Adjustable fluid-filled lens assembly and method for assembling the same

Abstract

A method of assembling an adjustable fluid-filled lens assembly comprising biaxially tensioning an elastomeric membrane to a surface tension of greater than 180 N/m, typically greater than 1000 N/m; thermally conditioning the tensioned membrane, e.g., for one hour at a temperature of about 80° C., to accelerate relaxation of the membrane; mounting the membrane to a peripheral support structure whilst maintaining the tension in the membrane; assembling the mounted membrane with one or more other components to form an enclosure with the membrane forming one wall of the enclosure; and thereafter filling the enclosure with a fluid. The membrane may be formed from an aromatic polyurethane, and the fluid may be a phenylated siloxane. In some embodiments, the membrane is able to hold a substantially constant surface tension of at least 180 N/m for a period of at least 12 months.

Claims

1. An adjustable fluid-filled lens assembly comprising an enclosure, one wall of which is formed by a thermally conditioned and bi-axially tensioned thermoplastic polyurethane membrane having a thickness in the range 100 to 300 μm and an elastic modulus in the range 10 to 200 MPa which is mounted to a peripheral support structure, and which is filled with a fluid; wherein the membrane is saturated with said fluid, is coated on its outer face with a barrier layer to said fluid, and the membrane holds a substantially constant surface tension of at least 180 N/m.

2. The adjustable fluid-filled lens assembly of claim 1, wherein the membrane absorbs up to about 20% fluid by weight of the membrane.

3. The adjustable fluid-filled lens assembly of claim 1, wherein the membrane holds a substantially constant surface tension of at least 180 N/m for a period of at least 12 months.

4. The adjustable fluid-filled lens assembly of claim 1, wherein the membrane is non-circular.

5. The adjustable fluid-filled lens assembly of claim 1, wherein the enclosure is compressible.

6. The adjustable fluid-filled lens assembly of claim 5, wherein the peripheral support structure for the membrane comprises one or more bendable rings that are arranged to hold the membrane around its edge.

7. A pair of spectacles comprising at least one adjustable fluid-filled lens assembly of claim 1.

8. An adjustable fluid-filled lens assembly comprising an enclosure, one wall of which is formed by a thermally conditioned and bi-axially tensioned thermoplastic polyurethane membrane having a thickness in the range 100 to 300 μm and an elastic modulus in the range 10 to 200 MPa which is mounted to a peripheral support structure, and which is filled with a fluid; wherein the membrane is coated on its inner face with a barrier layer to said fluid, and the membrane holds a substantially constant surface tension of at least 180 N/m.

9. The adjustable fluid-filled lens assembly of claim 8, wherein the membrane is free of said fluid.

10. The adjustable fluid-filled lens assembly of claim 8, wherein the membrane holds a substantially constant surface tension of at least 180 N/m for a period of at least 12 months.

11. The adjustable fluid-filled lens assembly of claim 8, wherein the membrane is non-circular.

12. The adjustable fluid-filled lens assembly of claim 8, wherein the enclosure is compressible.

13. The adjustable fluid-filled lens assembly of claim 12, wherein the peripheral support structure for the membrane comprises one or more bendable rings that are arranged to hold the membrane around its edge.

14. A pair of spectacles comprising at least one adjustable fluid-filled lens assembly of claim 8.

Description

(1) Following is a description by way of example only of embodiments of the present invention.

(2) FIG. 1 is a schematic perspective view of an adjustable fluid-filled lens assembly according to the present invention.

(3) FIG. 2 is a schematic perspective view of the adjustable fluid-filled lens assembly of FIG. 1 shown in cross-section.

(4) FIG. 3 is a schematic exploded view showing in cross-section the components of the lens assembly of FIG. 1.

(5) FIG. 4 is a schematic perspective view of a thin sheet of viscoelastic material mounted on a circular clamp and a press for stretching the sheet, shown in cross-section.

(6) FIGS. 5A-5L show a sequence of steps for assembling an adjustable fluid-filled lens assembly with a biaxially tensioned membrane in accordance with the present invention.

(7) FIGS. 6A, 6B and 6C are illustrative graphs based on empirical data showing respectively the change in thickness (in μm), tension (in N/m) and stress (in MPa) of the membrane during the principal manufacturing steps.

