Patent Publication Number: US-2020285077-A1

Title: Tunable non-round spectacles with immersed lens shaper

Description:
The present invention relates to an optical device, particularly in the form of a lens, particularly for a spectacle. Further, particularly, the invention relates to spectacles comprising such lenses. 
     More particularly, such a lens is at least in part fluid- or liquid-filled and has an adjustable focal length. 
     More particularly, the present invention relates to designs and methods of how to use and control such dynamic lenses. 
     The present invention is not only applicable to spectacle lenses but also to other lenses that may be used in a variety of different applications such as but not limited to diopter control in viewfinders, virtual reality (VR) and augmented reality (AR) systems, particularly spectacles for VR or AR. 
     Myopia (nearsightedness) refers to the condition of defect vision of distant objects. Hyperopia (farsightedness) refers to the condition of defect vision of near objects. Both Myopia and Hyperopia are related to the refractive power of eye with respect to the size of the eyeball and are constant over the life. 
     Prespiopia (farsightedness) is a condition caused by loss of elasticity of the lens of the eye. It reduces the ability of the human eye to accommodate, i.e. to alter the focal length of the natural eye. It is occurring typically in middle and old age. 
     Accommodation in human beings is reduced to 3 D (diopters) or less at an age range of 35-45 years. At that point, reading glasses or some other form of near vision correction becomes necessary for the human eye to be able to focus on near objects. 
     Having to switch between spectacles with different optical power can be prevented by using either bifocal, multifocal or progressive spectacle lenses or contact lenses. In case of progressive lenses, a “corridor” of optimum lens power runs vertically down each progressive lens. In contrast to bifocals and trifocals a smooth change of focus from distance to near occurs without image jump. 
     A large segment of the population requires a different visual correction for each eye. These people, known as anisometropes, require different visual correction for each eye for maximum visual comfort. 
     Adjustable optical lens systems comprising fluids are ideally suited for spectacles because of their compact size, low weight, and continuous adjustment of optical power. In case of an electrically controlled lens the fast switching speed and the low electrical power are key benefits. 
     Adjustable optical lens systems comprising fluids are known from the prior art. 
     WO07049058 is directed to a lens with a variable focus, which comprises a rigid ring to which a flexible membrane is attached. A rigid transparent front cover is attached to the flexible membrane and a rigid rear cover on the rear surface of the ring. A cavity is formed between the flexible membrane and the rear cover which is filled with a liquid. The amount of liquid in the cavity can be adjusted to vary the curvature of the flexible membrane and so vary the optical characteristics of the lens. A second flexible membrane can be positioned between the rear cover and the ring. 
     Further, WO06011937 is directed to a fluidic adaptive lens device with at least one flexible membrane (indicated as first partition). The adaptive lens includes a first flexible and optically transparent membrane. A second partition, which is coupled to the flexible membrane, is at least partially optically transparent. A first cavity is formed in between the flexible membrane and the second partition. The lens device comprises a fluid within the cavity. Furthermore, the device comprises means, e.g. a Teflon coated screw, to control the pressure or the volume of the fluid in the chamber. When the parameter of the fluidic medium changes, the membrane flexes and the optical property of the lens changes. 
     Further, US2003095336 describes a lens arrangement mainly for a corrective or a prescription lens. The prescription lens is adjacent to a fluid cell which has a flexible membrane and a base. In that fluid is pumped into or out of the fluid cell the corrective power of the entire lens arrangement is varied. 
     Furthermore, fluid lenses have also been proposed for ophthalmic applications (see, e.g. U.S. Pat. No. 7,085,065). 
     Furthermore, fluid lenses designed for the purpose of tunable spectacles are described in the subsequent paragraphs. 
     U.S. Pat. No. 8,414,121 B2 describes non-round tunable fluid lens assembly where the thickness of the membrane includes thickness contours to partially cancel out asphericity (especially astigmatism) at a particular stage of inflation of the membrane. In consequence a complicated fitting and optimization procedure is required for each specific shape of spectacle frame. 
     Further, US 2012/0087014 describes a liquid actuation mechanism integrated into the brackets of the spectacles. Fluid is pumped from the reservoir inside the bracket into the optical aperture via a flexible tubing. 
     Furthermore, US 2012/0287512 A1 describes different actuator mechanism for an adjustable fluid-filled lens, including magnetic, mechanical and thermal, all integrated into the bracket of the spectacles. In some embodiments, an adjustable fluid-filled lens includes a septum configured to be pierceable by a needle and automatically and fluidly seal a chamber after withdrawal of the needle. 
     Furthermore, US 2012/0087015 A1 describes an embodiment of a piezo-electrically controlled fluid reservoir which is integrated into the perimeter of the lens module. 
     Based on the above, the problem underlying the present invention is to provide a versatile optical device for vision correction. 
     This problem is solved by an optical device having the features of claim  1 . 
     Preferred embodiments of the optical device are stated in the corresponding sub claims and are described below. 
     According to claim  1 , an optical device is disclosed, comprising: at least a first lens having an adjustable focal length, wherein the first lens comprises a container comprising at least one reservoir volume and a lens volume which are in fluid flow communication via a channel and filled with a transparent fluid (e.g. a liquid), and wherein the container comprises a stretchable transparent membrane and a transparent lens shaper contacting the fluid and connected to the membrane, so that the lens shaper defines a curvature-adjustable area of the membrane, and wherein the container comprises a transparent back wall facing the membrane, wherein the fluid is arranged between the membrane and the back wall. According to the present invention, the reservoir volume is covered by a wall, wherein a plunger for being attracted by an actuator part (e.g. an electropermanent magnet) is arranged in the reservoir volume and connected to the wall so that fluid is pumped from the lens volume into the reservoir volume when the plunger is attracted by the actuator part (e.g. electropermanent magnet) whereby the curvature of the curvature-adjustable area and therewith said focal length is changed. 
     According to a preferred embodiment, said wall is a stretchable wall. Particularly, in all embodiments, the transparent fluid can be a transparent liquid (and vice versa). 
     Due to the fact, that the membrane can be elastically deformed for adjusting the curvature of said area, said container and the fluid residing therein form a focus adjustable (or tunable) lens. 
     Particularly, the fact that the lens shaper contacts the membrane can mean that the lens shaper contacts the membrane directly or indirectly via another material layer (e.g. formed by a glue etc.). The lens shaper can further be attached to the membrane by bonding it directly to the membrane or via another material layer such as a glue layer. 
     Particularly, the notion according to which the lens shaper defines an area of the membrane that has an adjustable curvature may mean that the lens shaper delimits, by being attached to the membrane or by contacting the latter, an elastically expandable (e.g. circular) area of the membrane, wherein particularly said area extends up to an (e.g. circumferential) inner edge of the opening formed in the lens shaper. 
     When the pressure of the fluid residing in the lens volume changes due to fluid being pumped in or out of the lens volume the curvature-adjustable area of the membrane changes its curvature accordingly. Particularly, said area of the membrane may change its curvature from a concave state via a flat state to a convex state. 
