highly linear response. Moreover with the exception of the first response 706, there is only a small amount of overshoot, and the progress frora time of stimulus to steady state is remarkably similar in both form and timing for each partial clearing pulse.

The invention is not restricted to the described embodiments,

Claims

1... A method of operating a liquid crystal device having a liquid crystal composition with smectic-A properties, the device being responsive to a first waveform to optically clear the device so that it is substantially transparent to visible light and to a second waveform to disorder the material of the liquid ctystal composition to afford a strongly light-scattering state, wherein the first waveform has a higher frequency than, the second waveform, the method comprising applying a third waveform partially to scatter at least a portion of the device from a cleared state.

2. A method according to claim 1, comprising modifying the second waveform to provide the third waveform.

3. A method according to claim 1, wherein the second waveform is a repetitive dc balanced waveform consisting of a quasi-continuous series of cycles, the number of cycles being at least equal to a predetermined number, wherein the predetermined number fully scatters the device, and the ste of applying a third waveform comprises applying a number f cycles of the second waveform fewer than the predetermined number.

4. A method according to claim 1 , comprising determining a number of cycles of the second wa veform sufficient to fully scatter the device, and applying a fewer number of cycles, as said third waveform, to partially scatter the device,

5. A method according to claim 1, wherein the first, waveform is dc- balanced and has constant amplitude.

6. A method according to claim 1, wherein the second waveform is dc balanced and constant amplitude.

7. A method according to any preceding claim, wherein, the second

waveform and the third waveform are at least substantially sinusoidal.

8. A method according to claim 1 , wherein the step of applying a modified waveform comprises applying one or more cycles of a waveform having the frequency of the second waveform and an amplitude less than the amplitude of the second waveform.

9. A method according to claim 1, comprising providing a third waveform having varying amplitude.

10. A method according to claim 1, wherein the second waveform has a predetermined poise width, and the step of applying a third waveform comprise at least one cycle having the frequency of the second waveform and having a pulse width different to the predetermined pulse width,

11. A liquid crystal device has a smectie-A liquid crystal composition, wherein which a first waveform is applied to optically clear the device so that it is substantially transparent to visible light and a second waveform is applied to disorder the material of the liquid crystal composition to afford a strongly light- scattering state, wherein the first waveform has a higher frequency than the second waveform, the device further comprising circuitr for applying a modified waveform to partially scatter at least one portion of the device from the clear state

12. A device according to claim 1 1, wherein the circuitry is configured to respond to a control input for selecting a desired degree of scattering of the at least one portion of the device,

13. A device according to cl im 11 wherein the circuitry is controlled to take into account the temperature of the liquid crystal composition.