Optical assembly

Abstract

The optical assembly includes a support structure, several lens elements configured to provide overall cylindrical and spherical power. At least one actuating unit 8 is configured to laterally shift a

Description

Field The present application relates to an optical assembly comprising one or more actuating units configured to laterally shift a first lens element relative to a second lens element so as to adjust an overall spherical power and an overall cylindrical power of the optical assembly. Background Head-mounted devices, including extended reality (XR) devices like virtual reality (VR), augmented reality (AR), and mixed reality (MR) headsets, typically include optical assemblies through which a user views a display. To maximize the field of view in these devices, it is advantageous to position the optical assemblies as close to the user's eyes as possible. However, this placement often leaves insufficient space for users who require prescription eyewear, such as glasses. To address this, some companies offer prescription inserts that fit into head-mounted devices to correct vision issues like near-sightedness, far-sightedness, and astigmatism. However, these custom inserts are often costly and time-consuming to design, produce, and fit. The optical assemblies described herein offer a simpler solution, enabling easy, adjustable vision correction. Alvarez and Lohmann lenses are variable focus optical devices based on lateral shifts of two lenses with cubic-type surface profiles, although the profiles of the cubic surface are different between Alvarez and Lohmann lenses. Summary According to an aspect of the present invention, there is provided an optical assembly comprising: a support structure; a plurality of lens elements, collectively configured to provide an overall spherical power and an overall cylindrical power; one or more actuating units configured to laterally shift a first lens element of the plurality of lens elements relative to a second lens element of the plurality of lens elements so as to adjust the overall cylindrical power to comprise a selected power at a selected angle.. The plurality of lens elements may comprise two pairs of lens elements: a first pair of lens elements comprising the first lens element and a third lens element, and a second pair of lens elements comprising the second lens element and a fourth lens element; wherein the first pair of lens elements provides a cylindrical power with a first orientation, and the second pair of lens elements provides a cylindrical power with a second orientation; wherein the first and second pairs of lens elements are configured such that the first and second orientations of cylindrical power are at 45 degrees relative to each other and the one or more actuating units are configured to shift the first lens element and the second lens element so as to adjust the overall cylindrical power to comprise the selected power at the selected angle. The orientation of the first pair of lens elements may at 45 degrees relative to the second pair of lens elements. Having the orientation of the cylindrical power provided by the first lens at 45 degrees relative to the orientation of the cylindrical power provided by the second lens element, enables the relative lateral shift movement between the first and second lens elements to adjust the orientation and angle of the overall cylindrical power. With such an arrangement, the orientation of the overall cylindrical power can be adjusted without the need to rotate the plurality of lens elements about the overall optical axis of the optical assembly. The cubic thickness profile of each lens element may be of the form 1 z, = A^y^2 + -¾3) + Bx}2 + Cx^ + Dy-t2 + Ex± + Fy± + G The first and second lens elements may be movable and the third and fourth lens elements may be static. The one or more actuating units may be configured to laterally shift the first lens element relative to the third lens element, and configured to laterally shift the second lens element relative to the fourth lens element so as to adjust the overall spherical power and the overall cylindrical power. The first pair of lens elements may be identical to the second pair of lens elements. Optionally and alternatively, the plurality of lens elements may comprise a pair of lens elements comprising the first lens element and the second lens element, and the first lens element may have a first cubic thickness profile and the second lens element may have a second cubic thickness profile such that when the first lens element is moved relative to the second lens element in a first direction an optical power is produced with a cylindrical component in a first axial direction, and when the first lens element is moved relative to the second lens element in a second direction an optical power is produced with a cylindrical component in a second axial direction, where the first and second axial directions are at 45 degrees relative to each other. The second direction may be perpendicular to the first direction and an optical axis of the optical assembly. The cubic thickness profile of the plurality of each lens elements may be of the form 1 z2 = A(x2y22 -~x23) + Bx22 + Cx2y2 + Dy22 + Ex2 + Fy2 + G wherex2 and y2 respectively represent coordinates relative to an x-axis that extends perpendicular to the optical axis of the second lens element and a y-axis that extends perpendicular to this x-axis and the optical axis of the second lens element, and A, B, C, D, E, F, and G are coefficients. The plurality of lens elements may each be at a fixed rotation relative to each other and relative to the support structure. The first and second lens elements may be configured such that the orientation of the cylindrical power provided by the first lens element is fixed at 45 degrees relative to the orientation of the cylindrical power provided by the second lens element. The plurality of lens elements may overlap each other, and be arranged such that