Device for controlling transmittance of light

Abstract

A liquid crystal device with non-uniformly spaced, substantially elongated and plural electrodes (512 and 510, fig 8) within each electrode layer. The LCD device may further comprise a second, third a

Description

The present application relates to a device for controlling transmittance of light, an assembly comprising the device and at least one optical element, an apparatus comprising the device, a processor, and a storage comprising instructions for controlling the device, and a method of operating such a device and a method of manufacturing such a device. Control of the divergence cone shape and angle of light illumination is desirable in many applications. Technically this can be achieved by mechanically moving refractive lenses. While functionally effective, such solutions are comparatively bulky or require precise control of individual lenses within the illumination system. This results in a relatively complex and costly system. By way of example only, examples are described in detail below, with reference to the accompanying drawings, in which: Fig. 1 shows a representation of an example of a single LC cell; Fig. 2 shows a representation of an example of an electrode layer in the example LC cell of Fig. 1; Figs. 3(a) and 3(b) show a representation of an example operation of LC molecules of the LC cell of Fig. 1 in respective first and second states; Fig. 4 shows a representation of an example of a dual LC cell; Fig. 5 shows a representation of an example of a further electrode layer in the example dual LC cell of Fig. 4; Figs. 6(a) and 6(b) show a representation of an example method of manufacture of an example dual LC cell as shown in Fig. 4; Fig. 7(a) show a representation of an example of an IPS layered structure before folding; Fig. 7(b) show a representation of an example of an IPS layered structure after folding; Fig. 7(c) show a representation of an example of an FFS layered structure before folding; Fig. 8 shows a representation of an example of a second electrode layer in the example LC cell of Fig. 4; Fig. 9 shows a sectional representation of an example LC cell incorporating non-uniform electrode pacing; Fig. 10 shows a planar representation of an example LC cell incorporating non-uniform electrode pacing; Figs. 11(a) to 11(d) show a representation of an example of beam spreading in orthogonal axes; Figs. 12(a) to 12(d) show a representation of an example of electrode shapes; Fig. 13 shows a representation of an example headset incorporating a device; Fig. 14 shows a representation of an example of a system for operating the headset of Fig. 113; and Fig. 15 shows schematically an example of an apparatus. There is provided an optical device stack comprising: a liquid crystal, LC, layer positioned between a wave plate and an electrode layer, the optical stack being folded such that a first portion of the wave plate is parallel to a second portion of the wave plate. The wave plate may be a quarter wave plate, and the optical stack may be folded such that in combination the parallel portions of the quarter wave plate form a half wave plate. The optical stack may be folded such that a first portion of the LC layer is parallel to a second portion of the LC layer, the first and second portions of the LC layer defining first and second cells, and a first portion of the electrode layer is parallel to a second portion of the electrode layer, the first and second portions of the electrode layer respectively being associated with the first and second portions of the LC layer. The optical device stack may further comprise an adhesive layer to affix the first portion of the wave plate to the second portion of the wave plate. Each of the first and second portions of the electrode layer may comprise first and second electrode arrays, each of the first and second electrode arrays comprising first and second pluralities of interleaved, substantially elongated electrodes, the interleaved, elongated electrodes of the first portion of the electrode layer being substantially orthogonal to the interleaved, substantially elongated electrodes of the second portion of the electrode layer. The optical device stack may further comprise a further electrode layer, the optical stack being folded to fold the further electrode layer to provide first and second further portions of the further electrode layer for the first and second cells, first and second portions of the electrode layer comprising first and second electrode arrays, each of the first and second electrode arrays comprising a plurality of interleaved, substantially elongated electrodes, the interleaved, substantially elongated electrodes of the first portion of the electrode layer being substantially orthogonal to the interleaved, substantially elongated electrodes of the second portion of the electrode layer, the first and second portion of the further electrode layer providing a common electrode for the respective first and second pluralities of interleaved, elongated electrodes. The electrode layer may contain electrical connections for the first and second portions of the electrode layer, an electrical connection extending from the first portion to the second portion. In the folded structure the electrical connection for the first and second electrodes portions may be provided in-plane with one of the first and second electrode portions. The optical device stack may further comprise a first substrate layer adjacent the electrode layer, the optical stack being folded such that the first substrate layer provide an outer substrate of stack. The optical device stack further may comprise a second substrate layer between the LC layer and the quarter wave layer, the optical stack being folded such that first and second portions of the second substrate layer are positioned between the first and second portions of the quarter wave plate. The spacing of the interleaved, elongated electrodes in the first portion and / or the second portion may be uniform. The spacing of the interleaved, elongated electrodes in the first portion and / or the second portion may be non-uniform. incident light may be spread about substantially perpendicular axes by the respective first and second portions of the electrode layer. The optical stack may be folded twice, to form a four-cell LC layer stack, which optical device stack causes non-polarised incident light to be spread in two substantially perpendicular axes. There is provided an assembly comprising: an optical stack comprising a liquid crystal, LC, layer positioned between a wave plate and an electrode layer, the optical stack being folded such that a first portion of the wave plate is parallel to a second portion of the wave plate. The at least one further optical element may comprise at least one of: a waveguide, a luminance adjustment component, a lens, an image generation device, a reflection-reduction layer, or a protective layer. There is provided apparatus comprising: an optical device stack comprising a liquid crystal, LC, layer positioned between a wave plate and an electrode layer, the optical stack being folded such that a first portion of the wave plate is parallel to a second portion of the wave plate; at least one processor; and at least one storage comprising instructions, the instructions configured to, with the at least one processor, cause the apparatus to control one or more properties of the first and / or second portions of the LC layer. The apparatus may be configured to be mounted on a human head with the optical device cell stack positioned in a field of view of an eye of the human head. The apparatus may further comprise a first lens comprising a first one of the optical device stack and a second lens comprising a second one of the optical device stack. The field of view of the eye may be a first field of view of a first eye, and the first lens may be configured to be positioned in the first field of view, in use, and the second lens is configured to be positioned in a second field of view, of a second human eye of the human head, in use. The apparatus may be at least one of an augmented reality display device, a virtual reality display device or a mixed reality display device. There is provided a method of controlling a liquid crystal, LC, layer positioned between a wave plate and an electrode layer, the optical stack being folded such that a first portion of the wave plate is parallel to a second portion of the wave plate, the method comprising selectively applying a potential to the first and / or second portions of the electrode layer to selectively spread light in substantially perpendicular axes. Each of the first and second portions of the electrode layer may comprise first and second electrode arrays, each of the first and second electrode arrays comprising first and second pluralities of interleaved, substantially elongated electrodes, the interleaved, elongated electrodes of the first portion of the electrode layer being substantially orthogonal to the interleaved, substantially elongated electrodes of the second portion of the electrode layer, The method may further comprise selectively applying a potential across the first and second electrode arrays of the first portion or selectively applying a potential across the first and second electrode arrays of the second portion to selectively spread light in substantially perpendicular axes. The method may further comprise a further electrode layer, the optical stack being folded to fold the further electrode layer to provide first and second further portions of the further electrode layer for the first and second cells, first and second portions of the electrode layer comprising first and second electrode arrays, each of the first and second electrode arrays comprising a plurality of interleaved, substantially elongated electrodes, the interleaved, substantially elongated electrodes of the first portion of the electrode layer being substantially orthogonal to the interleaved, substantially elongated electrodes of the second portion of the electrode layer, the first and second portion of the further electrode layer providing a common electrode for the respective first and second pluralities of interleaved, elongated electrodes, The method may further comprise selectively applying a potential between the first electrode arrays of the first portion and the common electrode of the first portion of the further electrode, or selectively applying a potential between the second electrode array of the second portion and the common electrode of the second portion of the further electrode, to selectively spread light in substantially perpendicular axes. There is provided a method of manufacturing an optical device comprising: forming a stack comprising a liquid crystal, LC, layer positioned between a wave plate and an electrode layer, the electrode layer defining a first and second portion of substantially parallel and substantially elongate electrodes, the elongate electrodes in each portion being substantially parallel to each other; folding the optical stack, such that: a first portion of the wave plate is parallel to a second portion of the wave plate, in combination the parallel portions forming a half wave plate: a first portion of the LC layer is parallel to a second portion of the LC layer, the first and second portions of the LC layer defining first and second cells; and a first portion of the electrode layer is parallel to a second portion of the electrode layer, the first and second portions of the electrode layer respectively being associated with the first and second portions of the LC layer, the substantially parallel and substantially elongate electrodes of the first portion of the electrode layer being substantially orthogonal to the substantially parallel and substantially elongate electrodes of the second portion of the electrode layer. The method of manufacturing an optical device may further comprise: forming each of the first and second portions of the electrode layer with first and second pluralities of substantially elongated, substantially parallel and interleaved electrodes, each plurality for connection between a potential difference. The method of manufacturing an optical device may further comprise: forming a further electrode layer, the optical stack being folded to fold the further electrode layer to provide first and second further portions of