Optical assembly

Abstract

An optical assembly includes an optical element 12 defining a primary axis and an actuator assembly 14 which deforms the optical element. The actuator assembly includes a first part 22 coupled to the optical element, a support structure 18, a moving part 20 and a shape memory alloy (SMA) element. The moving part moves relative to the support structure and is coupled to the first part. The shape memory alloy (SMA) element drives movement of the moving part relative to the support structure effecting movement of the first part and thus deforming the optical element. The optical assembly retains the first part in position with respect to the support structure when the SMA element is unpowered. The actuator assembly may drive movement of the first part in a direction parallel to the primary axis. The actuator assembly may rotate the first part about an axis perpendicular to the primary axis. The moving part may include two friction surfaces 25 and 28,36 engaging with a friction force to resist movement of the moving part with respect to the first part and / or support structure. A virtual reality headset, augmented reality headset or pair of glasses may include the optical assembly.

Description

Field 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. Background 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. 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. Virtual reality (VR) and augmented reality (AR) headsets allow a user to experience a simulated, or partially, 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. 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. 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. 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. Summary 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; 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 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. 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. 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. In the case that the deformable optical element is a deformable lens, the primary axis may be an optical axis of the deformable lens. 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. 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. 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. 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. 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. 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 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. 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. The movable part may comprise a first friction surface. The first friction surface may be engaged with a second friction surface so as to generate a friction force that resists movement of the movable part with respect to the first part and / or the support structure. 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. The use of a first and second friction surface may allow the provision of "zero hold power", whereby the movable part is held in position relative to the first part even after the SMA element has been powered down and 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. The use of friction surfaces to provide zero hold power greatly reduces the power consumption of the assembly. The surface of the friction surfaces may be treated to control the friction coefficient thereof. The first part may comprise the second friction surface. The SMA element may be configured to drive movement of the first friction surface over the second friction surface to move the first part relative to the support structure and deform the deformable optical element. At least one of the first friction surface and second friction surface may define an angled surface which is at an acute, non-zero angle to the primary axis of the deformable optical element. The angle 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. 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. 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. 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) 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. 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. 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. 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. The angle 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. 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. 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 introduction of a second angled surface arranged at an opposing angle to the first 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. The first friction surface and / or 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 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 noncircular lens is required. The first friction surface and second friction surface may be biased into engagement by at least one of an external biasing element and the deformable optical element itself. 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. 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). This movement may be opposed by a biasing force provided by an external 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. In an alternative arrangement, the movement of the first part may be effected by means of a rotating linkage, which may also be referred to as a flexure bearing. The rotating linkage may be configured to drive the first part in a direction perpendicular to the movement of the movable part - that is, the first part may be driven parallel to the primary axis. 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 . 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. The movable part may be configured to extend around at least part of a periphery of the deformable optical element. The movable part may be configured to extend around the whole periphery of the deformable optical element. The movable part may be a singular part, or a plurality of parts collectively forming the movable part. The movable part may be located near the circumference of the deformable optical element. The movable part may be substantially annular. 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. 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. 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 opposing directions and / or senses. 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) or inherent elasticity of the surrounding parts of the device. The opposing SMA elements may be arranged substantially parallel to each other. The opposing SMA elements may cross over each other when viewed along the primary axis of the deformable optical element. 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. The optical assembly may comprise one or more SMA elements located around a periphery of the deformable optical element. Locating SMA elements around the periphery of the deformable optical element minimises visual obstruction provided by the elements. The optical assembly may further comprise one or more guide elements arranged to guide the SMA element around the periphery of the deformable optical element. 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. 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. 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. The actuator assembly may be located inside the deformable optical element. Locating the actuator assembly within the deformable optical element may provide a particular 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. Alternatively, the actuator assembly may be located on the outside of the deformable optical element. The actuator assembly may be configured to adjust the focal length of the deformable optical element, e.g. of the deformable lens. 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. 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 support structure may comprise a seal configured to seal a fluid or gel in at least part of the deformable optical element. The support structure may comprise a biasing means for biasing the first part and movable part into engagement. 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. Further according to an aspect of the present invention is a virtual reality headset comprising an optical assembly as described anywhere herein. Further according to an aspect of the present invention is an augmented reality headset comprising an optical assembly as described anywhere herein. 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. Further according to an aspect of the present invention is a pair of glasses comprising an optical assembly as described anywhere herein. Brief description of the drawings Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic view of an optical assembly according to the disclosure; Figure 2 is a cross-section of the optical assembly of Figure 1; Figure 3 is a further cross-section of the optical assembly of Figure 1; Figures 4A and 4B are enlarged partial schematic views of optical assemblies; Figure 5 is a schematic view of an actuator assembly; Figure 6 is a schematic view of the actuator assembly of Figure 5; Figure 7 is a schematic view of an alternative actuator assembly; Figures 8A to 8F are schematic views of actuator assembly arrangements; and Figures 9A to 9C are schematic views of optical assemblies. Detailed description 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 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. 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. The optical assembly 10 has 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 8F. 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. 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. 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. 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 8F). 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. 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. 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 18. The membrane 16 is rotated about an axis perpendicular to the optical axis OA of the deformable lens 12. In this configuration, the stiffness of the membrane 16 causes deformation of the membrane 16 and deformable lens 12. 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. 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. 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. 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 numberof different ways. In the present example, as discussed in more detail with reference to Figure 6, the movable part 20 comprises a friction surface configured to engage corresponding surfaces on each of the first part 22 and second part 24. 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. 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 22 of the present example comprises a series of angled teeth projecting from the surface of the deformable lens towards the movable part 20. 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. 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. 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 that of the SMA element. 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 of Figure 6 further defines a fourth friction surface 27 on its side adjacent the second part 24. The fourth friction surface 27 is configured to engage the third friction surface 32, 40. 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 only one of the angled friction surfaces were present. In other examples according to the disclosure, the movable part 20 may comprise only a single angled friction surface. In some embodiments, the movable part 20, first part 22 and (optionally) second part 24 are biased into engagement. 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. 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 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. 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 SMA element and movement of the movable part, even after the SMA element is powered down. 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. 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. 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 member 120 and first part 122 causes the linkage 142 to rotate, urging the movable member 120 and first part 122 apart. Figures 8A to 8F show different arrangements of the optical assembly 10. 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 member 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 member 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. 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. 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 217 primarily extend around the circumference of the deformable lens 212, i.e. into the plane of the page. 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. 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. 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. 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. 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. In the examples discussed, the movable part 20 is formed of a single, 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. 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. 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). In Figure 9B, both pairs of SMA elements 21a, 21b and 21c, 21d are arranged sequentially around the perimeter of the lens 12. 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 SMA 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. 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. 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. 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 5 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