(8) FIG. 7 is a scattergram of the measured line tensions with time for sixty-six individual polyurethane membranes that have been tensioned and thermally conditioned in accordance with the present invention and held in continuous contact with a body of silicone oil.

(9) FIGS. 1, 2 and 3 show schematically an adjustable fluid-filled lens assembly 10 of the kind known in the art. The lens assembly 10 of FIGS. 1 to 3 is of the “compression type” referred to above, in that it comprises a body of incompressible fluid 60 of fixed volume, and the focal power of the lens is controlled by compression of the assembly 10 in the manner described below to redistribute the fluid behind a thin, elastic membrane 12 membrane to cause the membrane to distend or contract thereby changing its curvature. The present invention is equally applicable to adjustable fluid-filled lens assemblies of the “fluid injection type”, which also comprise a similar membrane.

(10) Only the parts of the assembly that are directly relevant to the present invention are shown in the interests of brevity. Additional features, such for example as the control mechanism for selectively controlling the refractive power of the assembly 10 are briefly mentioned below, but are omitted from the drawings.

(11) As shown in FIGS. 2 and 3, the membrane 12 has an outer front face 14 and an inner rear face 16 and is stretched and mounted between a front ring 18 and a rear ring 20, which serve as a peripheral support structure for the membrane 12, holding the membrane 12 under tension around its edge as described in more detail below.

(12) The membrane 12 comprises a sheet of a thermoplastic polyurethane. In the present embodiment, the membrane comprises a sheet cf a polyether polyurethane formed from diphenylmethane-4,4′-diisocyanate (MDI), polytetramethylene glycol and 1,4-butanediol having a Shore A hardness of about 86, a density of about 1.12 g/cm.sup.3, a tensile strength of about 33 MPa and a tear strength of about 105 N/mm. This material is commercially available from BASF under the trade mark Elastollan® 1185A 10. The sheet has an initial thickness of about 380 μm, but in the finished assembly has a thickness of about 220 μm, as described in more detail below. Other grades of thermoplastic polyurethanes may be used; for instance, a polyether polyurethane in which the relative proportions (stoichiometry) of the isocyanate, polyol and chain extender components are varied to afford a different a Shore hardness. Alternatively, the membrane may comprise a polyether polyurethane made from a different isocyanate, polyol and/or chain extender. More generally, the membrane may be formed from any suitable thermoplastic polyurethane material or a different viscoelastic polymer material, provided it is optically clear, has a glass transition temperature below the usual operating range of the lens, typically below about −5° C., an elastic modulus in the range 10-200 MPa, is inert and non-toxic, exhibits low microbial growth and is capable of being bonded to the rings 18, 20.

(13) In the present embodiment the outer face 14 of the membrane 12 is coated with a protective layer of a barrier material (not shown) for the purpose described below. Any suitable hydrophobic coating material may be used, e.g. a fluorinated polymer. The coating material should be capable of adhering well to the membrane 12. It should be non-yellowing, and the barrier layer should be as thin as possible. In some embodiments, the barrier layer may have a thickness of about 10 nm, but those skilled in the art will appreciate that the thickness may be varied according to the nature of the coating material used and the desired properties of the lens assembly 10. In one embodiment, a fluorine based polymeric homologue of PTFE that is commercially available from Canon Optron, Inc. under the trade mark OF 210 is used.

(14) In another embodiment, the barrier material comprises a layer of cross-linked polyurethane acrylate, which may optionally include a nanoparticulate silica filler as described in more detail below. In this other embodiment, the layer has a thickness of about 1 μm, but again this may be varied according to the nature of the coating material and the desired properties of the lens assembly 10. Thus, in alternative embodiments, the barrier layer comprising a silica-filled acrylic-modified polyurethane may have a thickness in the range 0.5-1.5 μm.

(15) The membrane 12 is shaped and dimensioned as a lens, with the outer surface 14 of the membrane 12 serving as an optical surface of the lens. The membrane 12 can be any shape as desired. In some embodiments, the lens 10 may be used in a pair of spectacles, in which case the membrane 12 will be suitably shaped and dimensioned for that application. For instance, the membrane 12 may be circular, or it may be generally oval or rectangular. Numerous different lens shapes for spectacles are known in the art. In the present embodiment, the membrane 12 is generally rectangular, having rounded corners. Only about half of the assembly 10 is shown in FIGS. 2 and 3.