an object, e.g. a display, is viewable through the plurality of lens elements. Laterally shifting the first lens element relative to the second lens element involves translationally moving the first and second lens elements relative to each other in a manner that maintains the optical axes of the lens elements parallel to each other. The relative movement between the first and second lens elements may be limited to translational, non-rotational, 2D, planar movement. The optical axes of the plurality of lens elements may be aligned when the plurality of lens elements are moved into an aligned configuration by the one or more actuating units. The optical axes of the plurality of lens elements may be offset in parallel when the plurality of lens elements are moved into an offset configuration by the one or more actuating units. Each of the plurality of lens elements may comprise a planar (e.g., nominally planar or minimally curved) front optical surface, and a planar back optical surface. Optionally, at least one of the first and second lens elements is a diffraction-based lens element, such as a meta lens element or a diffractive lens element. Providing the optical assembly with diffraction-based lens element, rather than refractive-based lens elements, may enable the optical assembly to be lighter and more compact. It may also help reduce the energy that needs to be provided by the actuating units to drive the relative movements between the lens elements. Optionally, at least one of the first and second lens elements is a meta lens element. Optionally, at least one of the first and second lens elements is a diffractive lens element. Diffractive lens elements are also referred to as Diffractive Optical Elements (DOEs). Optionally, the optical assembly comprises only two lens elements consisting of the first and second lens elements. In other words, the plurality of lens elements may consist of two lens elements, the first and second lens elements. Optionally, the plurality of lens elements comprises two pairs of lens elements: a first pair comprising the first lens element and a third lens element, and a second pair comprising the second lens element and a fourth lens element; the first pair (collectively) provides a cylindrical power with a first orientation, and the second pair (collectively) provides a cylindrical power with a second orientation; and the first and second pairs are configured such that the first and second orientations of cylindrical power are at 45 degrees relative to each other. The first and second pairs may be configured such that the first and second orientations of cylindrical power are fixed at 45 degrees relative to each other. Each pair of lens elements may collectively form an effective Alvarez lens or Lohmann lens. Optionally, the first pair of lens elements is identical to the second pair of lens element, except that the second pair is rotated by 45 degrees relative to the first pair. Optionally, the one or more actuating units are configured to laterally shift the first lens element relative to the third lens element, and configured to laterally shift the second lens element relative to the fourth lens element, so as to adjust the overall spherical power and the overall cylindrical power; and the first and second orientations of cylindrical power are at 45 degrees relative to each other such that the relative lateral shift movements enable adjustment of the orientation of the overall cylindrical power. The first and second orientations of cylindrical power may be fixed at 45 degrees relative to each other. Optionally, at least one of the actuating units comprises an SMA (shape memory alloy) element configured, when powered, to drive the relative lateral shift movement between the first and second lens elements. Optionally, the one or more actuating units are configured to move the first lens element relative to the second lens element to any position within a range of movement; and the optical assembly comprises a brake mechanism configured, when the actuating units are unpowered, to hold the first lens element in any position within the range of movement - e.g. against the force of gravity for any orientation of the support structure, any orientation of the optical assembly, and / or any orientation of a head-mounted device the optical assembly is provided within. Optionally, the optical assembly is configured to correct vision by adjusting the overall spherical power, the overall cylindrical power, and / or the orientation of the overall cylindrical power. Optionally, the one or more actuating units comprise four actuating units arranged so as to be capable of moving the first lens element relative to the second lens element in any direction in a movement plane without applying any net torque to the first lens element about a first axis perpendicular to the movement plane. Optionally, the one or more actuating units comprises a plurality of actuating units arranged such that, for each direction along each axis of a Cartesian coordinate system, there is at least one actuating force with a non-zero component along that direction. Optionally, there is provided a head-mounted device comprising: an optical assembly as described above; and a controller configured to generate drive signals for controlling the one or more actuating units so as to adjust the overall spherical power, the overall cylindrical power, and the orientation of the overall cylindrical power. The head-mounted device may be provided with a further optical assembly as described above, and the controller may also be configured to generate drive signals for controlling the one or more actuating units of the further optical assembly so as to adjust the overall spherical power, the overall cylindrical power, and the orientation of the overall cylindrical power of the further optical assembly. The two optical assemblies may be provided at locations suitable for the eyes of the user of the head-mounted device. Optionally, the head-mounted device comprises a display, and the optical assembly is configured to primarily correct vision (i.e. long-sightedness, short-sightedness, astigmatism) for the primary colours (e.g. red, green, and blue light) emitted by the display, rather than to correct the entire visible spectrum. This may be important for both diffractive and meta lens elements, as these lens elements can exhibit strong chromatic aberration, meaning their focal length changes significantly with wavelength. 