Patent Publication Number: US-2019196216-A1

Title: Speckle reduction

Description:
The invention relates to a device and method for speckle reduction comprising a liquid crystal. 
     Speckle is a well-known phenomenon that occurs when coherent laser light is reflected diffusely from an optically rough surface and imaged by an intensity detector with finite aperture, such as the eye. Any point in the observation plane may be subject to incident light from numerous spatially separated positions on the surface. The incident wave-fronts have a fixed phase relationship as a result of their coherence, which causes constructive and destructive interference. A stationary detector will observe a time invariant, high contrast, high spatial frequency speckle pattern superimposed upon the intended image. This phenomenon is often unwanted, particularly when laser sources are used in projection displays since important information in the intended image is often obscured. Lasers are a necessary requirement in applications such as holography, which is fundamentally a coherent imaging technique. There has been a considerable body of research dedicated to finding new techniques with which to reduce speckle. 
     In order to quantify the degree of speckle within an image, the speckle contrast parameter is typically used, which is defined as: 
     
       
         
           
             C 
             = 
             
               
                 
                   σ 
                   i 
                 
                 
                   I 
                   _ 
                 
               
               = 
               
                 
                   
                     ( 
                     
                       
                         〈 
                         
                           I 
                           2 
                         
                         〉 
                       
                       - 
                       
                         
                           〈 
                           I 
                           〉 
                         
                         2 
                       
                     
                     ) 
                   
                 
                 
                   I 
                   _ 
                 
               
             
           
         
       
     
     where σ i  is the standard deviation of the intensities, I is the intensity of an individual pixel and Ī is the mean intensity of the image. The speckle parameter C can take any value between 0 and 1, with 1 corresponding to fully developed speckle. A perfectly monochromatic laser with linear polarisation that is scattered from a depolarising white screen would have speckle contrast of 1/√2. It has been reported that speckle contrast values of &lt;0.04 are unperceivable to the eye, which provides a standard against which speckle reduction methods can be judged. 
     Ultimately, the phenomenon of speckle may not be possible to completely eliminate. Instead, the most commonly used methods of speckle reduction take advantage of the finite temporal response and spatial resolution of the detector to reduce observable speckle. The traditional approach to speckle reduction is the time averaging of N statistically independent speckle patterns, implying that the patterns have to be both non-interfering and uncorrelated. In the ideal case of N independent patterns, the speckle contrast can be reduced by a factor of 1/√N. Creating independent patterns can be achieved through the diversification of properties such as incident angle, time, frequency, and polarization. A known method of reducing speckle is to use a rotating ground glass diffuser that creates a time-varying speckle pattern which, when averaged over a sum of N independent patterns, results in the smearing out of the effects of speckle. Other techniques for reducing the speckle contrast parameter include the use of a rotating rectangular light pipe, a vibrating diffractive beam shaper and fast scanning micromirrors. 
     These prior art methods all achieve a reduction in speckle contrast to C=0.04-0.075, but typically at the cost of additional bulky mechanical parts or expensive components, making them unsuitable or undesirable for applications that require a compact and vibration free solution such as laser-based pico-projectors. 
     An alternative approach that utilises the electro-optical properties of certain Liquid Crystal (LC) phases has recently been reported. For example, Furue, Hirokazu, et al. “ Control of laser speckle noise using liquid crystals.” Japanese Journal of Applied Physics  50.9S2 (2011) reported a 40% speckle reduction as a result of applying an alternating square-wave electric field to a ferroelectric LC, combined with a wedge cell and polariser. In this disclosure a LC device is switched between a first and second state. In the first state the laser light is passed substantially without modification, and in the second state the plane of linearly polarised light is rotated by 90 degrees. The resulting speckle pattern for a linearly polarised laser passing through the switching LCD is a combination of the two speckle patterns that are produced for each polarisation state. This approach emphasises maintaining coherence of the laser light by avoiding optically diffusive effects. The same authors have more recently reported a 25% speckle reduction with an electrically addressed polymer-dispersed nematic LC device (Furue, Hirokazu, et al. “ Control of Laser Speckle Noise by Using Polymer - Dispersed LC.” Molecular Crystals and Liquid Crystals  612.1 (2015): 245-250). 
     Andreev et al have reported 50% speckle reduction with the application of more complicated waveforms to a helix-free ferroelectric LC (Andreev, Alexander L., Tatiana B. Andreeva, and Igor N. Kompanets. “31.1 : Speckle Reduction Due to using the Electro - optical Cell with Helix - free FLC.” SID Symposium Digest of Technical Papers. Vol.  45. No. 1. 2014). In this disclosure the mechanism of speckle reduction is based on electrically controlled formation of small-scale randomly-distributed gradients of the refractive index in the ferroelectric LC layer. 
     Although these previous efforts to solve the problem of speckle show promise, considerable room for improvement remains. 
     According to a first aspect, there is provided a device for speckle reduction, comprising an optical cell, comprising: a first and second cell wall spaced apart by a gap, a first and second electrode for causing an electric field within the gap and a liquid crystal composition in the gap, comprising mobile ions. The liquid crystal composition is selected to exhibit fluid turbulence due to movement of the mobile ions in response to the electric field. The fluid turbulence in the liquid crystal results in time varying spatially random multiple scattering of light transmitted through the liquid crystal composition. 
     According to a second aspect, there is provided a device for speckle reduction, comprising an optical cell and a controller, wherein:
         the optical cell comprises a first and second cell wall spaced apart by a gap, and a liquid crystal composition in the gap;   the controller is configured to cause fluid turbulence in the liquid crystal composition;   wherein the fluid turbulence in the liquid crystal results in time varying spatially random scattering of light transmitted through the liquid crystal composition.       