(16) In embodiments, such as the present embodiment illustrated in FIGS. 1 to 3, in which the membrane is non-round, the rings 18, 20 should be bendable out of the plane of the membrane 12, as described in WO 2013/144533, the contents of which are incorporated herein by reference, to cause or allow the membrane to distend or contract spherically in use, or in accordance with another Zernike polynomial of the kind typically prescribed for ophthalmic use. In the present embodiment, the rings 18, 20 are fabricated from a sheet of stainless steel; the front ring 18 has a thickness of about 0.25 mm and the rear ring 20 a thickness of about 0.15 mm. In embodiments in which the membrane 12 is circular, the rings 18, 20 are not required to be bendable; the membrane may be held by a rigid peripheral support structure, which is more convenient for holding the membrane 12 under tension.

(17) The membrane 12 is glued between the front and rear rings 18, 20. Suitable adhesives are known to those skilled in the art such, for example, as light curable adhesives. In the present embodiment, Delo® MF643 UV curing epoxy adhesive is used.

(18) The rear ring 20 is glued to a peripheral lip 22 of a dish-shaped receptacle 24. The same adhesive may be used as for attaching the rings 18, 20 to the membrane 12. The dish-shaped receptacle 24 comprises a rear wall 26 having a shape that corresponds to the shape of the membrane 12 and a peripheral side wall 28 that extends forwardly of the rear wall and terminates in said peripheral lip 22. The dish-shaped receptacle is made of a flexible, transparent thermoplastic polyurethane such, for example, as Tuftane® (available from Messrs. Permali Gloucester Ltd, Gloucester, UK) and is about 50 μm thick; other similar transparent materials may be used, e.g., DuPont® boPET (biaxially-oriented polyethylene terephthalate) and the thickness adjusted accordingly.

(19) In some embodiments, the assembly 10 may comprise an annular support disc (not shown) of the kind described in WO 2013/143630, the contents of which are incorporated herein by reference, that is interposed between the rear ring 20 and the lip 22 for reinforcing the rings 18, 20 against unwanted “in-plane” buckling under the tension in the membrane 12. In the other embodiment mentioned above, in which the membrane 12 is coated with a layer of silica-filled cross-linked polyurethane acrylate, the annular supporting disc may be omitted.

(20) The rear wall 26 of the dish-shaped receptacle 24 has a rear face 30 (see FIG. 3) that is bonded to a planar front face 32 of a rear lens 34 of fixed refractive power. The rear lens 34 is a meniscus lens having a concave opposite rear face 36. The rear face of the dish-shaped receptacle 24 is bonded contiguously to the front face 32 of the rear lens 34 by means of a transparent pressure-sensitive adhesive (PSA) such, for example, as 3M® 821 1 adhesive. In the present embodiment, a layer of PSA about 25 μm thickness is used, but this may be varied as required.

(21) The dish-shaped receptacle 24, rear ring 20 and membrane 12 thus form a sealed enclosure 54. The enclosure 54 is filled with an incompressible fluid 60 through a fill-port (not shown) let into the side wall 28 of the dish-shaped receptacle 24. In the present embodiment, the fluid is pentaphenyltrimethyltrisiloxane, which is a phenylated siloxane, but other suitable silicone oils and other fluids are available to those skilled in the art. The fluid should be colourless, with a high refractive index of at least 1.45 or 1.5. In the present embodiment, the fluid has a refractive index of about 1.58±0.02; it should have low toxicity and low volatility; it should be inert and exhibit no phase change above about −10° C. or below about 100° C. The fluid should be stable at high temperatures and exhibit low microbial growth. Generally the fluid has a density of about 1 g/cm3. As described in detail below, the enclosure 54 is filled with the fluid 60 under vacuum to ensure no air is present. Further, the enclosure 54 may be over-filled to distend the membrane 12 slightly to ensure the enclosure 54 is filled completely with the fluid 60, such that the fluid contacts whole of the inner face 16 of the membrane 12 continuously, with no gaps between the membrane 12 and the fluid 60.

(22) The filled enclosure 54 is compressible owing to the flexibility of the side wall 28 of the dish-shaped receptacle 24 and the elasticity of the membrane 12. Compressing the enclosure against the rear lens 34 causes the side wall 28 of the dish-shaped receptacle 24 to buckle, which in turn causes the membrane to distend outwardly to accommodate the incompressible fluid 50, thereby changing the curvature of the membrane as disclosed, for example, in WO 2013/144533.

