Liquid crystal cell

Abstract

There is disclosed a refractive index changing cell, comprising: a first substrate; a second substrate; a refractive index changing material between the first and second substrates, the refractive index changing material creating a negative pressure in the space defined by the gap between the first and second substrates.

Description

[0001] LIQUID CRYSTAL CELL

[0002] The present application relates to a device comprising a liquid crystal cell, an assembly comprising the device and at least one optical element, an apparatus comprising the device, a processor, a storage comprising instructions for controlling the device, and a method of manufacturing such a device.

[0003] In the formation of liquid crystal cells, for example one drop fill (ODF) techniques for liquid crystal displays (LCDs) and liquid crystal (LC) cells in general, overfill of the liquid crystal layer with liquid crystal material can occur.

[0004] An improved technique is provided.

[0005] Figs. 1 (a) and 1 (b) show a cross-sectional representation of an example cell formed on a glass substrate;

[0006] Figs. 2(a) and 2(b) show a cross-sectional representation of an example cell formed on a plastic substrate;

[0007] Fig. 3 shows a planar representation of an example cell formed on a plastic substrate;

[0008] Fig. 4 shows a representation of an example of a headset incorporating a liquid crystal, adaptive optical lens;

[0009] Fig. 5 shows a representation of an example of a system for operatingthe headset of Fig. 4; and

[0010] Fig. 6 shows schematically a representation of an example apparatus.

[0011] There is provided a refractive index changing cell, comprising: a first substrate; a second substrate; a refractive index changing material between the first and second substrates, the refractive index changing material creating a negative pressure in the space defined by the gap between the first and second substrates.

[0012] The space may be defined by an area of the opposing planar surfaces of the first and second substrates, and the distance between the opposing planar surfaces of the first and second substrates. The refractive index changing material may comprise less than 100% of the space, approximately 87% to 97% of the space, preferably approximately 87% to 93% of the space, preferably approximately 87% to 90% of the space, to thereby create the negative pressure in the space.

[0013] The refractive index changing cell may further comprising a plurality of spacers extending between opposing surfaces of the first and second substrate, defining a distance between the first and second substrates, the spacers preferably being dual height spacers.

[0014] The space may be defined by the area of the opposing planar surfaces of the first and second substrates, and the distance between the opposing planar surfaces of the first and second substrates, less the volume of the plurality of spacers.

[0015] The refractive index changing cell may further comprise a sealant between opposing surfaces of the first and second substrate, defining a perimeter of the cell.

[0016] At least one of the first and second opposing surfaces may exhibit tenting.

[0017] At least one surface of the first and second opposing surfaces adjacent the refractive index changing material may be non-planar.

[0018] The first and second plastic substrates may be sucked in around the spacers due to the negative pressure, thereby holdingthe first and second plastic substrates together and stabilisingthe refractive index changing cell.

[0019] At least one of the first and second opposing surfaces may exhibit tenting between at least two spacers.

[0020] The refractive index changing material may be a liquid electro optic material, and preferably being a being an LC material forming an LC cell.

[0021] The first and second substrates may be one of: plastic substrates; flexible substrates; flexible plastic substrates; rigid substrates; and glass substrates.

[0022] There is provided an assembly comprising: a first substrate; a second substrate; a refractive index changing material between the first and second substrates, the refractive index changing material creating a negative pressure in the space defined by the gap between the first and second substrates; and at least one further optical element.

[0023] 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.

[0024] There is provided apparatus comprising: a first substrate; a second substrate; a refractive index changing material between the first and second plastic substrates, the refractive index changing material creating a negative pressure in the space defined by the gap between the first and second substrates; 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 refractive index changing layer.

[0025] 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.

[0026] The apparatus may further comprise a first lens comprising a first one of the refractive index changing layer and first electrode and a second lens comprising a second one of the refractive index changing layer and first electrode.

[0027] 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 may be configured to be positioned in a second field of view, of a second human eye of the human head, in use.

[0028] The apparatus may be at least one of an augmented reality display device, a virtual reality display device or a mixed reality display device.

