FOCUS LENS

5 Various types of variable focus lenses are known. One particularly successful type is

fluid filled lenses in which a fluid filled cavity includes a deformable membrane such that the shape of the lens can be altered by adjusting the volume of fluid within the lens to deform the membrane.

10 One problem with fluid filled lenses is that it is difficult to make lenses of a non-round

shape. If a non-round frame member is used, the membrane attached to the frame member does not stretch uniformly when the volume of fluid in the lens is increased or decreased. This non-uniform stretch introduces cylindrical and other components to the lens which hinder its usefulness for correcting vision. In other words, it is

15 possible to make a non-round fluid-filled variable focus membrane lens with a

uniform thickness membrane, but such a lens will be astigmatic, and the astigmatism will vary as the power changes. One way of reducing the astigmatism in such a lens is to make the membrane non-uniform in thickness.

20 One method of using non-round frames with fluid filled lenses is described in US

201 1/0085243, the contents of which are incorporated herein in their entirety by

reference. The non-round membrane is formed with a carefully prescribed thickness profile, for example with a circular or elliptical disc at the centre so that when the volume of fluid changes and the membrane stretches, the central disc is able to

25 stretch to a certain degree in isolation with respect to the rest of the non-round

membrane. With this central disc placed in front of the pupil in an eyeglass lens, an essentially spherical correction can be provided to the user for most incident light.

The great advantage to this arrangement is that it allows a far greater flexibility in the design of the eyeglass frames, including non-round frame members. This facilitates

30 both the manufacture of visually appealing frames and also allows wider frames to be

manufactured without the unnecessary increase in weight that would be required if circular lenses were to be used. The quantity of fluid can be reduced which in turn reduces the weight of the lens.

35 However the cost to manufacture these lenses is increased by the need to create the

complex thickness profile of the membrane. Currently, the membranes are molded so as to provide the desired profile. To provide suitable optical quality on the molded surfaces of the membrane a very high quality mold is required, typically costing tens of thousands of Euros for example. A new mold is required every time a new lens shape is designed. Therefore the cost associated with the membrane manufacture is high.

Cost is an important factor in fluid filled variable focus lenses due to the fact that one of the major markets for such lenses is in the developing world where vision correction is still not widely available. The cost of eyeglasses is one of the biggest barriers to bringing vision correction to the developing world and fluid filled lenses can be made cheaply and in bulk, while each pair is capable of being adjusted to the appropriate prescription. This is a major advantage compared with the cost of providing traditional ground plastic lenses which either have to be ground to order or provided in a large number of different predetermined powers. According to one aspect of the invention there is provided a method of making a flexible membrane for a variable focus lens, the method comprising: shaping a first surface of the membrane using a process with non-optical quality.

Normally all surfaces of a lens would have to be shaped with a suitable optical quality process so as to ensure the desired optical characteristics of the lens, for example ensuring sufficient transmission of light and sufficiently low distortion in images produced by the lens. Such processes typically have to be highly accurate and are more expensive than lower quality processes. Using a non-optical quality process (e.g. one with lower spatial resolution) to shape one surface of the membrane is possible with fluid filled lenses because in the assembled lens, the non-optical quality surface can be used on the inside of the fluid chamber and the fluid can be selected so that its refractive index matches that of the membrane. The fluid flows to match the irregularities in the non-optical quality surface of the membrane and because the refractive indices on both sides of the interface are substantially equal, the interface between the fluid and the membrane will not then adversely affect the optical characteristics of the lens by causing distortion through refraction. Therefore, according to this invention a fluid-filled lens membrane can be shaped to a desired profile by a less expensive process, thus reducing the manufacturing cost of the membrane, the fluid-filled lenses in which the membrane is used and any optical apparatus using the fluid-filled lenses (such as a pair of spectacles). One of the main purposes foreseen for this invention is in the manufacture of non- round fluid-filled lenses e.g. using a process similar to that described in