(23) The rear lens 34, dish-shaped receptacle 24, rings 18, 20 and membrane 12 are accommodated within a housing 40 comprising a front retainer 48 and a rear retainer 46 that are made of metal and glued together at 47 to form an internal recess in which the rear lens 34, dish-shaped receptacle 24, rings 18, 20 and membrane 12 are received. The rear retainer 46 has a circumferential side wall 43 having an inner surface 44 that is formed with an intermediate step 42. The rear lens 34 is glued to the inner surface 44 towards a rear end of the rear retainer 46, such that the front face 32 of the rear lens 34 is level with said step 42, where the inner surface 44 of the side wall 43 is stepped outwardly to provide a clearance between the side wall 28 of the dish-shaped receptacle 24 and the inner surface 44 forwards of the step 42 to accommodate the side wall 28 as it buckles in use, as well as parts of a control mechanism (omitted from the drawings for simplicity) for selectively compressing the filled enclosure 54 against the rear lens 34 in the manner described above.

(24) The front retainer has a turned-in front rim 50 that is spaced forwardly of the rings 18, 20 and membrane 12 to allow the membrane to distend forwardly in use.

(25) Depending on the shape of the membrane 12, the rings 18, 20 may be hinged to the housing 40 at one or more hinge points as disclosed in WO 2013/144533 or WO 2013/144592, the contents of which are incorporated herein by reference. The control mechanism may include one or more actuators that are mounted to the housing 40 in engagement with the rings 18, 20 (or parts attached to the rings) at predetermined control points around the rings 18, 20 to move the rings towards or away from the rear lens 34 at the control points, as disclosed in WO 2013/144592 or WO 2015/044260, the contents of which are incorporated herein by reference. In this way, the assembly may be selectively actuated to cause the membrane to distend outwardly or contract inwardly in relation to the enclosure to control the curvature of the outer face 14 of the membrane 12.

(26) The assembly 10 thus forms a compound lens with a number of internal and external optical surfaces. The total refractive power of the assembly 10 is determined by the curvature of the rear surface 36 of the fixed rear lens 24 and the curvature of the outer face 14 of the membrane 12. Preferably the materials for the membrane 12, the dish-shaped component 24 and fluid 60 are selected to have as closely as possible the same refractive index, so that the interfaces between the membrane 12 and the fluid 60, and between the fluid 60 and the rear wall 26 of the dish-shaped component are almost invisible to the eye when viewed through the assembly 10.

(27) The membrane 12 is held under tension to stabilise it against deformation. An untensioned or inadequately tensioned membrane would be susceptible to external vibrations, to inertial effects when subjected to acceleration or deceleration in use, and to external forces such as gravity. When used in a pair of eyeglasses, for example, the membrane 12 is subject to continual movement and is worn in in a generally upright orientation which gives rise to a hydrostatic pressure gradient in the fluid 60. In order to minimise distortion of the optical surface provided by the membrane 12, and any consequential optical aberration, it is necessary to hold the membrane 12 under tension between the front and rear rings 18, 20. In accordance with the present invention the membrane 12 is held at a surface tension of at least about 180 N/m, preferably at least 200 N/m.

(28) Further, as mentioned above, the surface tension in the membrane 12 should be stable enough over the working life of the assembly 10 and environmental conditions to provide a substantially constant load in the balance of forces between the tension in the membrane 12 on the one hand and the beam bending reaction force of the rings 18, 20, the pressure of the fluid 60, the force at the control points and/or hinge points mentioned above and any parasitic forces (such as from the receptacle 24, or friction).

(29) FIGS. 4 and 5 illustrate schematically a method in accordance with the present invention for pre-tensioning the membrane 12 to a tension of at least 180 N/m, conditioning the membrane 12 such that it holds this load stably for an extended period of time and assembling the assembly 10 incorporating the pre-tensioned membrane 12. In some embodiments using the method of the invention, the membrane 12 may hold a substantially constant surface tension of at least 180 N/m for a period of at least 12 months.