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 cross-sectional view of an optical assembly; Figure 2 is a schematic cross-sectional view of an alternative optical assembly; Figure 3 is a schematic plan view of an arrangement of four actuating units; and Figure 4 is a schematic perspective view of an arrangement of eight actuating units. Detailed description Figs. 1 and 2 each show variations of an optical assembly 1. These optical assemblies 1 are suitable for being provided within head-mounted devices (e.g. extended reality, XR, devices) as optical assemblies through which users view e.g. a display. One such optical assembly 1 may be provided for each eye of the user of the head-mounted device. First optical assembly The optical assembly 1 of Fig. 1 includes a support structure 10, and a plurality of lens elements 22, 23, 31, 32 arranged as two pairs of lens elements 20, 30: a first pair 20 comprising a first lens element 21 and a third lens element 22, and a second pair 30 comprising a second lens element 31 and a fourth lens element 32. The lens elements 21, 22, 31, 32 of Fig. 1 are arranged one behind the other such that an object, e.g. a display, is viewable through the plurality of lens elements 21, 22, 31, 32. In other words, the plurality of lens elements 21, 22, 31, 32 are generally aligned along an axis (e.g. a primary axis P of the optical assembly 1), such that they overlap each other when viewed along said axis. The plurality of lens elements 21, 22, 31, 32 may each be at a fixed rotation relative to each other and at a fixed rotation relative to the support structure 10. Each of the plurality of lens elements 21, 22, 31, 32 are capable of providing a non-zero spherical power and a non-zero cylindrical power and are collectively configured to provide an overall spherical power and an overall cylindrical power. The optical assembly 1, the support structure 10 and / or the plurality of lens elements 21, 22, 31, 32 may extend predominantly in a direction perpendicular to the primary axis P. In other words, the extent of the optical assembly 1, the support structure 10 and / or the plurality of lens elements 21, 22, 31, 32 along the primary axis P may be less than the extent thereof along any direction perpendicular to the primary axis P. The first and third lens elements 21, 31 are moveable. The second and fourth lens elements 22, 32 are static. The optical assembly 1 further includes one or more actuating units (not shown) configured to laterally shift the first lens element 21 relative to the third lens element 22, and the second lens element 31 relative to the fourth lens element 32, e.g. as illustrated by the arrows of Fig. 1. This lateral shifting of the first and second lens elements 21 relative to the second lens element 31 involves translationally moving the first and second lens elements 21, 31 perpendicular to the primary axis P and / or the overall optical axis O of the optical assembly 1, in a manner that maintains the optical axes of the lens elements 21, 31 parallel to each other. This lateral shift movement provided by the one or more actuating units is purely translational, i.e. non-rotational. The optical axis of each of the plurality of lens elements 21, 22, 31, 32 may be aligned with each other when the plurality of lens elements 21, 22, 31, 32 are moved into an aligned configuration by the one or more actuating units, as shown in Figure 1. The optical axis of each of the plurality of lens elements 21, 22, 31, 32 may be offset in parallel when the plurality of lens elements 21, 22, 31, 32 are moved into an offset configuration by the one or more actuating units (not shown). Each of the first and second pairs of lens elements 20, 30 collectively form an effective Alvarez or Lohmann lens. In other words, the first and third lens elements 21, 22, forming the first pair 20, are configured such that: (i) relative lateral shift movement between the first and third lens elements 21, 22 in a first direction perpendicular to the primary axis P and / or the overall optical axis O, enables adjustment of the spherical power collectively provided by the first pair of lens elements 20, and (ii) relative lateral shift movement between the first and third lens elements 21, 22 in a second direction perpendicular to the first direction, and perpendicular to the primary axis P and / or the overall optical axis O, enables adjustment of the cylindrical power collectively provided by the first pair of lens elements 20. Moreover, the second and fourth lens elements 31, 32, forming the second pair 30, are configured such that: (i) relative lateral shift movement between the second and fourth lens elements 31, 32 in a third direction perpendicular to the primary axis P and / or the overall optical axis O, enables adjustment of the spherical power collectively provided by the second pair of lens elements 30, and (ii) relative lateral shift movement between the second and fourth lens elements 31, 32 in a fourth direction perpendicular to the third direction, and perpendicular to the primary axis P and / or the overall optical axis O, enables adjustment of the cylindrical power collectively provided by the second pair of lens elements 30. The spherical power provided by the first pair 20 and the spherical power provided by the second pair 30 combine to provide the overall spherical power of the optical assembly 1, and the cylindrical power provided by the first pair 20 and the cylindrical power provided by the second pair 30 combine to