[0029] There is provided a method of manufacturing a refractive index changing, cell, comprising: forming a first substrate; forming a second substrate; forming a layer of refractive index changing material between the first and second substrates, the refractive index changing material creating a negative pressure in the space defined by the gap between the first and second substrates. The space may be defined by an area of the opposing planar surfaces of the first and second substrates, and the distance between the opposing planar surfaces of the first and second substrates.

[0030] The refractive index changing material may comprise approximately 87% to 97% of the space, preferably approximately 87% to 93% of the space, preferably approximately 87% to 90% of the space, to thereby create the negative pressure in the space.

[0031] The method may further comprise forming a plurality of spacers extending between opposing surfaces of the first and second substrates, defining a distance between the first and second substrates, preferably forming dual height spacers, wherein the space is defined by the area of the opposing planar surfaces of the first and second substrates, and the distance between the opposing planar surfaces of the first and second substrates, less the volume of the plurality of spacers.

[0032] The method may further comprising forming a sealant between opposing surfaces of the first and second plastic substrate, defining a permitter of the cell.

[0033] The method may further comprise forming the refractive index changing material prior to forming the cell or injecting the refractive index changing material after the first and second plastic substrates are positioned to form the cell.

[0034] 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.

[0035] Various other aspects are also described in the following detailed description and in the attached claims.

[0036] Figs. 1 (a) and 1 (b) illustrate an exemplary operation for filling a liquid crystal, LC, cell on a glass substrate.

[0037] As shown in Fig. 1 (a), a first half of a cell comprises a glass substrate 2, and a second half of the cell comprises a glass substrate 4. A first end of a plurality of spacers 8a, 8b, 8c are formed on a surface 14 of the second substrate 4. The spacers 8a, 8b, 8c define a height of the LC cell to be formed. A surface 12 of the first glass substrate 2 is moved toward the surface 14 of the glass substrate 4, as denoted by the arrow 10, into contact with a second end of the plurality of spacers 8a, 8b, 8c.

[0038] A cell sealant is denoted by pillars 6a, 6b, formed at the perimeter of the LC cell. The cell sealant illustrated by pillars 6a, 6b may be a continuous layer around the edge of the cell, and the illustration of a pillar shape is due to the cross-section nature of the figure.

[0039] As shown by Fig. 1 (b), a glass substrate cell 20 is formed when the surface 12 of the glass substrate 2 is in contact with a second end of the plurality of spacers 8a, 8b, 8c, and a liquid crystal material has been injected into the space which is defined by the two glass substrates and the sealant denoted by pillars 6a, 6b to form LC layer 16.

[0040] The volume of LC material injected into the space can be termed a full fill volume, defined by the surface area of the cell defined by the cell sealant, multiplied by the spacer height - to give the volume of the space, less the volume due to the spacers themselves.

[0041] The above-described technique may be used for forming an LC cell on a plastic substrate.

[0042] An improved technique for forming an LC cell on a flexible plastic substrate is illustrated with respect to Figs. 2(a) and 2(b).

[0043] As shown in Fig. 2(a), a first half of a cell comprises a flexible plastic substrate 22, and a second half of the cell comprises a flexible plastic substrate 24. A first end of a plurality of spacers 28a, 28b, 28c are formed on a surface 34 of the second substrate 24. The spacers 28a, 28b, 28c define a height of the LC cell to be formed. A surface 32 of the first flexible plastic substrate 22 is moved toward the surface 34 of the flexible plastic substrate 34, as denoted by the arrow 30, into contact with a second end of the plurality of spacers 28a, 28b, 28c.

