Abstract:
A variable focus lens comprises first and second transparent, flexible membranes ( 2 ) which are tensioned and which define a cavity ( 30 ) in which a transparent, silicone oil ( 4 ) is sealed. Adjustment of the pressure or volume of the oil in the cavity changes the focal length of the lens. To fabricate the lens, the periphery of each membrane ( 2 ) is engaged between two rings ( 8, 10; 10, 12 ) of a set of three interengaging rings ( 8, 10, 12 ). The interengaged rings are crimped together to tension the membranes ( 2 ) and seal the cavity ( 30 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of fabricating a variable focus lens and to the variable focus lens so constructed. 
     2. Related Art 
     It is known to provide a variable focus lens in which one or more transparent flexible membranes are tensioned to define a cavity therebetween in which a transparent fluid is received. Adjustment of the pressure and/or volume of the fluid in the cavity causes variations in the shape of the membranes, and the structure defines a lens of variable focal length. 
     However, whilst it is known to use such a construction to provide a variable focus lens, no current proposals for fabricating such a lens provide a lightweight structure which can easily be incorporated into spectacles, for example. 
     The present invention seeks to provide a new method of fabricating a variable focus lens and to provide a new structure for such a lens. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a method of fabricating a variable focus lens comprising the steps of interengaging first and second rings to engage a first transparent, flexible membrane and to retain said first flexible membrane across the first ring, interengaging a third ring with said first and second rings to engage a second transparent, flexible membrane and to retain said second flexible membrane across the first ring, whereby the first and second flexible membranes define a cavity therebetween, and holding said first, second and third rings together such that said first and second flexible membranes are each tensioned across said first ring. 
     A method of an embodiment of the invention enables a variable focus lens to be assembled quickly and easily. 
     A transparent fluid is to be sealed within the cavity defined between the two flexible membranes. In an embodiment, a duct is provided through said rings, which duct is in communication with said cavity. The duct may be formed from pre-formed bores in said first, second and third rings. However, in a preferred embodiment, said duct is drilled, or otherwise formed, through said rings. 
     In an embodiment said first ring receives said second and third rings therein and defines the periphery of a peripheral, annular frame for the lens, which annular frame is defined by said first, second and third rings. This annular frame may readily be supported within lens apertures provided in spectacles, for example. 
     The peripheral shape of the peripheral annular frame, and hence of the individual rings, may be chosen as is required. For example, the annular frame may have a circular, oval, elliptical, or other closed curve, peripheral shape. 
     Any method for holding the first, second and third rings together to retain the flexible membranes under tension may be used. 
     In a preferred embodiment, the method further comprises turning, crimping or otherwise deforming the first ring so that the second and third rings are retained within said first ring. 
     Additionally and/or alternatively, said annular frame may be retained within the frame of a pair of spectacles. In this case, the frame of the spectacles may act to hold the first, second and third rings together. Of course, any other retaining means may additionally and/or alternatively be provided to secure the first, second and third rings in their interengaged position. 
     According to a further aspect of the present invention there is provided a variable focus lens having a cavity containing a transparent fluid defined between first and second transparent, flexible membranes, wherein each of said first and second flexible membranes is tensioned across and held by a peripheral annular frame, and wherein said annular frame is formed from first, second and third interengaged rings, said first ring receiving the second and third rings therein and defining the periphery of said annular frame. 
     A variable focus lens of an embodiment of the invention is simple and relatively inexpensive to assemble. Furthermore, the lens may be supported readily within the lens aperture in a pair of spectacles, for example. 
     The peripheral shape of the peripheral annular frame, and hence of the individual rings, may be chosen as is required. For example, the annular frame may have a circular, oval, elliptical, or other closed curve, peripheral shape. 
     Any method for holding the first, second and third rings together to retain the flexible membranes under tension may be used. 
     In an embodiment, the annular frame is arranged to be retained within the frame of a pair of spectacles. In this case, the frame of the spectacles may act to hold the first, second and third rings together. Of course, any other retaining means may additionally and/or alternatively be provided to secure the first, second and third rings in their interengaged position. 
     It is important that the transparent fluid within said cavity cannot leak. Accordingly, it is preferred that the interengagements between said rings cause the periphery of an appropriate flexible membrane to change direction more than once. This aids in sealing the periphery of the cavity. 
     It would be possible to construct the lens from a single flexible membrane web appropriately arranged to define two flexible membranes. However, in a preferred embodiment, two separate flexible membranes are provided with the periphery of the first flexible membrane being held by the interengagement of the first and second rings, and the periphery of the second flexible membrane being held by the interengagement of the second and third rings. This provides a simple, yet effective sealed structure for the cavity. 
     Each of said first and second membranes has a continuous periphery which is held by the respective rings so that it is bent to change direction more than once. 
     Preferably, each of two adjacent rings has a complementary annular step therein for interengaging the two rings, said interengaged steps also guiding an interposed flexible membrane around two direction changes. 
     In a preferred embodiment, each of said first and second flexible membranes is of high grade Mylar. 
     The first, second and third rings may be of any suitable material. Preferably, each of the rings is of a material which is sufficiently rigid to positively interengage with the adjacent ring and to ensure that two interengaged rings can maintain the flexible membrane under tension. The material of the rings is also required to be lightweight to facilitate the incorporation of lenses of the invention in spectacles. Thus, the rings may be made, for example, of a high impact resistant plastics material, or of aluminium, or of titanium. 
     The transparent fluid retained within the cavity may be any suitable fluid. In a preferred embodiment, the fluid is a silicone oil, for example, an oil of the silicone 703 type. 
