Optical assembly

Abstract

Optical assembly (10) comprising a deformable optical element (215), defining a primary axis, and an actuator assembly (14) configured to deform the deformable optical element. The actuator assembly comprises a first part (22, 122) coupled to the deformable optical element, a support structure (18), a movable part (20, 120) configured to move relative to the support structure and which is coupled to the first part, and a shape memory alloy (SMA) element (21) configured to drive movement of the movable part relative to the support structure to effect movement of the first part and thereby deform the deformable optical element. The optical assembly is configured so as to retain the first part in position with respect to the support structure when the SMA element is unpowered.

Description

[0001] Optical assembly

[0002] Field

[0003] The present application relates to optical assemblies, specifically optical assemblies comprising a deformable optical element, such as a deformable lens or mirror. Such optical assemblies may be used in glasses or virtual or augmented reality headsets, for example.

[0004] Background

[0005] To see clearly the eye needs to be able to focus on an object. To do so, light from the object must be focused on the retina of the eye. The lens of the eye is primarily responsible for focusing and to focus on objects at different distances, the lens must adjust its focal length. Errors to the focus can come from presbyopia (an error in the focus of the eye usually corrected with glasses) and also from an inability to adjust focus for objects at different distances.

[0006] The ability of the human eye to focus on objects at different distances reduces with age and people over a certain age may have a restricted range of distances over which they can focus. It is therefore desired to provide an optical assembly that assists the eye in focusing on objects at different distances and that can adjust its focal length to do so.

[0007] Virtual reality (VR) and augmented reality (AR) headsets allow a user to experience a simulated, or partially simulated, experience by looking through a headset which replaces or supplements the visual environment around them. To do so, VR and AR headsets present virtual objects to the user. In order for the simulation to be effective, it is desirable that the eye should have to focus on these objects at the correct distance. Given that objects presented in the VR or AR simulation will need to give the impression of being at different distances, VR and AR headsets require an optical assembly that can adjust its focal length or other optical properties based on the simulated distance of the object, requiring the eye to adjust and focus on the simulated object as it would in real life. Such optical assemblies may include deformable lenses, deformable mirrors or other deformable optical elements.

[0008] VR and AR headsets are electronic devices worn on the head. As with all worn electronics, space and weight are at a premium and thus it is advantageous for any optical assembly included in such devices to be as compact and lightweight as possible. Furthermore, given that some devices may be battery powered, it is beneficial for all components of VR and AR headsets to require as little power to operate as possible.

[0009] Standard glasses and VR / AR headsets are provided in a range of different shapes and sizes and it is thus important for optical assemblies for use in such devices to also accommodate a range of different sizes.

[0010] It is therefore desirable to provide an optical assembly that can change its focal length and / or other optical properties, that is light weight, that need not be circular and consumes very little power.

[0011] Summary

[0012] According to an aspect of the present invention, there is an optical assembly comprising: a deformable optical element defining a primary axis; an actuator assembly configured to deform the deformable optical element, the actuator assembly comprising: a first part coupled to the deformable optical element; a support structure; a movable part configured to move relative to the support structure and which is coupled to the first part; and a shape memory alloy (SMA) element configured to drive movement of the movable part relative to the support structure to effect movement of the first part relative to the support structure and thereby deform the deformable optical element; wherein the optical assembly is configured so as to retain the first part in position with respect to the support structure when the SMA element is unpowered.

[0013] It has been found that such an optical assembly provides significant advantages, including the ability to adjust the focal length or other optical properties of the deformable optical element with a high degree of precision, while maintaining a lightweight design that consumes very little power.

[0014] The optical assembly may be configured so as to retain the first part in position with respect to the support structure when the SMA element is unpowered. This allows power consumption of the optical assembly to be reduced, as the SMA element do not need to be provided with continuous energization to hold or retain the movable part in position relative to the support structure and given positions within a range of movement. This also reduces the risk of damage to the movable part compared to situations in which the movable part is free to move when the SMA elements are unpowered.

[0015] Said retained position of the first part with respect to the support structure may be any position within a range of potential positions of the first part with respect to the support structure. The range of potential position of the first part with respect to the support structure may be defined by the range of movement of the first part with respect to the support structure or the range of movement of the first part with respect to the support structure which may be effected by the SMA element driving movement of the movable part relative to the support structure.

[0016] Said retained position of the first part with respect to the support structure may be any position within a continuous range of movement of the first part with respect to the support structure. The continuous range of movement of the first part with respect to the support structure may be defined by the range of movement of the first part with respect to the support structure or the range of movement of the first part with respect to the support structure which may be effected by the SMA element driving movement of the movable part relative to the support structure.

[0017] Said retained position of the first part with respect to the support structure may be the position of the first part with respect to the support structure which may be effected when the SMA element drives movement of the movable part relative to the support structure.

[0018] Said retained position of the first part with respect to the support structure may be the position of the first part with respect to the support structure such that the deformable optical element (and hence optical assembly as a whole) is in a certain or selected configuration. Said certain or selected configuration may provide certain or selected optical properties.

[0019] Said retained position of the first part with respect to the support structure may be any given position.

[0020] The optical assembly may be configured so as to retain the first part in any position with respect to the support structure within a range of potential positions of the first part with respect to the support structure, when the actuator is unpowered. The movable part may be configured to move relative to the support structure and relative to the first part. The SMA element may be configured to drive movement of the movable part relative to the support structure and relative to the first part to effect movement of the first part relative to the support structure and thereby deform the deformable optical element.

[0021] The term deformable optical element as used herein describes any optical element capable of deforming in order to adjust its optical properties, for example a deformable lens or deformable mirror. Examples of such deformable lenses may include liquid lenses, or lenses using a gel to allow deformation, however the disclosure is not limited as such. The deformable optical element may define a primary axis.

[0022] The primary axis may be defined in a number of ways. In the case that the deformable optical element is a deformable lens, the primary axis may be an optical axis of the deformable lens, for example when the deformable lens is in a neutral position, i.e. when the lens is undeformed. The primary axis may be defined as being colinear with a line joining the centre of curvature of a deformable surface of the lens when the lens is minimally deformed and the centre of curvature of the deformable surface of the lens when the lens is maximally deformed. The terms minimum and maximum here refer to the minimum deformation (i.e. no deformation) and maximum deformation of the deformable optical element achievable by the optical assembly.

[0023] In any case, the deformable optical element may be generally planar and / or define a plane and the primary axis may be perpendicular to that plane. In embodiments in which the deformable optical element is a deformable lens, a plane may be defined by the deformable lens when in an undeformed state and the primary axis may be perpendicular to that plane.

[0024] The primary axis may be defined as being perpendicular to a plane defined by the average position of a number of points around the edge of the deformable optical element, e.g. along a direction in which they are driven to move on actuation of the SMA element(s). In other words, a plane may be defined by n points (where n>3) around the edge of the deformable optical element, wherein the plane is the best-fit plane of the n points. The best-fit plane may be defined such that the summed squared distance to all points is minimised and may be calculated using planar regression. The plane may be defined by the n points when the deformable optical element is in a deformed state or a 'neutral' undeformed state. The primary axis may be perpendicular to such a plane. The primary axis may pass through a centre of the deformable optical element (the centre being defined, for example, by an outer circumference of the deformable optical element).

[0025] In embodiments in which the deformable optical element is a deformable lens, the primary axis may be parallel to or colinear with the principal axis of the lens, for example when the lens is in a neutral (e.g. undeformed) state.

[0026] Alternatively, the primary axis may be defined with reference to the support structure. For example, the support structure may comprise a frame located around the periphery of the deformable optical element for supporting the parts of the optical assembly and the primary axis may be perpendicular to a plane defined by the frame. When viewed along the primary axis, the support structure may form a closed loop around the first axis.

[0027] As mentioned above, the optical assembly comprises a first part coupled to the deformable optical element. The first part may be integrally coupled, or mechanically or chemically attached to the deformable optical element. The first part may form part of the deformable optical element. The coupling may be such that movement of the first part deforms the deformable optical element, with such deformation adjusting the optical properties of the deformable optical element. The movement of the first part may be any movement that would effect deformation of the deformable optical element.

[0028] The actuator assembly may be configured to drive movement of the first part in a direction parallel (or substantially parallel) to the primary axis of the deformable optical element. The actuator assembly may be configured to drive movement of the first part in a direction perpendicular to the direction in which the movable part is driven by the SMA element.

[0029] The actuator assembly may be configured to drive movement of the first part in a direction having at least a (non-zero) component which is parallel to the primary axis of the deformable optical element. The actuator assembly may be configured to drive movement of the first part in a direction having at least a (non-zero) component which is perpendicular to the direction in which the movable part is driven by the SMA element.

[0030] The movement of the first part may be a translation of the first part, for example in a direction parallel to or substantially parallel to the primary axis or otherwise (e.g. as set out above). Such a movement of the first part may directly adjust the shape, and hence optical properties, of the deformable optical element and, as such, may be suitable for adjusting the deformable optical element regardless of the physical properties of the surface of the deformable optical element to which the first part is coupled.

[0031] The actuator assembly may be configured to rotate the first part about an axis perpendicular to the primary axis of the deformable optical element. The axis may lie in a generally tangential direction (or may be parallel to a tangential direction), defined with reference to an outer periphery of the deformable optical element. The axis may lie substantially along the length of the first part (or may be parallel to the length of the first part), such that the first part rotates longitudinally.

[0032] The actuator assembly may be configured to rotate the first part about an axis parallel to or collinear with the direction in which the movable part is driven by the SMA element.

[0033] Rotation of the first part may use the bending stiffness of the surface to which the first part is attached to effect a deformation of the deformable optical element. Depending on the bending stiffness, rotating the first part may allow a greater degree of overall deformation than a linear movement of the first part. Furthermore, the bending stiffness can be selected to tailor the deformation and hence optical property adjustment.

[0034] The first part may be coupled to a membrane of the deformable optical element. The bending stiffness of the membrane may effect deformation of the deformable optical element. The membrane may comprise a variable bending stiffness across its surface. That is, the membrane may comprise a first bending stiffness at a first location and a second bending stiffness (different to the first bending stiffness) at a second location. The membrane may have a higher stiffness at its edges than at its centre.

[0035] The use of a membrane with a variable stiffness may facilitate the use of the assembly with non- spherical optical surfaces, or to correct errors within the optical element (e.g. due to gravity- induced sag). It may also allow the assembly to be used with lenses that are not circular.