     The optical cell may comprise a first and second electrode for causing an electric field within the gap; and a liquid crystal composition in the gap comprising mobile ions. 
     Further electrodes may be provided for causing an electric field within the gap. In some embodiments, interdigitated electrodes are disposed on at least one of the first and second cell wall for causing (or contributing to) a field in the gap. An array of such electrodes may be used to apply an electric field between the elements of the array. 
     The controller may be configured to apply a voltage between the first and second electrodes to cause an electric field that results in fluid turbulence in the liquid crystal composition due to movement of the mobile ions in response to the electric field. 
     The controller may be a fluid pump that is configured to cause fluid turbulence in the liquid crystal composition by pumping the liquid crystal composition. The fluid pump may be a piezoelectric fluid pump, for example comprising a diaphragm actuated by a piezoelectric material. 
     At least one of the first and second cell wall may be optically transmissive. At least one of the first and second cell wall may be at least partially reflective. In embodiments with transmissive first and second cell walls, light may pass through the first cell wall, through the liquid crystal composition, and through the second cell wall. In embodiments with a reflective cell wall and a transmissive cell wall, light may pass through the transmissive cell wall, through the liquid crystal composition, and then reflect off the reflective cell wall to pass back through the liquid crystal composition and transmissive cell wall. This double pass through the liquid crystal composition may enhance scattering. 
     The liquid crystal composition may have anisotropic electrical conductivity. For example, in the case of a nematic liquid crystal or a chiral nematic liquid crystal the conductivity may be greater parallel to the director than perpendicular to the director. This means that mobile ions can flow more easily along the director than perpendicular to it. Redistribution of charge caused by the movement of the mobile ions may set up new components of the electric field that impose a torque on the local director, resulting in local instabilities in the director profile, causing fluid turbulence. 
     The fluid turbulence may result from electrohydrodynamic instability in the liquid crystal composition. 
     The liquid crystal composition may comprise a dopant to enhance ionic concentration and reduce the electric field required for fluid turbulence (or electrohydrodynamic instability). 
     The dopant may comprise at least one of: an ionic dopant, an electron donor, or an electron acceptor. 
     The dopant may comprise cetyltrimethylammonium bromide, CTAB. 
     CTAB may be is present in the liquid crystal composition at a concentration of between 0.05 and 0.2% by weight. 
     The liquid crystal composition may have positive dielectric anisotropy. The liquid crystal composition may have negative dielectric anisotropy, or may have isotropic dielectric properties. 
     The liquid crystal composition may have a chiral nematic phase. References to a phase in this disclosure may mean a phase at standard temperature and pressure conditions of 20° C. and 101.325 kPa. 
     The liquid crystal composition may comprise a chiral dopant to induce chirality. 
     The liquid crystal composition may be inherently ionic. 
     The liquid crystal composition may comprise at least one of: a smectic A phase; a blue phase; an achiral nematic phase. 
     The controller may be configured to apply a time varying electric field. 
     The controller may be configured to apply a time varying voltage to the first and second electrode. This time varying voltage may be in the form of a square wave. Where there are additional electrodes, the controller may be configured to apply voltages between the electrodes to cause an appropriate field in the gap. 
     The frequency of the time varying electric field may be less than 1 kHz. The frequency of the time varying electric field may be between 50 Hz and 500 Hz. 
     The first and second cell wall may be substantially parallel. The gap between the first and second cell wall may be between 2 μm and 50 μm or 2 μm to 1 mm. 
     The first and/or second cell wall may have a structured surface adjacent to the liquid crystal composition. The structured surface may be a patterned or etched surface. 
     The device may additionally comprise a static diffuser. The static diffuser may be ground glass or may comprise a polymer. The static diffuser may be a portion of the first and/or second cell wall. The static diffuser may be separate from the first and/or second cell wall. 
     The first electrode may be disposed on the first cell wall, and the second electrode may be disposed on the second cell wall. Alternatively, first and second electrode may be disposed on the first cell wall, in a lateral electrode configuration. There may be multiple first electrodes and/or multiple second electrodes. 
     The first and second electrode may comprise a transparent conductive material, such as transparent conductive oxide or a transparent conducting polymer. Any suitable material can be used, e.g. ITO, IGZO, ZnO etc. 
     Alternatively, the first and second electrode may comprise a transparent conductive polymeric material, or the first and second electrode may comprise graphene. 
     The device may be used in a reflective configuration. The first or second cell wall may additionally comprise a reflective element. The reflective element may be the first or second electrode. The reflective first or second electrode may be silver, aluminium or gold. 
     The liquid crystal composition may comprise nanoparticles or microparticles to increase scattering. The nanoparticles or microparticles may have a different refractive index to the bulk liquid crystal composition. The refractive index of the nanoparticles or microparticles may be higher or lower than the bulk liquid crystal composition. The nanoparticles or microparticles may be opaque. The nanoparticles may comprise titanium dioxide. A nanoparticle may be defined as a particle with a volume equivalent median diameter of less than 1 μm. A microparticle may be defined as a particle with a volume equivalent median diameter of less than 1 mm. 
     The liquid crystal combination may comprise localised polymer networks to increase scattering. 
     The liquid crystal composition may comprise a viscosity reducing agent. The liquid crystal composition may comprise a redox dopant to improve lifetime of the liquid crystal composition. 
     The liquid crystal composition may comprise a chiral nematic phase with a pitch of 200 nm to 400 nm or a pitch of 50 nm to 1000 nm. The liquid crystal composition may comprise a chiral nematic phase with a pitch of 200 μm to 600 nm. The liquid crystal composition may comprise a liquid crystal with a pitch of less than 600 nm, or less than 400 nm, or less than 200 nm. 
     At least one of the first and second cell walls may comprise an anti-reflective coating. 
     The device may comprise a plurality of cells. The cells may be driven from the same controller. Static diffusers may be interspersed between the cells. 
     The device may comprise a temperature controlled mount/stage for the optical cell for controlling the operating temperature of the optical cell. 
     The device may further comprise a controller configured to operate the temperature controlled mount/stage to keep the liquid crystal composition at an operating temperature (or range of operating temperatures). The operating temperature (or range of operating temperatures) may be nearer to an upper transition temperature of the chiral nematic phase than a lower transition temperature of the chiral nematic phase. 
     According to a third aspect, there is provided a display projection system, comprising a coherent light source and the cell or device according to the first or second aspect, the cell or device configured to reduce speckle contrast in an image projected by the system. 
     According to a fourth aspect, there is provided a method of speckle reduction, comprising: transmitting a coherent light beam through a liquid crystal composition in a liquid crystal cell; causing fluid turbulence in the liquid crystal composition, the fluid turbulence causing dynamic scattering of the coherent light beam. 
     The method may comprise transmitting the beam of light through the liquid crystal cell. Where the liquid crystal cell comprises reflective cell wall, the method may comprise transmitting a beam of light through at least part of a liquid crystal cell. 
     Causing fluid turbulence may comprise applying an electric field to the liquid crystal composition. Applying an electric field to the liquid crystal composition may comprise causing electrohydrodynamic instability in the liquid crystal composition due to movement of mobile ions in the liquid crystal composition. 
     Causing fluid turbulence may comprise pumping the liquid crystal composition. 
     The method may comprise using a cell or device according to either of the first or second aspect to reduce speckle. 
     Applying an electric field may comprise applying a voltage between a first and second electrode of the liquid crystal cell. 
     Applying an electric field may comprise applying a time varying voltage between the first and second electrode. 
     The time varying voltage may have a frequency of between 50 Hz and 500 Hz. The applied field may be less than 20V/μm. The applied voltage may be less than 300V. 
     Each and every feature of each aspect may be combined with each and every feature of each other aspect, as appropriate. For example, features described with reference to a device may be included in the cell, and also in the method. A device or cell may be configured to perform a feature described with reference to a method, and a method may use a device or cell including any features described herein. 
    