     Generally, the membrane can be made of at least one of the following materials: a glass, a polymer, an elastomer, a plastic or any other transparent and stretchable or flexible material. For example, the membrane may be made out of a silicone-based polymer such as poly(dimethylsiloxane) also known as PDMS or a polyester material such as PET or a biaxially-oriented polyethylene terephtalate (e.g. “Mylar”). 
     Further, the membrane can comprise a coating. Such coating can for example reduce the reflection loss at the membrane-air interface. It can also have a different function such as an anti-fog function. Further, the membrane can also be structured, e.g. comprises a structured surface or have a variable thickness or stiffness across the membrane. 
     Further, said fluid residing in the container preferably is or comprises a liquid metal, a gel, a liquid, a gas, or any transparent, absorbing or reflecting material which can be deformed. For example, the fluid may be a silicone oil (e.g. Bis-Phenylpropyl Dimethicone). Additionally, the fluid may include fluorinated polymers such as perfluorinated polyether (PFPE) inert fluid. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the (e.g. stretchable) wall is formed by said membrane (e.g. by a region of said membrane), which covers said reservoir volume and said lens volume. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the optical device comprises an actuator unit comprising said plunger and said actuator part (e.g. electropermanent magnet) for interacting with said plunger, wherein the plunger comprises a counter member (e.g. a magnetic flux guiding countermember) attractable by the actuator part , so that fluid is pumped from the lens volume into the reservoir volume when the counter member is attracted by the actuator part whereby the curvature of the curvature-adjustable area and therewith said focal length is changed. 
     Furthermore, according to an embodiment, the counter member is a permanent magnet. 
     Furthermore, according to an alternative embodiment, the counter member is a magnetic flux guiding counter member. 
     Furthermore, according to an embodiment, the actuator part is an electropermanent magnet. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the counter member faces the electropermanent magnet. 
     Further, according to an embodiment of the present invention, the electropermanent magnet is configured to generate an external magnetic field for attracting said counter member for adjusting (particularly decreasing) the curvature of said curvature-adjustable area of the membrane, e.g. for changing the curvature of said area from a convex state to a less convex state or even to a flat or concave state. 
     Particularly, said external magnetic field can be turned on or off by applying a corresponding electrical current pulse to a coil of the electropermanent magnet, which coil encloses a (e.g. second) magnet of the electropermanent magnet, whose magnetization can be switched by a magnetic field generated by the coil upon applying said current pulse to the coil. 
     Further, according to an embodiment of the present invention, the electropermanent magnet comprises a first magnet having a first coercivity (e.g. a “hard” magnetic material) and a first magnetization, and wherein the electropermanent magnet further comprises a second magnet having a second coercivity (e.g. a “soft” or “semi hard” magnetic material) and a second magnetization, wherein the first coercivity is larger than the second coercivity, and wherein the electropermanent magnet further comprises a coil encompassing the second magnet such that by applying a corresponding current to the coil the second magnet can be partially magnetized to generate an external magnetic field of pre-defined strength for achieving a continuous adjustment of the curvature of said curvature-adjustable area or such that by applying a corresponding current the second magnetization of the second magnet can be switched from a parallel state where the two magnetizations are parallel to an antiparallel state where the two magnetizations are antiparallel (and vice versa), wherein when the second magnetization is in the parallel state the electropermanent magnet generates said external magnetic field, and wherein when the second magnetization is in the antiparallel state said external magnetic field vanishes. 
     In other words, in case the magnetically hard and soft materials (first and second magnet) have opposing magnetizations the magnet produces no net external field across its poles, while when their direction of magnetization is aligned, the electropermanent magnet produces an external magnetic field, which attracts the respective counter member. Further, as described above, it is not necessary to fully magnetize the second magnet, but one can also merely partially magnetize the magnet to adjust the force of the respective electropermanent magnet in a continuous fashion. This operation is also denoted as analog mode. Further, according to an embodiment, the electropermanent magnet comprises two pole members, particularly consisting of a soft magnetic material, namely a first pole member arranged at a first end of the first magnet and at a first end of the second magnet, and a second pole member arranged at a second end of the first magnet and at a second end of the second magnet. 
     Because the pole members have a higher permeability than air, they concentrate the magnetic flux of the magnets. Particularly, when the magnetizations are antiparallel, the magnetic flux is short-circuited at the ends of the magnets via the respective pole member. In case the magnetizations are parallel, the magnetic flux is guided from one pole member to the associated counter member and back to the other pole member. 
     Furthermore, according to an embodiment, the actuator part comprises a single magnetizable member (e.g. formed out of a magnetically soft material) and a coil encompassing the magnetizable member, wherein the actuator unit is configured to magnetize the magnetizable member by applying an electrical current pulse to the coil such that the magnetizable member is magnetized and attracts said counter member for adjusting the curvature of said curvature-adjustable area, and wherein particularly the actuator unit is configured to apply an electrical current pulse to the magnetizable member to demagnetize the magnetizable member. 
     Furthermore, according to an embodiment, the actuator part comprises a coil, wherein the actuator unit is configured to apply an electrical current to the coil such that the coil generates a magnetic field that attracts or repels the counter member for adjusting the curvature of said curvature-adjustable area. Here particularly, said counter member is a permanent magnet and the actuator unit particularly forms a voice coil motor. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the optical device comprises a frame for holding said container, wherein the actuator part (e.g. electropermanent magnet) is arranged on a portion of the frame so that the actuator part faces the associated counter member. 
     Furthermore, according to an embodiment of the optical device according to the present invention, said portion of the frame faces the reservoir volume in a direction running parallel to the optical axis of the at least one first lens, particularly such that said portion of the frame covers the reservoir volume. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the reservoir volume comprises an elongated shape and extends along a longitudinal axis. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the plunger comprises an elongated shape and extends along said longitudinal axis. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the plunger comprises a further (e.g. magnetic flux guiding) counter member for interacting with a further actuator part (e.g. electropermanent magnet) comprised by the actuator unit, so that fluid is pumped from the lens volume into the reservoir volume when the two counter members are attracted by the respective actuator part whereby the curvature of the curvature-adjustable area and therewith said focal length is changed. 
     Furthermore, according to an embodiment of the optical device according to the present invention, said counter member and said further counter member face each other in the direction of said longitudinal axis. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the further actuator part is arranged on said portion of the frame, too, so that the further actuator part faces the associated further counter member. 
     Furthermore, according to an embodiment, the further actuator part is one of: an electropermanent magnet (e.g. configured as described above); an actuator part comprising a single magnetizable member and a coil encompassing the magnetizable member (e.g. configured as described above). 
     Further, in an embodiment, the further counter member is one of: a magnetic flux guiding counter member; a permanent magnet. 