(30) With reference to FIG. 5A, a sheet 112 of polyether polyurethane Elastollan® 118510A as mentioned above, having a sheet thickness of about 380 μm is held in a circular clamp 114 to define a circular area of the sheet within the clamp. The clamp is fixedly secured by a jig (not shown) directly beneath a selectively operable press 101 with the sheet 112 arranged horizontally. The press 101 is releasably fitted with an annular inner carrier ring 102 having a cylindrical outer surface 103 that is formed with an intermediate, circumferential rib 111 (best seen in FIGS. 5A-C; omitted from FIG. 4 for clarity) and carries a first PTFE O-ring 104 at its lower extremity. The outer diameter of the first O-ring 104 is approximately half the inner diameter of the clamp 114, although this ratio is not important; it is only necessary that the first O-ring should fit through the middle of the clamp 101 and protrude through it sufficiently for the following steps to occur. The inner carrier ring 102 also has a cylindrical inner surface 110.

(31) With the clamp 114 and sheet 112 in position, the press 101 is operated to move the press downwards in the direction of arrow Z in FIG. 5B (see also FIG. 4) first to engage with the sheet 112 and then to stretch the sheet 112. The stretching of the sheet 112 is facilitated by the first O-ring 104, which is suitably made from a low friction material such, for example, as PTFE, to ensure the sheet slides easily over the press 101 and is tensioned uniformly. The press 101 is moved downwards against the sheet 112 until the sheet is strained by about 40% to a biaxial tension of about 1200 N/m at the end of the stroke of the press. The sheet 112 becomes thinner as it stretched, reaching a thickness of about 220 μm corresponding to a stress of about 6 MPa, as shown in FIG. 5C. FIG. 6A shows how the thickness (in μm) of the sheet 112 changes as it is strained to about 40%. FIGS. 6B and 6C show respectively the corresponding changes of stress (in MPa) and load (in N/m). The plot-lines in FIGS. 6B and 6C have a plurality of distinct legs. Leg I represents the change in stress/load during the tensioning of the sheet as described above.

(32) Once the sheet 112 is stretched to its target tension, the inner carrier ring 102 is engaged with an outer carrier ring 105, as shown in FIG. 5D. The outer carrier ring 105 is annular having an inner surface 106 that is slightly greater than the outer diameter of the inner carrier ring 102, so that the inner carrier ring 102 forms snug fit inside the outer carrier ring 105. The outer carrier ring 105 is held fixedly by the jig such that the inner carrier ring 102 is entered into the outer carrier ring 105 as the press 101 is moved downwards. The inner surface 106 has a circumferential groove 107 that accommodates a second friction fluoroelastomer (e.g. Viton®) or Nitrile rubber O-ring 108 having an inner diameter that is slightly smaller than the outer diameter of the circumferential rib 111 formed on the outer surface 103 of the inner ring 102, so that on engaging the outer ring 105 with the inner ring 102 at the end of the stroke, the second O-ring 108 bumps over the ridge 111 to trap the membrane 112 between the second O-ring 108 and the inner carrier ring 102. An end stop 109 on the outer ring 105 prevents the inner and outer carrier rings 102, 105 from separating from one another.

(33) The portion of the sheet 112 that is held by the inner and outer carrier rings 102, 105 is then severed from the remainder of the sheet as shown in FIG. 5E. The inner and outer carrier rings 102, 105, with the trimmed sheet 112 held firmly between them under tension are then transferred to an oven having a temperature of about 80° C. The sheet 112 is conditioned in the oven for about 1 hour, during which time the macromolecular structure of polyurethane material comprising the sheet 112 relaxes. As shown by leg II in FIGS. 6B and 6C, during this step the stress in the sheet relaxes to about 2 MPa and the tension falls to about 440 N/m. The temperature and duration of the thermal conditioning step may be altered, provided that the sheet 112 is caused or allowed to undergo stress relaxation. It has been found that after this step the sheet is surprisingly able to hold a substantially constant line tension of about 200 N/m for a period of several years. Temperatures above about 90° C. should be avoided as the polyurethane material may begin to degrade.