provide the overall cylindrical power of the optical assembly 1. As such, it can be said that, the first and second pairs of lens elements 20, 30 are configured such that: (i) relative lateral shift movement between the respective lens elements 21, 22; 31, 32 for each pair 20; 30 in a first direction perpendicular to the primary axis P and / or the overall optical axis O, enables adjustment of the overall spherical power of the optical assembly 1, and (ii) relative lateral shift movement between the respective lens elements 21, 22; 31, 32 for each pair 20;30 in a second direction perpendicular to said first direction, and perpendicular to the primary axis P and / or the overall optical axis O, enables adjustment of the overall cylindrical power of the optical assembly 1. The first pair of lens elements 20 may be identical to the second pair of lens elements 30, except that the second pair 30 is rotated by 45 degrees relative to the first pair 20. The first and second orientations of cylindrical power is thus fixed at 45 degrees relative to each other, and the cylindrical power provided by the first lens element 21 may be fixed at 45 degrees relative to the orientation of the cylindrical power provided by the second lens element 31. When the first lens element 21 in the first pair of lens elements 20 is actuated in the first direction (defined relative to the first pair) the spherical power is adjusted. When the first lens element 21 in the first pair of lens elements 20 is actuated in the second direction (defined relative to the first pair) the cylindrical power is adjusted. Similarly, when the third lens element 31 in the second pair of lens elements 30 is actuated in the first direction (defined relative to the second pair) the spherical power is adjusted. When the third lens element 31 in the second pair of lens elements 30 is actuated in the second direction (defined relative to the second pair) the cylindrical power is adjusted. Since the second pair of lens elements 30 is rotated at 45 degrees to the first pair of lens elements 20 the axis of this cylindrical power is at 45 degrees to the axis of the cylindrical power induced by the first pair of lens elements 20. The first pair of lens elements 20 collectively provides a cylindrical power with a first orientation, and the second pair of lens elements 30 collectively provides a cylindrical power with a second orientation. The first and second orientations of cylindrical power are at 45 degrees relative to each other. This enables the orientation of the overall cylindrical power of the optical assembly 1 to also be adjusted by the relative lateral shift movement between the first lens element 21 and the third lens element 22, and the relative lateral shift movement between the second lens element 31 and the fourth lens element 32. It can also be said that the cylindrical power provided by the first lens element 21 is at 45 degrees relative to the orientation of the cylindrical power provided by the second lens element 31, such that the relative lateral shift movement between the first lens element 21 and the second lens element 31 adjusts the orientation of the overall cylindrical power. With such an arrangement, the orientation of the overall cylindrical power can be adjusted without the need to rotate the plurality of lens elements 21, 22, 31, 32 of the optical assembly 1 about the overall optical axis O. This means that by combining the two cylindrical powers that are at 45 degrees to each other, an overall cylindrical power can be created with a selected power at a selected angle. Each of the plurality of lens elements 21, 22, 31, 32 may have the form of a refractive lens element in a traditional Alvarez or Lohmann lens. In other words, each of the plurality of lens elements 21, 22, 31, 32 may have a planar optical surface and an opposing curved optical surface, such that the thickness of the lens element in a direction parallel to the optical axis of the lens element varies in a direction perpendicular to said optical axis; and said curved optical surface is configured such that the thickness of the lens element in a direction parallel to the optical axis of the lens element. The thickness of each lens has a profile of the form defined by the following equation: 1 z, = A^y^2 + -x^) + Bx}2 + Cx^ + Dy-t2 + Ex± + Fy± + G wherein: x± and y± respectively represent coordinates relative to an x-axis that extends perpendicular to the optical axis of the lens element and a y-axis that extends perpendicular to this x-axis and the optical axis of the lens element, and A, B, C, D, E, F, and G are coefficients. The plurality of lens elements 21, 22, 31, 32 may not necessarily have the form of a refractive lens element. Instead of a refraction-based lens element, at least one of (e.g. each of) the plurality of lens elements 21, 22, 31, 32 may be a diffraction-based lens element configured to effectively function like a refractive lens element in a traditional Alvarez or Lohmann lens e.g. formed using the first equation. Such a diffraction-based lens element may have a planar (e.g., nominally planar, or minimally curved) front optical surface and a planar back optical surface, rather than a planar optical surface and an opposing curved optical surface. Such a diffraction-based lens element may be a diffractive lens element comprising a pattern or etching on an optical surface configured to cause diffraction of light. Alternatively, such a diffraction-based lens element may be a meta lens element comprising subwavelength structures (i.e. metasurfaces) configured to cause diffraction of light. Providing the optical assembly 1 with diffraction-based lens element, rather than refractive-based lens elements, may enable the optical assembly 1 to be lighter and more compact. It may also help reduce the energy that needs to be provided by the actuating units to drive the relative movements between the lens elements. Second optical assembly The optical assembly 1 of Fig. 2 includes a