U.S. patent application number 15/784626 was filed with the patent office on 2018-04-12 for deformable non-round membrane assemblies.
The applicant listed for this patent is Adlens Ltd.. Invention is credited to Derek Paul Forbes Bean, Roger Brian Minchin Clarke, Peter Lee Crossley, Alex Edginton, Benjamin Thomas Tristram Holland, Richard Leefe Douglas Murray, Dijon Pietropinto, Daniel Paul Rhodes, Robert Edward Stevens, Edwin James Stone.
Application Number | 20180101028 15/784626 |
Document ID | / |
Family ID | 46087214 |
Filed Date | 2018-04-12 |
United States Patent
Application |
20180101028 |
Kind Code |
A1 |
Stevens; Robert Edward ; et
al. |
April 12, 2018 |
Deformable Non-Round Membrane Assemblies
Abstract
A deformable membrane assembly comprises an at least partially
flexible fluid-filled envelope, one wall of which is formed by an
elastic membrane that is held around its edge by a resiliently
bendable supporting ring, a fixed support for the envelope and
selectively operable means for causing relative movement between
the supporting ring and the support for adjusting the pressure of
the fluid in the envelope, thereby to cause the membrane to deform.
The bending stiffness of the ring varies round the ring such that
upon deformation of the membrane the ring bends variably to control
the shape of the membrane to a predefined form. The moving means
comprise a plurality of ring-engaging members that we arranged to
apply a force to the ring at spaced control points. There are at
least three control points, and there is a control point at or
proximate each point on the ring where the profile of the ring that
is needed to produce the predefined form upon deformation of the
membrane exhibits a turning point in the direction of the force
applied at the control point between two adjacent points where the
profile of the ring exhibits an inflection point or a turning point
in the opposite direction.
Inventors: |
Stevens; Robert Edward;
(Oxford, GB) ; Edginton; Alex; (Oxford, GB)
; Holland; Benjamin Thomas Tristram; (Oxford, GB)
; Rhodes; Daniel Paul; (Oxford, GB) ; Pietropinto;
Dijon; (Oxford, GB) ; Bean; Derek Paul Forbes;
(Oxford, GB) ; Clarke; Roger Brian Minchin;
(Melbourn, GB) ; Crossley; Peter Lee; (Melbourn,
GB) ; Murray; Richard Leefe Douglas; (Melbourn,
GB) ; Stone; Edwin James; (Melbourn, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Adlens Ltd. |
Eynsham |
|
GB |
|
|
Family ID: |
46087214 |
Appl. No.: |
15/784626 |
Filed: |
October 16, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15617563 |
Jun 8, 2017 |
9791720 |
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15784626 |
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14388741 |
Jun 9, 2015 |
9709824 |
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PCT/GB2012/051426 |
Jun 20, 2012 |
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15617563 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 3/14 20130101; G02C
7/085 20130101 |
International
Class: |
G02C 7/08 20060101
G02C007/08; G02B 3/14 20060101 G02B003/14 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2012 |
GB |
1205394.8 |
Claims
1-33. (canceled)
34. A deformable membrane assembly comprising; an at least
partially flexible fluid-filled envelope, one wall of which is
formed by a distensible membrane that is held around its edge by a
resiliently bendable supporting ring; and a selectively operable
adjuster for adjusting the pressure of fluid in the envelope to
cause the membrane to deform; wherein the bendable supporting ring
comprises one or more sapphire ring elements.
35. The deformable membrane assembly as claimed in claim 34,
wherein the adjuster is configured to compress the envelope.
36. The deformable membrane assembly as claimed in claim 34,
wherein the adjuster is configured to expand the envelope.
37. The deformable membrane assembly as claimed in claim 34,
wherein the adjuster is arranged to act on the resiliently bendable
supporting ring at one or more actuation points for active
displacement of the supporting ring relative to a support.
38. The deformable membrane assembly as claimed in claim 34,
wherein the distensible membrane is continuously adjustable between
an un-actuated state and a fully deformed state, and at each
position between the un-actuated and fully deformed states the
resiliently bendable supporting ring is displaced at the or each
actuation point by a distance required to achieve a predefined form
of the membrane.
39. The deformable membrane assembly as claimed in claim 34,
further comprising one or more hinge points, wherein the supporting
ring is held stationary relative to a support.
40. The deformable membrane assembly as claimed in claim 39,
wherein the resiliently bendable supporting ring is free to bend
passively relative to the support between the hinge points.
41. The deformable membrane assembly as claimed in claim 34,
wherein the resiliently bendable supporting ring comprises two or
more ring elements, and the membrane is sandwiched between two
adjacent ring elements.
42. The deformable membrane assembly as claimed in claim 34,
wherein upon actuation the membrane adopts a spherical form or a
form defined by one or more Zernike polynomials Z.sub.j.sup..+-.k
(k.ltoreq.j).
43. The deformable membrane assembly as claimed in claim 34,
wherein the supporting ring is formed with a protruding tab at the
or at least one of the actuation points for engaging the supporting
ring with the selectively operable adjuster.
44. The deformable membrane assembly as claimed in claim 34,
wherein the membrane is pre-tensioned on the supporting ring.
45. The deformable membrane assembly as claimed in claim 34,
wherein a reinforcing diaphragm is provided that is fastened to the
supporting ring, which diaphragm has a greater stiffness in a plane
of the supporting ring than in a direction of bending of the
ring.
46. The deformable membrane assembly as claimed in claim 45,
wherein the reinforcing diaphragm is fastened to the supporting
ring uniformly around the supporting ring so that a tension in the
membrane is transmitted uniformly to the reinforcing diaphragm.
47. The deformable membrane assembly as claimed in claim 45,
wherein within the plane of the supporting ring, the membrane is
longer in one dimension than it is in another dimension, and the
reinforcing diaphragm has a lower stiffness in the one dimension
than it has in the other dimension.
Description
[0001] The present invention provides improvements in or relating
to deformable non-round membrane assemblies in which the shape of a
membrane is controllably adjustable by altering the fluid pressure
across the membrane. The invention has particular reference to
assemblies in which the membrane is selectively deformable
spherically or according to another Zernike polynomial. In some
embodiments the assembly may be a variable optical power
fluid-filled lens in which the membrane is transparent and forms
one optical surface of the lens whose curvature can be adjusted
over substantially the entire lens with minimal optical distortion
that would otherwise be caused by the non-round character of the
lens. In other embodiments, the membrane may be mirrored and/or
opaque. Other applications of the membrane assembly include
acoustic transducers and the like.
[0002] Variable focus fluid-filled lenses are known in the art.
Such lenses generally comprise a fluid-filled transparent envelope,
the opposite optical surfaces of the lens being formed by two
spaced opposing walls of the envelope, at least one of which walls
comprises a flexible transparent membrane. For example, U.S. Pat.
No. 1,269,422 discloses a lens with spaced opposed walls of arcuate
formation that are merged together at their circumferential edges
and which may be adjusted towards or away from each other, and a
liquid body between the walls. The pressure of the fluid within the
envelope is adjustable to change the degree of curvature of the
membrane, thereby adjusting the power of the lens. In some
examples, the volume of the envelope may be adjusted, as in U.S.
Pat. No. 1,269,422 or WO 99/061940 A1. Alternatively the amount of
fluid within the envelope may be adjusted, as in U.S. Pat. No.
2,576,581, U.S. Pat. No. 3,161,718 and U.S. Pat. No. 3,614,215. In
either case, an increase in the fluid pressure within the envelope
causes deformation of the flexible membrane.
[0003] Whilst various applications of adjustable lenses are
possible--for example in cameras and other optical equipment --,
one use is in eyewear. An adjustable lens is particularly useful
for correction of presbyopia--a condition in which the eye exhibits
a progressively diminished ability to focus on close objects with
age. An adjustable lens is advantageous because the wearer can
obtain correct vision through a range of distances from
long-distance to near vision. This is more ergonomic than bifocal
lenses in which near-vision correction is provided in a bottom
region of the lens, thereby only allowing the user to see close
objects in focus when looking downwardly.