(34) The inner and outer rings 102, 105 are then removed from the oven and the front and rear rings 18, 20 are glued to the front and rear surfaces 14, 16 of the sheet respectively using a light curable epoxy adhesive as mentioned above. Each of the rings 18, 20 is fabricated integrally with a respective circular lead frame 118, 120 and is attached to the rest of the lead frame by severable tabs 122, as shown in FIG. 5G. Each of the lead frames 118, 120 has an outer diameter that is slightly smaller than the diameter of the Inner surface 110 of the inner carrier ring such that it fits snugly within the inner carrier ring 102, as shown in FIG. 5F, to locate the rings 18, 20 accurately with respect to the sheet 112 and each other. The lead frames 118, 120 are provided with location features 124 to assist further in positioning them accurately within the inner carrier ring 102. For convenience the epoxy adhesive applied to the whole of the lead frames 118, 120 and then cured after being positioned in contact with the sheet 112. For the epoxy adhesive used, a two-stage curing process is needed; after initiation with UV light, the adhesive is then subjected to a secondary thermal curing step in an oven at about 40° C., for about 12 hours to develop the adhesive's full strength. If an alternative adhesive is employed then it should be cured according to the manufacturer's instructions.

(35) In some embodiments, the rear ring 20 may be attached to an annular support disc (not shown) of the kind described in WO 2013/143630 for reinforcing the rings 18, 20 against the tension in the sheet 112 in the plane of the sheet. The support disc is not shown here for clarity. Typically the rings 18, 20 have protruding tabs (not shown) at predetermined locations around the rings 18, 20 for connecting the rings at those locations to the housing 40 at hinge points, or to the control mechanism at actuation points, as described in WO 2013/144533, WO 2013/144592 or WO 2015/044260. The tabs are also omitted from the drawings for simplicity.

(36) In the one embodiment, the outer face 14 is then coated with a thin layer (not shown) of the fluorinated polymer barrier material (OF 210™, Canon Optron, Inc) to form a protective layer as described above. The barrier material is coated onto the outer face 14 under vacuum by physical vapour deposition (PVD) to a thickness of about 10 nm.

(37) A fluorinated polymer barrier layer that is coated onto the outer face 14 by PVD deposition is satisfactory for use in many situations, but a disadvantage is that there are no interfacial bonds between the outer face 14 of the membrane and the polymer coating. As a result, the PVD coating may be fragile with a risk of wearing off, for instance by touch. In the other embodiment mentioned above, the outer face 14 of the mounted pre-tensioned membrane 12 is coated with a layer of silica-filled cross-linkable polyurethane acrylate material instead of the fluorinated polymer material. The use of a barrier material that is compatible with the membrane material allows the formation of strong interfacial bonds between the barrier layer and the membrane 12 as result of interactions at the molecular level. An aromatic polyurethane acrylate material, for example, may be suitable for coating a thermoplastic aromatic polyurethane membrane 12. The inclusion of acrylate moieties within the barrier material allows the barrier material to be cross-linked after coating onto the membrane 12 for increased stiffness and hardness. The inclusion of a small amount of photoinitiator within the material allows curing to proceed by exposure to UV light.

(38) Suitable acrylic-modified polyurethane materials include UV1 and Ceranoshield, which are commercially available from PPG Industries, Inc. Barberton Speciality Chemicals Plant, Barberton Ohio, and G-NT200 which is available from Lens Technology International of La Mirada Calif.

(39) The inclusion of silica nanoparticles affords added stiffness and scratch resistance. The concentration of nanoparticulate silica included in the barrier material may be varied according to the desired properties of the coating, but typically the barrier material contains 50-60% wt. silica. In one embodiment, the acrylic-modified polyurethane material may include about 52% wt. silica. If a thinner barrier layer is desired, the silica-filled polyurethane barrier material may be diluted with a suitable solvent such, for example, as an acetate or alcohol, prior to application to the face of the membrane 12 as described below, which may reduce the concentration of silica particles to the range 7-10% wt. which would still be sufficient to impart a degree of hardness to the coating. Generally in accordance with the invention, the nanoparticles may have an average diameter in the range 50-200 nm, typically about 50-100 nm.

(40) The acrylate-modified polyurethane barrier material may be applied to the outer face 14 of the mounted membrane 12 by spin coating, but preferably ultrasonic spray coating is used, which has been found to achieve a thickness well below about 1 μm. The use of ultrasound causes the polyurethane barrier material to be atomised into tiny droplets, which are then sprayed over the face 14 of the membrane 12 in the form of a thin film.