support structure 10, and a plurality of lens elements 2, 3 comprising a first lens element 2 and a second lens element 3. The plurality of lens elements 2, 3 are arranged one behind the other such that an object, e.g. a display, is viewable through the plurality of lens elements 2, 3. In other words, the plurality of lens elements 2, 3 are generally aligned along an axis (e.g. a primary axis P of the optical assembly 1), such that they overlap each other when viewed along said axis. The plurality of lens elements 2, 3 may each be at a fixed rotation relative to each other and at a fixed rotation relative to the support structure 10. Each of the plurality of lens elements 2, 3 are capable of providing a non-zero spherical power and a non-zero cylindrical power and are collectively configured to provide an overall spherical power and an overall cylindrical power. The primary axis P is defined relative to the optical assembly 1 and / or the support structure 10. The primary axis P extends through the optical assembly 1, e.g. through the centre of the optical assembly 1. In some examples, the optical assembly 1, the support structure 10 and / or the plurality of lens elements 2, 3 extend predominantly in a direction perpendicular to the primary axis P. In other words, the extent of the optical assembly 1, the support structure 10 and / or the plurality of lens elements 2, 3 along the primary axis P is less than the extent thereof along any direction perpendicular to the primary axis P. The primary axis P may be the longitudinal axis of the optical assembly 1 and / or the support structure 10. As shown in Fig. 1, the primary axis P may be parallel to the overall optical axis O defined by the plurality of lens elements 2, 3 and / or may coincide with said overall optical axis O. The optical assembly 1 further includes one or more actuating units (not shown) configured to laterally shift the first lens element 2 relative to the second lens element 3, e.g. as illustrated by the arrows of Fig. 1. This lateral shifting of the first lens element 2 relative to the second lens element 3 involves translationally moving the first and second lens elements 2, 3 relative to each other, perpendicular to the primary axis P and / or the overall optical axis O, in a manner that maintains the optical axes of the lens elements 2, 3 parallel to each other. This lateral shift movement provided by the one or more actuating units is purely translational, i.e. non-rotational. The optical axis of each of the plurality of lens elements 2, 3 may be aligned with each other when the plurality of lens elements 2, 3 are moved into an aligned configuration by the one or more actuating units, as shown in Fig. 1. The optical axis of each of the plurality of lens elements 2, 3 may be offset in parallel when the plurality of lens elements 2, 3 are moved into an offset configuration by the one or more actuating units (not shown). Importantly, the first and second lens elements 2, 3 are configured such that the relative lateral shift movement between the first and second lens elements 2, 3 enables adjustment of the overall spherical power and the overall cylindrical power of the optical assembly 1. Specifically, as with Alvarez and Lohmann lenses, the first and second lens elements 2, 3 are configured such that: (i) relative lateral shift movement between the first and second lens elements 2, 3 in a first direction perpendicular to the primary axis P and / or the overall optical axis 0, enables adjustment of the overall spherical power, and (ii) relative lateral shift movement between the first and second lens elements 2, 3 in a second direction perpendicular to the first direction, and perpendicular to the primary axis P and / or the overall optical axis 0, enables adjustment of the overall cylindrical power. Moreover, the first and second lens elements 2, 3 are configured such that the orientation of the cylindrical power provided by translation of the first lens element 2 relative to the second lens element is at 45 degrees relative to each other. This enables the orientation of the overall cylindrical power of the optical assembly 1 to also be adjusted by the relative lateral shift movement between the first and second lens elements 2, 3. With such an arrangement, the orientation of the overall cylindrical power can be adjusted without the need to rotate the plurality of lens elements 2, 3 of the optical assembly 1 about the overall optical axis 0. The first lens element 2 and the second lens element 3 have the form of a refractive lens element in a traditional Alvarez or Lohmann lens. In other words, the first and second lens elements 2, 3 may have a planar optical surface and an opposing curved optical surface, such that the thickness of the first lens element 2 in a direction parallel to the optical axis of the first lens element 2 varies in a direction perpendicular to said optical axis. In this second embodiment, the cubic profiles of each lens element are different and have the form: 1 z2 = A(x2y22 -~x23) + Bx22 + Cx2y2 + Dy22 + Ex2 + Fy2 + G wherein: x2 and y2 respectively represent coordinates relative to an x-axis that extends perpendicular to the optical axis of the respective lens element 2,3 and a y-axis that extends perpendicular to this x-axis and the optical axis of the respective lens element 2,3 and A, B, C, D, E, F, and G are coefficients equivalent to those mentioned above in relation to the first equation. Notably, the second equation differs from the first equation described in relation to the first optical assembly described above in that 1 Q the-x term within the parentheses in the expression multiplied by / 1 has a negative sign, instead of a positive sign. Thus, the first lens element 2 has a first cubic thickness profile and the second lens element 3 has a second cubic thickness profile such that when the first lens element 2 is moved relative to the second lens element 3 in a first direction, an optical power is produced with a