[0004] A disadvantage of the fluid-filled lenses disclosed by the
documents mentioned above is that they need to be circular, or at
least substantially circular, with a rigid boundary, in order to
maintain the membrane spherical; otherwise unwanted optical
distortion occurs. However, circular is not a preferred shape for
certain applications, including eyewear, because it is not
considered to be aesthetically appealing for those applications.
Round lenses may also be unsuitable or unpractical for certain
applications, such as in optical instruments.
[0005] It is desirable therefore to provide an adjustable non-round
lens, in which the lens is not distorted as the optical power of
the lens is increased.
[0006] U.S. Pat. No. 5,371,629 discloses a non-circular variable
focal length lens which includes a rigid lens to provide the
wearer's distance correction, and a liquid-filled lens bounded by a
distensible stretched elastomeric membrane to provide a variable
near addition. The liquid, which has a fixed volume, is stored in
the field of view between the elastic membrane and the rigid lens.
Variation of the optical power of the liquid-filled lens is
achieved by displacement of the membrane support to which the outer
periphery of the stretched elastomeric membrane is attached. U.S.
Pat. No. 5,371,629 claims that the shape of the distended membrane
is substantially spherical, despite the circumference of the
membrane being non-circular, by allowing the membrane support to
bend in a predetermined controlled manner as it is moved.
Specifically, the thickness of the membrane support varies around
the circumference of the membrane support. U.S. Pat. No. 5,371,629
asserts that by properly proportioning the moment of inertia of the
section of the membrane support around its circumference, the shape
of the membrane support, when deflected, can be made such as to
result in a substantially spherical membrane, despite the fact that
the free membrane shape is not circular. The configuration of the
membrane support required to result in the desired deformation for
any particular lens can be calculated using the method of finite
element analysis or in other ways. However, the liquid-filled lens
of U.S. Pat. No. 5,371,629 is unpractical for various reasons and
was never commercialised. In particular, despite its teachings,
U.S. Pat. No. 5,371,629 fails to disclose a liquid-filled lens that
avoids unwanted distortion when the membrane is distended, and the
degree of distortion encountered in the liquid-filled lens of U.S.
Pat. No. 5,371,629 renders the lens unusable.
[0007] WO 95/27912 A1 proposes a workaround that comprehends the
use of a non-round membrane supporting ring having a circular
central opening, but this does not provide a true non-round lens
and is a cumbersome arrangement that is also sub-optimal from an
aesthetic point of view.
[0008] Similarly, it is desirable to be able to adjust controllably
the shape of a membrane for other non-optical applications. For
example, a surface of controllably variable sphericity or some
other Zernike polynomial would be useful in the field of acoustics
for the creation of non-round transducers, such as loudspeakers.
Many products would benefit from non-round drivers owing to space
constraints and the typical geometry of the product, e.g.
televisions, mobile phones. Maintaining the sphericity of a
membrane of variable curvature would be beneficial in the
production of drivers, since spherical deformation would ensure the
emitted waves behave as though they originated from a point source,
thereby avoiding interference patterns in the emitted pressure
waves. However, the unmodified deformed shape of a non-round
membrane that is held at its edges is not spherical. Hence
providing a selectively adjustable non-round surface would be
desirable for improving the performance of non-round drivers for
acoustic use.
[0009] In one aspect of the present invention therefore there is
provided a deformable membrane assembly as claimed in claim 1
below.
[0010] The present inventors have realised that in a deformable
membrane assembly such, for example, as a fluid-filled lens in
which the flexible envelope contains a fixed volume of fluid and
the membrane is distended to adopt a predefined form by adjusting
the volume of the envelope, to alter the pressure of the fluid
therein, the control points where the force is applied to the
membrane supporting ring for adjusting the envelope volume must be
carefully positioned. By controlling carefully the control points
at which the force is applied to the membrane supporting ring and
allowing the membrane supporting ring to bend freely between the
control points, semi-active control over the shape of the membrane
is achieved. The bending stiffness of the supporting ring varies
around the ring so that when actuated the ring conforms to the
desired profile to produce a membrane shape of the predefined form.
Suitably the bending stiffness may be varied round the ring by
varying the second moment of area of the ring.
[0011] The means for causing relative movement between the
supporting ring and the support for the envelope for adjusting the
volume of the envelope may comprise means for moving the supporting
ring or support. Said moving means may be configured for
compressing the envelope to reduce its volume, thereby to increase
the pressure of the fluid within the envelope and to cause the
membrane to distend outwards relative to the envelope in a convex
manner. Thus in some embodiments the moving means may be configured
for compressing the envelope in a first direction against the
support to increase the pressure of the fluid therein to cause the
membrane to distend outwardly in a second opposite direction.
[0012] In another aspect of the present invention therefore there
is provided a deformable membrane assembly as claimed in claim 6
below.
[0013] Alternatively the means for moving the supporting ring or
support for adjusting the volume of the envelope may be configured
for expanding the envelope to increase its volume, thereby to
reduce the pressure of the fluid within the envelope and to cause
the membrane to distend inwards in a concave manner.
[0014] The means for moving the supporting ring or support for
adjusting the envelope volume may suitably comprise a selectively
operable device comprising one or more components arranged to act
between the membrane supporting ring and the support to move the
supporting ring and/or the support, the one relative to the other,
to adjust the volume of the envelope.
[0015] Suitably the flexible envelope may comprise the one wall
defined by the membrane and another opposing rear wall that is
joined to the edge of the membrane in such a manner as to close and
seal the envelope. In some embodiments, the opposing walls may be
joined directly to one another. Alternatively the envelope may
comprise a peripheral side wall intermediate the two opposing
walls. The side wall may be flexible to allow the opposing walls to
be moved towards or away from each other for adjusting the volume
of the envelope. The rear wall may be rigid or substantially rigid
or may be supported stably at least round a peripheral edge.
[0016] The means for moving the supporting ring or support may be
configured to act between the membrane supporting ring and the rear
wall. In some embodiments, the rear wall may form part of the
support for the envelope, in that the rear wall may afford a stable
part for the adjusting means to react against.
[0017] The invention is especially applicable to non-round
membranes in which the edge of the membrane is planar in the
un-actuated state and deviates from the planar when the assembly is
actuated. However the invention is equally applicable to round
membranes where, by dint of the shape of the predefined form, the
edge of the membrane similarly deviates from the planar when the
assembly is actuated. In particular the invention is also concerned
with round membranes where the predefined form is
non-spherical.
[0018] To produce the predefined membrane form when actuated, the
supporting ring must adopt an actuated profile in which one or more
regions of the ring are displaced in one direction away from a
planar datum defined by the ring in the un-actuated state and/or
one or more regions must be displaced from the planar datum in
another opposite direction. To achieve the desired actuated profile
a force is applied to the supporting ring at each control point.
The inventors have realised that there should be at least one
control point within each sector of the supporting ring, whereby a
"sector" is meant a portion of the ring lying between two adjacent
inflection points or minimal points in the profile, said minimal
points being local minimums of displacement of the ring in the
direction of the force applied at the control point, e.g. in the
first direction inwards relative to the envelope, when the membrane
is deformed. Since a minimal point is defined as being a local
rather than a global minimum in the direction of the applied force
at the adjacent control points (and thus a local rather than a
global maximum in the direction opposite to the direction of the
applied force, e.g. in the second direction outwards relative to
the envelope) it will be understood that at these points the ring
may actually be displaced in either direction, or not displaced at
all, from the planar datum. In general the ring may at all points
move in either direction from, or remain stationary at, the planar
datum, depending on the perimeter shape, surface profile and
actuation required. In some embodiments where forces having
opposite directions are applied at adjacent control points to
achieve a desired ring profile when the membrane is deformed, a
control point may be positioned between two inflection points in
the profile of the supporting ring. However the forces applied at
the control points will usually all be in the same direction, such
that a sector of the ring is defined between adjacent local minima
as described above.