[0036] Movement of the movable part relative to the support structure may cause relative movement between an angled surface which is at an acute, non-zero angle to the primary axis and a second surface to drive movement of the first part relative to the support structure.

[0037] The second surface may be an angled surface which is at an acute, non-zero angle to the primary axis. The angled surface may be defined by the movable part or the first part.

[0038] The movable part may comprise a first friction surface. The first friction surface may be arranged to move over a second friction surface during movement of the movable part relative to the support structure. The optical assembly may be arranged such that the first and second friction surfaces are biased together so as to generate a friction force therebetween that resists movement of the movable part with respect to the first part and / or support structure. For example, the biasing together of the first and second friction surfaces may be provided by one or a combination of: one or more resilient elements (e.g. springs), one or more magnets and gravity.

[0039] The friction force may be sufficient to retain the movable part in position with respect to the first part (and / or the first part in position with respect to the support structure) when the SMA element is unpowered.

[0040] The use of a first friction surface and a second friction surface may allow the provision of "zero hold power" functionality, whereby the movable part is held in position relative to the first part even after the SMA element has been powered down and, optionally, is no longer exerting a force on the movable part. The movable part being held in position means the first part is consequently held in position. The static friction between the first and second friction surfaces is configured to hold the first part in position relative to the support structure, such that the deformable optical element (and hence optical assembly as a whole) is maintained in a certain configuration - providing certain optical properties - even in the absence of any power being applied to the SMA element.

[0041] The use of friction surfaces to provide zero hold power greatly reduces the power consumption of the assembly.

[0042] The surface of the friction surfaces may be treated to control the friction coefficient thereof.

[0043] The first part may comprise the second friction surface.

[0044] Where the first part comprises the second friction surface, the same surface may provide the zero hold power and deformation-inducing movement of the first part.

[0045] The support structure may comprise the second friction surface. At least one of the first friction surface and second friction surface may define the angled surface(s).

[0046] The angle of the angled surface may be configured to move the first part relative to the movable part in a direction perpendicular to the direction of force of the SMA element / direction of movement of the movable part. This may act to drive the centres of the first part and movable part apart as the movable part moves relative to the first part.

[0047] The use of an angled surface in this manner allows the direction of force and movement to be altered within the device. That is, a linear movement of the movable part perpendicular to the primary axis can be efficiently converted to a linear movement parallel to the primary axis to effect deformation of the deformable optical element.

[0048] This is of particular use given the geometry of an optical assembly, which are typically much smaller in a dimension parallel to the primary axis than in other dimensions. The degree of movement provided by an SMA element is directly proportional to its length and so in certain circumstances it is preferable to use longer lengths of SMA element. It may not be possible to align such a length of SMA element with the primary axis of the deformable optical element, and so it becomes desirable to align the SMA element perpendicular to primary axis, but effect movement parallel to the primary axis. The use of angled surfaces in this manner achieves this objective.

[0049] Furthermore, the choice of angle of the angled surface with respect to the primary axis determines the degree of movement amplification provided - that is, the amount the movable part and first part are effectively "driven apart" (i.e. driven in a direction perpendicular to the direction of force of the SMA element and / or direction of movement of the movable part) for each unit of movement of the movable part. The use of angled surfaces allows fine tuning and accurate control of the movement of the first part - and hence deformation of the deformable optical element. It also allows the amount of first part movement (and hence optical element deformation) to be varied around the circumference of the deformable optical element, as will be discussed below.

[0050] The optical assembly may comprise a second part. The second part may comprise a third friction surface. The movable part may further comprise a fourth friction surface. The fourth friction surface may engage with the third friction surface so as to generate a friction force that resists movement of the movable part with respect to the first part and / or support structure. The friction force (or a combination of the friction forces between the first and second friction surfaces and between the third and fourth friction surfaces) may be sufficient to retain the movable part in position with respect to the first part (and / or the first part in position with respect to the support structure) when the SMA element is unpowered.

[0051] The second part may be coupled to the support structure or deformable optical element. The introduction of a second pair of friction surfaces (i.e. the third friction surface and fourth friction surface) may increase the friction and thus improve the zero hold power performance of the optical assembly.

[0052] At least one of the third friction surface and fourth friction surface may define an angled surface which is at an acute, non-zero angle to the primary axis of the deformable optical element.

[0053] The angle of the angled surface may be configured to move the second part relative to the movable part in a direction perpendicular to the direction of force of the SMA element / direction of movement of the movable part. This may act to drive the centres of the second part and movable part apart as the movable part moves relative to the second part.

[0054] If the second part is coupled to the support structure, the angle of the angled surface may be configured to move the first part relative to the support structure in a direction perpendicular to the direction of force of the SMA element / direction of the movement of the movable part. This may act to drive the centres of the first part and the second part apart as the movable part moves relative to the second part. The movement produced by the angle of the angled surface of the first friction surface and the second friction surface may be added to the movement produced by the angle of the angled surface of the third friction surface and the fourth friction surface to provide increased cumulative movement of the first part relative to the support structure.

[0055] The acute, non-zero angle of the at least one of the first friction surface and second friction surface may be defined in an opposite direction with respect to the primary axis to the acute, non-zero angle of the at least one of the third friction surface and fourth friction surface. In other words, the angled surfaces of the first and / or second friction surfaces may slope in a different direction to the angled surfaces of the third and / or fourth friction surfaces relative to the primary axis. For example, the angled surfaces of the first and / or second friction surfaces may slope 'upwards' and the angled surfaces of the third and / or fourth friction surfaces slope 'downwards' when viewed along a direction perpendicular to the primary axis of the deformable optical element.

[0056] The second friction surface and the third friction surface may be spaced apart. The movable part may be disposed between the first part and second part. The movable part may be disposed between the first part and second part when viewed along a direction perpendicular to the primary axis.

[0057] The introduction of a second angled surface arranged at an opposing angle to the first angled surface may improve the movement amplification performance of the optical assembly. More specifically, the distance that the first part can be driven (and hence the deformation that is imposed on the deformable optical element) is increased for a unit moved by the movable part compared to with only a single angled surface.

[0058] The second and third friction surfaces may be interleaved.

[0059] The second and third friction surfaces may collectively define interleaved peaks around at least part of a circumference of the deformable optical element.

[0060] The second and third friction surfaces may comprise similar surface profiles but offset with respect to the circumference of the deformable optical element. Each of the second and third friction surfaces may comprise a series of peaks. The peaks of the second and third friction surfaces may be offset from each other in the direction of the circumference of the deformable optical element.

[0061] Using interleaved friction surfaces (e.g. with an undulating movable part arranged therebetween) reduces the thickness of the actuator assembly.

[0062] One or both of the first friction surface and the second friction surface may comprise a first angled surface at a first acute, non-zero angle to the primary axis of the deformable optical element and a second angled surface at a second acute, non-zero angle to the primary axis of the deformable optical element. The first and second angled surfaces may be at different positions around the periphery of the deformable optical element. The first angle may be different to the second angle. Adjusting the acute angle of the angled surfaces adjusts the amount the first part moves in a direction parallel to the primary axis (or the amount by which the first part is driven to rotate about an axis perpendicular to the primary axis, as explained above) and hence the deformation effected. As such, providing differently angled surfaces around the periphery of the deformable optical element adjusts the deformation imposed on the deformable optical element around the periphery of the deformable optical element. This can be useful, for example, when the deformable optical element is non-circular. In particular, it may be useful when spherical deformation of a non-circular lens is required.

[0063] A component of the optical assembly may comprise a plurality of teeth each defining a respective angled surface. For example, such angled surfaces may be those described above or any other angled surface described herein, for example one or more of the first, second, third and fourth friction surfaces

[0064] The component comprising the plurality of teeth may be the first part, the movable part or the second part, for example. More than one component may comprise a plurality of teeth, for example two or more of the first, second and movable parts. The component(s) may comprise ten or more teeth, optionally twenty or more teeth, optionally fifty or more teeth. The angled surfaces may each be at an acute, non-zero angle to the primary axis. The angled surfaces on a given component may all be at the same angle to the primary axis or at different angles to the primary axis.

[0065] In a particular example, the movable part comprises a plurality of teeth, each defining a respective angled surface which is at an acute, non-zero angle to the primary axis. The plurality of teeth may comprise ten or more, twenty or more or fifty or more teeth, for example. The teeth may engage with corresponding teeth on the first part, for example. The plurality of teeth may be provided on a first side of the movable part. A further plurality of teeth may be provided on a second side of the movable part, the second side being spaced from the first side along the primary axis. The further plurality of teeth may each define a respective angled surface at an acute, non-zero angle to the primary axis. The angled surfaces of the further plurality of teeth may be angled in an opposite direction with respect to the primary axis, as compared to the angled surfaces of the plurality of teeth on the first side of the movable part. The further plurality of teeth may engage with corresponding teeth on the second part, for example (in embodiments where a second part is provided). The further plurality of teeth may comprise ten or more, twenty or more or fifty or more teeth, for example. The first friction surface and second friction surface may be biased into engagement by at least one of a biasing element and the deformable optical element itself. The biasing element may be external to the other components of the actuator assembly. In embodiments in which the deformable optical element is a deformable lens, if the components of the actuator assembly are inside the external envelope of the deformable lens, the biasing element may be outside the external envelope of the deformable lens. Alternatively, if the components of the actuator assembly are outside the external envelope of the deformable lens, the biasing element may be inside the external envelope of the deformable lens. The envelope of the deformable lens may refer to the extent of the deformable lens when viewed along the primary axis and / or the extent of the deformable lens when view in a direction perpendicular to the primary axis.

[0066] The biasing element may be passive, i.e. not driven by any actuator. The biasing element may comprise one or more of the following: one or more resilient elements (e.g. a spring or deformable material) and one or more magnets.

[0067] Biasing the first and second friction surfaces (and optionally the third and fourth friction surfaces) into engagement increases the static friction and can provide a more reliable zero hold power performance. It can also create a more robust mechanism that can more reliably operate even when the optical assembly is subjected to external forces or shocks.

[0068] The SMA element may be arranged such that a normal force acting between the first friction surface and second friction surface remains substantially constant on actuation of the SMA element.

[0069] The SMA element may be arranged to apply a force to the movable part with a component orthogonal to the first friction surface that reduces a normal force between the first friction surface and the second friction surface and with a component parallel to the first friction surface so as to drive movement of the movable part relative to the support structure across the second friction surface.

[0070] The actuator assembly may comprise a rolling bearing assembly associated with the movable part.