the further electrode layer for the first and second cells, and forming each of the first and second portions of the electrode layer with a plurality of substantially elongated, substantially parallel and interleaved electrodes, each plurality and a portion of the further electrode for connection between a potential difference. The method of manufacturing may further comprise: further folding the folded structure to define third and fourth cells, each cells being associated with a portion of the electrode layer, and there being provided two pairs of half wave plates, each pair in combination providing a half-wave plate for a pair of cells. There is disclosed a device comprising: a first liquid crystal, LC, layer; a first electrode layer, the electrode layer comprising: a first plurality of substantially elongated, substantially parallel electrodes, the spacing between each electrode being non-uniform. The first electrode layer may further comprise: a second plurality of substantially elongated, substantially parallel electrodes interleaved with the first plurality of substantially elongated-substantially parallel electrodes, the spacing between each electrode of each of the first and second plurality being non-uniform. The first plurality of substantially elongated, substantially parallel electrodes and the second plurality of substantially elongated, substantially parallel electrodes may be for connection between a first potential difference, in a first state the potential difference is zero, and incident light passes through the first LC layer unchanged, in a second state the potential difference is non-zero, and incident light passing through the first LC layer is spread about a first axis, the extent of spreading being dependent on the size of the first potential difference. The device may further comprise: a second liquid crystal, LC, layer; a second electrode layer, the electrode layer comprising: a third plurality of substantially elongated, substantially parallel electrodes; and a fourth plurality of substantially elongated, substantially parallel electrodes interleaved with the third plurality of substantially elongated-substantially parallel electrodes, the spacing between each electrode of each of the third and fourth plurality being non-uniform. The third plurality of substantially elongated, substantially parallel electrodes and the fourth plurality of substantially elongated, substantially parallel electrodes may be for connection between a second potential difference, in a third state the potential difference is zero, and incident light passes through the second LC layer unchanged, in a fourth state the potential difference is non-zero, and the incident light passing through the second LC layer is spread about a second axis, the extent of spreading being dependent on the size of the second potential difference. The first and second substantially elongate electrodes may be substantially orthogonal to the third and fourth substantially elongate electrodes, the first and second axes being substantially orthogonal. The first electrode layer may be adjacent the first LC layer, and further comprising substrate layers either side of the first LC layer and the first electrode layer. The second electrode layer may be adjacent the second LC layer, and further comprising substrate layers either side of the second LC layer and the second electrode layer. The device may further comprise: a second electrode layer, the first electrode layer comprising the first plurality of substantially elongated, substantially parallel electrodes and the second electrode are for connection between a first potential difference, in a first state the potential difference is zero, and incident light passes through the first LC layer unchanged, in a second state the potential difference is non-zero, and incident light passing through the first LC layer is spread about a first axis, the extent of spreading being dependent on the size of the first potential difference. The device may further comprise: a second LC layer; a third electrode layer, the third electrode layer comprising a second plurality of substantially elongated, substantially parallel electrodes; and a fourth electrode layer, the third electrode layer and the fourth electrode layer for connection between a second potential difference, in a first state the potential difference is zero, and incident light passes through the first LC layer unchanged, in a second state the potential difference is non-zero, and incident light passing through the first LC layer is spread about a second axis, the extent of spreading being dependent on the size of the first potential difference. The substantially elongate electrodes of the first and third electrode layers may be substantially orthogonal, the first axis and the second axis are substantially orthogonal. The non-uniform spacing of the first electrode and the non-uniform spacing of the second electrode may be independent. The non-uniform spacing of the first electrode and the non-uniform spacing of the third electrode may be independent. The non-uniform spacing may be pseudo-random. There is provided an assembly comprising: a first liquid crystal, LC, layer; a first electrode layer, the electrode layer comprising: a first plurality of substantially elongated, substantially parallel electrodes, the spacing between each electrode being non-uniform; and at least one further optical element. The at least one further optical element may comprise at least one of: a waveguide, a luminance adjustment component, a lens, an image generation device, a reflection-reduction layer, or a protective layer. There is provided apparatus comprising: a first liquid crystal, LC, layer; a first electrode layer, the electrode layer comprising: a first plurality of substantially elongated, substantially parallel electrodes, the spacing between each electrode being non-uniform at least one processor; and at least one storage comprising instructions, the instructions configured to, with the at least one processor, cause the apparatus to control one or more properties of the LC layer. The apparatus may be configured to be mounted on a human head with the optical device cell stack positioned in a field of view of an eye of the human head. The may further comprise a first lens comprising a first one of the LC layer and first electrode and a second lens comprising a second one of the LC layer and first electrode. The field of view of the eye may be a first field of view of a first eye, and the first lens may be configured to be positioned in the first field of view, in use, and the second lens is configured to be positioned in a second field of view, of a second human eye of the human head, in use. The apparatus may be at least one of an augmented reality display device, a virtual reality display device or a mixed reality display device. There is provided a method of operating a device comprising: a liquid crystal, LC, layer; a first plurality of substantially elongated, substantially parallel electrodes disposed in a first electrode layer; and a second electrode, the method comprising: applying a potential difference between the first plurality of substantially elongated, substantially parallel electrodes and the second electrode to control broadening of a light beam passing through the LC layer about a first axis. The second electrode may comprise a second plurality of substantially elongated, substantially parallel electrodes disposed in the first electrode layer, interleaved with the first plurality of substantially elongated, substantially parallel electrodes. The second electrode may comprise an electrode disposed in a second electrode layer substantially parallel the first. In the above, many different aspects have been described. It should be appreciated that further aspects may be provided by the combination of any two or more of the aspects described above. Various other aspects are also described in the following detailed description and in the attached claims. A non-mechanical approach to control the light beam's shape is provided. This approach provides electrically tuneable liquid crystal (LC) lenses, and specifically microlens arrays to achieve uniform and wide-angle beam broadening. With reference to Fig. 1 there is illustrated an example of a single cell arrangement 100, comprising a first substrate layer 102, a liquid crystal (LC) layer 104, an electrode layer 106, and a second substrate layer 108. The electrode layer 106 is adjacent to one side of the LC layer 104, and the first and second substrate layers 102, 108 are the outside layers of the single cell arrangement 100. Reference numeral 101 denotes incident light on the single cell arrangement 100. The substrates 102, 108 are preferably a bendable, non-birefringent, and optically clear plastic, for example, cellulose triacetate with a thickness of approximately 50 pm. The two outside substrates 102 and 108 could be separated with spacers to form a cell gap ranging from ~10-100 pm. The LC layer 104 could comprise a vertically aligned LC with a high birefringence and positive dielectric anisotropy. With reference to Fig. 2, there is illustrated an example of the electrode layer 106, through the cross-section A-A of Fig. 1. The electrode layer 106 is formed of an array of electrodes, comprising at least one first linear electrode 110, preferably a first plurality of parallel linear electrodes 110, and at least one second linear electrode 112 preferably, a second plurality of parallel linear electrodes 112. The plurality of linear electrodes 110 and 112 are interleaved, with each linear electrode of the first plurality 110 being adjacent linear electrodes of the second plurality 112, and each linear electrode of the second plurality 112 being adjacent linear electrodes of the first plurality 110 - with the exception of those linear electrodes being toward the edges of the electrode layer. In the illustrated example of Fig. 1, the first and last electrodes of the second plurality 112 are the extreme linear electrodes of the overall electrode array, and have an adjacent linear electrode of the first plurality only on the inner side. Each linear electrode of the first and second pluralities 110,112 extends from one side of the electrode layer 106 to the other side of the electrode layer 106. Each linear electrode of the first plurality 110 is connected together, preferably by each being connected to a common connection 114 at an edge of the electrode layer. The common connection 114 is preferably connected to an electrical connector 118. Each linear electrode of the second plurality 112 is connected together, preferably by each being connected to a common connection 116 at an edge of the electrode layer 106, preferably the opposite side of the electrode layer 106 than the first plurality 110. The common connection 106 is preferably connected to an electrical connector 120. In the illustrated example of Fig. 2 the cross-section of the electrode layer 106 is shown as a circular layer, but the electrode layer may be formed of any shape suitable for an implementation. The electrode layer 106 comprises two electrode arrays 110 and 112. In the illustrated example of Fig. 2, the spacing between each linear electrode is uniform. Each linear electrode of each of the first and second pluralities 110,112 is uniformly spaced from the adjacent electrodes of the other of the first and second pluralities 110,112. Depending on the focal length required for the application the spacing can be tens to hundreds of micrometres. With reference to Figs. 3(a) and 3(b), there is illustrated a portion of the LC layer 104, showing an electrode llOn of the first plurality of electrodes 110, and an electrode 112n of the second plurality of electrodes 112. Fig. 3(a) represents a first state, in which state no electrical potential is applied to the two pluralities of electrodes 110 and 112, such that no potential difference is applied between the two pluralities of electrodes 110 and 112. This may also be termed an 'off' state. Reference numeral 122 denotes the LC molecules of the LC layer 104. The LC molecules of the LC layer are generally oriented in a direction from one side of the LC layer to the other (homeotropic alignment). In the illustrative example where the top and bottom sides of the LC layer are horizontal, the LC molecules are generally vertical. Light is incident on the LC layer