    
     
       Embodiments of the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a device according to an embodiment comprising a transmissive cell; 
         FIG. 2  is a schematic diagram of a device according to an embodiment comprising a reflective cell wall; 
         FIG. 3  is a schematic diagram of a device according to an embodiment comprising two cells arranged in series; 
         FIG. 4  illustrates the mechanism of operation of a device according to an embodiment; 
         FIG. 5  is a diagram of a test arrangement for evaluating speckle reduction; 
         FIG. 6  is a plot of a speckle contrast parameter C as a function of field amplitude E for a first example embodiment at 25° C., comparing the parameter C obtained according to an embodiment with those for a rotating ground glass diffuser and for no speckle reduction device; 
         FIG. 7  is a) microscope images of a cell according to a first example embodiment at 25° C. at different applied electrical fields, b) corresponding images of the speckle pattern recorded by the LCD in the test arrangement, c) plots of intensity along a central line of pixels across the width of the images; 
         FIG. 8  is a plot of speckle contrast C, as a function of electrical field E, at 25° C., for a first example embodiment; 
         FIG. 9  is a plot of speckle contrast C, as a function of camera exposure time for a first example embodiment at 25° C., with E=10V/μm; 
         FIG. 10  is a plot of speckle contrast as a function of time at 25° C. for a first example embodiment under constant electric field conditions for 2 hour periods; 
         FIG. 11  is a plot of inverse chiral pitch length as a function of wt. % chiral dopant concentration for a second example embodiment; 
         FIG. 12  is a speckle contrast map showing speckle contrast at varying applied electric field strengths and varying applied electric field frequencies for a second example embodiment at 25° C.; 
         FIG. 13  ( a - c ) is a number of speckle contrast maps showing speckle contrast at varying applied electric field strengths and varying applied electric field frequencies for a second example embodiment at temperatures of 25° C., 40° C. and 55° C.; 
         FIG. 14  ( a - d ) is a number of plots showing parameters (speckle contrast, transmission, applied electric field strength, applied electric field frequency) as a function of temperature for a first example embodiment; 
         FIG. 15  is a plot of inverse chiral pitch length as a function of wt. % chiral dopant concentration for a third example embodiment; 
         FIG. 16  is a speckle contrast map showing speckle contrast at varying applied electric field strengths and varying applied electric field frequencies for a third example embodiment at 25° C.; 
         FIG. 17  ( a - d ) is a number of plots showing parameters (speckle contrast, transmission, applied electric field strength, applied electric field frequency) as a function of chiral pitch length for the second and third example embodiments at 25° C.; and 
         FIG. 18  is a schematic of an image projector according to an embodiment. 
     
    
    