[0071] The bearing assembly may be configured to reduce the friction force exerted on the movable part. The bearing assembly may be located between the movable part and any one, two, or all of the first part, the second part and the support structure. The bearing assembly may be located between the first and second friction surfaces and / or the third and fourth friction surfaces.

[0072] The bearing assembly may comprise a ball bearing assembly to reduce friction between abutting surfaces.

[0073] The rolling bearing assembly may be located between the movable part and only one of the first part and the second part.

[0074] In other examples the rolling bearing assembly may be located between the movable part and both of the first and second parts.

[0075] The bearing assembly may reduce the friction between abutting surfaces. The bearing assembly may facilitate the use of surfaces with smaller angles with respect to the primary axis. Smaller angles (i.e. steeper angles) may facilitate greater movement of the first part relative to the support structure in a direction parallel to the primary axis (or greater rotation of the first part about an axis perpendicular to the primary axis), per unit movement of the movable part in a direction perpendicular to the primary axis. Increasing the movement of the first part substantially parallel to the primary axis may increase the deformation and change of optical characteristic that can be achieved by the optical assembly.

[0076] The bearing assembly may be located between only one of the movable part and the first part (e.g. first and second friction surfaces) or the movable part and the second part (e.g. third and fourth friction surfaces). This may allow the other pair of parts (i.e. those without such a bearing assembly) to retain the optical assembly in the chosen configuration when the SMA element is unpowered (i.e. by preventing movement of the first part with respect to the movable part or support structure). In this way, one interface (e.g. an interface between the movable part and the first part) may comprise steeper-angled surfaces (relative to a plane perpendicular to the primary axis) and a rolling bearing assembly and another interface (e.g. an interface between the movable part and the second part) may comprise a plain bearing for the provision of friction for zero hold power functionality. This second interface may comprise angled surfaces (e.g. with shallower angles relative to a plane perpendicular to the primary axis than those of the first interface).

[0077] Movement of the movable part over another part of the assembly may be supported by the rolling bearing assembly at one or more points around the circumference of the deformable optical element and by a plain bearing at one or more different points around the circumference of the deformable optical element. The other part of the assembly may be the support structure, the first part or the second part.

[0078] The bearing assembly may be located between only a portion of the contact area between the movable part and the first part (e.g. first and second friction surfaces) and / or only a portion of the contact area between the movable part and the second part (e.g. third and fourth friction surfaces).

[0079] Part of the first and second friction surfaces may comprise a bearing assembly (e.g. comprising one or more ball bearings) therebetween and part of the first and second friction surfaces may be in direct contact (without a bearing assembly). Alternatively, or additionally, part of the third and fourth friction surfaces may comprise a bearing assembly therebetween (e.g. comprising one or more ball bearings) and part of the third and fourth friction surfaces may be in direct contact (without a bearing assembly).

[0080] Using the combination of a rolling bearing and plain bearing can allow increased transverse movements where required, but still allow the zero hold power functionality described elsewhere herein.

[0081] The part of the friction surfaces with a bearing assembly may be areas that require greater displacement (e.g. at a greater radius from the optical centre). The use of the bearing assembly facilitates the greater deformation, as outlined above. The part of the friction surfaces without a bearing assembly may provide the friction necessary to retain the optical assembly in the chosen configuration when the SMA element is unpowered.

[0082] At least one of the first friction surface and second friction surface may define a (non-zero, acute) angle with a plane of the deformable optical element that varies around the circumference of the optical element. Such an angled surface may be configured to cause rotation of the first part about an axis perpendicular to the primary axis (e.g. as described above) as the movable part moves relative to the support structure.

[0083] That is, one of the first friction surface and second friction surface may have a varying camber around the circumference of the optical element. This may effect rotation of the first part about an axis parallel to or collinear with the direction in which the movable part is driven by the SMA element, which may use the bending stiffness of the surface or membrane of the optical element to deform the optical element.

[0084] At least one of the first friction surface and second friction surface may define an undulating profile configured to rotate the first part about an axis perpendicular to the primary axis of the deformable optical element as the movable part moves relative to the support structure.

[0085] As noted above, the first part may be moved in a variety of different ways to effect deformation of the deformable optical element. The first part may be rotated, e.g. by means of an undulating friction surface - e.g. a friction surface with a varying camber. Rotating the first part by means of a varying camber may provide the benefits of rotationally deforming the optical element, as discussed above.

[0086] The movable part may be coupled to the first part by a rotating linkage or flexure bearing, the rotating linkage or flexure bearing being arranged such that the first part is urged away from the movable part as the movable part moves relative to the support structure (e.g. in a first direction).

[0087] This movement may be opposed by a biasing force provided by a biasing element and / or by the deformable optical element itself such that when the movable part moves relative to the support structure in a second direction, opposite to the first direction, the first part is urged back towards the movable part.

[0088] The movable part may extend around at least part of a periphery of the deformable optical element (e.g. when viewed along the primary axis). The movable part may extend around the whole periphery of the deformable optical element (e.g. when viewed along the primary axis).

[0089] The movable part may be a singular part, or a plurality of parts collectively forming the movable part.

[0090] The movable part may be located near the circumference of the deformable optical element (e.g. when viewed along the primary axis). The movable part may be substantially annular (when viewed along the primary axis). The movable part may be formed of a plurality of parts, either separate or coupled together. Alternatively, the movable part may be a single annular part. It may extend around part, or all of the periphery of the deformable optical element. Such arrangements of the movable part may ensure that the movable part is located away from the primary lines of vision through or via the deformable optical element near the primary axis. The visible obstruction of the movable part is therefore minimised.

[0091] The first part and / or movable part may be spaced from the outer circumference of the deformable optical element. Spacing the first part and / or movable part from the outer circumference of the deformable optical element may define an outer annular region of the deformable optical element. The outer annular region may comprise fluid in communication with an inner or centre region of the deformable optical element. The spacing and / or width of the outer annular region may be substantially between 1 and 15 mm, 1 and 10 mm or 1 and 5 mm.

[0092] The annular region may be arranged such that movement of the first part changes the volume of the annular region. Fluid may be communicated with the centre region of the optical element as the volume of the annular region changes. This may increase the deformation achievable. Increasing the possible deformation may increase the range of focal lengths and other changes in optical characteristics that can be achieved.

[0093] The first part and / or movable part may define a width that extends radially with respect to the deformable optical element. The width of the first part and / or movable part may be substantially between 1 and 15 mm, 1 and 10 mm or 1 and 5 mm. Increasing the width of the first part and / or movable part may increase the surface area over which the movement of the first part relative to the support structure acts. The larger area may increase the deformation that can be achieved - for example by increasing the volume displaced by the movement. Increasing the possible deformation may increase the range of focal lengths and other changes in optical characteristics that can be achieved.

[0094] The actuator assembly may be configured to rotate the movable part about an axis parallel to or collinear with the primary axis of the deformable optical element.

[0095] The actuator assembly may comprise at least one pair of opposing SMA elements, the opposing SMA elements being configured to drive the movable part relative to the support structure in opposite directions and / or senses.

[0096] SMA elements are configured to exert a tension force. They are therefore only capable of exerting forces in a single direction. In devices such as optical devices, which benefit from adjustment in two directions, it is advantageous to employ SMA elements in opposing pairs, to effect movement in both directions. In alternative arrangements, it may be able to effect movement in a second direction using a bias - e.g. provided by a separate biasing element (e.g. a resilient element such as a spring), the deformable optical element itself or inherent elasticity of the surrounding parts of the device.

[0097] The opposing SMA elements may be arranged substantially parallel to each other.

[0098] The opposing SMA elements may cross over each other when viewed along the primary axis of the deformable optical element.

[0099] The specific arrangement of the SMA elements may vary and can be selected to ensure a compact arrangement, accurate and efficient movement control and to minimise obstruction provided by the SMA elements.

[0100] The optical assembly may comprise one or more SMA elements located around a periphery of the deformable optical element (e.g. when viewed along the primary axis). In other words, the SMA element referred to above and optionally one or more other SMA elements may be located around a periphery of the deformable optical element (e.g. when viewed along the primary axis).

[0101] Locating SMA elements around the periphery of the deformable optical element minimises visual obstruction provided by the elements.

[0102] The optical assembly may further comprise one or more guide elements arranged to guide the SMA element(s) around the periphery of the deformable optical element.

[0103] Guide elements may be employed around which the SMA elements can be diverted. This allows a longer SMA element to be used than would otherwise be possible. It can also allow SMA elements to be diverted around the periphery of the deformable optical element to avoid the SMA elements impeding on a user's vision through or via the deformable optical element.

[0104] The movable part may comprise a plurality of parts. The optical assembly may comprise a plurality of SMA elements of different lengths associated with the plurality of parts of the movable part. The different lengths of the SMA elements may be configured to drive the respective part of the movable part by different amounts. Put differently, the optical assembly may comprise a plurality of SMA elements of different lengths associated with the plurality of parts of the movable part. The plurality of SMA elements may be configured to drive the respective parts of the movable part by different amounts on contraction of the SMA elements.

[0105] Non-circular deformable optical elements may require accommodation (e.g. non-symmetrical deformations) in order to suitably adjust optical performance. This may be achieved by moving different sub-parts of the movable part by different amounts around the periphery of the deformable optical element. Moving the movable part by different amounts around the periphery of the deformable optical element varies the amount that the first part moves at the corresponding locations. This, in turn adjusts the amount of deformation of the deformable optical element at that location. To achieve this, different lengths of SMA element may be associated with different parts of the movable part.

[0106] The actuator assembly may be configured to resist rotation of any of the first part, support structure and / or second part relative to any of the others of the first part, support structure and / or second part, about the primary axis.

[0107] The actuator assembly may comprise a flexure arranged to resist relative rotation of the first part and support structure (and optionally second part) about the primary axis. The flexure may be a mechanical linkage between the two parts, arranged to permit movement in one dimension, but resist movement in another.

[0108] The actuator assembly may be located inside the deformable optical element. For example, the actuator assembly may be located inside the deformable optical element when viewed along the primary axis and / or when viewed along a direction perpendicular to the primary axis.

[0109] Locating the actuator assembly within the deformable optical element may provide a particularly compact arrangement. Locating the actuator assembly within the deformable optical element may require the actuator assembly to completely overlap with the deformable optical element when viewed in a direction perpendicular to the primary axis and / or in a direction parallel to the primary axis.