[0019] In some embodiments, the ring may be non-round and the
predefined form may have a centre. In such embodiments, the minimal
points of minimal displacement may also be minimal points in the
sense that the distance between the supporting ring and the centre
of the predefined form of the membrane when distended is a local
minimum. It will be understood that the position of the centre will
depend on the shape of the predefined form. In some embodiments the
centre may be at or close to the geometric centre of the membrane.
Alternatively the centre of the predefined form may be at a
different location from the geometric centre of the membrane.
Typically, when deformed, the membrane will have a vertex (i.e. a
point of global maximum displacement) and the centre may be located
at the vertex. This is particularly the case in optical
applications where the membrane forms an optical surface of the
lens. Generally the centre of the defined form will be positioned
somewhere within the body of the membrane away from the supporting
ring.
[0020] In practice, according to the shape of the membrane, some
regions of the ring may be supported to reduce the flexibility of
the ring in those regions. Accordingly, the inventors have realised
that there should be at least one control point within each sector
of the ring between unsupported minimal points. It will be
appreciated that the number of such minimal points will depend upon
the shape of the ring. In some embodiments, the number of minimal
points may be determined by the number of corners of the ring. For
instance, a quadrilateral ring with four corners has four minimal
points generally equidistant between the corners where the centre
of the predefined form of the membrane is at or towards the
geometric centre of the quadrilateral. In practice, the centre may
be positioned asymmetrically between opposite sides, and such an
arrangement may be particularly suitable for a rectangular optical
lens. In some embodiments, in a quadrilateral shaped ring, the
centre of the defined form may be positioned generally
symmetrically between one pair of opposite sides and asymmetrically
between the other pair of opposite sides.
[0021] In a generally rectangular ring with two long sides and two
short sides there will normally be four such minimal points where
displacement of the ring from the planar datum in the direction
opposite to the direction of the force applied to the supporting
ring at the adjacent control points is a local maximum, one on each
of the sides between two adjacent corners, but in some embodiments,
especially where the short sides are substantially shorter than the
long sides, the short sides of the ring may be reinforced to reduce
their flexibility, so that in practice the ring along each short
side does not bend substantially as the membrane is distended, in
which case there are only two unsupported minimal points along the
two long sides. In such a rectangular ring, for optical
applications, the centre of the defined form may be positioned
further from one short side than from the other.
[0022] The inventors have also realised that there should be at
least three control points, regardless of the number of minimal
points and sectors in order to define the plane of the
membrane.
[0023] Further, the inventors have realised that within each
sector, a control point should be positioned at or close to a
maximal point where the displacement of the ring in the actuated
state away from the planar datum in the direction of the force
applied at the control point in that sector is a local maximum,
e.g. in the first direction inwards relative to the envelope to
achieve compression of the envelope. It will be understood that
where the rest of the ring within a given sector is displaced in
the opposite direction when actuated, e.g. in the second direction
outwardly relative to the envelope, the maximal point within that
region may be a point at which the ring is stationary, i.e. is
subject to no or substantially no displacement away from the planar
datum. Further, a maximal point may be a point at which the ring is
actually displaced in the opposite direction from the planar datum,
e.g., outwardly relative to the envelope, less far than the rest of
the ring within the same sector. In other words a point of locally
maximal displacement in the direction of the force applied at the
control point is equivalent to a point of local minimum
displacement from the planar datum in the opposite direction.
[0024] In embodiments where the ring is non-round and the
predefined form has a centre, a maximal point may be a point on the
ring between adjacent inflection or minimal points where the
distance between the ring and the centre of the predefined form of
the membrane when distended is a maximum. If this were not the case
then within a sector there would be a portion of the ring that was
further away from the centre than the control point(s) within the
sector and which would therefore be uncontrolled, leading
potentially to unwanted distortion and shape of the membrane when
distended.
[0025] In some embodiments, one or more of said control points may
be actuation points, where the ring-engaging members are configured
actively to displace the supporting ring relative to the support.
Said supporting ring may be formed with a protruding tab at the or
at least one of the actuation points for engaging the ring with the
ring engaging element.
[0026] The membrane may be continuously adjustable between an
un-actuated state and fully distended state. The supporting ring
may be planar when un-actuated.
[0027] At each position between the un-actuated and fully distended
states the supporting ring may be displaced at the or each
actuation point by the distance required to achieve the profile
required to produce the predefined form of the membrane. This is
important so that at each position between the un-actuated and
fully distended states, the ring is positioned at the or each
actuation point at its desired location within the overall desired
profile of the ring. It will be understood that if the actuation
point were to be held in a different position by the ring engaging
member at that point then local distortion in the desired profile
of the ring would occur at that point leading potentially to
unwanted distortion in the shape of the membrane.
[0028] In some embodiments, one or more of said control points may
be hinge points, where the ring-engaging members are configured to
hold the supporting ring stationary relative to the support. The
supporting ring is required to remain stationary at the or each
hinge point to achieve the required actuated ring profile to
produce the predefined form of the membrane at each position
between the un-actuated and fully distended states. Thus, in the
same way as the actuation points, the ring must be held at each
hinge point by the ring-engaging member at that point in a position
that corresponds to the desired overall profile of the ring at each
state of the ring between the un-actuated and fully distended
states. Since the ring is not displaced at each hinge point, it
follows that the position of the ring at each hinge point must be
the same for each state of the ring between the un-actuated and
fully distended states. Where the predefined form has a centre,
there may be a plurality of hinge points that are substantially
equidistant from the centre of the predefined form.
[0029] In some embodiments two adjacent hinge points may define a
tilting axis, in which case there is suitably at least one
actuation point where the ring engaging member is configured
actively to displace the supporting ring relative to the support
for tilting the ring relative to the support about said tilting
axis in the first direction for compressing or the second direction
for expanding the envelope.
[0030] For some applications, the supporting ring may be generally
rectangular, having two short sides and two long sides. In such
cases, at least one actuation point may be located on one of the
short sides, and two adjacent hinge points may be located on or
proximate to the other short side. The predefined form may have a
centre which may be located off-centre with respect to the
membrane, being closer to the other short side than it is to the
one short side. The one short side may generally follow the arc of
a circle that is centred on the centre of the defined form. The at
least one actuation point may be located substantially centrally on
said one short side.
[0031] The supporting ring should be free to bend passively
relative to the support intermediate the control points. However,
in some embodiments it may be desirable to control the bending of
the ring by means of stiffening elements for stiffening one or more
regions of the supporting ring.
[0032] Advantageously the supporting ring may comprise two or more
ring elements, and the membrane may be sandwiched between two
adjacent ring elements.
[0033] According to another aspect of the present invention
therefore there is provided a deformable membrane assembly as
claimed in claim 18 below.
[0034] Suitably, the membrane may be pre-tensioned on the membrane
supporting ring. The inventors have realised that by sandwiching
the membrane between two adjacent ring elements, the torsional
forces applied by the membrane to the ring can be balanced out
resulting in no or substantially no net torsional force. It will be
appreciated that it is desirable to avoid torsional forces on the
ring which might lead to unwanted distortion in the shape of the
ring and thus in the shape of the membrane when distended. Thus, in
some embodiments, the membrane supporting ring may consist of two
ring elements. In some embodiments more than two ring elements may
be provided. However, the arrangement should be such that when the
membrane is pre-tensioned between the two adjacent ring elements,
the torsional forces on the ring elements above and below the
membrane cancel each other out or substantially cancel each other
out.
[0035] The means for adjusting the pressure within the envelope may
comprise a selectively operable device comprising one or more
components arranged to adjust the fluid pressure in the envelope.
In some embodiments, the means for adjusting the pressure of a
fixed volume of fluid within the membrane may comprise means for
compressing or expanding the envelope as mentioned above. Suitably,
a fixed support may be provided, and means may be provided for
compressing or expanding the envelope against the support to
increase or decrease the pressure of the fluid therein.