     Furthermore, in an embodiment, the optical device may also comprise a plurality of reservoir volumes, wherein each reservoir volume is in flow connection with the lens volume via a separate channel. Here, according to an embodiment, a plunger comprising a magnetic flux guiding counter member or formed by a magnetic flux guiding counter member, is arranged in each reservoir volume and connected to a (e.g. stretchable) wall of the respective reservoir volume. According to an embodiment, each (e.g. stretchable) wall is formed again by a region of the membrane that also covers the lens volume. The individual reservoir volume can be designed as described herein with regard to the at least one reservoir volume. Further, each counter member faces an associated electropermanent magnet (that is particularly arranged in said frame that is arranged in front of the reservoir volumes), wherein the optical device is configured to control each electropermanent magnet independently from the other electropermanent magnets. Thus, the optical device is configured to generate a plurality of different curvatures of the said curvature-adjustable area of the membrane (and therewith a plurality of corresponding focal lengths of the first lens) even when the respective electropermanent magnet merely moves the associated counter member between two stable states corresponding to a convex state of the respective (e.g. stretchable) wall (membrane region), in which the respective counter member is closest to the associated electropermanent magnet and in which the respective reservoir volume has maximal size, and a flat state of the respective wall/membrane region corresponding to a smaller volume value of the respective reservoir volume. 
     Further, according to an embodiment, the reservoir volumes have different volume values (e.g. with respect to a flat state of the respective reservoir volume) so as to increase the number of different focal lengths that can be selected/adjusted by the optical device. The operation mode in which the counter members are only moved between said two positions, respectively, is also denoted as bistable operation. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the back wall may form a rigid lens. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the back wall may form a free-form optics, such as a coma-compensation plate. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the lens shaper comprises an (e.g circular) opening forming at least a portion of the lens volume, wherein said opening is covered by the membrane, wherein said curvature-adjustable area covers said opening. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the lens shaper comprises an e.g. elongated recess forming at least a portion of the reservoir volume, wherein said recess is covered by the membrane, too, see also above. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the material of the lens shaper, the membrane, and the fluid each comprise a refractive index, wherein the absolute value of the difference of any two refractive indices of these three refractive indices is smaller than 0.1, preferably smaller than 0.02. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the container (including the plunger arranged in the reservoir volume) forms a (e.g. semi-finished) lens-blank having a circumferential boundary region that is configured to be at least one of: formed, shaped, machined, cut, sanded, milled, in order to form an outer contour of the container that fits a desired frame for holding the container. 
     Furthermore, according to an embodiment of the present invention, the container forms a semi-finished lens-blank that comprises a curved shape or is configured to be formed into a curved shaped. In case of an optical device in the form of spectacles (e.g. for vision correction), this allows one to adapt the container(s) more easily to a frame of the spectacles for holding the container(s). 
     Furthermore, according to an embodiment of the optical device according to the present invention, the optical device comprises a transparent front wall arranged in front of the membrane for protecting the membrane. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the front wall can be a rigid lens, e.g. for providing a base refractive power. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the back wall comprises a recess forming a part of the reservoir volume, and/or wherein the back wall comprises a recess forming a part of the lens volume. Particularly, these two recesses can form one continuous recess. 
     Further, according to an embodiment, the channel connecting the at least one reservoir volume to the lens volume is formed by a recess of the back wall. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the container comprises an intermediary layer arranged between the back wall and the lens shaper, wherein said intermediary layer comprises a recess forming a part of the reservoir volume, and/or wherein intermediary layer comprises a recess forming a part of the lens volume. 
     Particularly, in case an intermediary layer is used, the channel connecting the at least one reservoir volume and the lens volume can also be formed by a recess of the intermediary layer. 
     Generally, it is also possible to form a recess into the lens shaper for forming a channel connecting the at least one reservoir volume to the lens volume, but such a recess is not preferred since it leads to a smaller optical quality of the lens due to deformations of the membrane in the region of this recess when the membrane is connected to the lens shaper. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the container comprises a circumferential lateral inner side delimiting the lens volume, wherein said inner side comprises a rounded shape (particularly in a cross section perpendicular to the back wall). Furthermore, particularly, the inner side comprises a curvature having an inflection point. Furthermore, preferably a portion of the rounded shape is formed by the back wall, and wherein an adjacent portion of the rounded shape is formed by the lens shaper. Particularly the two portions form roundings of opposite curvature. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the optical device comprises a sensor arranged on the wall covering the at least one reservoir volume (e.g. region of the membrane), wherein the sensor is configured to measure a curvature of said wall or a strain of said wall, wherein particularly the optical device is configured to use an output signal of the sensor as a feedback signal for controlling the actuator unit of the optical device. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the wall comprises a bellows structure. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the container comprises a further (e.g. passive) reservoir volume connected to the at least one reservoir volume via flow connection, and wherein the further reservoir volume is connected to the lens volume via a further channel. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the further reservoir volume is covered by a further wall (can also be a stretchable wall formed by a region of the membrane), wherein particularly the further wall comprises a bellows structure. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the channel, the further channel, and said flow connection are each configured to be opened or closed by means of an associated valve, wherein the actuator unit is configured to control said valves (e.g. for pumping fluid into the lens volume or out of the lens volume using e.g. the actuator part(s) and counter member(s)). 
     Furthermore, according to an embodiment of the optical device according to the present invention, the at least one reservoir volume and the lens volume are in flow communication via a plurality of channels. Said channels may extend side by side, particularly parallel. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the fluid comprises a refractive index that is larger than 1.45, preferably larger than 1.55, and/or wherein the lens shaper comprises a refractive index that is larger than 1.45, preferably larger than 1.55. 
     Furthermore, in an embodiment, the fluid (e.g. transparent liquid) and the immersed lens shaper have equal Abbe numbers. 
     Furthermore, according to an embodiment, a spacer is arranged between the plunger and the membrane, which spacer comprises a surface via which the spacer is connected to the membrane, wherein this surface of the spacer is smaller than a surface of the plunger facing the membrane. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the optical device comprises a further first lens having an adjustable focal length, wherein a waveguide is arranged between the first lens and said further first lens. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the optical device comprises a second lens having an adjustable focal length. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the optical device comprises a further second lens having an adjustable focal length, wherein a further waveguide is arranged between the second lens and said further second lens. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the optical device is formed as a pair of spectacles, particularly for virtual reality or augmented reality, or a headset, particularly for virtual reality or augmented reality, wherein the respective lens is held by a frame that can be worn by a user such that the respective lens is arranged in front of an associated eye of the user. 
     Furthermore, according to an embodiment of the optical device according to the present invention, the optical device is configured to adjust a focal length of the first lens and a focal length of the second lens simultaneously. 
     Furthermore, according to an embodiment, the optical device is configured to adjust a focal length of the first lens and a focal length of the further first lens simultaneously. Particularly, this adjustment is conducted such that the total focal power of the first lens and the further first lens stays constant. 
     Furthermore, according to an embodiment, the optical device is configured to adjust a focal length of the second lens and a focal length of the further second lens simultaneously. Particularly, this adjustment is conducted such that the total focal power of the second lens and the further second lens stays constant. 