[0110] Alternatively, the actuator assembly may be located on the outside of the deformable optical element. For example, the actuator assembly may be located on the outside of the deformable optical element when viewed along the primary axis and / or when viewed along a direction perpendicular to the primary axis. The actuator assembly may be configured to adjust the focal length of the deformable optical element, e.g. of the deformable lens.

[0111] The actuator assembly may be configured to deform the deformable optical element so as to adjust the optical properties of the deformable optical element. The focal length is one example of such an optical property, however the actuator may be configured to adjust other optical properties of the deformable optical element.

[0112] The deformation of the optical element may move the lens centre. That is, by deforming the optical element, the optical centre of the lens may move within the plane of the optical element. This may be achieved by adjusting the degree of deformation around the circumference of the optical element, as described herein - for example, having a larger average deformation on one side of the optical element compared to the other. This may be beneficial when looking at objects at different distances, since objects that are closer are often located at different locations within a user's field of view - for example objects that are closer may be closer to the viewer's nose than those at a distance, e.g. on the horizon. The deformation of the optical element may comprise a combination of an adjustment of the focal length and the location of the optical centre of the lens.

[0113] The support structure may comprise a frame located around the periphery of the deformable optical element for supporting the parts of the optical assembly. The frame may be located around a periphery of the deformable optical element when viewed along the primary axis.

[0114] The support structure may comprise a seal configured to seal a fluid or gel in at least part of the deformable optical element.

[0115] The support structure may comprise a biasing means, e.g. a biasing element, for biasing the first part and movable part into engagement.

[0116] The optical element may be a deformable lens and the primary axis may be an optical axis. The optical element may be a deformable mirror.

[0117] Various parts of the optical assembly may be referred to more generally. In particular, the support structure and the movable part may be referred to more generally as, for example, second and third parts respectively. In this way, an optical assembly is disclosed, the optical assembly comprising: a deformable optical element defining a primary axis; an actuator assembly configured to deform the deformable optical element, the actuator assembly comprising: a first part coupled to the deformable optical element; a second part; a third part configured to move relative to the second part and which is coupled to the first part; and a shape memory alloy (SMA) element configured to drive movement of the third part relative to the second part to effect movement of the first part relative to the second part and thereby deform the deformable optical element; wherein the optical assembly is configured so as to retain the first part in position with respect to the second part when the SMA element is unpowered.

[0118] It will be appreciated that there are no particular additional restrictions on the functions of the first, second and third parts. For example, the second part may not necessarily act as a support structure. The optional features and description above apply to any optical assembly described herein, including where more general terms (e.g. second and third parts in place of support structure and movable part) are used. Where more general terms are used, the second part comprising a third friction surface described above may be referred to as a fourth part.

[0119] Optical assemblies have been described above as comprising an SMA element. However, a different (non-SMA) actuator may be used instead. Accordingly, an optical assembly is disclosed, the optical assembly comprising: a deformable optical element defining a primary axis; an actuator assembly configured to deform the deformable optical element, the actuator assembly comprising: a first part coupled to the deformable optical element; a second part; a third part configured to move relative to the second part and which is coupled to the first part; and an actuator configured to drive movement of the third part relative to the second part to effect movement of the first part relative to the second part and thereby deform the deformable optical element. The optical assembly may be configured so as to retain the first part in position with respect to the second part when the actuator is unpowered.

[0120] Said retained position of the first part with respect to the second part may be any position within a range of potential positions of the first part with respect to the second part. The range of potential position of the first part with respect to the second part may be defined by the range of movement of the first part with respect to the second part or the range of movement of the first part with respect to the second part which may be effected by the actuator driving movement of the third part relative to the second part.

[0121] Said retained position of the first part with respect to the second part may be any position within a continuous range of movement of the first part with respect to the second part. The continuous range of movement of the first part with respect to the second part may be defined by the range of movement of the first part with respect to the second part or the range of movement of the first part with respect to the second part which may be effected by the actuator driving movement of the third part relative to the second part.

[0122] Said retained position of the first part with respect to the second part may be the position of the first part with respect to the second part which may be effected when the actuator drives movement of the third part relative to the second part.

[0123] Said retained position of the first part with respect to the second part may be the position of the first part with respect to the second part such that the deformable optical element (and hence optical assembly as a whole) is in a certain or selected configuration. Said certain or selected configuration may provide certain or selected optical properties.

[0124] Said retained position of the first part with respect to the second part may be any given position.

[0125] The optical assembly may be configured so as to retain the first part in any position with respect to the second part within a range of potential positions of the first part with respect to the second part, when the actuator is unpowered.

[0126] The second part may be referred to as a support structure and the third part may be referred to as a movable part.

[0127] The actuator may be of any suitable type. For example, the actuator may comprise one or more voice coil motors (VCM), one or more SMA elements and / or one or more piezoelectric actuators. In the optical assemblies described above, the movable part may be described as an intermediate part which is driven to move in order to drive movement of the first part. Optical assemblies are also disclosed which do not comprise such an intermediate part but which comprise a part which is coupled to a deformable optical element and which is driven (e.g. directly driven) to move relative to a support structure in order to deform the deformable optical element. Such optical assemblies are now described.

[0128] Further according to an aspect of the present invention is an optical assembly comprising: a deformable optical element defining a primary axis; an actuator assembly configured to deform the deformable optical element, the actuator assembly comprising: a support structure; a first part which is coupled to the deformable optical element and which is movable relative to the support structure; and a shape memory alloy (SMA) element configured to drive movement of the first part relative to the support structure to deform the deformable optical element.

[0129] Any of the features described herein (e.g. in the context of the optical assemblies described above) may be applied to such an optical assembly. Additionally, the following optional features are disclosed.

[0130] The optical assembly may be configured so as to retain the first part in position with respect to the support structure when the SMA element is unpowered. This may reduce the power consumption of the optical assembly, as compared to a situation in which it is necessary to continuously power the SMA element in order to hold the first part in position with respect to the support structure.

[0131] The first part may be fixed relative to at least a portion of the deformable optical element. For example, the first part may be attached to or integrally formed with at least a portion of the deformable optical element. In other words, the first part is configured so as not to move relative to at least a portion of the deformable optical element on actuation of the SMA element.

[0132] The SMA element may be connected to the first part. The SMA element may be attached to the first part or hooked over a portion of the first part. The optical assembly may comprise a bearing arrangement configured to guide movement of the first part with respect to the support structure. The SMA element may be configured to drive movement of the first part in a direction perpendicular to the primary axis which is converted to movement along a direction having at least a component along the primary axis by the bearing arrangement. The direction perpendicular to the primary axis may aligned with or parallel to an outer edge of the deformable optical element. In other words, the direction perpendicular to the primary axis may be a tangential direction, for example, defined with reference to an outer edge of the deformable optical element. The axis may lie substantially along the length of the first part (or may be parallel to the length of the first part), such that the first part rotates longitudinally.

[0133] The bearing arrangement may comprise one or more angled surfaces which are at an acute, non-zero angle to the primary axis. The one or more angled surfaces may each be engaged with a respective engagement surface. The SMA element may be configured to drive movement of the one or more angled surfaces over the respective engagement surface(s) to move the first part relative to the support structure and deform the deformable optical element. Such angled surfaces may guide movement of the first part along a direction having at least a component along the primary axis (or along a direction parallel to the primary axis).

[0134] The one or more angled surfaces may be provided on the first part and the engagement surface(s) may be provided on the support structure. In another example, the one or more angled surfaces may be provided on the support structure and the engagement surface(s) may be provided on the first part. The engagement surface(s) may also be angled surfaces in that they may be arranged at an acute, non-zero angle to the primary axis.

[0135] The first part may be coupled to a first deformable surface of the deformable optical element. Movement of the first part relative to the support structure may deform the first deformable surface. The optical assembly may comprise a second part which is coupled to a second deformable surface of the deformable optical element (different to the first deformable surface) and which is movable relative to the support structure. Movement of the second part relative to the support structure may deform the second deformable surface.

[0136] The SMA element may be configured to also drive movement of the second part relative to the support structure to deform the second deformable surface of the deformable optical element. In this way, the first and second parts may be driven together relative to the support structure. In other embodiments, the optical assembly may comprise a further actuator (e.g. a separate SMA element or some other actuator) which is configured to drive movement of the second part relative to the support structure to deform the second deformable surface of the deformable optical element. The deformable optical element may be a deformable lens, e.g. a liquid lens, for example. The first and second deformable surfaces may be spaced from one another along the primary axis.

[0137] In this way, two parts which are each movable with respect to the support structure (either together or independently) may drive deformation of two different surfaces of the deformable optical element.

[0138] Further according to an aspect of the present invention is a virtual reality headset comprising an optical assembly as described anywhere herein.

[0139] Further according to an aspect of the present invention is an augmented reality headset comprising an optical assembly as described anywhere herein.

[0140] The optical assembly of the present disclosure is particularly suited for use in wearable electronics, such as VR and AR headsets. This is due to the compact and lightweight design, reliable operation and low power consumption.

[0141] Further according to an aspect of the present invention is a pair of glasses comprising an optical assembly as described anywhere herein.

[0142] Brief description of the drawings

[0143] Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0144] Figure 1 is a schematic view of an optical assembly according to the disclosure;

[0145] Figure 2 is a cross-section of the optical assembly of Figure 1;

[0146] Figure 3 is a further cross-section of the optical assembly of Figure 1;

[0147] Figures 4A and 4B are enlarged partial schematic views of optical assemblies;

[0148] Figure 5 is a schematic view of an actuator assembly;

[0149] Figure 6 is a schematic view of the actuator assembly of Figure 5;

[0150] Figure 7 is a schematic view of an alternative actuator assembly;

[0151] Figures 8A to 8H are schematic views of actuator assembly arrangements; Figures 9A to 9C are schematic views of optical assemblies;

[0152] Figure 10 is a schematic view of an alternative actuator assembly;

[0153] Figure 11 is a schematic view of an alternative actuator assembly;

[0154] Figure 12 is a schematic view of a further alternative actuator assembly;

[0155] Figures 13A to 13C are schematic views of an actuator assembly with alternative zero hold power arrangements;

[0156] Figure 14A is a schematic view of an actuator assembly;

[0157] Figure 14B is a schematic view of the actuator assembly of Figure 14A; and

[0158] Figure 14C is a schematic view of an alternative actuator assembly.

[0159] Detailed description

[0160] Figure 1 schematically shows a front of an optical assembly 10. The optical assembly is suitable for use in glasses and electronic headsets - such as virtual reality or augmented reality headsets. The optical assembly 10 includes a deformable lens 12, which defines an optical axis OA (see Figure 2). The optical axis may be referred to as a primary axis.