[0036] Suitably the supporting ring may have a substantially
uniform depth and a variable width to control the second moment of
area round the ring and thus the bending stiffness of the ring.
Typically the supporting ring may be narrowest where it is required
to bend the most to achieve the predefined form when the membrane
is distended.
[0037] In some embodiments the predefined membrane shape may be
spherical or another form defined by one or more Zernike
polynomials. These have the general formula Z.sub.n.sup..+-.m.
Various shapes, as defined by Zernike functions or combinations of
more than one such function, are possible using the lens assembly
of the present invention. A priority for ophthalmic applications,
for instance, is to be able to achieve vision correction with a
linear superposition of Z.sub.2.sup..+-.2 (astigmatism) and
Z.sub.2.sup.0 (sphere for distance correction). Opticians typically
prescribe lenses based on these formulae. Higher order surfaces
with additional components Z.sub.j.sup..+-.j are also possible if
additional control points are provided on the edge of the membrane,
where j scales in similar magnitude to the number of control
points. Higher order surfaces with components Z.sub.j.sup..+-.k
(k.ltoreq.j) may also be possible where the shape of the membrane
edge permits.
[0038] Further, various linear superpositions of scaled Zernike
polynomials of the form Z.sub.n.sup..+-.m are possible: [0039]
Z.sub.2.sup..+-.2, Z.sub.2.sup.0, Z.sub.j.sup..+-.j,
Z.sub.j.sup..+-.k (k.ltoreq.j)
[0040] In general, except at their periphery, surfaces achievable
by deforming a membrane with pressure may have one or more local
maxima or one or more local minima, but not both, in addition to
saddle points. The shapes that are achievable are necessarily
limited by the shape of the periphery, which is stable in use.
[0041] Suitably, the required bending stiffness round the ring may
be determined by finite element analysis (FEA). In particular, FEA
may be used to calculate the required variation in bending
stiffness round the ring for the ring to adopt the desired profile
when actuated in order to produce a membrane shape of the
predefined form. For quasi-static or low frequency optical and
other applications, static FEA should be employed adequately.
However, where the surface is intended for acoustic applications,
dynamic FEA may be appropriate. FEA--whether static or
dynamic--involves numerous iterations performed using a computer
with the input of selected parameters to calculate the membrane
shape that would result in practice with an increasing force
applied at the control points. The element shape may be selected to
suit the calculation being performed. The selected parameters to be
input may include the geometry of the supporting ring, the geometry
of the membrane, the modulus of the membrane, the modulus of the
ring, including how the modulus of the ring varies round the ring
(which may be defined empirically or by means of a suitable
formula), the amount of pre-tension in any of the parts, the
temperature and other environmental factors. The FEA programme may
define how the pressure applied to the membrane increases as load
is applied to the rings at the control points.
[0042] Within each iteration of the FEA the calculated shape of the
membrane is compared with the predefined form, and any deviation
between the calculated shape the predefined form used to adjust the
bending stiffness round the membrane supporting ring for the next
iteration. Progressively, the bending stiffness of the supporting
ring is adjusted so that the calculated shape of the membrane
converges with the desired predefined form.
[0043] A reinforcing diaphragm may be provided that is fastened to
the supporting ring, which diaphragm has a greater stiffness in the
plane of the ring than in the direction of bending of the ring.
[0044] In yet another aspect of the present invention therefore
there is provided a deformable membrane assembly as claimed in
claim 28 below.
[0045] As mentioned above, the membrane is suitably pre-tensioned
on the membrane supporting ring. The reinforcing diaphragm serves
to stiffen the ring in the plane of the membrane in the un-actuated
state against the additional loading that is created by the
pre-tensioning within the membrane, while allowing the ring to bend
freely in the direction normal to the ring. Alternatively the
supporting ring itself may have a greater bending stiffness in the
plane of the membrane in the un-actuated state than out of the
plane of the membrane.
[0046] Suitably, the reinforcing diaphragm may be fastened to the
supporting ring uniformly round the ring so that the tension in the
membrane is transmitted uniformly to the diaphragm.
[0047] In some embodiments, in the plane of ring, the membrane may
be longer in one dimension than it is in another dimension. In such
cases, the reinforcing diaphragm may have a lower stiffness in the
one dimension than it has in the other dimension. Alternatively the
geometry of the assembly may itself serve to may be used to
compensate for the consequent differential strain in the
membrane.
[0048] The means for adjusting the pressure within the envelope may
comprise a selectively operable device comprising one or more
components arranged to increase or decrease the fluid pressure in
the envelope. Typically the means for adjusting the pressure within
the fluid-filled envelope, which may contain a fixed volume of
fluid, may comprise means for compressing or expanding the
envelope. The fluid-filled compressible envelope may comprise an at
least partially rigid rear wall that is spaced from the distensible
membrane and a flexible side wall between the membrane and the rear
wall.
[0049] In some embodiments, the membrane, rear wall and fluid are
transparent such that the membrane and rear wall form an adjustable
optical lens. Where provided, the reinforcing diaphragm may also be
transparent.
[0050] Suitably said rear wall may be shaped to provide a fixed
lens.
[0051] The assembly may further comprise a protective rigid front
cover over the membrane. The front cover may be transparent.
Suitably the front cover may be shaped to provide a fixed lens.
[0052] Thus, in some embodiments, the front cover and/or rear cover
may provide a fixed optical power for the correction of refractive
errors such as myopia and hyperopia. The adjustable optical lens of
the invention may be used to provide an additive (or subtractive)
optical power to the fixed optical power of the front or rear lens
for the correction of presbyopia. Suitably the front and/or rear
lenses may be shaped for the correction of astigmatism, and
similarly the predefined form of the distended membrane of the
adjustable optical lens of the invention may be adapted for the
correction of astigmatism.
[0053] In some embodiments the envelope may be housed within a
retaining ring.
[0054] In yet another aspect of the present invention there is
provided an article of eyewear comprising a deformable membrane
assembly in accordance with the invention.
[0055] Said article of eyewear may typically comprise a frame with
a rim portion; the deformable membrane assembly may be mounted
within the rim portion.
[0056] Following is a description by way of example only with
reference to the accompanying drawings of embodiments of the
present invention.
[0057] In the drawings:
[0058] FIG. 1 is a perspective view from above of the front of a
pair of eyeglasses comprising a frame that is fitted with two first
lens assemblies according to the invention;
[0059] FIG. 2 is a perspective view from above and to the left of
the left hand side of the eyeglasses of FIG. 1 showing how one of
the first lens assemblies is fitted to the frame;
[0060] FIG. 3 is a front elevation of the first lens assembly in
accordance with the invention in the un-actuated state;
[0061] FIG. 4 is a cross-section of the first lens assembly along
the line IV-IV of FIG. 3;
[0062] FIG. 5 is a cross-section of the first lens assembly along
the line V-V of FIG. 3;
[0063] FIG. 6 is a cross-section of the first lens assembly along
the line VI-VI of FIG. 3;
[0064] FIG. 7 is a perspective view from below and to the left of
the front of the first lens assembly of the invention which is
shown cut-away along the line VI-VI of FIG. 3;
[0065] FIG. 8 is an exploded view of the first lens assembly
showing the parts of the assembly;
[0066] FIG. 9 is a front elevation of the flexible membrane and
membrane supporting rings of the first lens assembly in the
un-actuated state, showing how the width of the rings varies round
the periphery of the membrane to control the second moment of area
of the rings;
[0067] FIG. 10 shows the membrane and rings of FIG. 9 in an
actuated state and projected onto a notional sphere of radius
R;
[0068] FIG. 11 is a cross-section of the first lens assembly which
corresponds to FIG. 4 but shows the assembly in an actuated
state;
[0069] FIG. 12 is a cross-section of the first lens assembly which
corresponds to FIG. 5 but shows the assembly in an actuated
state;
[0070] FIG. 13 shows the displacement of the membrane of the first
lens assembly in an actuated state, as calculated by static finite
element analysis (FEA);
[0071] FIG. 14 shows the uniformity of the optical power of the
first lens assembly in an actuated state, as calculated by FEA;
[0072] FIG. 15 shows the variation of pre-tension in the membrane
as calculated by FEA of a lens assembly in the un-actuated state
that is similar to the first lens assembly but omits the
reinforcing diaphragm;
[0073] FIG. 16 shows the variation of pre-tension in the membrane
as calculated by FEA of the first lens assembly of the invention in
the un-actuated state;
[0074] FIG. 17 shows the variation in the optical power as
calculated by FEA of a lens assembly in an actuated state that is
similar to the first lens assembly but omits the reinforcing
diaphragm;
[0075] FIG. 18 shows the variation in the optical power as
calculated by FEA of the first lens assembly of the invention;
[0076] FIGS. 19A-C show schematically in cross-section the first
lens assembly of the invention in the un-actuated state (FIG. 19A),
an actuated state (FIG. 19B) and a de-actuated state (FIG.