     Referring to  FIG. 1 , a device  100  for speckle reduction is shown, comprising a LC cell  110  and controller  130 . The LC cell  110  comprises a first cell wall  121  and second cell wall  122 , spaced apart by a gap. The first and second cell wall  121 ,  122  respectively comprise a first and second electrode  111   a ,  111   b  for applying an electric field in the gap. In this embodiment the electric field is applied perpendicular to the first and second wall, but other embodiments may comprise a lateral electrode arrangement, so that the field is applied parallel to the first wall  121  or second wall  122 . In some embodiments the first and second electrode may both be disposed on the first wall, or on the second wall. These electrodes may be in an in-plane configuration in which the two opposite polarity electrodes are interdigitated within the same plane. 
     In some embodiments the first and second electrode  111   a ,  111   b  may configured to contact the liquid crystal disposed in the gap  114 , to inject charge into the liquid crystal composition. The injected charge may result in an increased density of charge carriers or an increased average charge on mobile ions or molecules. The injection of charge may therefore enhance scattering and/or reduce the electrical field required to cause electrohydrodynamic instability in the LC composition. In some embodiments at least one further electrode may be provided for charge injection, independent of the electrodes controlling the electrical field in the gap (which may not be in contact with the liquid crystal). 
     The first and second cell wall  121 ,  122  are preferably planar and substantially parallel, but other arrangements may also be used. The first and second cell wall  121 ,  122  are both preferably substantially transparent at a wavelength of interest, and are at least transmissive at the wavelength of interest. Each of the first and second cell wall  121 ,  122  may comprise an electrode comprising substantially transparent electrically conductive material  111   a ,  111   b  (e.g. Indium Tin Oxide, ITO), disposed on a substantially transparent support layer  112  which may be non-conducting (e.g. glass). An alignment layer  113  (e.g. an anti-parallel rubbed polyimide layer) may optionally be provided on the interior faces of the first and second cell walls  121 ,  122 , but this is not essential. 
     A LC composition  114  is disposed in the gap between the first and second cell walls  121 ,  122 . The LC composition  114  is conductive and comprises mobile ions. 
     The controller  130  is configured to apply a voltage between the first and second electrodes (associated with the first and second walls respectively), so as to cause an electric field in the gap. The electric field is sufficient to cause a dynamic scattering state in the LC composition  114  due to electrohydrodynamic instability in the LC composition  114  resulting from movement of the mobile ions in response to the electric field. The movement of the mobile ions results in a spatial distribution of charge that imposes a torque on the LC molecules. The controller  130  is preferably configured to alternate the polarity of the electric field, to prevent build-up of the mobile ions adjacent to one of the first or second walls  121 ,  122 . The frequency of alternation may be from 50 Hz to 1 kHz, or from 100 Hz to 500 Hz. Speckle in a projected image formed using the coherent light beam  101  is reduced by passing the coherent light beam  101  through the cell  110 , to produce a scattered light beam  102 . 
       FIG. 2  shows an alternative embodiment, which includes all the features of  FIG. 1 , with the exception that the first electrode  111   a  in the embodiment of  FIG. 2  is at least partially reflective (e.g. comprising gold, aluminium etc), and the cell  110  is configured to operate in a reflective mode in which a coherent light beam  101  is reflected by the cell  110  as a scattered light beam  102 . This double pass through the LC composition may enhance scattering and facilitate a reduced gap (which in turn may reduce the voltage required to set up the necessary field). 
       FIG. 3  illustrates a further embodiment in which a first and second transmissive cell  110  are arranged in series, to produce a scattered light beam  102  from the interaction of a coherent light beam  101  from a light source  201  (e.g. a laser) with the first and second transmissive cell  110 . Arrangements with a plurality of cells  110  may be used to enhance scattering of the coherent beam, or to reduce the required voltage for a given degree of scattering (by reducing the gap in each cell, which may increase the applied field for a given voltage). The controller  131  in this example is configured to apply the same voltage difference between the electrodes of the first and second cells. This will produce the same field, should the first and second cells be of identical construction, which is not essential. In some configurations, one or more transmission mode cells  110  may be combined with a reflection mode cell  110 . 
     This mechanism is illustrated in  FIG. 4 , with reference to a chiral (right-handed) nematic LC composition with positive dielectric anisotropy. With no electrical field a), the LC composition is in a chiral nematic state, with the helical axes normal to the plane of the first and second cell walls  121 ,  122 . 
     When a sufficient electric field (E&gt;E t ) is produced b) between the first and second cell walls  121 ,  122  by applying a voltage between the first and second electrodes of the first and second cell walls  121 ,  122 , movement of the mobile ions leads to dynamic disruption of the chiral nematic phase of the LC composition. This disruption is dynamic due to the turbulence induced in the LC material caused by redistribution of the mobile ions in response to the field. This is different to prior art approaches to speckle reduction using LC cells, which rely either on complex time varying electrical fields, or which toggle rapidly between a number of fixed stable LC phases to achieve speckle reduction. In embodiments of the present invention, the dynamic scattering is caused by the movement of mobile ions in response to an applied field, which disrupts the orientation of the LC molecules. 
     The amount of applied field required to achieve dynamic scattering due to charge redistribution in the liquid crystal depends on the specifics of the LC composition (and for embodiments that employ charge injection, may also depend on the electrode properties), In general, more conductive LC compositions (with relatively high ion mobility, and/or with a relatively high concentration of ions) tend to be dynamically excited with a reduced electric field. In some embodiments, the conductivity of the LC composition may be enhanced by addition of a dopant, and in other embodiments the LC molecules may themselves be inherently ionic. Where a dopant is used to enhance conduction, the dopant may be inherently ionic, or may give rise to ions in the LC composition by accepting or donating electrons. A LC composition with low conductivity may still be operable in accordance with an embodiment, but with an increased electrical field (which may have cost implications). 