[0161] The optical assemblies of the present embodiments include a deformable lens having an optical axis OA, however the disclosure is not limited as such and it is understood that the present optical assembly can also be used with any alternative deformable optical element, such as a deformable mirror having a primary axis. The features outlined and discussed below would also apply equally to an embodiment including an alternative deformable optical element.

[0162] The deformable lens 12 of the optical assembly 10 may be any form of deformable lens 12, as discussed above. The deformable lens 12 is configured to be deformed to adjust its optical properties, such as its focal length. Typically, the profile of at least one of the front or rear surface of the lens will be made more concave or convex in order to adjust the focal length.

[0163] The optical assembly 10 comprises an actuator assembly 14 configured to deform the deformable lens 12 to adjust the focal length of the deformable lens 12. The actuator assembly 14 will be discussed in more detail with reference to Figures 4 to 8H and Figures 10 to 14C.

[0164] Although the present disclosure primarily discusses adjusting the focal length, it will be understood that the deformation of the deformable lens may instead, or additionally, be for adjusting other properties of the lens. In the example of Figure 1, the actuator assembly 14 is located around the circumference of the deformable lens 12 and optical assembly 10, however this disclosure is not limited as such and in other examples the actuator assembly 14 may be located elsewhere. Here, the actuator assembly 14 is located substantially within the outer envelope of the deformable lens 12, however in other examples the actuator assembly 14 may be located outside of the envelope of the deformable lens 12.

[0165] The deformable lens 12 of Figure 1 is substantially rectangular when viewed along the optical axis. The optical assembly 10 of the present disclosure may be used with a deformable lens of any shape.

[0166] Figures 2 and 3 are cross-sections in the plane 2' shown in Figure 1. Figure 2 shows the optical assembly 10 in a neutral state. Figure 3 illustrates the deformation of the deformable lens 12. A lower membrane 16 of the deformable lens 12 is schematically shown as moving between a concave and convex arrangement. As the membrane 16 deflects and the deformable lens 12 deforms, the optical properties of the deformable lens are adjusted - for example by adjusting the focal length.

[0167] In the present arrangement, the upper portion of the deformable lens 12 is glass and thus cannot be deformed, with a lower portion of the deformable lens 12 including the membrane 16 and being deformable (see Figures 8C to 8H). In other arrangements, the opposing (upper) surface / portion of the deformable lens 12 may be deformed or, alternatively, both portions / surfaces of the deformable lens may be deformed. An example of an arrangement in which both the upper and lower surfaces of the deformable lens will be described below with reference to Figures 14A-C.

[0168] It is understood that the nature and extent of the deformation of Figure 3 is schematic only. In embodiments of the disclosure, deformation of the deformable lens 12 may be less pronounced and the deflection of the membrane 16 may be between two different concave, or convex, profiles.

[0169] Turning now to Figures 4A and 4B, enlarged schematic views of the actuator assembly of Figure 3 are provided. Figures 4A and 4B schematically show two different deformation mechanisms, both of which change the shape of the membrane 16 of the deformable lens 12 and hence deform the deformable lens 12. In Figure 4A, the actuator assembly 14 is configured to rotate the edge of the membrane 16 about its edge 38. The membrane 16 is rotated about an axis perpendicular to the optical axis OA of the deformable lens 12. This axis is parallel to a tangential direction, defined with reference to an outer periphery of the deformable optical element, which in this case is the edge 18 of the membrane 16. In this configuration, the stiffness of the membrane 16 causes deformation of the membrane 16 and deformable lens 12. The axis may lie substantially along the length of the first part (or may be parallel to the length of the first part), such that first part rotates longitudinally. For example, in Figure 4A, the axis would be positioned into the page.

[0170] In Figure 4B, the actuator assembly 14 is configured to move the membrane 16 parallel to the optical axis OA, thus deforming the membrane 16 and deformable lens 12 and adjusting the optical properties of the deformable lens 12.

[0171] Figure 5 schematically illustrates an actuator assembly 14 in more detail. The actuator assembly comprises a support structure 18 configured to locate and support the other parts of the actuator assembly. In the present example the support structure 18 is located within the outer envelope of the deformable lens 12. The actuator assembly 14 further comprises a movable part 20. The movable part 20 is configured to move relative to the support structure 18. In the present example, the movable part 20 is configured to move perpendicular to the optical axis OA, in this case into the page.

[0172] The movable part 20 is driven by a shape memory alloy (SMA) element, e.g. a SMA wire, not shown in Figure 5. The SMA element can be controlled - e.g. by heating the element - to exert a tensile force. In this way, the SMA element, or elements, can be configured to act as actuators and to drive the movable part 20 relative to the support structure 18.

[0173] The movable part 20 in Figure 5 engages a first part 22 and a second part 24. The movable part 20 may engage the first part 22 and second part 24 in a number of different ways. In the present example, as discussed in more detail with reference to Figure 6, the movable part 20 comprises friction surfaces configured to engage corresponding surfaces on each of the first part 22 and second part 24.

[0174] The first part 22 of the optical assembly 10 is coupled to the deformable lens 12, in this case the membrane 16 thereof. In other examples, the first part 22 may be coupled (e.g. integrally coupled) to other parts of the deformable lens 12. The first part 22 is arranged such that, as the movable part 20 is driven by the SMA element, the movable part 20 moves relative to the first part 22. Turning briefly to Figure 6, it is shown that the first part 22 of the present optical assembly 10 comprises a series of angled teeth projecting from the membrane towards the movable part 20.

[0175] The second part 24 is also coupled to the deformable lens 12, in this case the surface opposing the membrane 16. As with the first part 22, the second part 24 is arranged such that, as the movable part 20 is driven by the SMA element, the movable part 20 moves relative to the second part 24. Turning briefly to Figure 6, it is shown that the second part 24 of the present example comprises a series of angled teeth projecting from the surface of the deformable lens towards the movable part 20.

[0176] In the present optical assembly 10 - and as will be discussed further with reference to Figure 6 - the movable part 20 is an elongate member of substantially rectangular cross section. The movable part 20 is located, e.g. sandwiched, between the first part 22 and second part 24 in a direction parallel to the optical axis OA.

[0177] Movement of the movable part 20 relative to the support structure 18 effects movement of the first part 22. Movement of the first part 22 causes the membrane 16 to deflect and hence the deformable lens 12 of the optical assembly 10 to deform.

[0178] Turning now to Figure 6, a schematic cross-section of an actuator 14 in plane 6' is shown. Generally speaking, the movable part 20, first part 22 and (optionally) second part 24 define angled surfaces such that, as the movable part 20 moves relative to the first part 22 and second part 24, the first part 22 is urged away from the second part 24. In this example, the first part 22 moves parallel to the optical axis OA. The use of angled surfaces provides a reliable method of changing the direction of force - whereby the tension applied to the movable part 20 in a first direction is converted to a relative movement of the first part 22 along an axis perpendicular to the direction of movement of the movable part 20 (which in this case is a direction parallel to the optical axis).

[0179] The movable part 20 defines a first friction surface 25 on its side adjacent the first part 22. The first part 22 comprises a second friction surface 28, 36. The first friction surface 25 is configured to engage the second friction surface 28, 36. The second part 24 comprises a third friction surface 32, 40. The movable part 20 of Figure 6 further defines a fourth friction surface 27 on its side adjacent the second part 24. The fourth friction surface 1 is configured to engage the third friction surface 32, 40.

[0180] In Figure 6, angled friction surfaces are provided on two opposing sides of the movable part 20. This provides a movement amplification effect, whereby the movement of the first part 22 relative to the second part 24 is doubled compared to if angled friction surfaces were present on only one side of the movable part 20. In other examples according to the disclosure, the movable part 20 may comprise only angled surfaces on only one side (e.g. the side facing the first part or the side facing the second part).

[0181] As illustrated in figure 6, the angled surfaces may be provided on a plurality of teeth. The moving part may comprise any number of teeth, for example 5 or more, 10 or more, 20 or more, 50 or more or 100 or more.

[0182] In some embodiments, the movable part 20, the first part 22 and (optionally) the second part 24 are biased into engagement with each other. In the examples of Figures 5 and 6, the movable part 20, first part 22 and second part 24 are urged into engagement by the elasticity of the deformable lens 12. Biasing the parts into engagement provides two benefits. First, a more robust and reliable contact is provided which improves the reliability of the optical assembly, in particular if subjected to external shocks or forces.

[0183] Second, urging the movable part 20, first part 22 and second part 24 together introduces friction between the respective friction surfaces. The presence of friction between the movable part 20, first part 22 and (optionally) second part 24 provides "zero hold power" functionality. That is, the movable part 20 is held in position relative to the first part 22 (and second part 24) by the static friction forces even after the SMA element has been powered down and, optionally, is no longer exerting a force on the movable part 20. In other words, the friction is sufficient to hold the movable part 20 - and hence optical assembly 10 as a whole - in the selected configuration without the need for any power to apply a holding tension to the movable part 20. This can greatly reduce the power requirements of the optical assembly 10.

[0184] In other examples not shown in the figures, the optical assembly 10 can achieve zero hold power by relying on the mechanical hysteresis of the SMA element. That is, the energy dissipated due to internal friction within the system means that there is reduced, or negligible, expansion of the SM A element and movement of the movable part, even after the SMA element is powered down.

[0185] As shown in Figure 1, the deformable lens 12 of the optical assembly 10 need not be circular. When a non-circular lens is used, it is necessary to cater for the asymmetry when deforming the lens 12. One way to achieve this is to adjust the interface of the movable part 20 and first part 22 (and optionally the movable part 20 and second part 24) around the circumference of the deformable lens. This is shown schematically in Figure 6. The second friction surface comprises two surfaces 28, 36. The movable part's first friction surface 25 comprises a first angled surface 26 and a second angled surface 34. Referring to the movable part's first friction surface 25, the first angled surface 26 and second angled surface 34 are at different inclinations - i.e. they define different angles with respect to the optical axis OA. The same is the case for the second part's third friction surface 32, 40 and the movable part's fourth friction surface 27.

[0186] The effect of varying the inclination of the friction surfaces is that the movement of the first part 22 relative to the movable part 20 and second part 24 varies around the circumference of the lens 12. This, in turn, causes the deformation of the lens to vary around the circumference of the lens 12. This allows effective control of optical properties in non-circular lenses. In particular, portions of the deformable lens which are further from the centre of the lens may be deformed by a greater amount, as compared to portions of the deformable lens which are closer to the centre of the lens.