[0044] A cell sealant is denoted by pillars 26a, 26b, formed at the perimeter of the LC cell. The cell sealant illustrated by pillars 26a, 26b may be a continuous layer around the edge of the cell, and the illustration of a pillar shape is due to the cross-section nature of the figure. As shown by Fig. 2(b), a flexible plastic substrate cell 39 is formed when the surface 32 of the plastic substrate 22 is in contact with a second end of the plurality of spacers 28a, 28b, 28c, and a liquid crystal material has been injected into the space which is defined by the two flexible plastic substrates and the sealant denoted by pillars 26a, 26b to form a space for an LC layer. The LC material 38 may be injected before the layer 22 is lowered onto the spacers, depending on the assembly method. Commonly a ‘one drop fill’ (ODF) method dispenses the LC before assembly. A capillary fill method injects the LC into a pre-assembled cell under vacuum.

[0045] As can be seen in the example illustration of Figs. 2(a) and 2(b), the surfaces 32 and 34 of the plastic substrates 22 and 24 are not planar. One or both of the surfaces may be non-planar. As such, in the formed cell 39 of Fig. 2(b), the cell gap between the substrates 22 and 24 is lowerthan the height of the spacers 28a, 28b, 28c, in at least some regions of the cell between the spacers 28a, 28b, 28c or between the spacers and the sealant.

[0046] The non-planar (or irregular) shape of the surfaces 32 and 34 may result in the cell being over-filled with LC material if the volume of LC material injected is the full fill volume as discussed above.

[0047] The volume of LC material injected into the space is therefore a reduced fill volume, defined by the surface area of the cell defined by the cell sealant, multiplied by the spacer height, less the volume due to the spacers themselves - to give the volume of the space, less a certain percentage. The reduced fill volume is thus a percentage of the full fill volume.

[0048] The reduced fill volume is preferably less than 100% of the full fill volume, and is preferably substantially or approximately 87-97% of the full fill fill volume, substantially or approximately less than or equal to 93% of the full fill volume, or preferably substantially or approximately less than or equal to 90% of the full fill volume. The reduced fill volume is thus typically 3%-13% less than the calculated fill ratio of the active area, determined by the full fill volume.

[0049] For a cell gap of greater than 3.5pm, i.e. spacer height of greater than 3.5pm, spacers in the range of no more than ±15% of the cell gap in the glue area may be used to maintain good cell integrity. Cell size can vary considerably depending on the application (from ~10mm2to metres across). Spacers are typically cylindrical in shape with diameter varying from ~5um diameter (largely limited by photolithography capabilities) to 20-30um diameter. Heights of spacers are commonly anything from 2- 3um up to 15 or20um. In some extreme examples spacer height may be greater than this. Spacer area as a % of the total cell commonly vary from a few percent down to a small fraction of a percent.

[0050] By filling the space using a reduced fill volume which is less than the full fill volume, the space is not overfilled.

[0051] By using the reduced fill volume the cell may be underfilled, and the substrate ‘sucked in’ around the spacers to stabilise the plastic cell and hold the top and bottom plate together. This is a further advantage. More specifically, by virtue of underfilling the cell with the reduced fill volume, a negative pressure is created within the cell. This negative pressure increase reliability in flexible liquid cells. The intentional underfill of flexible liquid crystal cells to create negative internal pressure is thus advantageous. The negative pressure results in the ‘sucked in’ effect.

[0052] Figs. 2(a) and 2(b) show a cross-section, and the cell may be any shape, such as circular.

[0053] With reference to Fig. 3, there is shown an example of a planar view of the cell 39. Fig. 2(b) is a cross-section through A-A of the planar view of Fig. 3.

[0054] As illustrated in the example of Fig. 3, the sealant 26 extends around the perimeter of the cell, and Figs. 2(a) and 2(b) show sections of this sealant which are denoted as pillars. The spacers 28a, 28b, 28c, are shown. Other spacers will be present, as denoted by the example spacer 28n. In the example of Fig. 3, the spacers have a substantially circular cross-section, and are cylindrical spacers.

[0055] The shape of the spacers may vary, the width or diameter (lateral dimension) of the spacer cross-section may vary, and the density of the spacers may vary (i.e. the total number of spacers, and / or the total cross-sectional area occupied by spacers). Dual height spacers may be used withing the cell. It may be advantageous to use dual-height spacers in combination with a reduced fill volume, forform a robust cell structure.