in a direction substantially perpendicular to the LC layer (as denoted by arrow 101 in Fig. 1), polarised in the plane of the page, and the light beam passes through the LC layer substantially parallel to the LC molecules of the LC layer. The passage of light through the LC layer is unchanged. Fig. 3(b) represents a second state, in which state an electrical potential is applied to each of the two pluralities of electrodes 110 and 112, such that a potential difference is applied between the two pluralities of electrodes 110 and 112. This may also be termed an 'on' state. Reference numeral 122 again denotes the LC molecules of the LC layer 104. With a potential difference being applied, an electric field is formed which at least some, and in some implementations a majority, of the LC molecules of the LC layer align to thanks to their dielectrically anisotropy. In areas 124a and 124c directly above the electrodes llOn and 112n, the LC molecules remain disposed generally perpendicular through the LC cell 104. In an area 124b, mid-way between the electrodes llOn and 112n, the LC molecules are now generally parallel. The LC molecules align with the electric field lines. Due to this re-orientation, regions of the LC layer having different refractive index are formed. Light incident on the device which is substantially perpendicular to the LC layer experiences different refractive index depending on the orientation of the LC molecules the light is incident to in the LC layer. Light is incident on the LC layer in a direction substantially perpendicular to the LC layer (as denoted by arrow 101 in Fig. 1), is linearly polarised in the plane of the page (as denoted by arrow 103 in Fig. 1) and the light beam passes through the LC layer substantially perpendicular to the LC molecules of the LC layer in area 124b. The passage of light through the LC layer is changed. The size of the potential difference between the first and second plurality of electrodes can vary, to vary the divergence applied to the passage of light through the LC layer. Thus the first and second pluralities of electrodes are controlled to control the direction of LC molecules in the LC layer 106. This control allows the beam spreading to be controlled in one axis (similar to a cylindrical lens). The beam spreading is controlled in the axis perpendicularto the optical axis, and perpendicular to the direction of the linear electrodes in electrode layer 106. The extent of beam spreading applied will be dependent on the size of the potential difference between the two pluralities of electrodes. The control of beam spreading is illustrated with further reference to Figs. 3(a) and 3(b). In the first state, the 'off' state of Fig. 3(a), light traverses the LC layer 104 unaffected. Three light rays 301a, 301b and 301c are illustrated, which are incident perpendicularly on one surface of the LC layer 104 (the lower surface as illustrated in the example), and linearly polarised in the plane of the page. These rays exit through the other surface of the LC layer 104 (the upper surface as illustrated) also perpendicular to that surface. The direction of the light rays is unaffected by the LC layer 104 in the first state. In the second state, the 'on' state of Fig. 3(b), light is affected as it traverses the LC layer 104. Two light rays 303a and 303b are illustrated, which are incident perpendicularly on one surface of the LC layer 104 (the lower surface as illustrated in the example), and linearly polarised in the plane of the page. These rays exit through the other surface of the LC layer 104 (the upper surface as illustrated) at an angle to that surface. The direction of the light rays is affected by the LC layer 104 in the first state, as the light beams traverse the LC layer. The light rays are spread by an angle. As illustrated, the light beams are spread about an axis perpendicular to the direction of the linear electrodes. The lensing refractive index profile in the 'on' state extends between the interdigitated electrodes 110 and 112. However, in the orthogonal direction (into / out of the page of Fig. 3) the depicted profile extends unchanged for the full span of the electrodes. This results in a cylindrical microlens array and light that is focused to lines and subsequently broadens in one direction in the far-field. Furthermore, the nematic LC molecules proposed in an example implementation are calamitic (rod-shaped), making them optically anisotropic. Hence, only the linear polarisation component of light co-aligned with the long axis of the LC molecules (in the plane of Figs. 3(a) and 3(b)) will experience the lensing refractive index profile. Cylindrical lenses and beam-broadening in one direction may be useful in some applications, but it is desirable to have the ability to change the beam shape in two orthogonal directions (a spherical microlens array). This can be achieved by adding another identical but 90°-rotated LC cell, to form a dual cell arrangement 130, as illustrated in Fig. 4. With reference to Fig. 4 there is illustrated an example of a dual cell (or doublet) arrangement 130. The dual cell arrangement comprises a first single cell arrangement 100 according to Fig. 1, stacked with a second single cell arrangement denoted 200. Similar to the first single cell arrangement 100 as described above with respect to Fig. 1, the second single cell arrangement 200 comprises a first substrate layer 202, a liquid crystal (LC) layer 204, an electrode layer 206, and a second substrate layer 208. The electrode layer 206 is adjacent to one side of the LC layer 204, and the first and second substrate layers 202, 208 are the outside layers of the second single cell arrangement 200. The first and second single cell arrangements 100 and 200 are stacked one on top of each other, and in the described example the first single cell arrangement is stacked on top of the second single cell arrangement. A layer 150, comprising a half wave plate (HWP) and an optically clear adhesive (OCA) is provided between the cell arrangements 100 and 200. As discussed below, the electrode directions in the electrode layers 106 and 206 are substantially orthogonal (or perpendicular) to each other. Before entering the second cell in a doublet arrangement, the light's polarisation direction needs to be adjusted to be perpendicular to the electrode direction. This can be achieved by sandwiching a correctly oriented optical retarder (waveplate) between the two LC cells, such that the linear polarisation direction isturned by 90° to match the orientation of the second cell. Wave plates or retarders are designed to alter the polarisation state of light passing through them. They achieve this by slowing down one component of light relative to another, changing the light's phase and polarisation. The layer 150 is provided to serve this optical purpose, while simultaneously mechanically joining the two single cells, in the example joining the substrate layer 108 of the first cell 100 to the substrate layer 202 of the second cell 200. After passing through the doublet cell described in Fig 4. when both cells are in the 'on' state, the light beam will be substantially broadened in both directions orthogonal to the light's propagation direction (as indicated by the rays 303a and 303b). Because of the halfwave retarder in the middle of the cell the light exiting the device will be linearly polarised perpendicular to the incoming light (out of the page as indicated by reference numeral 401). Whilst the electrode layers 106 and 206 are shown as full or complete layers, this is for illustration. In practice the electrode layers are formed adjacent to the LC layers as per Figs. 3(a) and 3(b). An electrode sheet is deposited, and then patterned according to the desired arrangement. The electrode layer 106 in the dual cell arrangement of Fig. 4 through the cross-section A-A is as shown in the example of Fig. 2. With reference to Fig. 5, there is illustrated an example of the electrode layer 206 through the cross-section B-B of Fig. 4. The electrode layer 206 is formed of an array of electrodes, comprising a third plurality of parallel linear electrodes 210 and a fourth plurality of parallel linear electrodes 212. The plurality of linear electrodes 210 and 212 are interleaved, with each linear electrode of the third plurality 210 being adjacent linear electrodes of the fourth plurality 212, and each linear electrode of the fourth plurality 212 being adjacent linear electrodes of the third plurality 210 - with the exception of those linear electrodes being toward the edges of the electrode layer. In the illustrated example of Fig. 5, the first and last electrodes of the fourth plurality 212 are the extreme linear electrodes of the overall electrode array, and have an adjacent linear electrode of the third plurality only on the inner side. Each linear electrode of the third and fourth pluralities 210, 212 extends from one side of the electrode layer 206 to the other side of the electrode layer 206. Each linear electrode of the third plurality 210 is connected together, preferably by each being connected to a common connection 214 at an edge of the electrode layer. The common connection 214 is preferably connected to an electrical connector 218. Each linear electrode of the fourth plurality 212 is connected together, preferably by each being connected to a common connection 216 at an edge of the electrode layer 206, preferably the opposite side of the electrode layer 206 than the third plurality 210. The common connection 206 is preferably connected to an electrical connector 220. In the illustrated example of Fig. 5 the cross-section of the electrode layer 106 is shown as a circular layer, but the electrode layer may be formed of any shape suitable for an implementation. The electrode layer 206 comprises two electrode arrays 210 and 212. In the illustrated example of Fig. 5, the spacing between each linear electrode is uniform. Each linear electrode of each of the third and fourth pluralities 210, 212 is uniformly spaced from the adjacent electrodes of the other of the third and fourth pluralities 210, 212. Depending on the focal length required for the application the spacing can be tens to hundreds of micrometres. The electrode layer 206 operates according to the same principles as the electrode layer 106, as described with reference to Figs. 3(a) and 3(b). In a third state (equivalent to the first state with respect to electrode layer 106) no potential difference is applied between the third and fourth pluralities of electrodes 210 and 212, and the LC molecules of the LC layer 204 are unchanged. In a fourth state (equivalent to the second state with respect to electrode layer 106) a potential difference is applied between the third and fourth pluralities of electrodes 210 and 212, and the passage of light through the LC layer is changed. Thus, the third and fourth pluralities of electrodes are controlled to control the direction of LC molecules in the LC layer 206, in the same way that first and second pluralities of electrodes control the direction of LC molecules in the LC layer 106. This control allows the beam spreading to be controlled in one axis. The beam spreading is controlled in the axis perpendicular to the direction of the linear electrodes in the electrode layer 206. The extent of beam spreading applied will be dependent on the size of the potential difference between the two pluralities of electrodes. A comparison can be made between Fig. 2 and Fig. 5 to note that whilst the two electrode layers 106 and 206 are structurally identical, one layer is rotated relative to the other. Preferably, one layer is rotated through substantially 90° relative to the other. This means that the first and second pluralities of electrodes in electrode 106 are substantially orthogonal to the third and fourth pluralities of electrodes in the electrode layer 206, when the electrode layers 106 and 206 are overlaid in the stack of Fig. 4. Noting that each electrode layer 106 and 206 allows beam spreading to be controlled in one axis, which axis is substantially perpendicular to the direction of the linear electrodes in that electrode layer, it can be understood that the dual-cell arrangement of Fig. 4 permits beam-spreading in two axes, which axes are substantially perpendicular (or substantially orthogonal) to each other. Each electrode layer 106 and 206 can be controlled independently, by