     Where the applied electric field is parallel with the quiescent (no field) orientation of the chiral nematic phase, positive dielectric anisotropy of the LC composition may assist in disrupting the orientation of the quiescent chiral nematic phase into a dynamic focal conic phase, in which micro-helices of chiral nematic LC material move turbulently due to electrohydrodynamic instability. In cells configured to provide an electric field perpendicular to the quiescent orientation of the chiral nematic phase, negative dielectric anisotropy may assist in disrupting the orientation of the chiral nematic phase. 
     At higher applied field c), above a critical field E C , the LC material may transition to a nematic phase, in which the molecules of LC material are orientated parallel with the applied electric field. This static state will not dynamically scatter to disrupt speckle. 
     To measure the speckle contrast C, a Helium-Neon (He—Ne) LASER (JDS Uniphase 1122P, λ=633 nm) was used as the coherent light source  201 , as shown in a diagram of the experimental apparatus in  FIG. 5 . A microscope objective  205  collects light transmitted through the cell and a CCD camera  207  operating in the linear regime then captures an image of the resulting spot that is projected onto the projection screen  206 . In the captured image, a selection of 200×200 pixels from the centre of the beam are then analysed to calculate C. A combination of signal generator and amplifier were used to apply a square wave ac electric field across the cell electrodes under test. The CCD  207  and signal generator were both computer controlled to ensure accurate timing of the electric field condition and image acquisition. 
     A range of neutral density filers  204  were used to maintain a near-constant mean intensity in each image. Histograms for every image recorded were checked to ensure no over- or under-exposure of the light occurred. A temperature controlled stage  202  was used to ensure a constant operating temperature for the LC cell throughout each measurement reported herein. Normal white printing paper was used as a projection screen  206  and the CCD  207  was placed at a distance of 160 mm from the screen  206  at an angle of 25°. A monochrome CCD was selected to maximise resolution as the Bayer filter layout used in colour CCDs renders up to 75% of the sensors obsolete when imaging monochromatic light. 
     Speckle measurements vary considerably with different experimental set-ups, making it somewhat difficult to compare directly the reported speckle contrast values. Nevertheless, a number of factors may be significant. These include 1) ensuring ambient lighting is less than the least significant bit of the dynamic range; 2) a minimal camera bit-depth of 6; 3) a ratio of pixel area to minimum speckle grain area of at least 3.5; and 4) a ratio of maximum speckle intensity to camera saturation of 0.3-1.0. All of these conditions were satisfied in the experimental set-up used to illustrate embodiments. 
     With applications such as imaging and holographic projection in mind, conditions were chosen to observe speckle in a way that closely matches ‘human perception’. A 3.2 mm iris in front of the CCD sensor  207  was used to match the average pupil size under similar lighting conditions. The integration time of the camera was set to 50 ms (with a range of exposure times also reported), which corresponds to the temporal integration time of the eye. The focussing lens used with the camera (focal length=33 mm) was selected to spatially sample incoming light with a camera pixel density equal to the density of cones in the eye, which corresponds to a pixel-speckle ratio of 3.5 as required above. 
     The cell walls  121 ,  122  used in all example embodiments described herein comprised two indium tin oxide-coated glass substrates with antiparallel rubbed polyimide alignment layers. In all cases, the gap between the cell walls  121 ,  122  was defined by the 20 μm diameter spacer beads that were dispersed throughout the cell. After the mixtures had been prepared, each cell was filled by capillary action at a temperature of T=80° C., before allowing them to cool to room temperature. 
     In other embodiments, an alternative material (that is substantially transmissive at the wavelength of interest) may be used for the cell walls, such as a polymeric material (e.g. polycarbonate). Any suitable electrode material may similarly be used, for example any transparent conducting oxide (ZnO, IGZO etc). Although spacer beads are a convenient way of spacing the cell walls apart, in other embodiments an alternative arrangement may be used. For example, the first and second cell walls may be portions of a monolithic cell arrangement. The spacer beads may be opaque to improve the scattering. 
     The alignment layer  113  may alternatively be a structured surface to disrupt the alignment of the liquid crystal molecules. This structured surface may be a patterned or etched glass, metal or polymer. The device may optionally incorporate a static diffuser to enhance the scattering. This static diffuser may be glass or polymer-based. The static diffuser may be incorporated into the cell walls or be external to the cell. 
     The experimental results shown in  FIGS. 6 to 10  were obtained for a first example device according to an embodiment. In the first example embodiment the chiral nematic LC composition  114  consisted of the wide temperature range nematic LC, BL006 premixed with chiral dopant R5011. This mixture formed a chiral nematic LC phase at room temperature with a right-handed helical structure, which was confirmed using optical polarizing microscopy. At a temperature of 25° C. the pitch was found to be 250 nm. A frequency of f=160 Hz was used in all experiments reported in  FIGS. 6 to 10 . 
     Although the first example LC composition is a chiral nematic LC composition, it is not essential that the composition should have a chiral nematic phase and any suitable LC composition may be used. For example, in other embodiments the LC composition may be nematic (non-chiral). In some embodiments the LC composition may have a Smectic A phase, or a blue phase. 
     The first example LC composition exhibits positive dielectric anisotropy, but this is not essential, and an LC composition with isotropic dielectric properties may be used, or a composition with negative dielectric anisotropy. 
     To enhance the conductivity properties of the mixture, an ionic species in the form of cetyltrimethylammonium bromide (CTAB) was added to the first example LC composition in selected cells at a concentration of 0.1 wt. %. To a first approximation, the conductivity of an isotropic liquid is determined by the ionic charge, the ionic concentration, and the mobility. By dispersing an ionic dopant (e.g. CTAB) into the host mixture, the ionic charge and the concentration are both increased, which in turn reduces the electric field amplitude required to induce electrohydrodynamic instability. In other embodiments an inherently ionic LC material may be used, or a different dopant may be added. Examples of inherently ionic dopants include: cetyltrimethylammonium bromide (CTAB), tetrabutylammonium picrate (TBAP), tetrabutylammonium bromide (TBAB), dodecyltrimethylammonium bromide (DTAB). Examples of acceptor dopants include tetracyanoethylene (TCE), tetracyanoquinodimethane (TCQM), and 2,3-dichloro-5,6-dicyanobenzoquinone (DCDCBQ). An example of a donor dopant is p-phenylenediamine (FDA). Other dopants can be used, and mixtures of different dopants are also envisaged. 