(41) Thermoplastic polyurethane of the kind used for the membrane 12 is hydrophobic in nature and has a contact angle ranging between 95-105°. A lower contact angle is generally needed to wet the surface uniformly to promote good adhesion between the face 14 of the membrane 12 and the barrier layer. In order to attain a lower contact angle and better adhesion, the outer face 14 of the membrane 12 is subjected to plasma treatment (air plasma) prior to coating with the barrier material. This serves to activate the surface and, as a result, the contact angle is reduced to the region of 78-83°. This may be tested using dyne ink, whereby the surface energy after plasma exposure is increased from 38-40 dynes/cm to about 48-52 dynes/cm.

(42) After activating the face 14 of the membrane 12 as described above, the pre-tensioned membrane 12, still mounted between the inner and outer rings 102, 105, is transferred to a coating chamber where the coating of silica-fill sd acrylate-modified polyurethane is sprayed onto the face 14 by ultrasound spray coating as mentioned above. After coating, the coating liquid on the membrane 12 is cured under UV exposure using Mercury vapour H-bulb. The mercury lamp has an output in the shortwave UV range between 220-320 nm, and a spike of energy in the long-wave range at 365 nm.

(43) A cured, silica-filled polyurethane coating of the kind described above provides a stiff, hard barrier layer on the outer face 14 of the pre-tensioned membrane 12 having elastic modulus of about 1 GPa. This provides an additional advantage as described in more detail below, in that as the tension in the membrane 12 decreases slightly during the subsequent assembly steps, the barrier layer is compressed.

(44) In other embodiments, the mounted membrane may be coated on its outer face additionally or instead with other coating materials known in the art such, for example, as single or multi-layer anti-reflection coatings.

(45) The dish-shaped receptacle 24 is pre-assembled with the rear lens 34 by bonding the front face 32 of the lens 34 to the rear face 30 of the receptacle 24 using a 25 μm layer of PSA as mentioned above. The pre-assembled lens 34 and receptacle 24 are then attached to the rear ring 20 as shown in FIG. 5H by bonding the peripheral lip 22 of the receptacle 24 to the rear ring 20 with the epoxy adhesive and curing the same. The sheet 112 is then trimmed between the rings 18, 20 and the lead frames 118, 120 as shown in FIG. 5I leaving the membrane proper 12 held between the rings 18, 20. At this stage, the rings 18, 20 are still attached to the lead frames 1 18, 120 by the tabs 122.

(46) With reference to FIG. 5J, the front and rear retainers 48, 46 are then assembled around the rear lens 34, dish-shaped receptacle 24 and rings 18, 20 and membrane 12 to enclose the rear lens 34, dish-shaped receptacle 24 and rings 18, 20 and membrane 12 as described above and to form said housing 40. The tabs 122 between the lead frames 118, 120 and the rings 18, 20 are then cut to detach the assembly 10 from the jig. as shown in FIG. 5K.

(47) Thereafter the enclosure 54 formed by the rear wall 26 of the dish-shaped receptacle 24, the membrane 12 and the rear ring 20 is filled under vacuum with pentaphenyltrimethyltrisiloxane as the fluid 60 through a fill-port (not shown) in the housing 40 and side wall 26 of the dish-shaped receptacle 24. As described above, alternative silicone oils may be used instead if desired. Filling is continued until the fluid 60 contacts the whole of the inner face 16 of the membrane 12 continuously as shown in FIG. 5L. Desirably the enclosure may be over-filled to a certain degree, causing the membrane 12 to distend outwardly. Suitably the enclosure may be over-filled to a membrane curvature of about +1.0 dioptres. This serves to stabilise the loaded membrane support structure comprising the rings 18, 20 and allows for absorption of some of the fluid 60 by the membrane 12.

(48) Over time the membrane 12 tends to absorb an amount of the fluid 60 from the enclosure via its inner face 16 which is in contact with the fluid 60. In the present embodiment, the membrane 12 may absorb up to about 15% of its weight of fluid. This causes the membrane 12 to swell-relax, further losing tension. Desirably this process may optionally be accelerated in accordance with the present invention by incubating the fluid-filled assembly 10 at about 50-51° C. for about 24 hours. This is shown in leg III of FIGS. 6B and 6C, with the final tension in the membrane being about 220 N/m and the final stress being about 1 MPa, which is equivalent to a strain reduction of about 5%. During this process the curvature of the membrane also decreases from about +1.0 dioptres, as mentioned above, to about +0.5 dioptres. In this way, the membrane tension in the finished assembly 10 is already substantially stabilised.