applying a potential difference to the electrical connectors 116 and 118, or the electrical connectors 216 and 218, respectively. As noted above, the potential difference applied to each electrical connector can be varied, in synchronism or independently. By controlling the potential difference applied to each electrode layer 106 and 206, beam shaping can be controlled in one or two axes, preferably substantially orthogonal axes. A device structure is provided which reduces weight, and is less bulky, in comparison to known implementations. This is achieved by the implementation of an electrode structure as described, which permits the substrate layers to be formed of plastic. A liquid crystal microlens array architecture can be provided on a flexible and conformable thin plastic substrate. Examples thus provide a single cell or dual cell device, including at least one electrode layer formed of parallel linear electrodes. Examples thus provide a method of controlling a single cell or a dual cell device, to control the beam shaping of light passing through an LC layer of the device. With reference to Figs. 6(a) and 6(d), there is illustrated an advantageous manufacturing technique for forming a dual cell allowing for beam spreading in two axes. As shown in Fig. 6(a), the electrode layers 106 and 206 are formed in a single plane, on a single planar layer 510. The single planar layer may be considered a substrate. The arrow 504 denotes the LC alignment direction for both the cells comprising electrode layers 106 and 206.Arrow 502 denotes the fast axis of the quarter wave plane (QWP) laminated on top of the finished LC cell. Since the single-layer device is built on a ductile plastic substrate 510, as denoted by arrow 500, one LC cell can now be folded over the other LC cell The thus folded-over electrode layers are shown in the example of Fig. 6(b), as a result of the fold denoted by arrow 506. The planar substrate layer 510 is folded. As can be seen in Fig. 6(b), the two electrode layers have their linear electrodes disposed substantially orthogonal relative to each other. Whilst Figs. 6(a) and 6(b) illustrated the folding of a layer in which electrode portions are formed. The preferable manufacturing process involves other layers. This is now described further with respect to Figs. 7(a) and 7(b). With reference to Fig. 7(a), there is illustrated the formation of a planar structure comprising a substrate layer 522 on which the electrode layer 510 is formed, an LC layer 524 formed on the electrode layer 510, a further substrate layer 528 formed on the LC layer 524, and a quarter wave plate, QWP, layer 528 formed on the further substrate layer 526. The QWP layer 528 preferably further comprises an optically clear adhesive, OCA. This adhesive may be a distinct layer. Arrow 500 denotes the folding od the planar structure of Fig. 7(a) according to a preferred manufacturing process. This folding results in a structure as shown in Fig. 7(b). The reference numerals of Fig. 7(b) are consistent with those of Fig. 4, to illustrate the dual cell structure which arises from folding the planar structure of Fig. 7(a). The single electrode layer 510 illustrated in Fig. 7(a) includes two distinct electrode portions 530 and 532, such that when the electrode layer is folded these portions form separate electrode layers in the device. Whilst the first and second electrode portions may correspond to the electrodes 106 and 206 respectively, other electrodes may be provided as the first and second electrode portions. For example one electrode portion may be a single patterned electrode, and the other electrode portion may be a common, un-patterned plate. As illustrated in Fig. 7(a), on top of the entire LC cell the QWP layer 528 is laminated, which when folded over, is a doubled-up structure and provides the retardance of a half-wave plate (HWP). The folding technique aids mass manufacturing, as it allows lamination of a whole sheet at once, rather than laminating individual sheets. The first and second LC layers may be formed from a continuous LC layer and the first and second electrode layers may be formed as distinct electrode elements (or portions) in an elongated linear strip which is also folded. As the LC cell assembly is completed, a quarter-wave plate can subsequently be laminated to one of sides of the LC cell. To finally assemble the full device, the continuous layer LC layer (containing both constituent cells) can be folded, resulting in the device structure outlined in Fig. 7(b). Assuming that the waveplate is attached with its fast axis parallel to the folding plane, the retardation of the doubled quarter wave plates is additive, and the two quarter wave plate layers combine to form a half-wave retarder in the device of Fig. 7(b). The first and further substrate layers 522 and 526 may be formed from continuous substrate layers respectively and the formed as substrate layers when folded. In an alternative, the further substrate layer 526 may not be utilised. The quarter wave plate layer, when folded, provides the separation between the two LC layers (formed when folded), in the stacked device. The electrode direction and the LC alignment direction are arranged to work with the fold. The electrode direction of each electrode layer is not important, in terms of the orientation of the electrodes. What is important is that the two electrode layers are orientated such that when they are folded over, the linear electrodes of each layer are substantially orthogonal relative to each other. The positioning of the electrode layers 106 and 206 in the planar substrate layer prior to folding is therefore important. Each ofthe plurality of the first, second, third and fourth plurality of electrodescan be controlled individually, by the connections 512, 514, 516 and 518. A liquid crystal microlens array architecture, based on an in-plane switching (IPS) drive structure, and constructed on a flexible and conformable thin plastic substrate is thus provided. A simplified method of manufacture is provided. IPS is possible with a device manufactured according to the described folding mechanism, as only one in-plane connection is required as exemplified by Fig. 6(a). However, the manufacturing technique could also apply to other similar LC drive schemes, such as fringe-field switching (FFS) where the in-plane linear electrodes are built on top of a sheet electrode. With reference to Fig. 7(c), there is illustrated the formation of a planar structure for an FFS arrangement rather than an IPS arrangement. The planar structure comprises the LC layer 524 formed on the patterned electrode layer 510, the substrate layer 528 formed on the LC layer 524, and the quarter wave plate, QWP, layer 528 formed on the further substrate layer 526 as illustrated in Fig. 7(a). In place of the substrate layer 522 shown in Fig. 7(a), a spacer layer 521 is formed and an additional electrode layer 523a, 523b is formed. The spacer layer 521 provides a spacer for the electrode layer 521 and the spacers layer comprising the electrodes 523a and 523b. When the structure of Fig. 7(a) is folded, it can be seen that a pair of electrode layers is formed the other side of the LC layer. The electrodes 523a,523b are common electrodes, which compliment the respective patterned electrodes in layer 510. A doublet cell as described above allows for independent control over two orthogonal divergence angles, allowing for a wide range of illumination patterns to be created. However, for unpolarised light incident on the doublet the linear polarisation component orthogonal to the one broadened in this doublet will perceive the homogenous refractive index environment of the LC's ordinary axis and will not be broadened. Polarisation-independent beam broadening is achievable though, if a doublet is followed by another identical cell. Since the doublet contains a half wave retarder (comprised of two QWPs after the fold), it flips both the incident linear polarization states and the second doublet thereby only acts on the opposite polarisation state from the first cell. As described above, in such an arrangement the presence of the further substrate is optional. To enable both axis-separate divergence control (combining two cylindrical lenses to make a spherical one) and polarisation-independent beam-broadening (one cell for each linear polarisation component), four LC cells are required. To build a single layer LC cell on thin plastic substrates and subsequently folding it into its final multilayer device structure (enabled by the electrode and waveplate design) significantly simplifies the manufacturing. Without the folding ability the device would need multiple additional process steps to be assembled. Furthermore, the more layers are added the more sequential process steps the folding technique avoids and the more beneficial it becomes. A four-cell structure may be constructed using the technique described with reference to Figs. 7(a) and 7(b), folding the structure twice, to form four LC cells. Such a structure provides for beam-broadening in two axes when the incident light is unpolarised. An exemplary arrangement is described in which a quarter wave plate is folded in order to give functionality of a half wave plate. In an alternative, an eighth wave plate could be folded to give functionality of a quarter wave plate. In general, a wave plate is folded to provide two parallel portions of the wave plate. In general, a wave plate providing beam spreading for polarised light about an axis is folded to provide beam spreading for polarised light about two axes. In general, a wave plate providing beam spreading for polarised light about an axis is folded to provide beam spreading for polarised light about two axes. In general, a wave plate is folded to provide beam spreading for non-polarised light about an axis, and is folded twice to provide beam spreading for non-polarised light about two axes. The examples set out, with reference on particularto Fig. 2 and Fig. 5, that the plurality of adjacent linear electrodes have a uniform spacing in the first and / or second electrode layers 106 and / or 206. In an alternative, the plurality of adjacent linear electrodes have a non-uniform spacing in the first and / or second electrode layers 106 and / or 206. In an example, the spacing of adjacent linear electrodes is pseudo-random. In another example, the non-uniform spacing may be selected. In a dual cell (doublet) arrangement, the spacing of the linear electrodes may be independent. This may mean that unform spacing may be provided in one electrode layer, and non-uniform spacing applied in the other electrode layer. Some individual spacings may match in an electrode layer, whilst other individual spacings may not. Fig. 8 shows an example of an electrode layer comparable to the electrode layer of Fig. 2, but with non-uniform spacing. With reference to Fig. 8, there is illustrated an example of an electrode layer 506, which may provide an alternative to either electrode layer 106 or 206. The electrode layer 506 is formed of an array of electrodes, comprising at least one first linear electrode 510, preferably a first plurality of parallel linear electrodes 510, and at least one second linear electrode 512 preferably, a second plurality of parallel linear electrodes 512. The plurality of linear electrodes 510 and 512 are interleaved, with each linear electrode of the first plurality 510 being adjacent linear electrodes of the second plurality 512, and each linear electrode of the second plurality 512 being adjacent linear electrodes of the first plurality 510 - with the exception of those linear electrodes being toward the edges of the electrode layer. In the illustrated example of Fig. 8, the first and last electrodes of the second plurality 512 are the extreme linear electrodes of the overall electrode array, and have an adjacent linear electrode of the first plurality only on the inner side. Each linear electrode of the first and second pluralities 510, 512 extends from one side of the electrode layer 506 to the other side of the electrode layer 506. Each linear electrode of the first plurality 510 is connected together, preferably by each being connected to a common connection 514 at an edge of the electrode layer. The common connection 514 is preferably connected to an electrical connector 518. Each linear electrode of the second plurality 512 is connected