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;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.

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

3. 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.

4. The optical assembly of any of the preceding claims, wherein the movable part comprises a first friction surface which is engaged with a second 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.

5. The optical assembly of claim 4, wherein:the first part comprises the second friction surface;the SMA element is configured to drive movement of the first friction surface over the second friction surface to move the first part relative to the support structure and deform the deformable optical element.

6. The optical assembly of claim 5, wherein at least one of the first friction surface and second friction surface defines an angled surface which is at an acute, non-zero angle to the primary axis.

7. The optical assembly of claim 5 or claim 6, 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.

8. The optical assembly of claim 7, 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.

9. The optical assembly of claim 8, whereinthe acute, non-zero 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 acute, nonzero angle of the at least one of the third friction surface and fourth friction surface.

10. The optical assembly of any of claims 7 to 9, 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.

11. The optical assembly of any of claims 4 to 10, 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.

12. The optical assembly of any of claims 4 to 11, wherein the first friction surface and second friction surface are biased into engagement by at least one of an external biasing element and the deformable optical element itself.

13. 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.

14. The optical assembly of any of claims 4 to 13, wherein at least one of the first friction surface and second friction surface defines an undulating profile configured to rotate the first part about the axis perpendicular to the primary axis as the movable part moves relative to the support structure .

15. 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.

16. The optical assembly of any preceding claim, wherein the movable part is a singular part, or a plurality of parts collectively forming the movable part.

17. 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 the movable part relative to the support structure in opposing directions and / or senses.

18. The optical assembly of claim 17, wherein the opposing SMA elements are arranged substantially parallel to each other.

19. The optical assembly of claim 17, wherein the opposing SMA elements cross over each other when viewed along the primary axis.

20. The optical assembly of any of the preceding claims, comprising one or more SMA elements located around a periphery of the deformable optical element.

21. 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.

22. The optical assembly of any of the preceding claims, whereinthe 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 different lengths of the SMA elements are configured to drive the respective part of the movable part by different amounts.

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

24. The optical assembly of any of claims 1 to 22, wherein the actuator assembly is located on the outside of the deformable optical element.

25. 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.

26. The optical assembly of any of the preceding claims, wherein the actuator assembly is configured to rotate the movable part about an axis parallel to or colinear with the primary axis.

27. The optical assembly of any of the preceding claims, wherein the support structure comprises a frame located around the periphery of the deformable optical element for supporting the parts of the optical assembly.

28. The optical assembly of any of the preceding claims, wherein the support structure comprises a seal configured to seal a fluid or gel in at least part of the deformable optical element.

29. The optical assembly of any of the preceding claims, wherein the support structure comprises a biasing means for biasing the first part and movable part into engagement.

30. 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.5 31. The optical assembly of any of claims 1 to 29, wherein the optical element is adeformable mirror.

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

033. An augmented reality headset comprising an optical assembly according to any of claims Ito 31.

34. A pair of glasses comprising an optical assembly according to any of claims 1 to 31.

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