    
    
     
       Further features, advantages and embodiments of the present invention will be described below with reference to the Figures, wherein 
         FIG. 1  shows the cross-section of an electrically tunable lens based on a liquid filled reservoir that is connected through a liquid channel with a single reservoir. The liquid-filled reservoir as well as the actual lens area are formed between membrane, lens shaper and back glass. A plunger containing of non-magnetic material having at least two inserts of soft magnetic material is placed inside the liquid reservoir. On the other side of the membrane there are at least two electro-permanent magnet (EPM) motors which are shown in the drawing in non-actuated state. The membrane is deflected to the outside, i.e. convex state. In this configuration the lens container consists of two layers: an etched/embossed transparent material that is bonded to the lens shaper. 
         FIG. 1A  shows the principle of an electropermanent magnet that can be used with the present invention. 
         FIG. 1B  shows a variation of  FIG. 1 , but with multiple reservoir volumes, channels and electropermanent magnets. 
         FIG. 2  shows the same configuration as in  FIG. 1  but with the EPM motors in actuated state, i.e. the magnetic metal pieces are pulled towards the EPM motor and the membrane is deflected negatively, i.e. concave shape. 
         FIG. 3  shows a similar configuration as in  FIG. 1  but with the lens container consisting of three layers: a bottom glass, an intermediate layer and the lens shaper. 
         FIG. 4  shows a similar configuration as in  FIG. 1  but with an additional spring structure made from non-magnetic material placed between the plunger and the EPM motors. It helps the lens to go quicker back to the nominal position. 
         FIG. 5  shows a practical implementation where the EPM actuator is integrated into the frame of a pair of spectacles with arbitrary shaped contours. The reservoir is hidden inside the frame. An additional correction lens or protection glass helps to protect the membrane from accidental touch. 
         FIG. 6  shows how several tunable lenses can be fabricated in a batch process. The sealed liquid containers with integrated plunger can be prefabricated and afterwards carved out with the desired contour and position of the lens shaper in such a way that the clear aperture of the lens is aligned with respect to the lens contour and the desired pupillary distance. 
         FIG. 7  illustrates the possibility to have a range of “templates” with different sizes of optical apertures as well as distances between the fluid reservoir and the optical aperture. This allows to accommodate different types of spectacle frame design and sizes. 
         FIG. 8  shows a configuration suitable to solve the vergence-accommodation conflict in augmented reality glasses that are based on the waveguide approach. In this configuration 2 tunable lenses are placed in series and the image generating waveguide is sandwiched between the tunable lenses of opposite optical power. 
         FIG. 9  shows the full 3D view of the augmented reality/mixed reality configuration with the various EPM motors for front lens/back lens and left eye/right eye integrated into a single bar. In this configuration the waveguide between front and back lens in not being displayed. 
         FIG. 10  shows a cross-section of further embodiment of an optical device according to the present invention comprising an electrically tunable lens based on a liquid filled reservoir that is connected through a liquid channel with a single reservoir. The liquid-filled reservoir as well as the actual lens area can be formed between membrane, lens shaper and back wall. A plunger containing of non-magnetic material having at least two inserts of soft magnetic material can be placed inside the liquid reservoir. On the other side of the membrane there can be at least two electro-permanent magnet (EPM) motors which are shown in the drawing in non-actuated state. Depending on the liquid fill level the membrane is deflected to the outside, i.e. convex state, in flat state or deflected to the inside, i.e. concave state. In this configuration the lens container can consist of two layers: an etched/embossed transparent material that is bonded to the lens shaper. In the displayed configuration the side walls that define the aperture can be rounded to avoid any light diffraction at sharp edges. The respective magnetic counter member can be formed by a permanent magnet. 
         FIG. 11  shows a similar configuration as in  FIG. 1  but with the liquid-filled reservoir volume being connected to the actual lens area/volume by multiple channels. As a modification, instead of an electropermanent magnet (EPM) motor the optical device comprises a coil wound around a soft magnetic material. 
         FIG. 12  shows a similar configuration as in  FIG. 1  but with an additional spacer placed between the plunger and the membrane. Particularly, the spacer is smaller than the plunger thus making it easier to stretch the actuation membrane. A strain sensor can be placed on the actuation membrane to give a feedback on the position of the plunger and thus the tuning state of the lens. 
         FIG. 13  shows a similar configuration as in  FIG. 1  but with a below structure in inside the reservoir instead of a flat membrane. 
         FIG. 14  shows a similar configuration as in  FIG. 1  but with two reservoirs that are connected to the actual lens area/volume but also between each other. One reservoir represents a membrane pump that allows to pump liquid from the actual lens area to a liquid storage. This stored liquid can quickly be released by opening the valve between the pressurized reservoir and the actual lens area. 
     
    
    
     While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the pertinent art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the present invention. It will be apparent to a person skilled in the pertinent art that this invention can also be employed in a variety of other applications, for example and not limited to virtual reality (VR) devices, augmented reality (AR) devices, mixed reality (XR) devices, progressive glasses, viewfinders. 
     It is noted that references in the specification to “one embodiment,” “an embodiment, an example embodiment,”etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure or characteristic is described in connection with an embodiment, it would be within the knowledge of one skilled in the art to effect such feature, structure or characteristic in connection with other embodiments whether or not explicitly described. 
     Any embodiment involving an actuator has implicitly the possibility to adjust the left and the right eye simultaneously, once the base correction for each eye has been set. 
     The embodiment shown in  FIG. 1  shows the cross-section of an optical device  1  comprising at least a first (e.g. electrically) tunable lens  100 , based on a fluid (e.g. liquid) filled reservoir volume  90  that is connected through at least one channel  92  to a single lens volume  91 . The liquid-filled reservoir volume  90  as well as the actual lens area/volume  91  are formed by a membrane  21 , a lens shaper  22  and back wall (e.g. back glass)  30 . While the main cavity (also denoted as lens volume)  91  is preferably circular or round in order to reduce optical aberrations, the at least one reservoir volume  90  has preferably an elongated non-round shape that may extend along a longitudinal axis L in order to hide inside a lens frame  10  containing an actuator component, here in form of at least one electropermanent magnet  80 . Thus, the reservoir volume  90  is advantageously not visible from the outside when looking along the optical axis A onto the first lens  100 . 
     Apart from the lens shaper  22 , at least a portion of the main cavity  91 , the (e.g. microfluidic) channel  92  and at least a portion of the reservoir volume  90  can be formed into the back wall  30  (e.g. in the form of corresponding recesses  91   a,    92 ,  90   a.  In a specific embodiment the back wall  30  is made from a glass and the cavities/recesses  90   a,    92 ,  91   a  are created by etching. In a different embodiment the back wall  30  is made from a highly transparent material that can be molded or embossed. A second optically transparent layer forms the lens shaper  22  and covers the channel  92 . It is e.g. made from a material that hermetically seals the back wall  30  and the membrane  21  and that particularly provides a very smooth and flat surface in order not to create optical aberrations. In a preferred embodiment the lens shaper layer  22  is made from a thin slab of a glass. In a further preferred embodiment the thickness of this glass slab is less than 0.5 mm. 