together, preferably by each being connected to a common connection 516 at an edge of the electrode layer 506, preferably the opposite side of the electrode layer 506 than the first plurality 510. The common connection 506 is preferably connected to an electrical connector 520. In the illustrated example of Fig. 8 the cross-section of the electrode layer506 is shown as a circular layer, but the electrode layer may be formed of any shape suitable for an implementation. Comparing the electrode layer 506 of Fig. 8 to the electrode layers 206 and 506, it can be seen that the spacing between linear electrics is non-uniform, and varies throughout the array. Some individual spacings may match, but the spacing between any electrode and it's adjacent electrodes is independent of the spacing between other electrodes. The provision of linear electrodes with non-uniform spacing is more generally applicable than to device arrangements in which a first electrode layer comprises two pluralities of linear electrodes which are interleaved, and a complimentary second electrode layer provides a common electrode for the plurality of linear electrodes, as discussed above. An example is illustrated with reference to Figs. 9 and 10. With reference to Fig. 8, there is illustrated a device 302 comprising a liquid crystal (LC) layer 304. First and second electrode layers 306, 310 are formed above the LC layer 12, separated by an insulating layer 303, with the first electrode layer 306 formed adjacent the LC layer 304. First and second substrate layers 312, 331 are formed, the first substrate layer 312 being formed adjacent the LC layer 304, and the second substrate layer 331 being formed adjacent the second electrode layer 310. Light is incident, as denoted by arrow 330, at the substrate layer 312 adjacent the LC layer 304. The electrode layers 306, 310 comprise a fringe-field switching (FFS) system, which controls the application of a voltage signal to the electrode layers 306, 310. In an alternative example, the electrode layers 306, 310 may comprise an in-plane switching (IPS) system, as described above. The second electrode layer 310 is, in this example, a single layer. In general, electrode layers may be metallic or plastic. The first electrode layer 306 comprises a plurality of linear strips 318, 320, 322, 324, 326, 328 extending from one edge of the electrode surface to an opposite edge of the electrode surface. With reference to Fig. 10, a further illustration of the first electrode layer 306 is shown. Fig. 10 shows a planer view of first electrode layer 306, though the cross-sectional line C-C of Fig. 9. In the illustrated arrangement of an FFS system, the linear electrodes in the electrode layer 306 are all connected to a common potential. A potential difference is created by applying a different potential to each of the electrode layers 306 and 310. In an alternative arrangement of an IPS system, each electrode layer contains a pair of linear electrodes, each of which is connected to a different potential, as described above. The number of linear strips shown in Figs. 9 and 10 is illustrative and not limiting. The shape of the electrode layer 306 shown in Fig. 9 is illustrative and not limiting. In Figs. 9 and 10 the linear strips are illustrated having a common, uniform width. As can be seen in both Figs. 9 and 10, one linear strip 318 is adjacent one edge of the first electrode layer 306, and the other edge of the electrode layer 306 is at a distance from the nearest linear strip. In general, a linear strip may be adjacent or separated from either edge of the electrode layer 306. In Figs. 9 and 10, it can be seen that there is a distance di between the edges of the linear strips 318 and 320, a distance d2 between the edges of the linear strips 320 and 322, a distance ds between the edges of the linear strips 322 and 324, a distance d4 between the edges of the linear strips 324 and 326, a distance ds between the edges of the linear strips 326 and 328, and a distance de between the edges of the linear strip 328 and the edge of the first electrode layer 306. In an example the distances di to de are pseudo random. Thus in an example the first electrode layer 306 comprises a plurality of linear strips at pseudo-random spacing. The provision of non-uniform spacing between the plurality of linear electrodes results in a microlens array with a plurality of focal lengths which produces a more uniform light distribution. At the same time, it minimises diffraction resulting from the beam spreading caused by the linear electrodes, which may otherwise detract from the desired homogenous spreading of light. In Figs. 11(a) to 11(d), the results from a diffraction simulation are displayed, demonstrating the diffraction improvement of using pseudo-random electrode spacings. In Fig. 11(a) a regular electrode grid with an electrode pitch (uniform spacing) of 8 pm and an electrode pitch of 50 pm is displayed, and in Fig. 11(b) the far-field diffraction intensity of 532 nm light is displayed. Note that the zero-order intensity is allowed to saturate to allow for visualisation of the higher order peaks. Clear diffraction maxima are seen for the regular spacing. Fig. 11(c) shows an electrode layer with pseudo-random electrode pitch (non-uniform spacing), in the example of between 30 and 100 pm. Fig. 10(d) displays the far-field diffraction intensity for 532 nm light. Compared to the diffraction peaks from the regular spacing case as shown in Fig. 11(b), the diffracted light spreads out, effectively reducing its visibility. Note that between the two diffraction maps the light intensity is adjusted to account for the difference in aperture ratio between the two electrode spacings. The example illustrated in Figs. 9 and 10 shows the applicability of pseudo-random spacing to an arrangement in which a single electrode layer comprising two interleaved linear electrodes is provided together with a single common electrode plate. Such an electrode configuration applies in an FFS system. The described technique is applicable to arrangements in which there is a provided a single electrode layer with interleaved linear electrodes, as for example as shown in Fig. 1, and examples in which there is a provided a pair of electrode layers each having interleaved linear electrodes, as shown in Fig. 4 or Fig. 7. Such an electrode configuration applies in an IPS system. Where the device is consistent with the examples described with reference to Figs. 1 to 7, with pseudo random or other spacing applied in one or more electrode layers, an IPS controller may be used in place of the FFS controller. By providing non-uniform spacing in two linear electrodes, as per the examples described with reference to Figs. 1 to 7, the light intensity distribution in the 'on' state can be homogenised and diffraction can be minimised in two substantially orthogonal beam axes. The device may be referred to as a beam-shaping device. A microlens is a small lens, generally with a diameter less than a millimetre, and often as small as 10 pm. A microlens array contains multiple microlenses, formed in a one-dimensional or two-dimensional array on a supporting substrate. A tuneable microlens array has multifocal capability. Voltage control of the microlens array allows its focal length to be varied. Whilst reference is made herein to linear electrodes, and an electrode layer comprising two linear electrodes, with linear electrodes of each electrode being interleaved, there is no limitation to the individual electrodes being of a linear shape. The individual electrodes may, for example, be a curved shape, or a zig-zag shape. In general, the electrodes are elongated electrodes, and the shape is not limited to being linear. Further reference can be made to Figs. 12(a) to 12(d), which illustrates example electrode arrays which may be utilised, in an IPS or an FFS system. These show examples of substantially parallel, elongated electrodes. Fig. 12(a) illustrated V-shaped electrodes. Fig. 12(b) illustrated square-pulse shaped electrodes. Fig. 12(c) shows stepped electrodes, with each step being a linear or straight-line shape, projecting at a different angle. Figs. 12(d) shows curved electrodes. In all examples shown, the electrodes are substantially parallel, the shape of each electrode being substantially identical. In each example, the electrodes are substantially elongate. The liquid crystal device described above may, for example, function as or be used within a switchable lens device or a beam steering device. For example, a device may be or comprise an adaptive optical lens comprising a liquid crystal device according to any of the examples herein. Such a device may be or comprise a headset, which may be referred to as a head-mounted display (HMD). The liquid crystal device described above is useful in a wide range of applications, including ophthalmic lenses (such as spectacle lenses), virtual reality (VR), mixed reality (MR) and augmented reality (AR) headsets; optical projectors; photographic devices; and communication devices. The LC optical lens device may be used for the push lens and / or the pull lens or a combined push / pulI lens of an augmented reality (AR) headset such as e.g. that shown in Fig. 13. The headset 40 comprises a support frame 42 supporting optical components arranged in optical series in front of the user eye. At least one optical component such as one or more of the optical components shown in Fig. 13 may be considered to correspond to or be part of an assembly, which may be considered to be a display stack, comprising at least one liquid crystal cell according to examples herein. In examples, such as that of Fig. 13, such an assembly includes a stack of liquid crystal cells according to examples herein. In the example of Fig. 13, the push lens 48a includes at least one stack of liquid crystal cells, the pull lens 48b includes at least one stack of liquid crystal cells, and the assembly includes the push lens 48a, the waveguide 50, the pull lens 48b, the variable dimmer device 46, which is an example of a luminance adjustment component, and the front window / lens 44. Liquid crystal cells of a stack may be aligned along a common optical axis. In some cases, though, optical axes of at least two of the liquid crystal cells of a stack may be offset from each other in a direction parallel to a plane of a radial electrode pattern of at least one of the liquid crystal cells, provided that light traversing the assembly traverses the liquid crystal cells of the stack. Fig. 13 only shows the optical components for one half of the headset for clarity of representation, but a matching set of optical components is also provided for the other half of the headset. The waveguides 50 of the headset respectively display left-and right perspectives of one or more virtual reality objects, by which the user perceives the one or more virtual reality objects as 3D objects. Alternatively, other mechanisms may be employed to display the left / right perspectives of the one or more virtual reality objects, such as e.g. laser projection. The degree to which the user's left and right eyes need to rotate relative to each other such that the left and right perspectives of a virtual reality object are simultaneously directed onto the foveas (which are the parts of the retina responsible for sharp central vision necessary for activities for which visual detail is of primary importance) of respective left and right eyes of the user determines the distance at which the user perceives the virtual reality object to be. This mechanism is referred to as vergence. The LC optical lens device described above may be used as an adaptive lens device to control the location at which the user's eyes perceive the left / right perspectives of a displayed virtual reality object in focus (i.e. not blurred), which location may be referred to as a focal plane. In other words, the LC optical lens device described above may be used as an adaptive lens device to control the degree to which the lenses in the user's eyes need to adapt to perceive the left and right perspectives of the virtual reality object in focus (i.e. not blurred). This adaptation mechanism of the lenses in the user's eyes is known as accommodation. The LC optical lens device described above may be used to produce optical images (real or virtual) of the left / right perspectives of a virtual reality object substantially