[0187] It will be appreciated that the optical assembly 14 may not comprise a second part 24. Instead, the movable part 20 could slide over a surface of the support structure 18.

[0188] Figure 7 shows an alternative actuator assembly 114, shown in a corresponding plane to Figure 6. In Figure 7, the inclined friction surfaces are replaced by a rotating linkage (e.g. a flexure bearing) 142. As the movable part 120 is driven relative to the support structure (not shown), the first part 122 is again urged in a direction parallel to the optical axis OA. In this example, relative movement of the movable part 120 and first part 122 causes the linkage 142 to rotate, urging the movable part 120 and first part 122 apart.

[0189] Figures 8A to 8H show different arrangements of the optical assembly 10.

[0190] Turning now to Figures 8A and 8B, an internal and external arrangement of the actuator assembly 14 are shown, respectively. In Figure 8A, the movable part 20, first part 22 and second part 24 are arranged inside - e.g. inside the external envelope - of the deformable lens 12. The movable part 20, first part 22 and second part 24 are considered to be located inside the deformable lens when they completely overlap with the deformable lens when viewed in a direction perpendicular to the optical axis and also in a direction parallel to the optical axis. An external biasing means 44 is located outside of the deformable lens 12. The support structure 18 surrounds the assembly such that the biasing means 44 urges the movable part 20, first part 22 and second part 24 into engagement. In Figure 8B, the component parts are the same as in Figure 8A, but the biasing means is located inside the envelope of the deformable lens 12 and the movable part 20, first part 22 and second part 24 are located outside of the lens 12. The biasing means 44 may be referred to as a biasing element.

[0191] Figures 8C to 8F show further arrangements of the optical assembly 10. In Figures 8C to 8F, further details of a specific example of a deformable lens are shown. The deformable lens 212 of the example of Figures 8C to 8F comprises an upper part 213 that is made of glass and has a convex surface. The deformable lens 212 also has a lower part 215 comprising a fluid - allowing the lens 212 to be deformed. The fluid may be a liquid or a gel. In other examples, the deformable lens may not comprise fluid and may instead be made of a deformable material.

[0192] In Figures 8C and 8D, the surface of the lower part 215 is a membrane 216. The first part 222 is coupled to the membrane 216. The membrane 216 has tension, but low bending stiffness. In both of Figures 8C and 8D, an elastomer seal 217 is provided around the outer circumferential edge of the deformable lens 212 to seal the fluid within the deformable lens 212. The movable part 220, first part 222 and second part 224 are located inside the deformable lens 212, with the fluid. Figures 8C and 8D also both schematically show part of an SMA element 221. As will be discussed below, the SMA elements 221 primarily extend around the circumference of the deformable lens 212, i.e. into the plane of the page.

[0193] In Figure 8C, the movable part 220, first part 222 and second part 224 are located spaced and separated from the outermost edge of the deformable lens 212 and elastomer seal 217. In Figure 8D the movable part 220, first part 222 and second part 224 are located adjacent the edge of the deformable lens 212. The relative location of these parts will determine the nature of deformation induced during use and thus the optical adjustments available. The distance between the outer circumferential edge of the deformable lens and at least one of the movable part 220, first part 222 and second part 224 may vary around the circumference of the deformable lens 212 so as to accommodate for non-circular lenses. In Figures 8E and 8F, the lowermost surface instead comprises a thin plate 219, which may be made of glass. The plate 219 of Figures 8E and 8F has higher bending stiffness than the membrane 216 of Figures 8C and 8D. The bending stiffness affects the deformations, and thus optical adjustments, available. The choice of bending stiffness enables different modes of operation to be considered.

[0194] In Figure 8E, the liquid is under a negative pressure such that the plate 219 is held in a concave arrangement, with the movable part 220, first part 222 and second part 224 adjacent an elastomer seal 217 at the outer edge of the deformable lens 212. The negative pressure of the fluid will cause the plate 219 to urge the movable part 220, first part 222 and second part 224 into engagement.

[0195] In Figure 8F, the movable part 220, first part 222 and second part 224 are located at the outermost circumferential edge of the deformable lens 212, with the elastomeric seal 217 located closer to the centre of the lens 212. The movable part 220, first part 222 and second part 224 of this embodiment are not located in the fluid of the deformable lens 212. This may simplify assembly and design of these parts.

[0196] Locating the elastomeric seal 217 between the movable part 220 and the centre of the lens 212 affects the deformation performance of the lens 212. This is because the seal 217 will a) resist deformation of the deformable lens 212 and b) at least partially act as a pivot point for loads applied by the movable part 220. The design of the movable part 220, first part 222 and (optionally) second part 224 are therefore adjusted to compensate for the presence of the seal 217.

[0197] Figure 8G shows a similar arrangement to Figure 8D, including corresponding features. In Figure 8G, the width "wi" of the first part 222, movable part 220 and second part 224 has been increased. The width is the dimension aligned with the radius of the deformable optical element. Increasing the width increases the area over which the movement of the first part 222 acts, increasing the volume within the deformable optical element 215 that is displaced 223, thus increasing the deformation achievable.

[0198] Figure 8H shows an arrangement in which the first part 222, movable part 220 and second part 224 are spaced from the outer circumference of the deformable optical element 215 to define an outer annular region 223. The spacing - and hence width of the outer annular region 223 - is "W2". The outer annular region 223 defines a volume of fluid within the deformable optical element 215. Spacing the first part 222, movable part 220 and second part 224 from the outer circumference of the deformable optical element 215 increases the area over which the movement of the first part 222 acts. As the first part 222 moves, fluid within the outer annular region 223 is displaced into the centre region of the deformable optical element, increasing the deformation achievable.

[0199] Figures 9A to 9C will now be discussed. As outlined above, the movable part 20 is moved relative to the support structure 18 by means of one, or multiple SMA elements 21. SMA elements generate a tension force when activated - e.g. by heating due to a current being applied to the SMA element.

[0200] In the examples discussed, the movable part 20 is formed of a single part, or multiple parts, that extend around the circumference of the deformable lens 12. The SMA elements 21 are arranged to drive the movable part 20 to rotate around the deformable lens, i.e. rotate about the primary (optical) axis.

[0201] Figures 9A and 9B schematically show a first two arrangements of SMA elements 21 within the optical assembly 10. In each case, the SMA elements 21 are arranged in pairs. This is advantageous as within each pair, a first SMA element 21a can drive the movable part 20 in a first direction, and the second SMA element 21b can drive the movable part 20 in the reverse direction, e.g. to reset the movable part 20.

[0202] In Figure 9A, the uppermost pair of SMA elements 21a and 21b are arranged to cross over each other when viewed along the optical axis of the deformable lens 12 (i.e. into the page). The lowermost pair of SMA elements 21c and 21d are arranged parallel to each other. It will be appreciated that both pairs could be parallel to each other (as the bottom pair are) or crossed over each other (as the top pair are).

[0203] In Figure 9B, both pairs of SMA elements 21a, 21b and 21c, 21d are arranged sequentially around the perimeter of the lens 12.

[0204] Figure 9C shows an alternative arrangement, in which an SMA element 21 runs around the periphery of the deformable lens 12 and optical assembly 10 and is deflected by a first guide or set of guides 23 located on at least one of the support structure 18, first part 22 and second part 24; and a second guide or set of guides 29 located on the movable part 20. The guides deflect the SM A element 21 so that it is held out of a central portion of the deformable lens 12 and does not obstruct a user's view.

[0205] Figures 10 through 13C show alternative actuator assemblies. In each, corresponding reference numerals are used to those referred to previously, albeit incremented by a multiple of 100. In the assemblies of Figures 10 through 13C, the first part 22 is shown at the top of the respective Figures.

[0206] Figure 10 is a further schematic of an actuator assembly. The actuator assembly includes a movable part 320 with a first angled surface 325 and a fourth angled surface 327. The first angled surface 325 faces a second angled surface 328 of a first part 322. The fourth angled surface 327 faces a third angled surface 332 of a second part 324. The second part 324 is fixed with respect to a support structure (not shown). An SMA element 321 is connected to the movable part 320 by means of a crimp 331. The SMA element 321 is also connected to the support structure (not shown).

[0207] The first, second, third and fourth angled surfaces operate in an analogous way to the first friction surface 25, second friction surface 28, third friction surface 32 and fourth friction surface 27 described with reference to Figure 6, in that the angled surfaces drive the first part 322 away from the second part 324 as the movable part 320 is driven to move by the SMA. As seen in figure 10, the movable part 320 is driven to move along the X direction and the configuration of the angled surfaces 325, 327, 328 and 332 cause the first part 322 to move along the Z direction.

[0208] A difference between the arrangements of Figures 6 and 10 is that in the arrangement of figure 10, the actuator assembly comprises a first bearing assembly 333 between the movable part 320 and the first part 322 and a second bearing assembly 335 between the movable part 320 and the second part 324. The bearing assemblies are rolling bearing assemblies. In other words, one or more ball bearings are disposed between the movable part 320 and the first part 322 and / or between the movable part 320 and the second part 324. The bearing assemblies 333, 335 allow the corresponding angled surfaces to define a more acute (i.e. smaller) angle with respect to the primary axis, such that the first part 322 can move further under the action of the SMA element and the amount of deformation of the deformable optical element can be increased. If such steep wedges were instead engaged directly with each other (i.e. arranged as a plain bearing as in e.g. Figure 6 as opposed to a rolling bearing), the normal force between the angled surfaces and hence the friction between the surfaces would be very high. The presence of a rolling bearing therefore facilitates the use of an angled surface which is at a steeper angle (i.e. a smaller angle with respect to the primary axis) than that which is possible with a plain bearing (as in Figure 6).

[0209] Figure 11 shows a further actuator assembly. In Figure 11, the second angled surface 428 and third angled surface 432 are offset relative to one another such that the peaks of each surface are offset. This provides an interleaved arrangement, which can reduce the overall thickness of the actuator assembly (in the z-direction shown in Figure 11).

[0210] Figure 12 shows a similar actuator assembly to that of Figure 11. In Figure 12, the actuator assembly further comprises a flexure 437. The flexure 437 connects the first part 422 and second part 424 to resist movement of the first part 422 in the circumferential direction of the optical element (e.g. in the x direction of Figure 12). In other words, the flexure 437 prevents or minimises relative rotation between the first part 422 and the second part 424 about the primary (optical) axis. The flexure 437 may also aid in biasing the various parts of the optical assembly together.