[0056] The reduced fill volume has particular advantages in application to cells with a flexible plastic substrate. These advantages are not limited though to cells with a flexible plastic substrates. The reduced fill volume may be advantageous applied to cells with flexible substrates in general, may be advantageously applied to cells with plastic substrates in general, and may be advantageously applied to cells formed of rigid substrates, including glass substrates. In one aspect, the reduced fill volume may be advantageously applied to any cell in which the creation of a negative pressure in forming the LC layer is advantageous.

[0057] The reduced fill volume is described for forming a cell with an LC layer with LC material. The reduced fill volume may be advantageous for forming a cell with a layer other than LC material. In general, a material with a switchable refractive index, or a refractive index changing material, or a liquid filled cell filled with a material with light changing properties forms this layer within the cell. The cell may be a liquid filled, liquid electro optic cell, of which an LC cell is an example.

[0058] 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).

[0059] 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.

[0060] The LC optical lens device may be used forthe push lens and / orthe pull lens or a combined push / pull lens of an augmented reality (AR) headset such as e.g. that shown in Fig. 4. The headset 40 comprises a support frame 42 supporting optical components arranged in optical series in front of the user eye.

[0061] At least one optical component such as one or more of the optical components shown in Fig. 4 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. 4, such an assembly includes a stack of liquid crystal cells according to examples herein. In the example of Fig. 4, 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.

[0062] 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. 4 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.

[0063] 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.

[0064] 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.

[0065] 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 vergence-accommodation 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.

[0066] 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.

[0067] In Fig. 4, 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. 4, 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.

[0068] 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).

[0069] 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 sub-area 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.

[0070] Although not shown in Fig. 4, it is to be appreciated that the headset 40 may be configured to obtain luminance data, e.g. from a light sensorof 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.

[0071] In the example of Fig. 4, 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 powerto be applied bythe push lens48a, 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.

[0072] 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 bythe 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 lens48b) 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 push and 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.

[0073] 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.

[0074] Fig. 4 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. 4, 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.

[0075] 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. 5, 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.

[0076] Although not shown in Fig. 5, 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. 5. 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. 6. 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.

[0077] 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.

[0078] Fig. 6 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. 6, 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. 6, 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. 4. In other examples, though, apparatus including a similar hardware architecture to the apparatus 60 of Fig. 6 may be configured for a different purpose, may include additional components and / or may omit at least one of the components illustrated in Fig. 6.

[0079] The apparatus 60 of Fig. 6 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.

[0080] 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. 4), 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. 4, 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. 6, 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.

[0081] A spatial arrangement of elements of the second display stack 62b in at least one layer of the stack may mirror the 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. 6.

[0082] 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 user to 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. 6, 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. 4. 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. 4) 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.

[0083] In the example of Fig. 6, 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.

[0084] 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.

[0085] In some examples, apparatus, such as the apparatus 60 of Fig. 6, 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.

[0086] 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).

[0087] 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. 5, 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).

[0088] 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. 6, 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] The apparatus 60 of Fig. 6 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.

[0093] 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.

[0094] 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.

[0095] Examples herein refer to a liquid crystal (LC) material. A liquid crystal material is an example of a liquid material with a switchable refractive index, or a refractive index changing material, or a material with light changing properties.

[0096] 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 embodiment examples may be made within the scope of the invention.

[0097] 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 embodiment may be made withingthe scope of the invention.

[0098] 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

CLAIMS1 . A refractive index changing cell, comprising: a first substrate; a second substrate; a refractive index changing material between the first and second substrates, the refractive index changing material creating a negative pressure in the space defined by the gap between the first and second substrates.

2. A refractive index changing cell according to claim 1 , the space being defined by an area of the opposing planar surfaces of the first and second substrates, and the distance between the opposing planar surfaces of the first and second substrates.

3. A refractive index changing according to claim 1 or claim 2, the refractive index changing material comprising less than 100% of the space, approximately 87% to 97% of the space, preferably approximately 87% to 93% of the space, preferably approximately 87% to 90% of the space, to thereby create the negative pressure in the space.