cylindrical component in a first axial direction, and when the first lens element 2 is moved relative to the second lens element 3 in a second direction that is perpendicular to both the first direction and the optical Axis O, an optical power is produced with a cylindrical component in a second axial direction, wherein the first and second axial directions are at 45 degrees relative to each other. Instead of a refraction-based lens element, the first lens element 2 may be a diffraction-based lens element configured to effectively function like a refractive lens element in a traditional Alvarez or Lohmann lens e.g. formed using the second equation. The first lens element 2 may thus have a planar (e.g., nominally planar, or minimally curved) front optical surface and a planar back optical surface, rather than a planar optical surface and an opposing curved optical surface. The first lens element 2 may be a diffractive lens element comprising a pattern or etching on an optical surface configured to cause diffraction of light. Alternatively, the first lens element 2 may be a meta lens element comprising subwavelength structures (known as metasurfaces) to cause diffraction of light. Similarly, the second lens element 3 may also be a diffraction-based lens element, instead of a refraction-based lens element, configured to effectively function like a refractive lens element e.g. formed using the first equation. The second lens element 3 may thus also have a planar (e.g., nominally planar, or minimally curved) front optical surface and a planar back optical surface, rather than a planar optical surface and an opposing curved optical surface. The second lens element 3 may be a diffractive lens element comprising a pattern or etching on an optical surface configured to cause diffraction of light. Alternatively, the second lens element 3 may be a meta lens element comprising subwavelength structures (i.e. metasurfaces) configured to cause diffraction of light. Providing the optical assembly 1 with diffraction-based lens element, rather than refractive-based lens elements, may enable the optical assembly 1 to be lighter and more compact. It may also help reduce the energy that needs to be provided by the actuating units to drive the relative movements between the lens elements. Head-mounted device The optical assemblies 1 of Figs. 1 and 2, e.g. when provided within a head-mounted device such as an XR device, may provide vision correction for near-sightedness, far-sightedness, and astigmatism by adjusting the overall spherical power, the overall cylindrical power, and the orientation of the overall cylindrical power. In other words, the optical assemblies 1 of Figs. 1 and 2 may be provided within XR devices, e.g. an AR or VR headset, for modulating the light from a display of the XR device into the user's eyes. The optical assemblies 1 of Figs. 1 and 2 may comprise a controller 8 configured to generate drive signals for controlling the one or more actuating units so as to adjust the overall spherical power, the overall cylindrical power, and the orientation of the overall cylindrical power. Wherein the optical assembly 1 forms part of a head-mounted device, the head-mounted device may be considered to comprise the controller 8. Wherein the head-mounted device is provided with two optical assemblies, e.g. one for each of the user's eyes, the controller 8 may be configured to generate drive signals also for controlling the one or more actuating units of the further optical assembly 1, so as to adjust the overall spherical power, the overall cylindrical power, and the orientation of the overall cylindrical power of the further optical assembly 1. The two optical assemblies 1 may be provided at locations suitable for the eyes of the user of the head-mounted device. Wherein the optical assembly 1 forms part of a head-mounted device, the head-mounted device may comprise a display, and the optical assembly 1 may be configured to primarily correct vision (i.e. longsightedness, short-sightedness, astigmatism) for the primary colours (e.g. red, green, and blue light) emitted by the display, rather than to correct the entire visible spectrum. This is particularly relevant where the plurality of lens elements comprise one or more diffractive or meta lens elements, as these types of lens elements can exhibit strong chromatic aberration, causing their focal length to vary significantly with wavelength. Actuating units At least one of (e.g. each of) the one or more actuating units of Figs. 1 and 2 may comprise a shape memory alloy (SMA) element, e.g. an SMA wire, configured, when powered, to cause one of the relative shift movements described above between the plurality of lens elements 2, 3, 21, 22, 31, 32. For example, at least one actuating unit of Fig. 1 may comprise an SMA element, e.g. an SMA wire, connected between the support structure 10 and the first lens element 2 of Fig. 1, and configured, when powered, to contract so as to cause the first lens element 2 to move relative to the support structure 10, and thus cause the first lens element 2 to move relative to the second lens element 3. The one or more actuating units of Figs. 1 and 2 may comprise four actuating units arranged so as to be capable of driving relative movement between two of the plurality of lens elements (e.g. between the first and second lens elements 2, 3 of Fig. 1, or between the first and second lens elements 21, 31 of Fig. 2) in any direction in a movement plane without applying any net torque to any of the two lens elements (e.g. without applying any net torque to the first lens element 2 of Fig. 1, or the first lens element 21 of Fig. 2) about a first axis perpendicular to the movement plane. For example, the one or more actuating units of Fig. 1 may include four actuating units configured to drive relative movement between the first and second lens elements 2, 3 by applying actuating forces F between the first lens element 2 and the support structure 10 as illustrated