US 2011/0085243, i.e. by shaping the membrane to form a section which can deform under pressure in a different manner to the rest of the membrane. However, other uses of the technology may also exist, e.g. for making lenses with different regions which deform differently under pressure, thus creating different optical effects. The invention could be used in both round and non-round lenses for the correction of astigmatisms or other refractive errors. However, in preferred embodiments, the membrane has a non-round shape. Such a membrane is designed for use in a non-round lens such as is commonly used in pairs of spectacles. Such lenses are typically wider in one direction so as to extend across a person's face while limiting the vertical extent (horizontal and vertical here being in reference to the vertical direction when in normal use on a person standing upright). Fluid-filled lenses benefit from such shapes as the volume of fluid required to fill a non-round lens is reduced when compared with a circular lens having the same horizontal dimension.

The flexibility in shape of the membrane allows inexpensive, yet still fashionable frames to be made with fluid filled lenses. In some preferred embodiments, a whole frame front could be made in one piece, with membranes bonded to the rear surface.

Nose pads and legs can then be attached to make inexpensive, fashionable glasses.

In particular, the use of an inexpensive manufacturing process permits a greater variety of frame shapes to be produced without incurring excessive expense. It is quite important in attractive / fashionable eyewear that not all pairs look the same, allowing the wearer to select an appropriate style or character and to have a certain individuality. This can facilitate widespread acceptance of eyewear and

consequently promote vision correction. The step of shaping may comprise forming a substantially circular or elliptical sub- region of different thickness to the rest of the membrane. This difference in thickness is what allows one region of the membrane to deform differently to the other region or regions. A circular sub-region can be used to define a region which deforms in a substantially spherical manner, similar to spherical lenses typically used for vision correction. A slightly elliptical sub-region can be used to counteract other forces which affect the lens such as gravity or other tensions introduced in the membrane, or to introduce cylindrical and other components into the deformed lens, e.g. to correct astigmatisms.

Preferably the sub-region is of a reduced thickness compared to the rest of the membrane. Such a reduced thickness region may be introduced either by cutting away material from a membrane preform or by depositing material to build up thickness either side of the reduced thickness region.

The membrane may be formed by starting from a preform and etching or otherwise removing material from the preform so as to form the desired shape. The preform may be a flat sheet of material. Such sheets are typically commercially available at low cost. The sheet preferably has at least one optical quality surface which can be used as the external surface of the membrane once used to form a fluid-filled lens.

This surface forms an air-membrane interface and thus needs to be of suitable optical quality in order to avoid significant distortions through refraction. Large sheets of plastic material can readily be formed with flat surfaces of suitable optical quality by standard low cost techniques.

As an alternative to etching, the membrane may be formed by an additive

manufacturing technique in which material is deposited, e.g. in successive layers to build up the required profile. Such techniques include various types of 3D printing and rapid prototyping such as fused deposition modeling or selective laser sintering (as non-limiting examples). The additive manufacturing technique can be used either to form the whole membrane or it can be used starting from a membrane preform to build up additional material. In the latter case, the newly added material is preferably the same as the material of the preform, or at least has the same refractive index. Preferably therefore the method comprises depositing material to build up the required shape. In other preferred embodiments the method comprises providing a membrane preform with at least one optical quality surface and the shaping step comprises depositing additional material onto the opposite surface of said

membrane. Preferably the additional material has the same refractive index as that of the membrane.

According to a further aspect, the invention provides a method of making a mold for molding a membrane for a variable focus lens, the method comprising: forming one surface of the mold to have optical quality; and shaping the opposite surface of the mold using a process with non-optical quality. According to yet a further aspect, the invention provides a method of making a variable focus lens comprising: forming a fluid filled cavity, wherein one wall of the cavity is a membrane as described above (with or without the described preferred features); providing a fluid reservoir with a variable volume in fluid communication with the cavity so that adjustment of the reservoir volume adjusts the volume of the cavity; and wherein the refractive index of the fluid is matched to the refractive index of the membrane. The invention also provides a membrane for a variable focus lens comprising: a first surface of optical quality; and a second surface with at least one area of non-optical quality; wherein the membrane has a substantially circular or elliptical sub-region of different thickness. The membrane may have a non-round shape. The sub-region of the membrane may be of reduced thickness.