     The fluid (e.g. liquid) F and the lens shaper  22  and the back wall material are preferably index-matched so that the channel  92  as well as the lens volume  91  are ideally non-visible. A plunger  94  that comprises e.g. two inserts of a soft magnetic material  81  that are e.g. inserted into a non-magnetic material of the plunger  94  is placed inside the reservoir volume  90 . The two inserts  81  form counter members of associated electropermanent magnets  80  to be described below. 
     In a non-actuated state a curvature-adjustable area  23  of the membrane  21 , which area  23  is defined by an (e.g. circular) opening  24  of the lens shaper  22  that is covered by the membrane  21  (particularly congruently by said area  23 ), is either flat or positively curved, based on the amount of fluid F inside the lens. In a particular embodiment a flexible, elastic membrane  21  (e.g. of a high optical transparency) is bonded onto the upper side of the lens shaper  22  (object side). The membrane  21  can be e.g. bonded either glue-free or with highly transparent colorless glue. For pumping fluid F from the reservoir volume  90  into the lens volume  91  and vice versa, the optical device  1  further comprises at least two electropermanent magnets (EPM)  80  that are arranged outside the at least one reservoir volume  90  in front of the membrane region  93  that covers the at least one reservoir volume  91 , particularly a recess  24   a  formed into the lens shaper  22 , wherein said EPMs  80  are aligned with said counter members  81  of the plunger  94  that is arranged in the reservoir volume and connected to the stretchable wall/membrane region  93  are. In comparison to other configurations this specific configuration minimizes the thickness of the optical device  1  by placing the plunger  94  with the magnetic material  81  inside the reservoir volume  90 . When the EPM motors  80  are not actuated, the membrane  21 , depending on the fluid (e.g. liquid) fill level, is either flat or positively deflected, i.e. in a convex state. 
     When the EPMs  80  are actuated as shown in  FIG. 2  the magnetic counter members  81  that are connected to the stretchable wall  93 , e.g. membrane region  93 , of the reservoir volume  90  are pulled towards the respectively associated EPM  80 . Thus, the reservoir volume  90  acts as a pump. The attractive force on the respective counter member  81  presses the stretchable wall  93  of the at least one reservoir volume upwards (the stretchable wall  93  is formed by a region  93  of the membrane  21  according to an embodiment), and consequently increases the effective volume inside the reservoir volume  90 . The resulting under-pressure displaces fluid F from the lens volume (main cavity)  91  into the reservoir volume  90  and creates an under-pressure which curves said area  23  of the membrane  21  inwards. 
     The lens volume  91  and thus the curvature of the area  23  of the membrane  21  can be adjusted by controlling the effective reservoir volume  90 . 
     By actuating all EPMs  80  symmetrically, the plunger  94  is pushed upwards symmetrically (i.e. without tilting the plunger  94 ). It is possible to actuate only a subset of all EPMs  80  in order to create an intermediate curvature between the minimum and maximum curvature. By driving the EPMs  80  in an analog fashion, an infinite number of states (and thus different curvatures of the area  23  and corresponding focal lengths of the first lens  100 ) can be created. 
     Particularly, the electropermanent magnets  80  can be formed and operated as shown in  FIG. 1A . Generally, an electropermanent magnet (EPM)  80  is a type of a permanent magnet in which the external magnetic field can be switched on or off by a pulse of an electric current in coil (e.g. a wire winding)  84  as indicated in  FIG. 1A . 
     The electropermanent magnet  80  consists of two sections or magnets  82 ,  83 , namely a first magnet  82  (of a “hard”/high coercivity magnetic material) and a second magnet  83  (of a “soft”/low coercivity magnetic material). The direction of the magnetization M′ of the latter piece  83  can be switched by a pulse of an electrical current in coil  84  surrounding the second magnet  83 . When the magnetically soft and hard materials  82 ,  83  have opposing magnetizations M, M′ the electropermanent magnet  80  produces no net external field across its poles, while when their direction of magnetization M, M′ is aligned, the electropermanent magnet  80  produces an external magnetic field. Two pole members  85  consisting of soft magnetic material are located on both ends of the two permanent magnets  82 ,  83 . Because the pole members  85  have a higher permeability than the air, they will concentrate the magnetic flux of the permanent magnets  82 ,  83 . This electropermanent magnet  80  is mechanically connected to the back wall or back lens  30 . 
     When the electropermanent magnet  80  is switched on and a counter member  81  of a soft magnetic material is placed in close proximity to the electropermanent magnet, the magnetic flux will flow confined in the soft magnetic material creating an attractive force. With the counter member  81  mechanically connected to the membrane above the reservoir volume, this attractive force increased the reservoir volume such that fluid F is transferred from the lens volume  91  into the reservoir volume. Due to this the stretchable curvature-adjustable area of the membrane  21  bulges less out and may even become flat or concave. 
     Particularly, as shown in  FIG. 1 , two electropermanent magnets  80  are arranged in the frame  10  of the optical device, wherein each electropermanent magnet  80  faces its associated counter member  81  that forms part of the plunger  94  connected to the membrane region  93  above reservoir volume  90 . Particularly, each pole member  85  of the respective electropermanent magnet  80  comprises a face side  85   a  that faces the associated counter member  81  and forms a gap  86  with the associated counter member  81 , When the respective electropermanent magnet  80  generates an external magnetic field, the respective counter member  81 , and therefore the membrane region  93  covering the reservoir volume  90 , is pulled towards the frame  10 . This movement draws fluid F from the lens volume  91  into the (now increased) reservoir volume  90  and thus changes the curvature of the curvature-adjustable area  23  of the membrane  21  accordingly since fluid F is displaced away from the curvature-adjustable area  23  which then bulges less outwards accordingly. When the respective external magnetic field is turned off, the membrane region covering the reservoir volume  90  returns to its initial position and the curvature-adjustable area  23  of the membrane  22  returns to its initial position as well as fluid F is pressed back to the lens volume  91  via the channel  92 . 
     This electropermanent magnet concept allows for fast diopter variations. A typical tuning speed can be in the order of 1 ms. Fast tuning is required in case the eyewear has built-in sensors that monitor the eye gaze distance with optical/electrical means. 
     Furthermore, as shown in  FIG. 1B , the optical device  1  may also comprise a plurality of reservoir volumes  90 , wherein each reservoir volume  90  is in flow connection with the lens volume  91  via a separate channel  92 . Furthermore, a plunger comprising a magnetic flux guiding counter member  81  (or formed by a magnetic flux guiding counter member  81 ), is arranged in each reservoir volume  90  and connected to a stretchable wall  93  of the respective reservoir volume  90 . Particularly, each stretchable wall  93  can be formed again by a region of the membrane  21  that also covers the lens volume (main cavity)  91 . 