in Fig. 3. The arrangement of actuating forces F of Fig. 3 corresponds to the arrangement of SMA wires described in WO2013 / 175197 Al, which is herein incorporated by reference to the maximum extent permissible by law. Additionally or alternatively, the one or more actuating units of Figs. 1 and 2 may comprise a plurality of actuating units arranged such that, for each direction along each axis of a Cartesian coordinate system, there is at least one actuating force F with a non-zero component along that direction. For example, the one or more actuating units of Fig. 1 may include eight actuating units configured to drive relative movement between the first and second lens elements 2, 3 by applying actuating forces F between the first lens element 2 and the support structure 10 as illustrated in Fig. 4. The eight actuating units may be arranged such that their actuating forces F are oriented or arranged in a manner equivalent to the orientation or arrangement of the forces applied by the eight SMA wires in the actuator assemblies disclosed in WO 2011 / 104518 Al, which is herein incorporated by reference to the maximum extent permissible by law. Controller The controller 8 may be implemented in an integrated circuit (IC) chip. The controller 8 generates drive signals for controlling the one or more actuating units. Wherein the one or more actuating units comprises SMA elements, the controller 8 may generate drive signals for controlling the SMA elements in particular. SMA material has the property that, on heating, it undergoes a solid-state phase change that causes the SMA material to contract. Thus, applying drive signals to the SMA elements, thereby heating the SMA elements by causing an electric current to flow, will cause the SMA elements to contract and thus actuate the actuating units comprising the heated SMA elements, so as to drive relative shift movement between the plurality of lens elements 2, 3, 21, 22, 31, 32. The drive signals are chosen to drive relative movement between the lens elements 2, 3, 21, 22, 31, 32 in a desired manner, for example, so as to provide a certain overall spherical power, a certain overall cylindrical power, and a certain orientation of the overall cylindrical power. SMA 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. Brake mechanism The optical assemblies 1 of Figs. 1 and 2 may be provided with a brake mechanism (not shown) configured to hold the first lens element 2, 21 in any position within its range of movement relative to the second lens element 3, 31 when the one or more actuating units are unpowered. The brake mechanism may be configured to hold the first lens element 2, 21 in said position against the force of gravity for any orientation of the support structure 10, any orientation of the optical assembly 1, and / or any orientation of a head-mounted device the optical assembly 1 is provided within. Such a brake mechanism may also be provided for the other lens elements which are configured to be moved by the one or more actuating units. For example, the optical assembly 1 of Fig. 2 may be provided with a brake mechanism configured to hold the second lens element 31 in any position within its range of movement relative to the fourth lens element 32 when the one or more actuating units are unpowered. Although Fig. 1 only shows arrows for the first lens element 2, and Fig. 2 only shows arrows for the first and second lens elements 21, 31, it will be appreciated that any of the plurality of lens elements 2, 3, 21, 22, 31, 32 may be configured to be moved by the one or more actuating units. For example, it will be appreciated that the one or more actuating units of Fig. 1 may drive the relative lateral shift movement between the first and second lens elements 2, 3 by moving the second lens element 3 instead of, or in addition to, moving the first lens element 2. Similarly, it will be appreciated that the one or more actuating units of Fig. 2 may drive the relative lateral shift movement between the first and third lens elements 21, 22 by moving the third lens element 22 instead of, or in addition to, moving the first lens element 21. It will also be appreciated that the one or more actuating units of Fig. 2 may drive the relative lateral shift movement between the second and fourth lens elements 31, 32 by moving the fourth lens element 32 instead of, or in addition to, moving the second lens element 31. As such, brake mechanisms may be provided for any of the plurality of lens elements 2, 3, 21, 22, 31, 32. WO 2020 / 120997 Al, which is herein incorporated by reference to the maximum extent permissible by law, discloses arrangements which may be suitable for use as the above-described brake mechanisms. Other variations It will be appreciated that there may be many other variations of the above-described examples. For example, the optical assemblies described herein may include different types of actuating units to those described above. Examples of such actuating units include a folded SMA wire arrangement as disclosed in WO 2021 / 111131 Al, a V-shaped SMA wire with a compliant connector as disclosed in WO 2013 / 121225 Al, a scissor jack arrangement as disclosed in WO 2021 / 156458 Al, a two-stage arrangement as disclosed in WO 2021 / 111181 Al, or an SMA actuating unit with a force-modifying mechanism as disclosed in WO 2022 / 084699 Al. The documents referred to in the preceding sentence are each herein incorporated by reference to the maximum extent permissible by law. The actuator assembly may have any number of different types of actuating units, and may have any suitable number of actuating units of each type. It will also be appreciated that the actuating units may comprise non-SMA actuating units, e.g. voice-coil motor (VCM) actuating units. Moreover, although the optical assemblies described herein have been discussed primarily in the context of head-mounted devices and vision correction, it will be appreciated that they may also be used for other applications.