The area of non-optical quality may be the sub-region of different thickness. As examples, this structure may be formed from a molding process where one whole surface of the membrane is of non-optical quality, or by a process in which deposition has taken place across the whole of one surface by a non-optical quality process, or by etching an original surface of the membrane with a non-optical quality process.

In other embodiments, the circular or elliptical sub-region of different thickness may be substantially flat. As an example, this structure may be formed by a non-optical quality deposition process that deposits additional material onto an originally flat membrane surface outside the sub-region. In such a process, where no deposition is performed within the sub-region, the original flat (and optical quality) surface is retained within the sub-region.

The invention extends to a variable focus lens comprising: a fluid filled cavity, wherein one wall of the cavity is a membrane as described above; a fluid reservoir with a variable volume in fluid communication with the cavity so that adjustment of the reservoir volume adjusts the volume of the cavity; wherein the refractive index of the fluid is matched to the refractive index of the membrane. The invention further extends to eyeglasses comprising two variable focus lenses as described above, connected by a nose piece. The nose piece may be a separate piece connecting two separate lens pieces. Alternatively, the nose piece may be integrally formed with one or both lenses, e.g. both lenses may be formed in a single frame member. Nose pads may optionally be added if they are not also integrally formed with the frame.

The ability to manufacture inexpensive, yet still fashionable eyeglasses is also particularly beneficial in combination with head mounted displays, such as Google Glass (and other similar products). It is not straightforward to attach such devices directly to existing vision correction eyewear, particularly with the large variety of different frames in use. Corrective eyewear is typically designed with general vision correction in mind, while head mounted display units are designed for general use, e.g. by users who require no vision correction. With adjustable lenses, an

attachment mechanism can be designed into the frame, e.g. in the region of the leg- attachment point. This may in some embodiments correspond to an area where fluid enters and exits the lens during the adjustment process as this may be a slightly thicker region of the frame. The adjustable eyewear is suitable for use by users who require vision correction as well as those who do not (as they can simply be set to a piano, zero Dioptre setting). Moreover, the adjustable lenses allow the user to set their own correction, thus avoiding the need to have separate prescription lenses prepared by an optician. A single product can then be manufactured and sold with applicability to a large number of users with a wide range of vision correction requirements. By far the majority of vision impairments can be adequately corrected with a simple spherical correction, i.e. without also requiring astigmatism correction. Moreover, as the user can adjust the lenses themselves, the correct lens power can be set without requiring a visit to an optician.

Preferably therefore the eyeglasses may further comprise a head mounted display unit. The eyeglasses may comprise a mount point for a head mounted display unit and the eyeglasses may further comprise a head mounted display unit attached to the mount point. The head mounted display unit may comprise a display or projector, a processor, memory and a battery amongst other components. In this document, references to optical quality surfaces mean surfaces sufficiently smooth to be used in ordinary vision correction eyeglasses without impairing the maximum visual acuity achievable. Inexpensive optical quality lenses have a surface roughness (R _a ) typically around 1-2 wavelengths, i.e. for visible light, typically around 1 micrometre or less. A typical optical quality membrane that has been used for variable focus fluid-filled lenses is Mylar™ (or more generally biaxially stretched PET) which is quoted as having a surface roughness (R _a ) of 38 nm.

The non-optical quality (or low optical quality) surfaces that can be used in the present invention may have a surface roughness of greater than 10 wavelengths, or in some embodiments greater than 20, greater than 30 or greater than 50

wavelengths. In some embodiments the surface roughness may even be much greater, e.g. greater than 100 wavelengths. A typical wavelength for the visual spectrum is about 550 nm.

The non-optical quality (or low optical quality) surfaces that can be used in the present invention may have a surface roughness of greater than 5 microns, greater than 10, greater than 20 or greater than 30 microns. In some embodiments the surface roughness may even be much greater, e.g. greater than 50 or 100 microns.