     Further, each counter member  81  faces an associated electropermanent magnet that is preferably embedded in said frame  10  for holding the container  2 , which frame  10  is arranged in front of the reservoir volumes  90  so as to cover them. 
     Here, particularly, the optical device  1  is configured to control each electropermanent magnet  80  independently from the other electropermanent magnets  80 . This allows the optical device  1  to generate a plurality of different curvatures of the said curvature-adjustable area  23  of the membrane  21  (and therewith a plurality of corresponding focal lengths of the first lens  100 ) even in case the respective electropermanent magnet merely moves the associated counter member  81  between two stable states corresponding to a convex state of the respective stretchable wall (membrane region)  93 , in which the respective counter member  81  is closest to the associated electropermanent magnet  80  and in which the respective reservoir volume  90  has maximal size, and a flat state (shown in  FIG. 1B ) of the respective stretchable wall/membrane region  93  corresponding to a smaller volume value of the respective reservoir volume  90 . Particularly, the reservoir volumes  90  can have different volume values (e.g. with respect to said flat state of the respective reservoir volume  90 ) so as to increase the number of different focal lengths that can be selected/adjusted by the optical device  1  when actuating each reservoir volume  90  independently. 
       FIG. 3  shows a variation of  FIG. 1  where the respective main portion of the fluid cavity/lens volume  91  and the reservoir volume  90  are not embossed into the back wall  30  as described above but are formed by an intermediate layer  95  that comprises recesses  95   a,    95   b  for forming at least a portion of the reservoir volume  90  and of the lens volume  91 , wherein a further recess  92  forms said channel connecting the reservoir volume  90  to the lens volume  91 . Particularly, said intermediary layer  95  is bonded both to the back wall  30  and to the lens shaper  22 , wherein the intermediate layer  95  acts as a spacer that defines portions of the reservoir  90 , the main cavity (lens volume)  91 , and the channel  92 . This configuration has certain advantages concerning fabrication since the different layers  30 ,  95 ,  22  can be cut out from sheets as 2-dimensional shapes. In one specific embodiment both the back wall  30  and the lens shaper  22  are made from a glass and the intermediate layer  95  is made out of a silicone-based polymer such as poly(dimethylsiloxane), also known as PDMS, or a polyester material such as PET or a biaxially-oriented polyethylene terephtalate (e.g. “Mylar”). In a different embodiment, the back wall  30 , intermediate layer  95  and lens shaper  22  are all made from glass layers that are bonded together by means of a glass-glass bonding process. 
     In yet another embodiment the back wall, intermediate layer and lens shaper are formed inside a monolythical block of photo-structurable glass. A femto-second laser locally changes the glass structure. In a subsequent etching process the exposed material is selectively etched away. Such a process would allow to create a fluid channel  22  that is completely embedded in a material (e.g. glass). 
     The embodiments depicted in  FIGS. 1 to 3  rely on the force of the EPM actuator(s)  80  to bring the membrane  21  into negative curvature and rely on the elastic force of the membrane  21  as a restoring force to bring the membrane  21 , particularly area  23 , back to a positive shape. In order to increase the dynamic of bringing the lens back to positive shape when the EPM  80  is turned off, an additional mechanical spring  96  can be arranged between the EPMs  80  and the magnetic counter members  81  as depicted in  FIG. 4 . Particularly, the spring  96  is made out of a non-magnetic material in order to avoid interference with the functioning of the EPM  80 . In a specific embodiment the spring  96  can be a bent sheet metal spring. However, any element that generates a restoring force upon mechanical compression can be used as spring structure  96 . This also includes having multiple springs, non-metallic springs, elastomer elements etc. 
       FIG. 5  shows how the previously described first lens  100  can be integrated into a frame  10  of spectacles. The reservoir volume  90  can be hidden inside an upper part of the spectacle&#39;s frame  10 . The lens contour  10   a  can be adjusted to the shape of the frame  10 . The container  2  including back wall (e.g. back glass or back lens)  30 , lens shaper  22 , and membrane  21  can be pre-fabricated and filled with fluid F. The EPMs  80  and the required driving electronics can be integrated into the frame  10  of the spectacles. The battery can be integrated into the frame  10  or into one or both of the spectacles earpieces. Having those earpieces exchangeable allows for a reserve charge in case battery levels get low during use. 
     Further, the membrane  21  can be mechanically protected by a front wall (e.g. front glass or front lens)  40 . Both front wall  40  and back wall  30  can include a spherical offset for myopia and hyperopia correction as well as provide correction of higher order aberrations such as astigmatism and gravity coma. In  FIG. 5 , for simplicity, no actuation is shown. In a specific embodiment both back wall  30  and front wall  40  can be formed by a flat glass, respectively. 
       FIG. 6  shows how several tunable lenses can be fabricated in a batch process using a substrate  98  that contains several tunable lens elements  97 . The sealed containers  2  with integrated plunger  94  can be prefabricated and afterwards carved out with the desired contour  10   a  and position of the lens shaper  22  in such a way that the lens shaper  22  is aligned with respect to the lens contour and provides a specific pupillary distance. 
       FIG. 7  illustrates the possibility to have a range of tunable lens “templates”  97  with different sizes of optical apertures as well as distances between the reservoir volume  90  and the opening  24  of the lens shaper  22 . This allows to accommodate different types of spectacle frame design and sizes. 
     In augmented reality (AR) or mixed reality applications waveguides are used to provide the user the illusion of an object located at a given distance from the eye. This can be achieved by coupling the projected image into a waveguide and coupling it out of the waveguide within the field-of view of the user. This sort of leaky waveguide has the advantage that the image is insensitive to the movement of the pupil and generally to misalignments. In other words waveguides provide a larger eye box. 
     While most AR systems use a fixed so called light-field image plane, it has proven to be problematic in terms of vergence. While different images provided to the left and to the right eye provide stereoscopic vision and thus the possibility to provide images at different image plans, the light physically originates always with the same divergence. The mismatch between the suggested light field plane and actual divergence of the light is well known as vergence-accommodation conflict and is responsible for example nausea and similar symptoms. 
       FIG. 8  shows a configuration with a first lens  100  followed by a waveguide  110  and a further first lens  100 ′ (such a configuration can e.g. be used for a single eye of a user). The waveguide as well as the two lenses  100 ,  100 ′ are transparent and allow the user to view its surrounding with an unobstructed field-of-view (FOV). 
     A light-field image is coupled into the waveguide  110  as known by somebody skilled in the art. The waveguide structure guides the light through total internal reflection and contains special out-coupling structures on the side towards the observer&#39;s eye where the light  111  is coupled out close to a collimated beam. It traverses then the further first lens  100 ′ after which a collimated or diverging light beam  111 ′ exits which is subsequently refocused inside the user&#39;s eye to form an image on the retina from an object placed at a given virtual distance. Depending on the curvature of the membrane  21 ′ the light-field image appears closer or further away from the user&#39;s eye. 