19C);
[0077] FIGS. 20A-C show schematically the front elevation of the
first lens assembly of the invention in the un-actuated state (FIG.
20A), an actuated state (FIG. 20B) and a de-actuated state (FIG.
20C);
[0078] FIGS. 21A-C show schematically in cross-section a second
square lens assembly of the invention in an un-actuated state (FIG.
21A), an actuated state (FIG. 21B) and a de-actuated state (FIG.
21C);
[0079] FIGS. 22A-C show schematically the front elevation of the
second lens assembly of the invention in the un-actuated state
(FIG. 22A), an actuated state (FIG. 22B) and a de-actuated state
(FIG. 22C);
[0080] FIG. 23 shows how the distance between the optical centre
and the membrane supporting rings varies in the first lens
assembly;
[0081] FIG. 24 shows how the distance between the optical centre
and the membrane supporting rings varies in the second lens
assembly of FIGS. 21A-C and FIGS. 22A-C.
[0082] FIG. 25 shows schematically in cross-section a flexible
membrane and single supporting ring in accordance with the
invention; and
[0083] FIG. 26 shows schematically in cross-section the flexible
membrane and supporting rings of the first lens assembly in
accordance with the invention.
[0084] With reference to FIG. 1, a pair of eyeglasses 90 (UK:
spectacles) comprise a frame 92 having two rim portions 93 and two
temple arms 94. The rim portions 93 are joined together by a bridge
95, and each is shaped and dimensioned to carry a respective first
lens assembly 1 in accordance with the present invention. One of
the first lens assemblies 1 is used for the right side of the
eyeglasses, and the other is used for the left side. As can be seen
from FIG. 1 the right-hand and left-hand lens assemblies 1 are
mirror images of one another, but their construction is otherwise
identical, and therefore only the left-hand side one is described
in detail below, but it will be appreciated that the construction
and operation of the right-hand side one is the same.
[0085] As best seen in FIG. 3, the first lens assembly 1 has a
generally rectangular shape with two opposing long sides 3, 5 and
two short sides 7, 9 and is designed to fit with the frame 92, but
it will be appreciated that the shape of the first lens assembly
shown is only one example of a suitable shape, and a lens assembly
in accordance with the invention may be given any shape that is
desired. The invention is especially suited for non-round shapes,
such as the one shown in FIGS. 1 and 3, but the teachings of the
invention may also be applied to round lenses. In round lenses, the
invention may be used, by way of example, for the correction of
aberrations in an optical system requiring more than spherical
wave-front correction.
[0086] As well as eyeglasses, the lens assembly of the present
invention is equally well applicable to other lens applications,
such as goggles, helmets and scientific and optical instruments of
various sorts. In the lens assembly 1 the optical parts as
described below are transparent, but the invention also comprehends
other kinds of deformable membrane assemblies which are constructed
and operate in a similar manner to provide a controllable
adjustable surface, and thus such membrane assemblies in accordance
with the invention may also find application in non-optical fields,
such as acoustics where a surface with a selectively and
controllably adjustable shape may be required.
[0087] The first lens assembly 1 is especially suitable for use in
the correction of presbyopia. In use, the first lens assembly 1 can
be adjusted for bringing into focus objects at a range of distances
from long distance to close distance. In this embodiment there is
no correction provided for long distance, but nevertheless, the
first lens assembly 1 allows a user to re-focus smoothly from a
far-away object to a near, reading-distance object.
[0088] The first lens assembly 1 comprises a pair of membrane
supporting rings 2, 10 of uniform thickness but variable width. The
design of these rings is explained in more detail below. A
retaining ring 6 holds the parts of the first lens assembly 1
together.
[0089] In FIG. 8, the component parts of the first lens assembly 1
can be seen in exploded view. The front of the first lens assembly
1 is shown at the top right of the figure, and the rear of the
assembly (which in use would be closest to the wearer's eye) is at
the bottom, although it will be appreciated that all the other
parts fit into the retaining ring 6, which forms an enclosing
housing for said other parts.
[0090] At the front of the first lens assembly 1 is a transparent
front cover plate 4, made of glass or a suitable polymeric
material. In the first lens assembly the front cover plate is about
1.5 mm thick, but this may be varied as mentioned below. Further,
in some embodiments, as described below, the front cover plate 4
may comprise a lens of fixed focal power(s), for example a single
vision (single power), multi-focal (two or more powers),
progressive (graded power) or even an adjustable element. As shown
in FIG. 4 for example, in the present embodiment, the front cover
plate 4 is plano-convex.
[0091] Behind the front cover plate 4 are disposed two stiffening
ribs 3a, 3b, which provide extra stiffness at the short sides 7, 9
of the first lens assembly 1, as described in more detail below.
Next is a front one of the pair of resiliently bendable supporting
rings 2. The rings may be made of stainless steel and, in the first
assembly, are about 0.3 mm thick, but other suitable materials may
be used and the thickness adjusted accordingly to provide the
desired stiffness as discussed below. Next is a transparent
non-porous, elastic membrane 8. In the first assembly the membrane
8 is made of Mylar.RTM. and is about 50 microns thick, but other
materials with a suitable modulus of elasticity may be used
instead. Behind the membrane 8 is disposed a rear one of the pair
of bendable supporting rings 10 of substantially the same geometry
as the front supporting ring 2. The flexible membrane 8 is
pre-tensioned as described below and attached to and sandwiched
between the front and rear supporting rings 2, 10, such that it is
stably supported around its edge, as shown in FIGS. 3-7 in which
the first lens assembly 1 is shown in its assembled condition. The
membrane 8 forms a fluid-tight seal with at least the rear
supporting ring 10.
[0092] The rear surface of the second supporting ring 10 is sealed
to a transparent reinforcing diaphragm 24. In the first embodiment
the reinforcing diaphragm 24 may comprise a sheet of polycarbonate,
but alternative materials that are suitable to provide the required
properties as described below may be used instead. Behind said
diaphragm is a dish-shaped part 12 having a flexible side wall 18,
a rear wall 19 and a forward scaling flange 20. In the first
assembly the dish-shaped part 12 is made of transparent DuPont.RTM.
boPET and is about 6 microns thick, but other suitable materials
for the dish-shaped part may be used and the thickness adjusted
accordingly. The forward sealing flange 20 of the dish-shaped part
12 is sealingly adhered to the rear surface of the diaphragm 24
with a suitable adhesive such, for example, as Loctite 3555.