[0211] In any of the arrangements disclosed herein (in particular those of Figures 6, 7, 10, 11, 12) various bearings between various parts which move relative to each other may be used, for example: plain bearings (i.e. direct contact between two respective surfaces of two parts which move relative to each other), rolling bearings (e.g. in which one or more rolling elements such as ball bearings are disposed between two parts which move relative to each other) a combination of rolling bearings and plain bearings.

[0212] For example, rolling bearings may be used at one interface between two parts that move relative to each other and a plain bearing may be used at another, different interface. The interface with the rolling bearing may provide the change of direction and / or amplification of movement as between the movable part and the first or second part (e.g. angled surfaces) where low friction is desirable. The interface with the plain bearing (i.e. friction surfaces) may be provided to provide zero-hold-power functionality. Such a plain bearing arrangement may comprise one or more friction surfaces which are at an acute, non-zero angle to the primary axis (e.g. as in Figure 6), one or more friction surfaces parallel to the primary axis (see Figures 13A-C) and / or one or more friction surfaces which are perpendicular to the primary axis. For example, the optical assembly may comprise a rolling bearing arrangement which guides relative movement between one of (i) the movable part and the first part and (ii) the movable part and the second part; and a plain bearing which guides relative movement between the other of (i) the movable part and the first part and (ii) the movable part and the second part.

[0213] In other embodiments, any one interface may comprise a combination of rolling bearings and plain bearings. For example, at the interface between the movable part and the first part and / or at the interface between the movable part and the second part, some angled surfaces may be configured as plain bearings and some may be configured as rolling bearings. In an example, portions of the deformable optical element which require a greater degree of deformation may be driven to move by angled surfaces which form a relatively small, acute respective angles with the primary axis and portions of the deformable optical element which require a lesser degree of deformation may be driven to move by angled surfaces which form relatively large, acute respective angles with the primary axis. Those surfaces which form a relatively small, acute angles with the primary axis may have an associated rolling bearing and those angled surfaces which form a relatively large, acute angle with the primary axis may be configured as plain bearings.

[0214] Optionally, zero-hold power functionality may be provided at an interface between the moving part and the support structure. This may be the case regardless of whether the other interfaces (between the movable part and the first part and / or between the movable part and the second part) are arranged as rolling bearings, plain bearings or a combination of the two. In other words, the zero-hold-power functionality may be provided by the support structure / movable part interface entirely or partially. Examples of an arrangement in which zero-hold functionality is provided at an interface between the moving part and the support structure are now described with reference to Figures 13A-C. Figure 13A through 13C are schematic views of actuator assemblies having alternative zero hold power arrangements.

[0215] The actuator assemblies of Figures 13A through 13C have a movable part 520, first part 522 and second part 524 as previously discussed. The first part 522 is not shown in Figure 13A. In these examples, the second part 524 is connected to the support structure 518. In Figures 13A through 13C, a friction surface 539 is provided on circumferential outer side of the movable part. This friction surface 539 is separate to, and in addition to or as an alternative to, any friction surface present on the angled surfaces that engage the first and / or second parts. The friction surface 539 is configured to abut an inner surface of the support structure 518 when the SMA element is in an un-energised state, as shown in Figure 13A. The contact between the friction surface 539 and the support structure 518 is configured to act as a brake and provide the zero-hold-power functionality discussed elsewhere herein - i.e. to resist movement of the movable part 520 with respect to the support structure 518. This friction force holds the optical assembly in its chosen state, even when power is not applied to the SMA element.

[0216] As can be seen from Figure 13A, when an SMA element contracts, it urges the movable part 520 away from the support structure 518, thus reducing the normal force and hence the friction force and allowing the movable part 520 to move with respect to the support structure 518. This arrangement therefore provides a variable, and controllable, friction force. When the SMA ceases to be powered (and hence relaxes and no longer provides a force on the movable part 520, the normal force is increased again which in turn increases friction, holding the movable part 520 stationary with respect to the support structure 518.

[0217] Figure 13B and 13C provide further arrangements. Figures 13B and 13C show cross-sectional views of an actuator assembly (the optical axis of the deformable lens is parallel to the Z axis). In Figure 13B, the actuator assembly is configured to reduce, but not remove, the contact force between the movable part 520 and support structure 518. As such, the movable part 520 (i.e. the friction surface 539) is always in contact with the support structure 518. In Figure 13C, the actuator assembly is configured such that the SMA element can pull the movable part 520 away from the support structure 518 so that the two parts are no longer in contact. In this arrangement, in order to maintain alignment of the movable part 520, a bearing surface 541 is provided. When the SMA element urges the friction surface 539 out of contact with the support structure 518, the other side of the movable part 520 contacts the bearing surface 541. The bearing surface 541 provides very little resistance to relative movement between the movable part 520 and the support structure 518. One or more rolling elements, such as ball bearings, may be disposed between the movable part 520 and the bearing surface 541. This keeps the movable part 520 in the right radial location, without applying any resistive force via the friction surface 539 during movement of the movable part 520.

[0218] In the variable-friction embodiments described herein, the force applied by the SMA to reduce or remove the normal force between two friction surfaces may be opposed by any suitable means, e.g. a biasing arrangement or by elasticity in one or more components of the assembly. Arrangements facilitating a variable friction force between the movable part 520 and the support structure 518 have been described with reference to Figures 13A-C. Alternatively, the optical assembly could be configured such that on contraction of the SMA, the normal force between the movable part 520 and the support structure 518 may be unchanged. In this sense, a constant friction force may be provided. Such an arrangement may be provided when the contraction of the SMA exerts a force on the movable part which is parallel to the surface across which the movable part moves, i.e. the SMA does not provide a component of force perpendicular to that surface.

[0219] With reference to Figure 14A and 14B, a further optical assembly is described. In contrast to other optical assemblies described herein, the optical assembly described with reference to Figures 14A and 14B deforms a deformable optical element by directly driving movement of a part which is coupled to the deformable optical element. Figure 14B shows a schematic crosssection of the actuator assembly of Figure 14A in plane 6'.

[0220] Figure 14A schematically illustrates an actuator assembly 14 which is configured to deform a deformable optical element, in this case a deformable lens 12. The deformable lens 12 comprises a first deformable surface 16a, which may otherwise be referred to as a first membrane 16a, and optionally a second deformable surface 16b, which may otherwise be referred to as a second membrane 16b.

[0221] The actuator assembly comprises a support structure 18 configured to locate and support the other parts of the actuator assembly. The actuator assembly 14 further comprises a first part 22a. The first part 22a of the optical assembly 10 is coupled to the deformable lens 12, in this case the first membrane 16a thereof. In other examples, the first part 22a may be coupled (e.g. integrally coupled) to other parts of the deformable lens 12. Turning briefly to Figure 14B, it is shown that the first part 22a of the present optical assembly comprises a series of angled teeth projecting towards the support structure 18. The support structure 18 comprises corresponding angled teeth. The overlap between the teeth of the first part 22a and the corresponding teeth on the support structure 18 is indicated by dotted lines in Figure 14A.

[0222] The first part 22a is configured to move relative to the support structure 18. The first part 22a is driven by a shape memory alloy (SMA) element, e.g. a SMA wire, not shown in Figure 14A or 14B. The SMA element can be controlled - e.g. by heating the element - to exert a tensile force. In this way, the SMA element can be configured to act as an actuator and to drive the first part 22a relative to the support structure 18. The direction of the force applied by the SMA element to the first part 22a is shown by the lower arrow in Figure 14B.

[0223] The actuator assembly 14 comprises a bearing arrangement which is configured to guide movement of the first part 22a with respect to the support structure 18. The bearing arrangement is a plain bearing arrangement and is provided by surfaces of the first part 22a and the support structure 18 respectively. As mentioned above, the first part 22a and the support structure 18 define angled teeth comprising angled surfaces such that, as the first part 22a is driven to move by the SMA element relative to the support structure 18 along a direction perpendicular to the optical axis OA (as shown by the lower arrow in Figure 14B), the first part 22a is displaced along a direction parallel to the optical axis OA (relative to the support structure 18). The use of angled surfaces provides a reliable method of changing the direction of force - whereby the tension applied to the first part 22a in a first direction is converted to a relative movement of the first part 22a along a direction parallel to the optical axis OA. Movement of the first part 22a causes the first membrane 16a to deflect and hence the deformable lens 12 of the optical assembly 10 to deform. In the present optical assembly, the first part 22a is an elongate member of substantially rectangular cross section.

[0224] In some embodiments, the first part 22a and the support structure (and optically other components of the actuator assembly 14) are biased into engagement. In the examples of Figures 14A and 14B, the first part 22a and the support structure 18 are urged into engagement by the elasticity of the deformable lens 12. Biasing the parts into engagement provides two benefits. First, a more robust and reliable contact is provided which improves the reliability of the optical assembly, in particular if subjected to external shocks or forces.

[0225] Second, urging the first part 22a and the support structure 18 together introduces friction between the respective angled surfaces of the first part 22a and support structure 18. The presence of friction between the first part 22a and the support structure 18 provides "zero hold power" functionality. That is, the first part 22a is held still relative to the support structure 18 by the static friction forces even after the SMA element has been powered down and is no longer exerting a force on the first part 22a. In other words, the friction is sufficient to hold the first part 22a - and hence the first membrane 16a - in the selected configuration without the need for any power to apply a holding tension to the first part 22a. This can greatly reduce the power requirements of the optical assembly. In other examples not shown in the figures, the optical assembly can achieve zero hold power by relying on the mechanical hysteresis of the SMA element. That is, the energy dissipated due to internal friction within the system means that there is reduced, or negligible, expansion of the SMA element and movement of the first part, even after the SMA element is powered down.

[0226] As described above, the first part 22a is driven to move in order to deflect the first membrane 16a. As mentioned above, the deformable lens 12 comprises a second deformable surface, i.e. a second membrane 16b. The optical assembly comprises a second part 22b which is coupled to the second membrane 16b. The second part 22b is driven to move in order to deflect the second membrane 16b, in an analogous way to the first part 22a and the first membrane 16a. However, it will be appreciated that the second part 22b and the second membrane 16b are not essential and instead, a second surface of the deformable lens may not be deformable and instead may be rigid (e.g. may be formed of glass).

[0227] In an analogous way to the first part 22a, the second part 22b comprises a series of angled teeth projecting towards the support structure 18. The teeth comprise angled surfaces which engage with corresponding angled surfaces of the support structure 18. The angled surfaces may act as friction surfaces in order to provide zero hold power functionality, as described above with reference to the first part 22a.