       FIG. 6  presents measurements of speckle contrast C as a function of the applied electric field amplitude at 25° C. for various conditions. A baseline value  401  of contrast C with no LC material is shown, along with a value of contrast C for a rotating ground glass disk (RGGD)  402 . The rotating ground glass diffuser gave a value of C=0.04, in good agreement with previously published results for similar devices. The speckle contrast parameter for the He—Ne alone was found to be 0.611±0.008, close to the theoretical value of 1/√2. 
     Also shown in  FIG. 6  are results from a cell according to an embodiment comprising the first example chiral nematic LC mixture with an ionic dopant  403  and without an ionic dopant  404 . For the range of electric field amplitudes considered here (up to 12 V/μm), no dynamic scattering and consequently no reduction in C was observed with no ionic dopant, although a higher excitation voltage could be used to cause electrohydrodynamic instability and consequently dynamic scattering. 
     In contrast, for a device according an alternative embodiment, comprising the same first example chiral nematic LC mixture (BL006 premixed with R5011) and with the addition of an ionic dopant, (in this example CTAB), a dynamic scattering state was observed with a relatively low electric field threshold of E=4 V/μm. A further increase in the electric field strength then resulted in an increase in the turbulence of the scattering state, which in turn served to reduce the magnitude of C to a value of 0.22 for E=12 V/μm. The speckle reduction was calculated as 64% for the example embodiment with ionic dopant at 12 V/μm. Notably, some embodiments enable control over the amount of reduction of speckle contrast C by varying the applied field E. 
       FIG. 7 a    shows microscope images of the CTAB-doped first chiral nematic example LC cell at 25° C. taken between uncrossed polarisers with: E=0V/μm  301 ; E=6V/μm  302 ; and E=12V/μm  303 . At E=0V/μm there is no observable turbulence in the cell, but above E=4V/μm dynamic scattering is observed, which increases with increasing electric field. The dynamic scattering in the first example LC composition  114  results in a marked drop in transmission to almost 13% at E=12V/μm. (In comparison, the RGGD was found to have a transmission of 33%.) Three 20 μm spacer beads are also present in each image.  FIG. 7 b    shows the 200×200 pixel, monochrome images of the speckle pattern that were recorded for three different magnitudes of the applied electric field. The mean intensity of each image has been normalised in order to allow for a direct comparison. A plot of pixel intensities through the central line of the image is provided in  FIG. 7 c   , where it can be seen that the contrast between the light and dark regions in the granular pattern has been significantly reduced due to dynamic scattering in the LC. 
       FIG. 8  presents results of speckle contrast C as a function of the applied electric field at 25° C. upon increasing and decreasing the amplitude for five consecutive cycles, with the electric field being held constant for 2 s at each level. The results for the first cycle up  405  are substantially identical to the results of the fifth cycle down  406 . In this example, the electric field was increased from E=0 V/μm to E=12 V/μm and subsequently decreased back to E=0 V/μm in increments of 0.1 V/μm. Clearly, there is little in the way of hysteresis between the first and the fifth cycle. 
     In addition, a study was performed of the speckle contrast dependency on frequency of the applied electric field at 25° C. It was found that C varies very little over a range of f=150-450 Hz, irrespective of E-field amplitude. A speckle contrast as low as 0.214 was achieved with E=12 V/μm and f=275 Hz. With a sufficiently high frequency (approximately &gt;20 kHz at 12V/μm, 25° C.), dynamic scattering disappears as a result of the transition from the ‘conductive’ regime, in which charge redistribution results in dynamic scattering, to a regime that is substantially dielectric (without sufficient charge redistribution over each cycle to cause dynamic scattering). At frequencies below 150 Hz, speckle contrast is again increased. This may be due to the ion transport time being shorter than the time period between the reversals in the polarity of the E-field, resulting in accumulation of charge. 
       FIG. 9  illustrates the speckle contrast  421  observed for the CTAB-doped first example cell, recorded as a function of camera exposure time (with a field E=10 V/μm) at 25° C. Increasing exposure time reduces speckle contrast, as more independent speckle patterns are averaged during the integration time. These results suggest that further speckle reduction could be achieved through a faster response of the dynamic scattering process. 
       FIG. 10  shows the results of investigating repeatability and stability of an example device. The example device was tested by applying a constant E-field amplitude to a CTAB-doped first example chiral nematic LC cell at 25° C. for two-hour periods and a measurement of C was taken every 1 s. Measurements were taken over two hours for each of 6, 8, 10 and 12 V/μm. The results are respectively plotted  411 ,  412 ,  413 ,  414  in  FIG. 10 , with each data point showing the average value of C measured over a five minute period.  FIG. 10  shows that the output is substantially constant over two hours for electric field amplitudes up to E=12 V/μm. 
     The experimental results shown in  FIGS. 11, 12 and 13  were obtained for a second example device according to an embodiment. In this second example embodiment the chiral nematic LC composition  114  consisted of the nematic LC E7 premixed with high twisting power chiral dopant BDH1281. This mixture formed a chiral nematic LC phase at room temperature. 
       FIG. 11  shows a plot of reciprocal chiral pitch length as a function of wt. % concentration chiral dopant BDH1281 for the mixture of E7 with BDH1281. The reciprocal chiral pitch of the chiral nematic LC composition  114  increases linearly with increasing chiral dopant concentration. The data presented includes both measured values  430  and estimated values  432 , the measured results being in agreement with theoretical predictions. 
       FIG. 12  shows a speckle contrast map for a chiral nematic LC composition  114  with a chiral pitch length of 317 nm. The speckle contrast map shows values of speckle contrast measured at varying applied electric field frequency (between 0 Hz and 200 Hz) and varying applied electric field strength (between 0 V/μm and 20 V/μm) at 25° C. for the chiral nematic LC composition  114 . The speckle contrast map is a “heat map” where values of speckle contrast are shown using a colour gradient. Low values of speckle contrast are shown in one colour (near 0 in the legend of  FIG. 12 ) and high values of speckle contrast are shown in another (near 0.6 in the legend of  FIG. 12 ). A minimum in speckle contrast is seen at an applied electric field strength of 13.6 V and an applied electric field frequency of 75 Hz. This minimum speckle contrast is shown by the outlined region  434 . The speckle contrast map of  FIG. 12  shows the speckle contrast increasing as the applied electric field strength and applied electric field frequency values are changed away from the region of minimum speckle contrast  434 . 