     The light coming from the surrounding needs to traverse both first lenses  100  and  100 ′ and the waveguide. In order for the user to see the surrounding without any refraction correction, the two lenses  100 ,  100 ′ need to compensate each other out. The refractive power of first lens  100  and further first lens  100 ′ are electrically adjusted in such a way to have the same refractive power but with opposite sign, i.e. the membranes  21  and  21 ′ have opposite deflection. 
       FIG. 8  shows that the EPMs  80  for both first lens  100  and further first lens  100 ′ are arranged all in the same common bar structure  101  in order to provide the most compact possible design. The minimum number of EPMs  80  per eye is  4 . In a specific embodiment the number is a multiple of two. For simplicity four EPMs  80  are depicted. The first lens  100  has magnetic counter members  81  in front of its associated two EPMs  80 . The further first lens  100 ′ also has magnetic counter members  81  in front of its two associated EPMs  80 ′. Thus, the counter members  81 ,  81  of both lenses are arranged offset. 
     Particularly, for this application, the fill-level of the respective lens  100 ,  100 ′ is chosen such that the respective membrane  21 ,  21 ′ is flat in its unactuated nominal position. 
     In a first situation a positive current pulse is sent to the EPMs  80  associated to the first lens  100  resulting in a negative curved first lens  100 . The further first lens  100 ′ is in its nominal flat position. This corresponds to the situation where the light-field is placed at infinity since the user&#39;s eye does not need to focus to see the virtual image. In a second situation a negative current pulse is sent to the EPMs  80  associated to the first lens  100  resulting in a zero magnetic force putting the first lens  100  back to its nominal flat position. Simultaneously, a positive pulse is sent to the EPMs  80  associated to the further first lens  100 ′ resulting in a negative curved further first lens  100 ′. This corresponds to the situation where the light-field is placed at a close distance since the eye needs to accommodate to short distance to create an image on the retina due to the incoming diverging beam. When switching from the first situation to the second situation the position of the light-field has shifted while the overall lens stack always keeps the same overall refracting power. 
     As mentioned previously the EPMs  80  can be controlled in an analog way by not fully magnetizing/demagnetizing the respective EPM  80  and thus arbitrary deflections of the respective membrane  21 ,  21 ′ within the possible deflection range can be achieved. This allows not only to produce the two extreme positions of the light field, but also all intermediate positions. 
     In a different embodiment the same configuration can be used to correct for small refractive errors of the users eye by adding a small offset to the first lens  100 . 
       FIG. 9  shows the full 3D view of the augmented reality/mixed reality configuration with the various EPM motors for front lens/back lens and left eye/right eye integrated into a single bar  101  or frame  101 . In this configuration, the waveguides  110 ,  210  between front and back lenses  100 ,  100 ′ (right), or  200 ,  200 ′ (left) are not being displayed. Particularly here, the first lens  100  and further first lens  100 ′ are associated to the right eye, while for the left eye a second lens  200  and a further second lens  200 ′ are provided. Particularly, the second lens is designed like the first lens  100  and the further second lens  200 ′ is designed like the further second lens. Further, the further waveguide  210  is positioned between the second and the further second lens  200 ,  200 ′ as described for the waveguide  110  in conjunction with the first lenses  100 ,  100 . 
     The embodiment shown in  FIG. 10  corresponds to a modification of the embodiment shown in  FIG. 1 . Particularly,  FIG. 10  shows the cross-section of an optical device  1  comprising at least a first (e.g. electrically) tunable lens  100 , based on a fluid F (e.g. liquid) filled reservoir volume  90  that is connected through at least one channel  92  to a single lens volume  91 . The liquid-filled reservoir volume  90  as well as the actual lens area/volume  91  are formed by a membrane  21 , a lens shaper  22  and back wall (e.g. back glass)  30 . While the main cavity (also denoted as lens volume)  91  is preferably circular or round in order to reduce optical aberrations, the at least one reservoir volume  90  has preferably an elongated non-round shape that may extend along a longitudinal axis L in order to hide inside a lens frame  10  containing an actuator component, here in form of at least one electropermanent magnet  80 . Thus, the reservoir volume  90  is advantageously not visible from the outside when looking along the optical axis A onto the first lens  100 . 
     Apart from the lens shaper  22 , at least a portion of the main cavity  91 , the (e.g. microfluidic) channel  92  and at least a portion of the reservoir volume  90  can be formed into the back wall  30  (e.g. in the form of corresponding recesses  91   a,    92 ,  90   a.  In a specific embodiment the back wall  30  is made from a glass and the cavities/recesses  90   a,    92 ,  91   a  are created by etching. In a different embodiment the back wall  30  is made from a highly transparent material that can be molded or embossed. A second optically transparent layer forms the lens shaper  22  and covers the channel  92 . It is e.g. made from a material that hermetically seals the back wall  30  and the membrane  21  and that particularly provides a very smooth and flat surface in order not to create optical aberrations. In a preferred embodiment the lens shaper layer  22  is made from a thin slab of a glass. In a further preferred embodiment the thickness of this glass slab is less than 0.5 mm. 
     The fluid (e.g. liquid) F and the lens shaper  22  and the back wall material are preferably index-matched so that the channel  92  as well as the lens volume  91  are ideally non-visible. A plunger  94  that comprises e.g. two inserts of a soft magnetic material  81  that are e.g. inserted into a non-magnetic material of the plunger  94  is placed inside the reservoir volume  90 . The two inserts  81  form counter members of associated electropermanent magnets  80  to be described below. 
     In a non-actuated state a curvature-adjustable area  23  of the membrane  21 , which area  23  is defined by an (e.g. circular) opening  24  of the lens shaper  22  that is covered by the membrane  21  (particularly congruently by said area  23 ), is either flat or positively curved, based on the amount of fluid F inside the lens. In a particular embodiment a flexible, elastic membrane  21  (e.g. of a high optical transparency) is bonded onto the upper side of the lens shaper  22  (object side). The membrane  21  can be e.g. bonded either glue-free or with highly transparent colorless glue. For pumping fluid F from the reservoir volume  90  into the lens volume  91  and vice versa, the optical device  1  further comprises at least two electropermanent magnets (EPM)  80  that are arranged outside the at least one reservoir volume  90  in front of the membrane region  93  that covers the at least one reservoir volume  91 , wherein said EPMs  80  are aligned with said counter members  81  of the plunger  94  that is arranged in the reservoir volume and connected to the stretchable wall/membrane region  93  are. In comparison to other configurations this specific configuration minimizes the thickness of the optical device  1  by placing the plunger  94  with the magnetic material  81  inside the reservoir volume  90 . When the EPM motors  80  are not actuated, the membrane  21 , depending on the fluid (e.g. liquid) fill level, is either flat or positively deflected, i.e. in a convex state. 