     In a preferred embodiment, the lens is additionally provided with two spaced, substantially parallel sheets of transparent polycarbonate between which the cavity defined by the flexible membranes is arranged. The polycarbonate sheets provide protection to the flexible membranes and are readily tinted, for example, if required. The annular frame may also be received between the polycarbonate sheets if required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 shows an axial section of a portion of a variable focus lens of the invention during construction thereof, and 
     FIG. 2 shows a section similar to that of FIG. 1 but showing the completed lens. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The lens described and illustrated herein may be utilised in an optical apparatus constructed and arranged to be adjusted as described and claimed in International application No. WO 96/38744. 
     The lens shown in FIG. 2 comprises first and second transparent, flexible membranes  2  which are tensioned and which define therebetween a cavity  30  in which a transparent fluid  4 , such as silicone oil, is contained. In the embodiment illustrated the membranes  2  are of Mylar, for example, 23 μm Type D Mylar. 
     The membranes  2  are held in tension by way of a peripheral annular frame  6 . The frame  6  defines the periphery of the lens, and of the cavity  30 , and is arranged to hold the flexible membranes  2  under tension. The frame  6  is also arranged to seal the lens. 
     As can be seen from FIG. 1, the annular frame  6  is formed by interengaging first, second and third rings  8 ,  10 ,  12  of aluminium. In this respect, the first ring  8  has a radially extending flange  14  which is stepped as indicated at  16 . The first ring  8  also has an axially extending rim  18 . The second ring  10  has a radially outwardly projecting portion  20  defining a first step  22  which cooperates with the step  16  of the first ring  8  and a second step  24  which cooperates with a step  26  of the third ring  12 . The third ring  12  has a recess  28  arranged radially outwardly therein. 
     Where the completed lens is to have a generally circular periphery, for example, a first membrane  2 , which is circular, is positioned to extend within the first ring  8  such that its periphery extends along the radial flange  14  thereof. The second ring  10  is then supported within the first ring  8  such that the complementary steps  16  and  22  thereof interengage. This interengagement positions the second ring  10  relative to the first ring  8 . The interengagement also bends the peripheral circumference of the first membrane  2  through two different directions as is clearly shown in FIG.  1 . 
     A second circular membrane  2  is then positioned across the first ring  8  such that its periphery extends along the upper radially extending surface of the second ring  10 . Thereafter, the third ring  12  is positioned such that the steps  24  and  26  of the second and third rings  10 ,  12  cooperate to retain the third ring  12  in position within the first ring  8 , and to bend the periphery of the second membrane  2  through two directions. The tolerances of the three rings  8 ,  10 ,  12  are chosen such that even when the arrangement is in the interengaged position shown in FIG. 1, the two membranes  2  are firmly held by the interengaged rings. 
     The construction has the considerable advantage that the circular membranes  2  do not have to be produced to close tolerances. Each circular membrane  2  can be cut to be generally circular and to have a diameter which is somewhat larger than is required. In its turn, each membrane  2  is positioned as described above and held by the interengagement of the corresponding ring  10 ,  12 . Any excess material of the membrane  2  can then be trimmed simply using a knife with the rings  10 ,  12  acting as guides. 
     To complete the construction of the lens it is necessary to ensure that the three rings  8 ,  10 ,  12  are secured together such that the peripheries of the membranes  2  are firmly retained to seal the lens and to prevent any leakage from the cavity  30  defined between the two membranes  2 . In the embodiment illustrated, the retention of the membranes  2  is achieved by crimping and deforming the first ring  8 . In this respect, the upstanding, axially extending, free end of the limb  18  of the first ring  8  is bent over to be received within the recess  28  of the third ring  12 . Any excess material deformed from the ring  8  during this crimping operation is received within an annular space  32  which is defined between the radially outwardly peripheries of the second and third rings  10  and  12 , and the inner periphery of the limb  18  of the first ring  8 . 
     Once the lens has been constructed, as shown in FIG. 1, and with the outer ring  8  deformed to secure the structure, a duct  34  is drilled through the frame  6  into the cavity  30 . The cavity  30  can then be filled with the transparent fluid  4  through the duct  34 . With  703  silicone oil as the fluid the resultant lens achieves a focal length range of at least −6 to +10 dioptres, and the range −10 to +10 dioptres should be achievable. 
     Fluid may be introduced into the cavity  30  by way of the duct  34  and retained therein by use, for example, of ducts, plugs and sealing means as described in International application No. WO 96/38744. In an embodiment, the lens is pre-filled at this stage with the fluid  4  and an appropriate material is provided in the duct  34  to seal the cavity  30 . However, the material sealing the duct  34  is preferably penetrable by a syringe, for example, so that the pressure of the fluid  4  within the cavity  30  can be adjusted. 
     In the embodiment shown, the two axially outer surfaces of the lens are each protected by a sheet  36  of a polycarbonate. Each polycarbonate sheet  36 , for example, may be about ½ millimeter thick. It will be appreciated that these sheets  36  are arranged to extend substantially parallel to each other. The circumferential periphery of each of the sheets  36  may be fixed to the annular frame  6  in any appropriate manner. For example, the sheets  36  may be adhered to the frame  6 . Additionally and/or alternatively, a plastics material frame (not shown) of a pair of spectacles, for example, may be arranged to receive the annular frame  6  in a manner to retain the sheets  36  against the annular frame  6  in the position of FIG.  2 . 
     It will be appreciated that alterations and modifications may be made to the embodiments described and illustrated herein within the scope of this application.