At the present time, high-end 3D printers that could be used for printing membranes or making molds for membranes have a layer thickness (z-axis) of about 16 microns with a horizontal (x, y-axes) resolution of more like 50-100 microns. Less expensive machines have a layer thickness (z-axis) more like 100 microns and are also suitable for the present invention.

Membranes according to the invention preferably have one relatively smooth surface and one relatively rough surface. In other words, the rough surface is rough in comparison with the smooth surface. As described above, the rough surface may have smooth areas and rough areas (the rough areas being for example additive depositions by a low resolution process or regions etched by a low resolution process. The smooth surface has a higher optical quality than the rough surface. The smooth (or relatively smooth) surface may have a surface roughness (R _a ) of no more than 5 microns, preferably no more than about 2 microns.

Preferred embodiments of the invention will now be described by way of example only, and with reference to the accompanying drawings in which:

Figs. 1a and 1 b shows an example of a membrane for a non-round lens in plan and cross-section; Fig. 2 shows an example of a non-round fluid filled lens;

Fig. 3 shows an example of a non-round fluid filled lens connected to a variable fluid reservoir;

Figs. 4, 5, 6 and 7 illustrate different methods of forming a membrane for a non-round fluid filled lens; and

Fig. 8 shows a pair of variable focus eyeglasses with a head mounted display unit.

Figs. 1a and 1 b show a membrane 10 having a first side 12 and a second, opposite side 14. Fig. 1 a shows the membrane 10 in plan view. Fig. 1 b shows the membrane 10 in cross-section taken through the line B-B. The first side 12 is flat and of good optical quality. The second side 14 has been profiled so as to create a thinner region 16. This thinner region 16 is approximately circular as shown in Fig. 1a and is intended to deform in preference to the rest of the membrane 10, thus allowing the non-round membrane 10 to produce a substantially spherical lens shape in region 16.

In one example, the first side 12 is of good optical quality (e.g. with a surface roughness (Ra) of less than 2 microns but the second side 14, which has been formed by 3D printing or a similar technique has a thinned portion 16 and is of poor optical quality. If the new membrane forms part of a fluid-filled variable focus lens but the side of poor optical quality 14 is in contact with the fluid in the lens, and the refractive index of the membrane material and the fluid are the same, then the optical performance of the fluid-filled lens will not be degraded by the poor optical quality of the surface 14.

Such a membrane may be made at very low cost and the use of 3D printing mean that many different shapes and sizes of membrane may easily be made, so this approach will very significantly reduce both the cost of manufacture, and the total time needed (i.e. from beginning tooling to having final parts) to manufacture such membranes, when compared with the more traditional plastic moulding methods. This approach may therefore be expected to greatly reduce the cost of eyeglasses with non-round fluid-filled variable power lenses, and also to reduce the cost, time and difficulty of prototyping different examples of such eyeglasses. Fig. 2 shows the membrane 10 in use in a variable focus lens 20. Variable focus lens 20 has a rigid non-round support ring 21 with a rigid front plate 22 attached to the front side thereof. The membrane 10 is attached to a rear side of the ring 21. The shaped side 14 of the membrane 10 faces in, i.e. it is on the inside of the fluid filled cavity 23 formed by the ring 21 , the membrane 10 and the front plate 22. The smooth side 12 of the membrane 10 faces out away from the fluid filled cavity 23. The fluid 23 has a refractive index that matches the refractive index of the membrane 10. This ensures that no refraction occurs at the boundary between the membrane 10 and the fluid 23. Therefore light passing through variable focus lens 20 is not influenced by the shape of the membrane/fluid boundary in the thin region 16 of the membrane 10. Thus the rough nature of the surface 14 does not matter.