(49) Where the membrane 12 carries a silica-filled, cross-linked polyurethane coating having elastic modulus of about 1 GPa on its outer face 14 as described above in relation to the other embodiment, the coating is compressed when the membrane and rings sub-assembly 12, 18, 20 is released from the lead frames 118, 120 by cutting tabs 122, and thus the elastic forces in the coating operate in the opposite direction to the elastic forces in the membrane 12 which is tensioned.

(50) The change in stress σ.sub.m of a biaxially strained membrane of modulus E.sub.m subject to a small strain ε is given by equation (I):
Δσ.sub.m=2E.sub.mε  (I)

(51) As the membrane 12 relaxes during incubation and swelling, it undergoes a negative “settling” strain that reduces its tension, while putting the coating into compression. The line tension in the membrane 12 is equal to the stress σ.sub.m in the membrane multiplied by its thickness T.sub.m. The negative strain serves to put the membrane 12 and coating into equal and opposite line tensions:
T.sub.m(σ.sub.in−2E.sub.mε)=2T.sub.cE.sub.cε  (II)

(52) where T.sub.c and E.sub.c are the thickness and modulus of the coating respectively.

(53) Applying equation (II) to a membrane 12 having a thickness T.sub.m of about 200 μm and a modulus E.sub.m of about 20 MPa at an initial biaxial stress of about 1 MPa and a coating having a modulus E.sub.c of about 1 GPa, the settling strain would be limited to about 1% with a coating having a thickness T.sub.c of about 6 μm. In this way, the force applied to the rings 18, 20 would be minimised to alleviate unwanted in-plane bending of the rings 18, 20 without the need for an annular support desk of the kind described in WO 2013/143630 for example. In the other embodiment, the coating has a thickness of about 1 μm, but even at this thickness, the compression of the coating is enough to have a significant effect on the mechanics of the membrane 12 such that it serves to prevent some or all of the tendency of the rings 18, 20 to in-plane collapse.

(54) The protective layer on the outer face 14 of the membrane 12 prevents the egress of absorbed fluid 60 from the front face of membrane. Such egress would be undesirable as the fluid 60 might form droplets on the surface of the membrane 12 thus impairing its optical properties.

(55) FIG. 7 shows a scattergram of measured line tensions over time for sixty-six individual polyurethane membranes that have been tensioned and thermally conditioned in accordance with the invention and held in continuous contact with a body of silicone oil. As can be seen, the membranes hold the tension substantially constant for an extended period of more than two years (FIG. 7 shows up to 796 days). It is likely that the membranes are able to hold the tension substantially constant for even longer, but that has not yet been measured.

(56) In yet another embodiment, the inner face 16 of the membrane 12 may be coated with a barrier layer (not shown) of a suitable hydrophobic coating material of the kind described above for use on the outer face 14. In this way, the ingress of fluid 60 into the membrane 12 may be prevented or at least retarded. In such a case, the manufacturing process would not need to accommodate swell-relaxing of the membrane 12 owing to the absorption of fluid 60 avoiding the need to incubate the filled assembly at an elevated temperature to accelerate swell-relaxing of the membrane and it may be possible to biaxially tension the membrane 12 to a slightly lower initial tension.

(57) The effective modulus E.sub.ef. of a membrane having a modulus E.sub.m carrying a coating having an elastic modulus E.sub.c is given by the equation:

(58) $\begin{array}{cc}{E}_{\mathrm{eff}}=\frac{E_m{T}_m+E_c{T}_c}{T_m+T_c}& \left(\mathrm{III}\right)\end{array}$

(59) The thickness T.sub.c of the coating may be measured optically, while the combined thickness T.sub.m+T.sub.C of the membrane and coating may be measured using a thickness gauge.

(60) The modulus of the membrane when coated and uncoated may be measured by holding the membrane around its edge in steel rings clamped to a sealed vessel that is pressurised to a pressure P. As a result of pressure in the vessel, the membrane bulges outwardly, and the maximum outward displacement h of the membrane can be measured using a laser height measuring system. From this, the tension, biaxial stress and strain on deforming the membrane from flat to nearly spherical, and hence the effective modulus of the coating and membrane, or of the membrane only, can be calculated.