       FIG. 13  shows speckle contrast maps for chiral nematic LC composition  114  with a chiral pitch length of 273 nm at temperatures of 25° C., 40° C. and 55° C. The speckle contrast map shown in  FIG. 13 c    shows values of speckle contrast measured at varying applied electric field strength (between 0 V/μm and 20 V/μm) and varying applied electric field frequency (between 0 and 240 Hz) at 55° C. A minimum in speckle contrast is seen at an applied electric field strength of approximately 13 V/μm and an applied electric field frequency of approximately 160 Hz. This minimum speckle contrast is shown by the outlined region  428 . The speckle contrast map of  FIG. 13 c    shows the speckle contrast increasing as the applied electric field strength and applied electric field frequency values are changed away from the region of minimum speckle contrast  428 . The trend across the speckle contrast maps of  FIGS. 13 a  to 13 c    shows that the range of applied electric field strengths and applied electric field frequencies where a low value of speckle contrast (comparable to the minimum speckle contrast) is observed is much broader for the sample trialled at 55° C. ( FIG. 13 c   ) than for the sample trialled at 25° C. ( FIG. 1 a   ). This indicates that a speckle reduction device  100  operated at a higher temperature may be more tolerant to variations in other operating parameters (e.g. applied electric field strength, applied electric field frequency). The minimum speckle contrast of  FIG. 13 c    is also lower than the minimum speckle contrast shown in  FIG. 13 a   , and achieved at a lower applied electric field strength. A lower minimum speckle contrast and a broader range of possible operating conditions indicate improved performance of speckle reduction device  100  at higher temperatures. 
     The experimental results shown in  FIG. 14  were obtained for an example device according to the first example embodiment. In this first example embodiment the chiral nematic LC composition  114  consisted of the nematic LC BL006 premixed with chiral dopant R5011. This mixture formed a chiral nematic LC phase at room temperature. 
       FIG. 14  includes a number of plots  420 - 427  showing minimum speckle contrast and other parameters (transmission, applied electric field strength, applied electric field frequency) at minimum speckle contrast as a function of temperature for a chiral pitch length of 273 nm measured at 25° C.). Each data point corresponds with the operating conditions (frequency, and field) that give either the minimum speckle contrast at that temperature, or the minimum Figure of Merit (FOM) at that temperature. The FOM is the product of speckle contrast and transmission loss. A minimum FOM value represents peak performance of the speckle reduction device  100 , where minimal speckle contrast is balanced against maximal transmission of light. 
       FIG. 14 a    shows plot  420  of applied electric field strength required for minimum speckle contrast as a function of temperature for a chiral pitch length of 273 nm (as measured at 25° C.). Plot  420  clearly shows a negative correlation between temperature and applied electric field strength required for minimum speckle contrast. A similar trend is seen for plot  421  showing the applied electric field strength required for minimum FOM as a function of temperature. 
       FIG. 14 b    shows plot  422  of applied electric field frequency required for minimum speckle contrast as a function of temperature for a chiral pitch length of 273 nm (as measured at 25° C.). Plot  422  clearly shows a positive correlation between temperature and applied electric field frequency required for minimum speckle contrast. A similar trend is seen for plot  423  showing the applied electric field frequency required for minimum FOM as a function of temperature. 
       FIG. 14 c    shows plot  424  of minimum speckle contrast as a function of temperature for a chiral pitch length of 273 nm (as measured at 25° C.). Plot  424  shows a negative correlation between temperature and minimum speckle contrast. A similar trend is seen for plot  425  showing the minimum FOM as a function of temperature. 
       FIG. 14 d    shows plot  426  of percentage light transmission at minimum speckle contrast as a function of temperature for a chiral pitch length of 273 nm (as measured at 25° C.). Plot  426  clearly shows a positive correlation between temperature and transmission. A similar trend is seen for plot  427  showing the transmission at minimum FOM as a function of temperature. 
     The results from  FIGS. 14 a - d    taken in context with one another indicate that increasing the temperature at which speckle reduction device  100  operates yields lower speckle contrast at lower applied electric field strengths with higher transmission. The change in properties may be due to a reduction in viscosity of the LC composition  114  with increasing temperature and/or reduced birefringence of LC composition  114  with increasing temperature. The temperature of operation of speckle reduction device  100  is therefore a parameter that may be important for improved performance. The temperature range suitable for operating speckle reduction device  100  is limited to the temperature range of the chiral nematic phase of chiral nematic LC composition  114  (i.e. the temperatures between an upper transition temperature and a lower transition temperature where composition  114  undergoes phase transformations from the chiral nematic phase). It may be optimal to choose a composition  114  with an upper transition temperature close to the operating temperature of speckle reduction device  100 . 
     The experimental results shown in  FIGS. 15 and 16  were obtained for a third example device according to an embodiment. In this third example embodiment the chiral nematic LC composition  114  consisted of the nematic LC E7 premixed with low twisting power chiral dopant R811. This mixture formed a chiral nematic LC phase at room temperature. 
       FIG. 15  shows a plot of reciprocal chiral pitch length as a function of wt. % concentration chiral dopant R811 for the mixture of E7 with R811. The reciprocal chiral pitch of the chiral nematic LC composition  114  increases linearly with increasing chiral dopant concentration. The data presented includes both measured values  440  and estimated values  442 . 
       FIG. 16  shows a speckle contrast map for chiral nematic LC composition  114  with a chiral pitch length of 518 nm. Again, the speckle contrast map shown in  FIG. 16  shows values of speckle contrast measured at varying applied electric field frequency (between 0 Hz and 100 Hz) and varying applied electric field strength (between 0 V/μm and 20 V/μm) at 25° C. A minimum in speckle contrast is seen at an applied electric field strength of 8.5 V and an applied electric field frequency of 30 Hz. This minimum speckle contrast is shown by the outlined region  444 . The speckle contrast map of  FIG. 16  shows the speckle contrast increasing as the applied electric field strength and applied electric field frequency values are changed away from the region of minimum speckle contrast  444 . 
       FIG. 17  includes a number of plots  450 - 464  showing minimum speckle contrast and other parameters (transmission, applied electric field strength, applied electric field frequency) at minimum speckle contrast as a function of chiral pitch length. The data presented shows data collected for the second example embodiment and the third example embodiment. Each data point corresponds with the operating conditions (frequency, and field) that give the minimum speckle contrast for that composition. The data from which the plots  450 - 464  in  FIG. 17  are generated is shown in Table 1 below. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                 Chiral 
                 Pitch 
                 Speckle 
                 Transmission 
                 Field 
                 Frequency 
               