at the distance from the user's eyes at which the user perceives the virtual reality object to be located through the vergence mechanism discussed above. This may allow the user to perceive a focussed 3D image of the virtual reality object without disrupting the vergenceaccommodation reflex, by which the focussing action of the lenses in the user's eyes (accommodation) is unconsciously linked to the above-mentioned rotation of the left and right eyes relative to each other (vergence). In other words, the LC optical lens device may be used to avoid or reduce the strain on the user's eyes that can arise from a conflict between the vergence and accommodation mechanisms (referred to as the vergence-accommodation conflict). For example, the LC optical lens device may be switchable between a positive focal power and a negative focal power. Hence, a liquid crystal device according to examples herein may provide a lower complexity and / or higher quality system to actively adjust focus to compensate for focal differences between a virtual object and a real-world environment visible to a user of a headset through the optical components mounted in front of each eye. This for example allows the perceived and actual image depth to be brought together in a consistent manner, improving user comfort. In Fig. 13, the headset 40 permits transmission of light from a real-world environment around the headset 40 at least partly through the optical components and into the user's eyes. In this example, the optical components are at least partly transparent. On a bright day, the luminance of the environment may be significantly higher outdoors than indoors, such as around 100 times higher. This can lead to a virtual object appearing washed out and difficult to see when the user operates the headset outdoors, unless the luminance of the light transmitted from the environment to the user is appropriately controlled. In Fig. 13, the variable dimmer device 46 controls the amount of light transmitted through the optical components and towards the eyes, e.g. so as to reduce the luminance of light from the environment transmitted towards the user in bright conditions, and may be used to provide ambient dimming to dim ambient light transmitted through the headset 40. The variable dimmer device 46 may provide so-called global dimming, in which the luminance of the light from the environment is adjusted by substantially the same amount within an extent of a plane of the variable dimmer device 46 facing the user (e.g. to reduce the luminance of the light by substantially the same amount across an entire surface area of the variable dimmer device 46). In other words, global dimming can allow the luminance of the light transmitted through the variable dimmer device 46 to be controlled in a substantially spatially uniform manner (e.g. so as to provide a substantially spatially uniform reduction in the luminance across a field of view of the user). The variable dimmer device 46 may also or alternatively provide local dimming, in which the variable dimmer device 46 is adjustable to control the luminance of the light transmitted from the environment on an area-by-area basis (where an area may correspond to a single pixel or a plurality of pixels). Local dimming may involve adjusting the luminance across less than all of the surface area of the variable dimmer device 46, such as within a subarea which is smaller than the surface area of the variable dimmer device 46. In other cases, though, local dimming may involve adjusting the luminance across the entire surface area of the variable dimmer device 46 but by different amounts in at least two portions of the surface area. Although not shown in Fig. 13, it is to be appreciated that the headset 40 may be configured to obtain luminance data, e.g. from a light sensor of the headset 40, indicative of the luminance of the light within the environment of the headset 40. For example, if a first side 49a of the headset 40 is configured to face the user, with the headset 40 mounted on the head of the user, the headset 40 may include a light sensor to detect the luminance of light at a second side 49b of the headset 40, opposite to the first side 49a. The variable dimmer device 46 may be controlled at least partly based on the luminance data, so as to adjust the luminance of light transmitted from the second side of the headset 40 towards the user, to improve the visibility of the virtual object displayed to the user by the headset 40. In the example of Fig. 13, a first lens comprising at least one liquid crystal cell stack of the examples herein (the push lens 48a) is located between the waveguide 50 and the eye, with the headset 40 in use. Light representative of the virtual object is generated and transmitted to the waveguide 50, which directs the light through the push lens 48a and into the eye. The push lens 48a has a focusing effect to focus the light representative of the virtual object so that the object appears in focus to the user. For example, the virtual object may be generated so that it is in focus at a focal plane of infinity. The push lens 48a may then bring the virtual object into focus at a focal plane which is closer to the user than infinity, to allow the user to focus on the virtual object more comfortably. The focal plane at which the virtual object is to be brought into focus, and hence the focusing power to be applied by the push lens 48a, may be determined based on eye tracking data, e.g. obtained by a suitable sensor as discussed further below, which is indicative of a direction in which the eye of the user is looking. Prior to use of the headset 40, the external environment may appear in focus to the user. However, in the absence of the pull lens 48b, light from the external environment would be at least partly transmitted through the waveguide 50 and through the push lens 48a and would therefore be subject to the focusing effect provided by the push lens 48a. This would distort the external environment as viewed by the user through the headset 40. To compensate for the distortion introduced by the push lens 48a, the headset 40 of Filg. 10 includes a second lens (the pull lens 48b) positioned at an opposite side of the waveguide 50 to the push lens 48a. The pull lens 48b applies an appropriate focusing effect to light from the environment traversing the pull lens 48b to at least partially compensate for or otherwise reduce the focusing effect introduced by the push lens 48a. For example, the push and pull lenses 48a, 48b may provide opposite focusing effects to each other, e.g. with substantially equal magnitudes but opposite signs. As an example, one of the pushand pull lenses 48a, 48b may provide a positive focusing power and the other one of the push and pull lenses 48a, 48b may provide a negative focusing power, which may be substantially equal in magnitude. In examples at least one lens of examples herein (such as at least one of the push lens 48a and the pull lens 48b, and in some cases both the push and pull lenses 48a, 48b) each includes a so-called doublet of liquid crystal cells according to examples herein. A doublet is a stack of two liquid crystal cells. The focusing effect of a liquid crystal-based lens may depend on the polarization of the light incident on the lens. Rather than using a separate polarizer component, using a doublet such as this may provide an appropriate focusing effect with improved light transmission; in some examples this is achieved by positioning one liquid crystal cell of the doublet orthogonal to the other liquid crystal cell of the doublet, with respect to the respective orientation of polarization that each liquid crystal cell is configured to modify light for. Fig. 13 shows an example of a push lens 48a and a pull lens 48b in combination with various other optical components. It is to be appreciated that a liquid crystal cell in accordance with examples herein can be used in combination with different optical component(s) than those shown in Fig. 13, to provide further flexibility in functionality. This may further reduce the size and / or weight of apparatus including the liquid crystal cell and / or improve optical performance of the apparatus. For example, an assembly, such as a display stack, including a liquid crystal cell in accordance with examples herein may include a reflection-reduction layer (such as an anti-reflection (AR) coating), which may be laminated to another optical component of the assembly, such as the front window / lens 44, and / or a protective layer (such as a hard coat) to protect the assembly from damage, e.g. due to abrasion, and / or wear due to exposure to environmental conditions. In examples, the liquid crystal device comprises electrical terminals electrically connected to the busbars. The electrical terminals for example allow a potential difference to be applied across the busbars, and thus across each set of concentric rings. As explained above, the electrical potential applied to an electrical terminal can be controlled by a suitable control system. With reference to Fig. 14, a system 55 according to some examples comprises a processor operating on the basis of computer program code stored in a memory 52 to control an image generation driver chip 53 to cause an image generation system to generate images of left / right perspectives of one or more virtual reality objects, by which the user may perceive 3D images of the virtual reality objects, and display the images via the waveguide 50. Although not shown in Fig. 14, it is to be appreciated that there may be two waveguides: one to display an image of a left perspective of a virtual reality object to a left eye and another to display an image of a right perspective of a virtual reality object to a right eye, as discussed further with reference to Fig. 14. There may further be two image generation systems: one to generate the image of the left perspective of the virtual reality object and another to generate the image of the right perspective of the virtual reality object (although in some cases a single image generation system may generate both images or an image generation system may generate a single image to be displayed to both eyes). An image generation system is discussed further below with reference to Fig. 15. Inputs from sensors feed into the processor to enable the processor to control positions at which the virtual reality objects are displayed by the waveguides 50, for seamless overlay of the one or more virtual reality objects into the user's view of the user's real environment. Based on inputs fed into the processor 51 from one or more sensors 54 sensing the movement of the user's eyes and / or based on the content being displayed by the waveguides 50, the processor 51 controls the adaptive lens driver chip 38 to achieve the optical focussing power (Dioptres) required to achieve the above-described generation of optical images of the display output of the waveguides at a distance from the user's eyes at which the virtual content that the user is determined to be looking at (e.g. through tracking of the user's eyes) is intended to be perceived by the user (through the vergence mechanism described above). A driver chip is an example of a controller, which may be implemented in hardware, e.g. via suitably configured circuitry. In some cases, a driver chip may include or be considered to implement at least one processor. Fig. 15 illustrates schematically hardware architecture of an apparatus 60 according to further examples. The apparatus 60 comprises at least one liquid crystal cell stack in accordance with examples herein. In Fig. 15, the apparatus 60 is configured to be mounted on human head, e.g. a head of a user, with a liquid crystal cell stack positioned in a field of view of an eye of the head, in use. In the example of Fig. 15, the apparatus 60 is an AR headset for displaying a virtual image to a wearer of the headset, and may be similar to or the same as the headset 40 of Fig. 13. In other examples, though, apparatus including a similar hardware architecture to the apparatus 60 of Fig. 15 may be configured for a different purpose, may include additional components and / or may omit at least one of the components illustrated in Fig. 15. The apparatus 60 of Fig. 15 includes an optical system 62, an image generation system 64, at least one processor 66, storage 68, at least one sensor 70, a user input / output