It will be appreciated that the rigid plate 22 could in other embodiments be a back plate with a membrane used as the front component of the variable focus lens 20. However, it is generally preferred to have the rigid plate 22 at the front to protect against damage. In use, the membrane then faces the user's eye. There is little risk of damage from this direction. In yet other embodiments, both the front and rear components of the cavity could be flexible membranes like the membrane 10 of Figs. 1 a and 1 b. Either or both such membranes 10 could be shaped. If only one membrane is shaped, the other can be a simple flat sheet. In yet further

embodiments, a rigid protective cover can be placed over the membrane, protecting it from damage, while still allowing sufficient movement for the membrane to deform, thus changing the power of the variable focus lens. Two membranes and two rigid covers can of course be used if desired.

Fig. 3 shows a variable focus lens 20 with a variable volume reservoir 30 of fluid. In Fig. 3, the reservoir 30 is shown as a syringe body 31 with a plunger 32. It will be appreciated that this is merely illustrative and any other form of variable volume reservoir can be used. Alternative controls for adjusting the volume of reservoir 30 include hand-operated screws or dials, or motor actuated plungers. Again these are merely examples and should not be construed as limiting on the scope of the invention. Optionally, a seal or valve can be used to hold the power of the lens once set so that the reservoir can be removed (temporarily or permanently).

The variable volume reservoir 30 is connected to the fluid filled cavity 23 by a flexible tube 33 or other suitable fluid carrying conduit. A bore 34 through rigid support ring 21 allows fluid communication between the cavity 23 and the reservoir 30. Four example processes for forming the membrane 10 will now be described with reference to Figs. 4, 5, 6 and 7.

Example 1 - Etching

Fig. 4 shows a membrane preform 10a being laser etched by laser 40. Membrane preform 10a starts out having two flat (optical quality) surfaces 12a, 14a. An example of a suitable membrane preform 10a is a flexible PET sheet, e.g. a biaxially stretched PET sheet. The laser etching process does not have to etch the surface to optical quality, but can etch a rough profile in thin region 16. Therefore the etching process does not need to be an expensive or high quality process with a high spatial resolution.

Example 2 - 3D printing onto a preform

Fig. 5 shows a membrane preform 10a (e.g. the same as the preform of Example 1) onto which layers of additional material 50 are deposited by nozzle 51 of a printing machine (not shown). In some preferred embodiments, the deposited material is ideally the same as the material of the membrane preform, or is different but has the same refractive index as the preform material. The spatial resolution of the printing machine need not be able to print an optical quality surface. Instead, rough surface 52 can remain as the surface that forms an interior surface of the cavity of a variable focus lens. No smoothing or polishing of this surface is normally required after deposition.

Example 3 - 3D printing the whole membrane

Fig. 6 is similar to Fig. 5 except that instead of depositing additional material onto a preform, the entire membrane is deposited by nozzle 61. The layers of deposited material 60 still produce a flat, optical quality surface on the lower (external) surface 62 if deposited onto a suitably flat substrate 63.

Example 4 - Molding the membrane

Fig. 7 shows a mold comprising a first mold half 70 and a second mold half 71. The first mold half 70 is flat and of optical quality with a low surface roughness. The flat mold half is easy and inexpensive to produce. The second mold half 71 contains the detailed surface shaping for the membrane, but the mold half 71 is formed by a low resolution process, i.e. one of non-optical quality and producing a relatively high surface roughness. The second mold half 71 may be formed by any suitable process such as by etching or 3D printing. As the second mold half 71 can be formed by a lower quality process, it can be formed inexpensively, thus allowing more frequent changes to the membrane profile without incurring excessive cost.

Fig. 8 shows a pair of eyeglasses 80 with a first adaptive lens (adjustable fluid-filled lens) 81 and a second adaptive lens 82 connected by a bridge / nose piece 83 and with legs 84 and 85. A head up display unit 86 is mounted to the frame of the glasses 80 in front of adjustable lens 82 where it displays images viewable to the wearer of eyeglasses 80. A microprocessor 87, memory 88 and a battery 89 are mounted on the leg 85 to power and control the head up display unit 86. Other inputs (not shown) such as buttons, switches, touch sensitive interfaces, etc. may also be provided on the eyeglasses 80 as part of the head up display system.