               
                 Dopant 
                 (nm) 
                 Contrast 
                 (%) 
                 (V/μm) 
                 (Hz) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 R811 
                 1977 
                 0.530 
                 30 
                 2.3 
                 20 
               
               
                   
                 1018 
                 0.235 
                 9 
                 4.5 
                 20 
               
               
                   
                 612 
                 0.200 
                 16 
                 7.1 
                 20 
               
               
                   
                 518 
                 0.178 
                 19 
                 8.5 
                 30 
               
               
                   
                 397 
                 0.209 
                 26 
                 9.8 
                 20 
               
               
                 BDH1281 
                 2730 
                 0.64 
                 46 
                 2.0 
                 20 
               
               
                   
                 1405 
                 0.44 
                 17 
                 3.2 
                 20 
               
               
                   
                 862 
                 0.24 
                 5 
                 4.8 
                 20 
               
               
                   
                 517 
                 0.14 
                 7 
                 8.4 
                 40 
               
               
                   
                 423 
                 0.16 
                 11 
                 10.0 
                 50 
               
               
                   
                 403 
                 0.15 
                 13 
                 10.4 
                 60 
               
               
                   
                 388 
                 0.16 
                 14 
                 11.0 
                 60 
               
               
                   
                 295 
                 0.15 
                 18 
                 14.8 
                 90 
               
               
                   
                 317 
                 0.14 
                 16 
                 13.6 
                 75 
               
               
                   
                 273 
                 0.14 
                 22 
                 16.2 
                 90 
               
               
                   
                 244 
                 0.13 
                 17 
                 15.8 
                 85 
               
               
                   
                 220 
                 0.14 
                 19 
                 18.0 
                 85 
               
               
                   
               
            
           
         
       
     
       FIG. 17 a    shows plots  450 ,  452  of speckle contrast as a function of chiral pitch length for the second and third example embodiments respectively. These plots clearly show a linear relationship between speckle contrast and pitch length for the chiral nematic LC compositions  114  of both the second and third example embodiments. LC compositions  114  with a pitch length in the range of approximately 200 nm to approximately 600 nm exhibited similar minimum speckle contrast values. 
       FIG. 17 b    shows plots  454 ,  456  of percentage light transmission as a function of chiral pitch length for the second and third example embodiments respectively. These plots show that light transmission approaches a minimum when the pitch length approaches the wavelength of light (633 nm) used to measure the speckle contrast properties of the chiral nematic LC compositions  114 . 
       FIG. 17 c    shows plots  458 ,  460  of applied electric field strength required for minimum speckle contrast as a function of chiral pitch length for the second and third example embodiments respectively. These plots show that the applied electric field strength required for minimum speckle contrast exponentially decreases as chiral pitch length increases. As with  FIG. 15 b   , a compromise between minimum speckle contrast and energy usage may be required for practical application. 
       FIG. 17 d    shows plots  462 ,  464  of applied electric field frequency required for minimum speckle contrast as a function of chiral pitch length for the second and third example embodiments respectively. These plots show a similar trend to  FIG. 15 c    in that the applied electric field frequency required for minimum speckle contrast decreases as chiral pitch length increases. 
       FIGS. 17 a - d    show that similar trends are observed for LC compositions  114  with either a high twisting power chiral dopant (second example embodiment) or low twisting power chiral dopant (third example embodiment). 
     The data in  FIG. 17  indicates that a smaller chiral pitch results in improved speckle contrast reduction, and for chiral pitch &lt;600 nm, smaller chiral pitch also improves light transmission. Against this, smaller chiral pitch results in a requirement for higher field strength to achieve optimal performance. However, it has also been shown that increased operating temperature acts to lower speckle contrast as well as reduce the field strength required for minimum speckle contrast. A compromise between chiral pitch length, operating temperature and applied electric field strength may be preferable to achieve optimal performance of speckle reduction device  100 . 
     Further speckle reduction should be possible using alternative embodiments in which the LC composition has improved properties, such as increased birefringence and conductive anisotropy and faster response time. 
     In some embodiments, multiple cells may be stacked adjacent to one another, increasing the reduction in speckle contrast without requiring the increased voltage that would be required to produce sufficient electrical field across a thicker gap/LC layer. 
     Embodiments of the invention have the potential to provide a low cost, lightweight and low power solution to the problem of laser speckle. Example applications for devices of the invention include (but are not limited to): laser projectors (pico-projectors, smartphone projectors, cinema projectors, home cinema projectors, business projectors, holographic displays); machine vision systems (assembly lines, robots, automated inspection, drones, security); microscopy (two photon, confocal etc.); laser machining (micromachining, nanomachining, laser marking); LIDAR (laser radar range finding); free space communications systems; holographic storage systems; and optical measurement systems (e.g. interferometers, low coherence interferometers). 
       FIG. 18  illustrates a projector  250  according to an example embodiment, comprising a coherent light source  201  (e.g. laser), a speckle reduction device  100 , objective lens  205 , spatial light modulator  211 , for projecting an image on a projection screen  206 . The speckle reduction device  100  may be as described herein. A temperature controlled mount  202  may be provided for controlling the temperature of the device  100 . A low cost, compact projector (e.g. a pico-projector) without perceptible speckle noise may be facilitated by the use of embodiments of the speckle reduction device disclosed herein. 
     It will be appreciated that speckle resulting from coherent light is not a problem restricted to light in the visible spectrum, and that the invention is applicable to non-visible wavelengths, such as UV and IR. 
     The cell may take any form that allows a volume of liquid crystal composition to be constrained. For example, it may take the form of a small square or circular element of a few square millimetres for positioning in front of a laser beam, or may be in the form of a large flat panel for a video screen. The cell may be a cavity within other optical elements and devices (for example, lenses, filters) or for example be integral to a laser housing. 
     Although in the example embodiments shown a square wave drive signal is used, the drive signal may take many forms, for example (without limitation) sinusoidal, triangular, sawtooth, a square wave (or sinusoidal etc.) which varies in frequency and/or amplitude over time, pseudorandom signals, and combinations of signals. 
     The skilled person will appreciate that other variations are possible, and the example embodiments are not intended to limit the scope of the invention, which should be determined with reference to the accompanying claims.