Claims

1. An optical assembly comprising:a support structure;a plurality of lens elements collectively configured to provide an overall spherical power and an overall cylindrical power;one or more actuating units configured to laterally shift a first lens element of the plurality of lens elements relative to a second lens element of the plurality of lens elements so as to adjust the overall cylindrical power to comprise a selected power at a selected angle.

2. An optical assembly according to claim 1, wherein the plurality of lens elements comprises two pairs of lens elements:a first pair of lens elements comprising the first lens element and a third lens element, anda second pair of lens elements comprising the second lens element and a fourth lens element;wherein the first pair of lens elements provides a cylindrical power with a first orientation, and the second pair of lens elements provides a cylindrical power with a second orientation;wherein the first and second pairs of lens elements are configured such that the first and second orientations of cylindrical power are at 45 degrees relative to each other and the one or more actuating units are configured to shift the first lens element and the second lens element so as to adjust the overall cylindrical power to comprise the selected power at the selected angle.

3. An optical assembly according to claim 2, wherein the orientation of the first pair of lens elements is at 45 degrees relative to the second pair of lens elements.

4. An optical assembly according to claim 2 or claim 3, wherein the cubic thickness profile of each lens element is of the form1z, = A^y^2 + -¾3) + Bx}2 + Cx^ + Dy-t2 + Ex± + Fy± + G5. An optical assembly according to any of claims 2 to 4, wherein the first and second lens elements are movable and the third and fourth lens elements are static.

6. An optical assembly according to any of claims 2 to 5, wherein the one or more actuating units are configured to laterally shift the first lens element relative to the third lens element, and configured to laterally shift the second lens element relative to the fourth lens element so as to adjust the overall spherical power and the overall cylindrical power.

7. An optical assembly according to any of claims 2 to 6, wherein the first pair of lens elements is identical to the second pair of lens elements.

8. An optical assembly according to claim 1, wherein the plurality of lens elements comprises a pair of lens elements comprising the first lens element and the second lens element, and the first lens element has a first cubic thickness profile and the second lens element has a second cubic thickness profile such that when the first lens element is moved relative to the second lens element in a first direction an optical power is produced with a cylindrical component in a first axial direction, and when the first lens element is moved relative to the second lens element in a second direction an optical power is produced with a cylindrical component in a second axial direction, where the first and second axial directions are at 45 degrees relative to each other.

9. An optical assembly according to claim 8, wherein the second direction is perpendicular to the first direction and an optical axis of the optical assembly.

10. An optical assembly according to any claim 8 or claim 9, wherein the cubic thickness profile of the plurality of each lens elements is of the form1z2 = A(x2y22 -~x23) + Bx22 + Cx2y2 + Dy22 + Ex2 + Fy2 + Gwherex2 and y2 respectively represent coordinates relative to an x-axis that extends perpendicular to the optical axis of the second lens element and a y-axis that extends perpendicular to this x-axis and the optical axis of the second lens element, and A, B, C, D, E, F, and G are coefficients.

11. An optical assembly according to any preceding claim, wherein at least one of the first and second lens elements is a diffraction-based lens element.

12. An optical assembly according to any preceding claim, wherein at least one of the first and second lens elements is a meta lens element.

13. An optical assembly according to any preceding claim, wherein at least one of the first and second lens elements is a diffractive lens element.

14. An optical assembly according to any preceding claim, wherein at least one of the actuating units comprises an SMA (shape memory alloy) element configured, when powered, to drive the relative lateral shift movement between the first and second lens elements.

15. An optical assembly according to any preceding claim, wherein the one or more actuating units are configured to move the first lens element relative to the second lens element to any position within a range of movement; andwherein the optical assembly comprises a brake mechanism configured, when the actuating units are unpowered, to hold the first lens element in any position within the range of movement.

16. An optical assembly according to any preceding claim, wherein the optical assembly is configured to correct vision by adjusting the overall spherical power, the overall cylindrical power, and / or the orientation of the overall cylindrical power.

17. An optical assembly according to any preceding claim, wherein the one or more actuating units comprise four actuating units arranged so as to be capable of moving the first lens element relative to the second lens element in any direction in a movement plane without applying any net torque to the first lens element about a first axis perpendicular to the movement plane.

18. An optical assembly according to any of claims 1 to 17, wherein the one or more actuating units comprises a plurality of actuating units arranged such that, for each direction along each axis of a Cartesian coordinate system, there is at least one actuating force with a non-zero component along that direction.

19. A head-mounted device comprising:an optical assembly according to any preceding claim; anda controller configured to generate drive signals for controlling the one or more actuating units so as to adjust the overall spherical power, the overall cylindrical power, and the orientation of the overall cylindrical power.

20. A head-mounted device according to claim 19, comprising a display, and wherein the optical assembly is configured to primarily correct vision for the primary colours emitted by the display.

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