[0093] A layer of a suitable transparent pressure-sensitive
adhesive (PSA) such, for example, as 3M.RTM. 8211 (not shown)
adheres the rear wall 19 of the dish-shaped part 12 to a front face
17 of a transparent rear cover plate 16 having a rear face 14. In
the first lens assembly 1 described herein the layer of PSA is
about 25 microns thick, but this may be varied as required. The
rear cover plate 16 may be made of glass or polymer and in the
first assembly 1 is about 1.5 mm thick, but again this may be
varied as desired. The rear cover plate 16 sits as the rearmost
layer within the retaining ring 6. As with the front cover plate 4,
in some embodiments, the rear cover plate 16 may form a lens of a
fixed focal power. In the present embodiment, as seen in FIG. 4 for
example the rear cover plate 16 is a meniscus lens.
[0094] The retaining ring 6 comprises a forwardly extending side
wall 13 having an inner surface 23, which side wall 13 terminates
in a front edge 15. The front cover plate 4 sits on and is bonded
to the front edge 15 of the retaining ring 6 so that the lens
assembly constitutes a closed unit. As best seen in FIGS. 4, 5, 11
and 12, the cover plate 4 is spaced forwardly of the front membrane
supporting ring 2 to provide a space within which the membrane 8
may distend forwardly in use as described below without impinging
on the front cover plate.
[0095] The dish-shaped part 12, membrane 8, second supporting ring
10 and diaphragm 24 thus define a sealed interior cavity 22 for
holding a transparent fluid. For optical applications, such as the
first lens assembly 1 described here, the membrane 8 and the rear
face 14 of the rear cover plate 16 form the opposite optical
surfaces of an adjustable lens. As described above the rear cover
plate 16 is a meniscus lens. In an un-actuated state, the membrane
is planar, so the lens has the fixed optical power afforded by the
rear cover plate 16, with zero addition from the membrane 8.
However, when actuated as described below, the membrane 8 is
inflated to protrude forwardly in a convex configuration and thus
adds positive optical power to the fixed meniscus lens. In some
embodiments, the membrane may distend inwardly in a concave
configuration such that in combination with the rear face 14 of the
rear cover plate 16, the lens 1 is biconcave. The greater the
curvature of the membrane 8, the greater the additional optical
power afforded by the membrane 8. For non-optical applications the
fluid, along with the other parts of the assembly, do not need to
be transparent.
[0096] The side wall 18 of the dish-shaped part 12 provides a
flexible seal between the rear wall 19 and the diaphragm 24, thus
forming the sides of the cavity 22. This flexible seal is provided
so that there can be relative movement between the supporting rings
2, 10 and the rear cover plate 16 when the first lens assembly 1 is
actuated to adjust the power of the lens. The deformable membrane 8
is adhered to the first 2 and second 10 supporting rings, for
example by Loctite.RTM. 3555.
[0097] The cavity 22 is filled during manufacture with a
transparent oil 11 (see FIG. 7), such for example as Dow Corning
DC705, which is chosen to have an index of refraction as close as
possible to that of the rear cover plate 16. The oil 11 is also
chosen so as to not be harmful to a wearer's eye in the event of a
leakage.
[0098] As shown in FIGS. 6 and 7, the first lens assembly 1 may be
received and seated snugly in a rear rim part 93b which is shaped
and dimensioned to mate with a front rim part 93a as shown in FIG.
2 to form one rim portion 93 of the frame 92 of the eyeglasses 90.
The front and rear rim parts 93a, 93b may be fixed together by any
suitable means available to the person skilled in the art. For
instance, the front and rear rim parts may be formed with matching
screw holes 97 that are adapted to receive small fixing screws for
holding the two rim parts securely together and to retain the lens
assembly 1 therebetween. In some embodiments, the rear rim portion
93b may be formed integrally with the retaining ring 6.
[0099] In some embodiments the reinforcing diaphragm 24 may be
omitted, in which case the sealing flange 20 of the dish-shaped
part 12 would be attached directly to the rear surface of the rear
supporting ring 10.
[0100] It will be appreciated that the present invention is not
limited to the particular materials and dimensions given above,
which are given only by way of example. Different types of
materials may suitably be used for the dish-shaped part 12 that are
optically clear, have low overall stiffness compared with the
supporting rings 2, 10 and are joinable to the diaphragm 24 or rear
supporting ring 10.
[0101] Various different materials may suitably be used for the
supporting rings 2, 10 provided they fulfil the criteria of: having
sufficiently high modulus to be able to be made thin relative to
the overall depth of the first lens assembly 1 (i.e. of the order
of 0.3 mm thickness); being joinable to the adjacent components;
having low creep (to continue to perform over multiple uses); and
being elastically deformable. Other possibilities are titanium,
glass and sapphire. By "joinable" is meant by joinable by adhesive,
crimping, laser welding or ultrasonic welding or any other means
that would be apparent and available to those skilled in the
art.
[0102] Different adhesives may suitably be chosen that arc able to
join the parts of the assembly durably, are creep resistant, are of
a suitable viscosity to be applied when constructing the lens
assembly and remain inert in the presence of the fluid in the lens.
Particular adhesives may be chosen in dependence on materials
selected for the various parts.
[0103] There are various other suitable materials that permit
sufficient flexing of the membrane 8, and various colourless oils
may be used, particularly in the family of high refractive index
siloxane oils for which there are a number of manufacturers. The
materials chosen for the various components need to be such that
they provide stability around the hinge and actuation points
(described below with reference to FIGS. 9 and 10).
[0104] The first lens assembly 1 provides an adjustable lens having
a focal power that can be adjusted by controlling pressure of the
fluid 11 within the cavity 22 and the shape of the bendable
supporting rings 2, 10, thereby controlling deformation of the
elastic membrane 8 into a desired profile. As mentioned above the
membrane 8 forms one of the optical surfaces of the lens, the other
one being the rear face 14 of the rear cover plate 16. Deformation
of the membrane 8 increases the curvature of the optical surface
provided by the membrane and changes the optical thickness of the
lens between the surfaces, thereby increasing the additional
optical power afforded by the membrane 8. Details of this operation
are given below.
[0105] As best seen in FIG. 9 the width of the supporting rings 2,
10 in the x-y plane normal to the front-rear z-axis of the lens
assembly 1 varies in a predetermined manner round the periphery of
the assembly 1. This is to provide for the desired deformation of
the supporting rings 2, 10 which in turn controls deformation of
the flexible membrane 8 and hence the power of the lens, as
explained in more detail below.
[0106] It can be seen from FIG. 8 that each of the supporting ribs
3a, 3b, the supporting rings 2, 10 and the reinforcing diaphragm 24
has a protruding tab 26 of similar shape and size which protrudes
outwardly of the first lens assembly 1 from one of the short sides
7 of the assembly 1. When assembled, the tabs 26 on these parts are
aligned with each other, and each is formed with one or more
closely adjacent holes 28a, 28b that align with the corresponding
holes in the other parts. These holes 28a, 28b define an actuation
point {circle around (A)} for attachment of an actuation device to
the lens assembly 1 to cause it to be compressed in use.
Compression of the lens 1 is described in more detail below. The
actuation device may be housed in the adjacent temple arm 94 of the
frame 92. In some embodiments the lens assembly may be expanded in
a similar manner to reduce the pressure of fluid 11 within the
cavity 22.
[0107] Adjacent the protruding tab 26 at the one short side 7 of
the assembly, the inner edge of each of the supporting rings 2, 10
deviates outwardly as best shown in FIG. 9 to form a generally
semi-circular recess 30. The side wall 18 of the dish-shaped part
12 has a similar, corresponding recess 30 which aligns with the
recesses 30 of the supporting rings 2, 10 when the lens is
assembled. The membrane 8 includes a corresponding semi-circular
protruding portion 31 that aligns with the recesses 30 to ensure
the closure of the seal afforded by the membrane. The reinforcing
diaphragm 24 is cut-out at 32a, which also aligns with the tabs 26.
This allows filling of the reservoir 22 after all the parts have
been assembled by protruding beyond the extent of the front and
rear cover plates 4, 16. Alternatively as shown in FIG. 10 a
separate hole 31' though the supporting rings 2, 10 may be provided
instead of said semi-circular recess 30.