4. A refractive index changing cell according to any one of claims 1 to 3, further comprisinga plurality of spacers extending between opposingsurfaces of the first and second substrate, defining a distance between the first and second substrates, the spacers preferably being dual height spacers.

5. A refractive index changing cell accordingto claim 4when dependent on claims 2 or 3, wherein the space is defined by the area of the opposing planar surfaces of the first and second substrates, and the distance between the opposing planar surfaces of the first and second substrates, less the volume of the plurality of spacers.

6. A refractive index changing cell according to any one of claims 1 to 5 further comprising a sealant between opposing surfaces of the first and second substrate, defining a perimeter of the cell.

7. A refractive index changing cell according to any one of claims 1 to 6, at least one of the first and second opposing surfaces exhibits tenting.

8. A refractive index changing cell according to any one of claims 1 to 7 at least one surface of the first and second opposing surfaces adjacent the refractive index changing material being non-planar.

9. A refractive index changing cell according to any one of claims 1 to 8, the first and second plastic substrates are sucked in around the spacers due to the negative pressure, thereby holding the first and second plastic substrates together and stabilising the refractive index changing cell.

10. A refractive index changing cell according to claim 7 to 9 when dependent on any one of claims 4 to 6, at least one of the first and second opposing surfaces exhibiting tenting between at least two spacers.

11. A refractive index changing cell according to any one of claims 1 to 10, the refractive index changing material being a liquid electro optic material, and preferably being a being an LC material forming an LC cell.

12. A refractive index changing cell according to any one of claims 1 to 11 , the first and second substrates being one of: plastic substrates; flexible substrates; flexible plastic substrates; rigid substrates; and glass substrates.

13. An assembly comprising: a first substrate; a second substrate; a refractive index changing material between the first and second substrates, the refractive index changing material creating a negative pressure in the space defined by the gap between the first and second substrates; and at least one further optical element.

14. The assembly of claim 13, 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.

15. Apparatus comprising: a first substrate; a second substrate; a refractive index changing material between the first and second plastic substrates, the refractive index changing material creating a negative pressure in the space defined by the gap between the first and second substrates; 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 refractive index changing layer.

16. The apparatus of claim 15, 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.

17. The apparatus of claim 16 further comprising a first lens comprising a first one of the refractive index changing layer and first electrode and a second lens comprising a second one of the refractive index changing layer and first electrode.

18. The apparatus of claim 17 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.

19. The apparatus according to any one of claims 15 to 18, the apparatus being at least one of an augmented reality display device, a virtual reality display device or a mixed reality display device.

20. A method of manufacturing a refractive index changing, cell, comprising: forming a first substrate; forming a second substrate; forming a layer of refractive index changing material between the first and second substrates, the refractive index changing material creating a negative pressure in the space defined by the gap between the first and second substrates.

21. A method according to claim 20, the space being defined by an area of the opposing planar surfaces of the first and second substrates, and the distance between the opposing planar surfaces of the first and second substrates.

22. A method according to claim 20 or claim 21 , the refractive index changing material comprising approximately 87% to 97% of the space, preferably approximately 87% to 93% of the space, preferably approximately 87% to 90% of the space, to thereby create the negative pressure in the space.

23. A method according to any one of claims 21 to 22, further comprising forming a plurality of spacers extending between opposing surfaces of the first and second substrates, defining a distance between the first and second substrates, preferably forming dual height spacers, wherein the space is defined by the area of the opposing planar surfaces of the first and second substrates, and the distance between the opposing planar surfaces of the first and second substrates, less the volume of the plurality of spacers.

24. A method according to any one of claims 20 to 23 further comprising forming a sealant between opposing surfaces of the first and second plastic substrate, defining a permitter of the cell.

25. A method according to any one of claims 20 to 24 further comprising forming the refractive index changing material prior to forming the cell or injecting the refractive index changing material afterthe first and second plastic substrates are positioned to form the cell.

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