     Furthermore, the container  2  comprises a circumferential lateral inner side  91   b  forming a side wall of the lens volume  91 , wherein said lateral inner side  91   b  that defines the aperture of the lens  100  of the optical device is rounded instead of extending simply vertical, i.e. perpendicular to the back wall  30 . The rounded shape prevents light diffraction at sharp edges. In the event of a non-perfect index matching, the rounded shape leads to an optical gradient and makes the edge completely invisible. The rounding can be in the back wall  30 , in the lens shaper  22  or in both. Particularly, the back wall  30  can comprise a concave curvature at the inner side  91   b  and the lens shaper  22  can comprise a convex curvature at the inner side  91   b,  such that particularly said inner side comprises an inflection point. 
     In a preferred embodiment the fluid F and the immersed lens shaper  22  have the same Abbe number, thus the same dispersion properties. 
     Furthermore, according to an embodiment the fluid F has a low viscosity to increase the flow rate through the channel  92  and thus the switching speed of the lens  100 . Also, in an embodiment, the fluid F and the lens shaper  22  comprise a high-refractive index to reduce the amount of fluid F that has to be displaced for a given focus change. This allows larger clear apertures, larger optical power ranges and lower switching times. According to a preferred embodiment, the refractive index of the fluid F and/or of the lens shaper  22  is larger than 1.45, preferably larger than 1.55. 
     Furthermore, while in the embodiment shown in  FIG. 1 , the respective counter member  81  is formed out of a soft magnetic material  81 , the magnetic counter members  81  according to the embodiment shown in  FIG. 10  are replaced by permanent magnets  81  that are used in conjunction with the respective EPM  80 . 
     This allows to operate the lens in push-pull and close to double the optical power tuning range of the lens. 
     Furthermore,  FIG. 11  shows a further embodiment of an optical device  1  according to the present invention which is a modification of the embodiment shown in  FIG. 10 . The tuning speed is directly related to the flow resistance in the channel  92 . A larger channel provides lower flow resistance and thus potentially higher speed. In a preferred embodiment there are multiple channels  92  connecting the reservoir volume  90  with the main lens area (i.e. the lens volume  91 ) instead of a single large channel to provide better mechanical stability. 
     In the depicted embodiment, the electropermanent magnet  80  that typically consists of two sections or magnets, namely said first magnet (of a “hard”/high coercivity magnetic material)  82  and said second magnet  83 (of a “soft”/low coercivity magnetic material) now comprises a single member  87  of “soft”/low coercivity magnetic material. The direction of the magnetization M of the latter piece can be switched by a pulse of an electrical current in coil  84  surrounding the single magnet  87 . Also here, two pole members  85  consisting of soft magnetic material can be located on both ends of the single magnet  87 . Because the pole members  85  have a higher permeability than the air, they will concentrate the magnetic flux of the single magnet  87 . Particularly, this electropermanent magnet  80  is mechanically connected to the back wall or back lens  30 . 
     When the member  87  is magnetized and a counter member  81  of a soft magnetic material (or a permanent magnet) is placed near member  87 , the magnetic flux will flow confined in the soft magnetic material creating an attractive force. With the counter member  81  mechanically connected to the membrane/wall  93  above the reservoir volume  90 , this attractive force increases the reservoir volume  90  such that fluid F is transferred from the lens volume  91  into the reservoir volume  90 . Due to this the stretchable curvature-adjustable area  23  of the membrane  21  bulges less out and may even become flat or concave. 
     While the EPM  80  using the two magnet  82 ,  83  has the advantage that switching to zero state is easier, the above described actuator version still can achieve neutral position (no magnetization) by applying the correct electrical current pulse to the single magnet  87  by means of the surrounding coil  84 . Furthermore, it is easier to manufacture and offers the same functionality. 
     Furthermore,  FIG. 12  shows an embodiment of an optical device  1  according to the present invention which is a modification of the embodiment shown in  FIG. 10 . Particularly, the difference is that there is a mechanical spacer  99  between the plunger  94  and the membrane  21  which is smaller than a surface of the plunger  94  facing the membrane  21 . As such the free-moving non-bonded membrane area is increased, the mechanical stress in the actuator membrane  21  is reduced. In consequence the actuation force is reduced, thus reducing the required switching power or potentially allowing for larger clear apertures. 
     This embodiment furthermore includes a sensor  88  that measures the curvature or the strain in the actuator wall  93  (which can be a region of the membrane  21 ) to provide an indirect feedback on the optical power setting of the lens  100 . This could be implemented by optical or electrical means. In a preferred embodiment a strain sensor  88  is placed towards the edge onto the wall  93  of the actuator. 
       FIG. 13  shows a further embodiment of an optical device  1  according to the present invention corresponding to a modification of the embodiment shown in  FIG. 10 , wherein here, in contrast to  FIG. 10 , said wall  93  (e.g. portion of membrane  21 ) covering the reservoir volume  90  comprises a bellow structure  25 . This increases the stroke of the actuator and thus allows to reduce the horizontal footprint of the actuator reservoir volume  90 . This is especially beneficial for spectacles due to aesthetic reasons. Furthermore, the bellow structure  25  reduces the required actuation force since there is no actuator membrane that needs to be stretched. Alternatively, when keeping the same horizontal footprint of the reservoir volume  90  it allows to increase the clear aperture and/or the optical power tuning range of the lens  100 . Also, in this embodiment the respective counter member  81  preferably is a permanent magnet to operate the lens in push-pull mode. 
       FIG. 14  shows yet another embodiment of an optical device  1  according to the present invention corresponding to a modification of the actuator shown in  FIG. 13 , wherein here, there are at least two reservoirs volumes  90   b,    90  that are connected to one another and to the lens volume  91  (main lens area) by means of channels  92 ,  92   b,    92   c.  At least one of the reservoirs  90  acts as an active micro-pump using an EPM motor  80  and a plunger  94  with a permanent magnet  81  as counter member. At least one further reservoir  90   b  acts as a non-actuated passive reservoir volume  90   b.    
     In a different embodiment the EPM motor  80  is replaced by a voice-coil actuator comprising e.g. a coil and a magnet. 
     Both, the active pump reservoir volume  90  and the passive reservoir volume  90   b  are covered by a wall  93 ,  93   b,  respectively, wherein these (e.g. stretchable) walls  93 ,  93   b  can beach be a region of the membrane  21 . In a preferred embodiment the wall  93  and/or the wall  93   b  comprises a bellows structure  25 ,  25   b.  In a preferred embodiment all reservoirs  93 ,  93   b  are fitted with a bellow structure  25 ,  25   b.    
     Furthermore, each of said channels  92 ,  92   b,    92   c  comprises a valve  89 ,  89   b,    89   c  that allows to control the liquid F flow electronically. An external electronics driver is preferably comprised by the optical device  1  that synchronizes the EPM movement together with the three valves  89 ,  89   b,    89   c.    
     One mode of operation is to pump liquid F from the main area/lens volume  91  to the passive reservoir volume  90   b.  The operation of the actuator and the valves  89 ,  89   b ,  89   c  is known by somebody skilled in the art. Once there is an overpressure of liquid F in the passive reservoir volume  90   b  it can be quickly released by opening the valve  89   c  between the passive reservoir  90   b  and the lens volume  91 . This could benefit a quicker response time.