[0108] The reinforcing diaphragm 24 affords significant
improvements over prior fluid-filled lenses by dint of its function
to stiffen the supporting rings 2, 10 in the plane defined by the
rings in the un-actuated state. It is desirable to pre-tension the
membrane 8 when assembling the parts, otherwise undesired wrinkles
or sag may appear in the membrane owing to temperature and
gravitational or inertial effects on the fluid pressure and the
like. One way to minimise the risk of such wrinkles or sag would be
to support the flexible membrane 8 on an inflexible supporting
ring, but this would be incompatible with the need for the
supporting rings 2, 10 to bend in use. The reinforcing diaphragm
24, which strengthens the supporting rings 2, 10 in the plane of
the membrane 8 to resist bending, but does not significantly add to
the stiffness of the rings transverse the membrane (z axis),
provides a solution to this problem.
[0109] In the first lens assembly 1 described herein, in which the
distance between the long sides 3, 5 is less than the distance
between the short sides 7, 9--making the first assembly generally
rectangular. The lens is thus wider in the E-W direction between
the short sides 7, 9 as shown in FIG. 9 than it is in the N-S
direction between the long sides 3, 5. The supporting rings 2, 10
are configured to bend more along the long sides. It will be
appreciated that, when actuated, the membrane 8 is stretched more
in the E-W direction than it is in the N-S direction, Since the
diaphragm 24 can only bend and not distend, it can only bend in one
direction, so it bends along the E-W axis of the lens. Bending a
beam brings the two ends of it slightly closer together, and this
compensates for the differential strain in the membrane 24.
[0110] In some embodiments, the diaphragm 24 may be made stiffer in
the E-W direction than in the N-S direction, and this directional
stiffness of the diaphragm 24 may be used to compensate for the
above-mentioned differential strain in the membrane 8.
[0111] In the first lens assembly 1, the reinforcing diaphragm 24
is made from a transparent material that is index-matched with the
membrane 8 and the fluid 11 within the cavity 22. It comprises a
flat sheet that is placed within the fluid of the lens between the
sealing flange 20 of the dish-shaped part 12 and the rear
supporting ring 10, so that it lies behind the flexible membrane 8
in the assembled lens 1, as best seen in FIGS. 4 and 5. The
diaphragm 24 is shaped similarly to the other parts of the lens
assembly 1, and in the first assembly is 0.55 mm thick, although
this thickness may be varied as desired. Since the diaphragm 24 is
attached to the dish-shaped part 12 and the rear supporting ring 10
round its edge, the stiffness of the supporting rings 2, 10 must be
adjusted accordingly such that they are still able to bend as
required in the z direction transverse the plane of the membrane
8.
[0112] The reinforcing diaphragm 24, in accordance with the
invention, has been found to work better than, for example,
localised support of the supporting rings 2, 10. In one embodiment,
the supporting ring size and stiffness may be reduced by
approximately 25% as compared with the size and stiffness of
similar supporting rings 2, 10 that are stiff enough by themselves
to prevent wrinkles without an associated diaphragm 24. The
necessary ability of the supporting rings 2, 10 to flex to control
the deformation of the flexible membrane 8 is not impaired. A
suitable material for the support disc 24 is polycarbonate, but
other materials may suitably be used. The reinforcing diaphragm 24
of the invention is equally suitable for use in round lenses as it
is for non-round lenses, but in such other embodiments the
diaphragm does not necessarily need to have differential stiffness
on different axes.
[0113] The design of the reinforcing diaphragm 24 is such that its
main effect is to increase the stiffness of the supporting rings 2,
10 in the in-plane direction normal to the front-rear axis of the
assembly (x-y plane in FIG. 10), but has only a small effect on the
bending stiffness in the z direction (i.e. normal to the rear wall
19). This z-direction effect is accounted for in the design of the
supporting rings 2, 10. Thus the stiffness of the assembly 1 is
increased for the purpose of maintaining tension in the flexible
membrane 8, but the supporting rings 2, 10 can still bend in the z
direction in use. This may be achieved by choosing for instance a
fibre material which has stiffness in the x-y plane but little
stiffness in the z-direction owing to the orientation of the
fibres. The diaphragm 24 is formed with a plurality of apertures
32a, 32b; in the first lens assembly 1 described herein there are
two--one adjacent the aforementioned tab 26, and the other in a
corner of the other opposite short edge 9 of the assembly. The
material surrounding the apertures 32a, 32b provides the stiffness,
but the apertures 32a, 32b allow fluid to pass through and hence
have little or no effect on deformation of the flexible membrane 8.
The precise number, size and arrangement of the apertures 32a, 32b
may be varied as desired--for example a plurality of smaller
apertures spaced across the diaphragm 24 may be provided. The
diaphragm 24 does not deform with the flexible membrane 8, and the
support it provides for the membrane 8 is not needed when the lens
is in an actuated state with the membrane distended as described
below. In the first lens assembly 1 the reinforcing diaphragm 24
comprises a continuous sheet that is formed with a number of
apertures 32a, 32b as described above, but in other embodiments,
the diaphragm may comprise a reticulated sheet or a mesh or the
like, as long as it is joined to the supporting rings 2, 10 round
substantially their whole extent in order to provide the desired
in-plane stiffness. The diaphragm may be connected to the rings 2,
10 substantially continuously or at spaced locations around its
periphery provided that the load is distributed uniformly without
giving rise to any significant local distortion of the rings or
membrane 8. In non-optical applications, there is no need for the
diaphragm to be transparent.
[0114] As best seen in FIG. 6 the inner surface 23 of retaining
ring 6 is formed with two spaced circumferential shelves 34, 36: a
rear shelf 34 and a forward shelf 36. The rear shelf 34 is disposed
proximate the rear of the retaining ring 6; the rear cover plate 16
is supported on said rear shelf. The forward shelf 36 is disposed
intermediate the front edge 15 of the retaining ring 6 and serves
to support the diaphragm 24 and front and rear supporting rings 2,
10. The side wall 18 of the dish-shaped part 12 is dimensioned such
that its front sealing flange 20 is supported on the forward shelf
36 when the lens is assembled.
[0115] At said other short side 9 of the first lens assembly 1, the
retaining ring 6 defines two hinge points {circle around (H)}1,
{circle around (H)}2--see FIG. 10. As shown in FIG. 4, the stacked
parts 2, 3b, 8, 10, 12, 24 are held in place within the retaining
ring 6 by means of formations 39 formed integrally with the
retaining ring 6 at the hinge points {circle around (H)}1, {circle
around (H)}2, such that they remain stable when the lens is
actuated as described below.
[0116] The supporting rib 3b provides additional stiffness for the
supporting rings 2, 10 in the region of the hinge points {circle
around (H)}1, {circle around (H)}2 and between them. In the first
lens assembly 1, the hinge points {circle around (H)}1, {circle
around (H)}2 and the region of the supporting rings 2, 10 between
them are approximately equidistant from the optical centre OC of
the lens when actuated (see FIG. 10), and so the rings 2, 10
intermediate the hinge points {circle around (H)}1, {circle around
(H)}2 are not required to bend much or at all. The other supporting
rib 3a similarly provides additional stiffness for the supporting
rings 2, 10 at the aforementioned actuation point {circle around
(A)} so that deformation of the membrane 8 is properly controlled,
as explained in more detail below. In some embodiments the
supporting ribs 3a, 3b may be omitted; they are generally useful
for regions of the supporting rings 2, 10 that are not required to
deform significantly during actuation of the assembly.
[0117] The shape of the first lens assembly 1 is suitable for the
eyeglasses 90 in terms of its aesthetic appearance. However, a
non-round lens gives rise to the problem of non-uniform, or
undesired, deviation from the desired shape of deformation of the
membrane, which would occur in the absence of a solution to the
problem. The means by which the present invention addresses and
solves this problem is explained below.