interface 72, a communications system 74 and at least one further hardware system 76. Components of the apparatus 60 are connected to each other via at least one bus 78, which may be or include any suitable interface or bus for transferring data between the illustrate components. The optical system 62 includes a first assembly and a second assembly, which in this example are a first display stack 62a and a second display stack 62b, respectively. The first display stack 62a comprises a first set of optical components, e.g. arranged as a stack of layers. The apparatus 60 is configured to permit at least partial transmission of light from an external environment through the first display stack 62a and towards a first eye of the user, with the apparatus 60 in use and mounted on the head. In other words, where the apparatus 60 has a first side configured to face the user, in use (e.g. the first side 49a of Fig. 13), the first display stack 62a is arranged for directing light from the second side towards the first eye (in this case, through the first display stack 62a). The first display stack 62a in this case includes the optical components shown in Fig. 13, i.e. the push lens 48a, the waveguide 50, the pull lens 48b (where the push and pull lenses 48a, 48b are each an example of a liquid crystal device according to examples herein), the variable dimmer device 46 and the front window / lens 44. The push lens 48a and / or the pull lens 48b of the first display stack 62a may be considered to be a first lens comprising a first at least one of the liquid crystal cell stacks according to examples herein. The first lens is configured to be positioned in a first field of view of a first eye, e.g. the first eye of a user, in use. In Fig. 15, the second display stack 62b comprises a second set of optical components, which in this example is the same as the first set of optical components but configured to transmit light towards a second eye of the user, with the apparatus 60 in use. In other words, the second display stack 62b is arranged to direct light from the second side of the apparatus 60 towards the second eye. Hence, in this example, the push lens and / or the pull lens of the second display stack 62b may be considered to be a second lens comprising a second at least one of the liquid crystal cell stacks according to examples herein. The second lens is configured to be positioned in a second field of view of a second eye, e.g. the second eye of the user, in use. It is to be appreciated that the first lens may be visible to solely the first eye or to both the first and second eye, in use, and the second lens may be visible to solely the second eye or to both the first and second eye, in use. A spatial arrangement of elements of the second display stack 62b in at least one layer of the stack may mirrorthe spatial arrangement of corresponding elements of the first display stack 62a in the corresponding layer of the stack of the first optical arrangement 62a as reflected in a sagittal plane of the apparatus 60 (which may be referred to as a longitudinal plane of the apparatus 60, and e.g. separates left and right sides of the apparatus, with the apparatus in use). In other cases, though, the first and second display stacks 62a, 62b may have a different structure from each other. It is to be appreciated that the optical system 62 may include further components, e.g. further optical components, not shown in Fig. 15. The apparatus 60 also includes an image generation system 64 to generate an image of a virtual object to be displayed to the user of the apparatus 60 so that the virtual object appears to the userto be overlaid on top of the external environment, which is at least partly visible to the user through the optical system 62. The image generation system 64 may be or include a display device to generate an image (e.g. of a virtual object) for display by the apparatus 60 to the user. The display device may be a liquid crystal display (LCD) device, a light emitting diode (LED) display device such as an organic light emitting diode (OLED) display device, an electroluminescent (EL) display device and so forth. In the example of Fig. 15, the image generation system 64 is in optical communication with the optical system 62. For example, the image generation system 64 may be housed by the support frame 42 if the apparatus 60 is in the form of the headset 40 of Fig. 13. Light generated by the image generation system 62 representing the virtual object may be transmitted to the optical system (e.g. to a waveguide such as the waveguide 50 shown in Fig. 13) either directly (e.g. without traversing another optical component) or via at least one further optical component. In some cases, the image generation system may include two display devices, a first one for the first eye and a second one for the second eye, e.g. if it is desired to display a first image to the first eye and a second image to the second eye. In other examples, a single display device may be used to generate an image to be displayed to both the first and second eyes. In the example of Fig. 15, the image generation system 64 is shown as a separate system from the optical system 62. In other examples, though, the image generation system may form part of the optical system. For example, an assembly, such as a display stack, of the optical system may include an image generation system, such as a display device. The at least one processor 66 of the apparatus 60 may be a single processor or a plurality of processors of one or more types. Components of the at least one processor 66 may be implemented using suitably programmed hardware, e.g. in the form of circuitry. The at least one processor 66 may include a central processing unit (CPU), a graphics processing unit (GPU) and / or a neural processing unit (NPU), which may be referred to as a neural network accelerator. In some examples, apparatus, such as the apparatus 60 of Fig. 15, includes driving circuitry connected to at least one electrical connection connected to the electrode patterns of the liquid crystal cell stack to apply a potential difference across one or more electrode sets of the liquid crystal cells of the liquid crystal cell stack. The potential difference applied (such as a magnitude and / or timing of the potential difference applied) may be determined by the at least one processor 66 and / or by the driving circuitry, such as by a controller implemented by at least a portion of the driving circuitry, based on the instructions stored in the storage. If the potential difference is determined by the driving circuitry, the determination of the potential difference may be instigated by instructions received from the at least one processor, such as instructions indicative that a virtual object is to be displayed and that one or more electrode sets are thus to be activated so that the virtual object appears in focus to the user. In this way, the driving circuitry may be agnostic to the at least one processor from which the instructions are received. In other words, the operation of the driving circuitry may for example be independent of the at least one processor used to control the driving circuitry, such that the same effect can be achieved irrespective of the at least one processor coupled to the driving circuitry (provided the at least one processor provides an appropriate indication to the driving circuitry to cause the driving circuitry to determine a suitable potential difference). The potential difference may be applied to the electrical connection(s) by at least one driver of the driving circuitry, such as the adaptive lens driver chip 38 of Fig. 11, which is an example of a driver. Application of a potential difference by the at least one driver may be considered to amount to so-called "driving" of the electrode pattern(s), via the electrical connection(s). The driving circuitry may be in the form of at least one system-on-a-chip (SoC). The storage 68 may be or include computer-useable volatile and / or non-volatile memory. The storage 68 may comprise random access memory (RAM) and / or read-only memory (ROM). The storage 68 may be removable or non-removable from the apparatus 60. The storage 68 stores instructions for controlling the apparatus 60 in accordance with examples herein, e.g. to activate one or more electrode sets of the liquid crystal cells of the liquid crystal cell stack. Activation of an electrode set for example refers to applying a potential difference between at least two connectors connected to the electrode set. The instructions may be in the form of computer-readable and / or executable instructions, e.g. computer program instructions. Although the storage 68 is shown as a separate component to the at least one processor 66 in Fig. 15, in some cases the storage 68 may be or include internal storage of the at least one processor 66, in which cases the at least one processor 66 and the storage 68 may be at least partly integrated into the same system or component. The at least one sensor 70 in this example is configured to obtain eye tracking data of the apparatus, in use, which for example indicates a direction in which at least one eye of the user is looking, as the skilled person will appreciate. Eye tracking data may be obtained for each eye, or the eye tracking data may be obtained for a single eye or for a combination of both eyes of the user. Suitable sensors for obtaining eye tracking data include a camera 70a for obtaining images of at least one eye of the user, an inertial measurement unit (IMU) 70b for determining an orientation of the apparatus 60 and at least one position sensor 70c such as a global positioning system (GPS) sensor to determine a location of the apparatus 60. As the skilled person will appreciate, an IMU 70b may include at least one accelerator or gyroscope for use in determining the orientation of the apparatus 60. The focusing effect of the at least one liquid crystal cell may be controlled based on the eye tracking data, e.g. so as to reduce user eye strain as described further above. The apparatus 60 also includes a user input / output interface 72 via which a user can interact with the apparatus 60 to control aspects of the apparatus 60. For example, the user input / output interface 72 may be or include an input device such as a button, a touchscreen, a slider, a controller or any other suitable device for communicating user requests to the apparatus 60 to control the apparatus 60. The apparatus 60 includes a communications system 74 for receiving data from a remote system, e.g. via a suitable telecommunications network, such as a wireless network, or via some other type of network or connection. The communications system 74 may include an input / output interface, such as a Bluetooth connector, a universal serial bus (USB) connector or a network connector, for receiving the data from the remote system. The apparatus 60 of Fig. 15 includes at least one further hardware system 76 such as a power source, e.g. a battery, for providing electrical power to the electrical components of the apparatus 60. Some examples have been described above for the example of an optical focussing device, but the same techniques have application in other areas such as e.g. beam steering optics. Further examples relate to a method of operating a liquid crystal device according to any of the examples herein. The term "substantially" used herein may be considered to mean that two elements that are "substantially" the same are: the same within manufacturing tolerances, the same within measurement uncertainties and / or are within 5% of each other. Examples herein refer to a liquid crystal (LC) material. A liquid crystal material is an example of a material with a switchable refractive index, or a refractive index changing material. The described device, assembly and apparatus has use in example implementations other than tuneable lens and optical components. Other example implementations include, but are not limited to: image generation systems, read only memory, network connections, USB, Bluetooth systems etc., methods of powering and associated techniques. In addition to any modifications explicitly mentioned above, it will be evident to a person skilled in the art that various other modifications of the described examples may be made within the scope of the invention. The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features.