[0228] The second part is driven to move relative to the support structure by an actuator, e.g. one or more SMA elements in an analogous way to the first part 22. Movement of the second part 22b drives deflection of the second membrane 16b. Additional SMA elements (or other actuators) may be present in order to drive movement of the first part 22a and / or second part 22b in a direction opposite to that indicated by the arrows in Figure 14B.

[0229] The first part 22a and the second part 22b are located, e.g. sandwiched, within the support structure 18 in a direction parallel to the optical axis OA. In other words, the first part 22a and the second part 22b may sit within an outer envelope of the support structure. Alternatively, the positions of the first and second parts and the support structure could be switched around, such that the support structure 18 sits in between the first and second parts (along a direction parallel to the optical axis). This is illustrated in Figure 14C. It will be appreciated that there may be many other variations of the above-described examples and that features from a first one of the above-described examples may be combined with features of a second example.

[0230] For example, the embodiments of figures 10, 11 and 12 may comprise all, none or some of the ball bearings shown. The embodiments of Figures 10, 11 and 12 may instead comprise plain bearings only, e.g. as illustrated in Figure 6. In other embodiments, some pairs of angled surfaces may have a ball bearing between them and others may not.

[0231] As noted above, the above embodiments are described as having a deformable lens for example only. Alternative optical assemblies according to the disclosure may include other deformable optical elements such as deformable mirrors. In such cases, the first part may be coupled to the deformable mirror, which may be deformed as the first part moves. For example, the optical assembly may be arranged as described above, with the membrane being a deformable mirror.

[0232] Some of the above-described SMA actuator assemblies comprise at least one SMA element. The term 'shape memory alloy (SMA) element' may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA element may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA element' may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, e.g. a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, e.g. by Joule heating, another heating technique or by applying a magnetic field.

Claims

CLAIMS:

1. An optical assembly comprising: a deformable optical element defining a primary axis; an actuator assembly configured to deform the deformable optical element, the actuator assembly comprising: a first part coupled to the deformable optical element; a support structure; a movable part configured to move relative to the support structure and which is coupled to the first part; and a shape memory alloy, SM A, element configured to drive movement of the movable part relative to the support structure to effect movement of the first part relative to the support structure and thereby deform the deformable optical element; wherein the optical assembly is configured so as to retain the first part in position with respect to the support structure when the SMA element is unpowered.

2. The optical assembly of claim 1, wherein the actuator assembly is configured to drive movement of the first part in a direction having at least a component which is parallel to the primary axis.

3. The optical assembly of any preceding claim, wherein the first part is coupled to a membrane of the deformable optical element that effects deformation of the deformable optical element; wherein the membrane has a variable bending stiffness across its surface.

4. The optical assembly of any of the preceding claims, wherein movement of the movable part relative to the support structure causes relative movement between an angled surface which is at an acute, non-zero angle to the primary axis and a second surface to drive movement of the first part relative to the support structure.

5. The optical assembly of claim 4, wherein the angled surface is defined by the movable part or the first part.

6. The optical assembly of any of the preceding claims, wherein the movable part comprises a first friction surface arranged to move over a second friction surface duringmovement of the movable part relative to the support structure and wherein the optical assembly is arranged such that the first and second friction surfaces are biased together so as to generate a friction force therebetween that resists movement of the movable part with respect to the first part and / or support structure.

7. The optical assembly of claim 6, wherein the first part comprises the second friction surface and / or the support structure comprises the second friction surface.

8. The optical assembly of claim 6 or claim 7 or claim 8 when dependent on claim 5, wherein at least one of the first friction surface and second friction surface defines the angled surface.

9. The optical assembly of any of claims 6 to 8 further comprising: a second part comprising a third friction surface; wherein the movable part further comprises a fourth friction surface which engages with the third friction surface so as to generate a friction force that resists movement of the movable part with respect to the first part and / or support structure.

10. The optical assembly of claim 9, wherein at least one of the third friction surface and fourth friction surface defines an angled surface which is at an acute, non-zero angle to the primary axis.

11. The optical assembly of claim 10 when dependent on claim 8, wherein the angle of the at least one of the first friction surface and second friction surface is defined in an opposite direction with respect to the primary axis to the angle of the at least one of the third friction surface and fourth friction surface.

12. The optical assembly of any of claims 9 to 11, wherein the second friction surface and the third friction surface are spaced apart and the movable part is disposed between the first part and second part.

13. The optical assembly of any of claims 9 to 12, when dependent on claim 7, wherein the second and third friction surfaces collectively define interleaved peaks around at least part of a circumference of the deformable optical element.

14. The optical assembly of any of claims 6 to 13, wherein at least one of the first friction surface and the second friction surface comprises a first angled surface at a first acute, non-zero angle to the primary axis and a second angled surface at a second acute, non-zero angle to the primary axis, wherein the first and second angled surfaces are at different positions around the periphery of the deformable optical element and wherein the first angle is different to the second angle.

15. The optical assembly of any of claims 6 to 14, wherein the first friction surface and second friction surface are biased into engagement by at least one of a biasing element and the deformable optical element itself.

16. The optical assembly of any of claims 6 to 15, wherein: the SMA element is arranged such that a normal force acting between the first friction surface and second friction surface remains substantially constant on actuation of the SMA element; or the SMA element is arranged to apply a force to the movable part with a component orthogonal to the first friction surface that reduces a normal force between the first friction surface and the second friction surface and with a component parallel to the first friction surface so as to drive movement of the movable part relative to the support structure across the second friction surface.

17. The optical assembly of any of the preceding claims, wherein the actuator assembly comprises a rolling bearing assembly associated with the movable part.

18. The optical assembly of claim 17 when dependent on claim 9, wherein the rolling bearing assembly is: located between the movable part and only one of the first part and the second part.

19. The optical assembly of claim 17 or 18, wherein movement of the movable part over another part of the assembly is supported by the rolling bearing assembly at one or more points around the circumference of the deformable optical element and by a plain bearing at one or more different points around the circumference of the deformable optical element.

20. The optical assembly of any preceding claim, wherein the movable part is coupled to the first part by a rotating linkage or flexure bearing, the rotating linkage or flexure bearing being arranged such that the first part is urged away from the movable part as the movable part moves relative to the support structure.

21. The optical assembly of any of the preceding claims, wherein the movable part: extends around at least part of a periphery of the deformable optical element; and / or is a singular part, or a plurality of parts collectively forming the movable part.

22. The optical assembly of any of the preceding claims, wherein the actuator assembly comprises at least one pair of opposing SMA elements, the opposing SMA elements being configured to drive movement of the movable part relative to the support structure in opposite directions and / or senses, optionally wherein the opposing SMA elements are arranged substantially parallel to each other or the opposing SMA elements cross over each other when viewed along the primary axis.

23. The optical assembly of any of the preceding claims, wherein the SMA element is located around a periphery of the deformable optical element.

24. The optical assembly of any of the preceding claims, further comprising one or more guide elements arranged to guide the SMA element around the periphery of the deformable optical element.

25. The optical assembly of any of the preceding claims, wherein: the movable part comprises a plurality of parts; the optical assembly comprises a plurality of SMA elements of different lengths associated with the plurality of parts of the movable part; andthe plurality of SMA elements are configured to drive the respective parts of the movable part by different amounts.

26. The optical assembly of any of the preceding claims, wherein the actuator assembly comprises a flexure arranged to resist relative rotation of the first part and support structure about the primary axis.

27. The optical assembly of any of the preceding claims, wherein the actuator assembly is located inside the deformable optical element.

28. The optical assembly of any of claims 1 to 26, wherein the actuator assembly is located on the outside of the deformable optical element.

29. The optical assembly of any of the preceding claims, wherein the actuator assembly is configured to: adjust the focal length of the deformable optical element and / or rotate the movable part about an axis parallel to or colinear with the primary axis.

30. The optical assembly of any of the preceding claims, wherein the support structure comprises at least one of: a frame located around the periphery of the deformable optical element for supporting the parts of the optical assembly; a seal configured to seal a fluid or gel in at least part of the deformable optical element; and a biasing means for biasing the first part and movable part into engagement.

31. The optical assembly of any of the preceding claims, wherein: the optical element is a deformable lens and the primary axis is an optical axis; or the optical element is a deformable mirror.

32. The optical assembly of any preceding claim, wherein a component of the optical assembly comprises a plurality of teeth each defining a respective angled surface which is at an acute, non-zero angle to the primary axis, optionally wherein the component is the first part orthe movable part, further optionally wherein the component comprises ten or more teeth, optionally twenty or more teeth, optionally fifty or more teeth.

33. The optical assembly of claim 1, wherein the actuator assembly is configured to rotate the first part about an axis perpendicular to the primary axis.

34. An optical assembly comprising: a deformable optical element defining a primary axis; an actuator assembly configured to deform the deformable optical element, the actuator assembly comprising: a support structure; a first part which is coupled to the deformable optical element and which is movable relative to the support structure; and a shape memory alloy (SMA) element configured to drive movement of the first part relative to the support structure to deform the deformable optical element.

35. The optical assembly of claim 34, comprising at least one of: configured so as to retain the first part in position with respect to the support structure when the SMA element is unpowered; the SMA element is attached to the first part or wherein the SMA element is hooked over a portion of the first part; comprising a bearing arrangement which is configured to guide movement of the first part with respect to the support structure, optionally wherein the SMA element is configured to drive movement of the first part in a direction perpendicular to the primary axis which is converted to movement of the first part along a direction having at least a component along the primary axis by the bearing arrangement; the bearing arrangement comprises one or more angled surfaces which are each at an acute, non-zero angle to the primary axis and which are each engaged with a respective engagement surface and wherein the SMA element is configured to drive movement of the one or more angled surfaces over the respective engagement surface(s) to move the first part relative to the support structure and deform the deformable optical element; the first part is coupled to a first deformable surface of the deformable optical element and wherein movement of the first part relative to the support structure deforms the firstdeformable surface and wherein the optical assembly comprises a second part which is coupled to a second deformable surface of the deformable optical element, different to the first deformable surface, and which is movable relative to the support structure, wherein movement of the second part relative to the support structure deforms the second deformable surface; and the deformable optical element is a deformable lens.

36. A virtual reality headset comprising an optical assembly according to any of the preceding claims.

37. An augmented reality headset comprising an optical assembly according to any of claims 1 to 35.

38. A pair of glasses comprising an optical assembly according to any of claims 1 to 35.

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