Claims

1. A device comprising:a first liquid crystal, LC, layer;a first electrode layer, the electrode layer comprising:a first plurality of substantially elongated, substantially parallel electrodes, the spacing between each electrode being non-uniform.

2. The device of claim 1, the first electrode layer further comprising:a second plurality of substantially elongated, substantially parallel electrodes interleaved with the first plurality of substantially elongated-substantially parallel electrodes, the spacing between each electrode of each of the first and second plurality being non-uniform.

3. The device of claim 2, the first plurality of substantially elongated, substantially parallel electrodes and the second plurality of substantially elongated, substantially parallel electrodes are for connection between a first potential difference, in a first state the potential difference is zero, and incident light passes through the first LC layer unchanged, in a second state the potential difference is non-zero, and incident light passing through the first LC layer is spread about a first axis, the extent of spreading being dependent on the size of the first potential difference.

4. The device of any one of claims 1 to 3, further comprising:a second liquid crystal, LC, layer;a second electrode layer, the electrode layer comprising:a third plurality of substantially elongated, substantially parallel electrodes; anda fourth plurality of substantially elongated, substantially parallel electrodes interleaved with the third plurality of substantially elongated-substantially parallel electrodes, the spacing between each electrode of each of the third and fourth plurality being non-uniform.

5. The device of claim 4, the third plurality of substantially elongated, substantially parallel electrodes and the fourth plurality of substantially elongated, substantially parallel electrodes are for connection between a second potential difference, in a third state the potential difference is zero, and incident light passes through the second LC layer unchanged, in a fourth state the potential difference is non-zero, and the incident light passing through the second LC layer is spread about a second axis, the extent of spreading being dependent on the size of the second potential difference.

6. The device of claim 5 when dependent on claims 3 and 4, the first and second substantially elongate electrodes being substantially orthogonal to the third and fourth substantially elongate electrodes, the first and second axes being substantially orthogonal.

7. The device of any one of claims 1 to 6, the first electrode layer being adjacentthe first LC layer, and further comprising substrate layers either side of the first LC layer and the first electrode layer.

8. The device of any one of claims 4 to 7, the second electrode layer being adjacent the second LC layer, and further comprising substrate layers either side of the second LC layer and the second electrode layer.

9. The device of claim 1 further comprising:a second electrode layer,the first electrode layer comprising the first plurality of substantially elongated, substantially parallel electrodes and the second electrode are for connection between a first potential difference, in a first state the potential difference is zero, and incident light passes through the first LC layer unchanged, in a second state the potential difference is non-zero, and incident light passing through the first LC layer is spread about a first axis, the extent of spreading being dependent on the size of the first potential difference.

10. The device of claim 9 further comprising:a second LC layer;a third electrode layer, the third electrode layer comprising a second plurality of substantially elongated, substantially parallel electrodes; anda fourth electrode layer,the third electrode layer and the fourth electrode layer for connection between a second potential difference, in a first state the potential difference is zero, and incident light passes through the first LC layer unchanged, in a second state the potential difference is nonzero, and incident light passing through the first LC layer is spread about a second axis, the extent of spreading being dependent on the size of the first potential difference.

11. The device of claim 9, the substantially elongate electrodes of the first and third electrode layers being substantially orthogonal, the first axis and the second axis are substantially orthogonal.

12. The device of any one of claims 2 to 8, the non-uniform spacing of the first electrode and the non-uniform spacing of the second electrode being independent.

13. The device of any one of claims 10 to 12, the non-uniform spacing of the first electrode and the non-uniform spacing of the third electrode being independent.

14. The device of any preceding claim, the non-uniform spacing being pseudorandom.

15. An assembly comprising:a first liquid crystal, LC, layer;a first electrode layer, the electrode layer comprising:a first plurality of substantially elongated, substantially parallel electrodes, the spacing between each electrode being non-uniform; andat least one further optical element.

16. The assembly of claim 15, wherein the at least one further optical element comprises at least one of: a waveguide, a luminance adjustment component, a lens, an image generation device, a reflection-reduction layer, or a protective layer.

17. Apparatus comprising:a first liquid crystal, LC, layer;a first electrode layer, the electrode layer comprising:a first plurality of substantially elongated, substantially parallel electrodes, the spacing between each electrode being non-uniform,at least one processor; andat least one storage comprising instructions, the instructions configured to, with the at least one processor, cause the apparatus to control one or more properties of the LC layer.

18. The apparatus of claim 17, configured to be mounted on a human head with the optical device cell stack positioned in a field of view of an eye of the human head.

19. The apparatus of claim 18 further comprising a first lens comprising a first one of the LC layer and first electrode and a second lens comprising a second one of the LC layer and first electrode.

20. The apparatus of claim 19 wherein the field of view of the eye is a first field of view of a first eye, and the first lens is configured to be positioned in the first field of view, in use, and the second lens is configured to be positioned in a second field of view, of a second human eye of the human head, in use.

21. The apparatus according to any one of claims 17 to 21, the apparatus being at least one of an augmented reality display device, a virtual reality display device or a mixed reality display device.

22. A method of operating a device comprising:a liquid crystal, LC, layer;a first plurality of substantially elongated, substantially parallel electrodes disposed in a first electrode layer; anda second electrode,the method comprising:applying a potential difference between the first plurality of substantially elongated, substantially parallel electrodes and the second electrode to control broadening of a light beam passing through the LC layer about a first axis.

23. The method of claim 22, the second electrode comprising a second plurality of substantially elongated, substantially parallel electrodes disposed in the first electrode layer, interleaved with the first plurality of substantially elongated, substantially parallel electrodes.

24. The method of claim 22, the second electrode comprising an electrode 5 disposed in a second electrode layer substantially parallel the first.s

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