[0118] FIG. 10 illustrates how a surface of the desired shape is
achieved using a membrane assembly of the invention. In FIG. 10,
the desired shape is spherical, but as described in more detail
below the assembly of the invention can be used to form other
shapes; for instance shapes defined by one or a combination of
Zernike polynomials. For non-optical applications, different shapes
may be required. The lens assembly 1 in an actuated state is shown
in FIGS. 11 and 12.
[0119] FIG. 10 thus shows the membrane 8 of the non-round first
lens assembly 1 in an actuated state projected onto an imaginary
sphere of radius R to afford a positive focal power. The actuation
point {circle around (A)} and hinge points {circle around (H)}1,
{circle around (H)}2 are shown. A force F may be applied at the
actuation point {circle around (A)} by means of an actuation device
connected via the holes 28a, 28b.
[0120] The lower half of FIG. 10 shows a section on the line b-b of
the upper half through the optical centre OC at the vertex of the
membrane 8 in the actuated state. The direction of application of
the force is shown (downwards in FIG. 10). The membrane 8 is
distended in a substantially part-spherical configuration, and the
edge of the membrane 8 defined by the supporting rings 2, 10 has a
profile that substantially follows the surface contours of the
sphere. In the un-actuated state the membrane 8 is flat, and the
edge of the membrane (and thus the supporting rings 2, 10) is also
flat--represented by line L in the lower half of FIG. 10. In the
actuated state, the membrane 8 substantially follows the surface of
the sphere, and its edge no longer lies in a plane (as it would do
if the lens were circular and the membrane formed a spherical cap).
This can be seen by comparing the edge of the membrane with the
line L. In the actuated state the membrane 8 is displaced at the
actuation point {circle around (A)} below the line L, representing
the plane of the membrane 8 in the un-actuated state, but where the
long sides 3, S of the membrane deviate (inwardly) from a round
shape, they are displaced above the line L, so that a major portion
of the edge of the membrane would fit contiguously against the
surface of a sphere of radius R.
[0121] In FIG. 10 the optical centre OC is located, according to
ophthalmic convention, at a predetermined distance from the centre
of the bridge 94 of the eyeglasses 94. This distance is half the
centration distance, which is the distance between the optical
centres of the two lenses 1 of the eyeglasses 90, which in turn is
the optimum distance for a wearer of the eyeglasses. With the shape
of lens illustrated, the point OC is approximately central between
the long sides 3, 5 of the lens assembly, but is positioned
leftwards of the visually observed geometric centre on the axis
between the short sides (i.e. from eye to nose when worn).
[0122] The lens assembly of the present invention is adapted to
provide a continuously adjustable lens power by a desired number of
dioptres D, typically 0 to +4D, which is additive with any lensing
power afforded by the front cover plate 4 and/or rear cover plate
16. In general, the power of a lens D is given by the product of
the difference in refractive index of the lens material and its
environment, and the curvature of the interface. Thus the formula
is:
D=(n-1)(1/R) (I)
[0123] Where n is the refractive index, 1 is taken as the
refractive index of air and R is the radius of the sphere of which
the lens forms part (as illustrated in FIG. 3b).
[0124] In the lower half of FIG. 10, the edge of the membrane 8 is
maximally displaced at the actuation point {circle around (A)} in
the direction of application of the force F. The hinge points
{circle around (H)}1, {circle around (H)}2 coincide with points on
the edge of the membrane 8 (as defined by the supporting rings 2,
10 in the first lens assembly 1) that involve substantially no
displacement upon deformation of the membrane 8. It can be seen
that these points in the actuated position have not moved from and
lie approximately on the line L. (Note they are out of plane of the
section shown in the lower half of FIG. 10). In order to control
optimally the deformation of the membrane 8, the hinge points
{circle around (H)}1, {circle around (H)}2 should be located where
minimal movement or no movement of the edge of the membrane 8 is
required, otherwise the profile of the edge of the membrane would
deviate at the hinge points {circle around (H)}1, {circle around
(H)}2 from the desired spherical (or other) shape, resulting in
unwanted distortion of the membrane. Suitably the hinge points
{circle around (H)}1, {circle around (H)}2 may be generally
equidistant from the optical centre OC as mentioned above, so that
they lie on the same circular contour of displacement when the lens
is actuated, i.e. a contour of no displacement. However, depending
on the shape and other parameters of the lens assembly 1 this may
not be possible, and some difference in the distances between the
respective hinge points {circle around (H)}1, {circle around (H)}2
and the optical centre OC can be tolerated, notwithstanding the
resulting distortion that will occur in the vicinity of one or both
hinge points {circle around (H)}1, {circle around (H)}2. In FIG.
10, it can be seen that one hinge point {circle around (H)}1 is
situated further from the centre OC than the other hinge point
{circle around (H)}2, leading to some distortion of the membrane in
the corners of the lens adjacent the hinge points {circle around
(H)}1, {circle around (H)}2, but this is tolerable, provided there
is a major zone around the centre OC where little or no distortion
occurs. This is best shown in FIG. 13.
[0125] It will be appreciated that maximal displacement of the
membrane 8 occurs at the actuation point {circle around (A)}, which
should always lie on the desired locus of displacement of the
membrane edge to define a spherical-fitting profile between the
un-actuated and maximum focal power positions. Since the edge of
the membrane 8 at the one short side 7 of the lens, which includes
the actuation point {circle around (A)}, happens to be
substantially circular should be it follows a circular contour of
displacement when actuated, but again some deviation from circular
can be tolerated. The actuation point should therefore be located
on the one short side 7 at the point furthest away from the optical
centre OC. Were the particular shape considered here not such that
a segment of its perimeter formed a circular are about the optical
centre, additional actuation point(s) (active or passive) may be
required to maintain the surface fidelity. It will be seen from
FIG. 10 that in the first lens assembly 1, the points furthest away
from the centre OC are in the corners of the membrane 8, between
the long sides 3, 5 and the one short side 7--identified as
positions {circle around (B)} and {circle around (C)} in FIG. 10.
However the actuation point {circle around (A)} is proximate to
these points and the stiffening rib 3a serves to distribute the
load applied at the actuation point {circle around (A)} along the
one short side 7 of the membrane 8 with an acceptable degree of
distortion of the membrane shape.
[0126] Those skilled in the art will understand that the optical
power of the first lens assembly 1 can be varied effectively by
varying the radius R of the sphere, which varies the curvature of
the optical surface provided by the flexible membrane 8 and hence
adjusts the power of the lens. As R is reduced, the optical power
of the lens increases because the curvature of the membrane is more
pronounced. This is achieved by greater deformation of the membrane
8, which in turn is effected by increasing the displacement of the
supporting rings 2, 10 at the actuation point {circle around (A)}
rearwardly towards the rear cover plate 16, resulting in greater
fluid pressure in the cavity and greater forwards distension of the
membrane.
[0127] The way that this variable deformation is achieved for the
first lens assembly 1 according to the invention is described in
greater detail below.
[0128] FIGS. 3-5 show the first lens assembly 1 is its un-actuated
state, and FIGS. 11 and 12 show an exemplary actuated state. In
practice the first lens assembly 1 is continuously adjustable
between the un-actuated state and its maximum deformation; the
actuated position of FIGS. 11 and 12 is just one deformed position
which is provided as an exemplar of all deformed positions. As
described above, the width of the supporting rings 2, 10 varies
round their extent, while their thickness in the z-direction
remains substantially constant. Specifically the rings 2, 10 are
widest at the short sides 7, 9 of the assembly 1 and become
progressively narrower away from those short sides towards the
middles of the long sides 3,5 as best seen in FIG. 9. They are
thinnest at points {circle around (D)} and {circle around (E)} on
the longer sides intermediate the short sides 7, 9 (see FIG. 10).
Note the thinnest points are not necessarily symmetrical as between
the two long sides; they are thinnest in this region because this
is where their bending needs to be greatest, as can be understood
with reference to FIG. 10 described above.
[0129] In operation, in order to increase the focal power of the
lens assembly 1, an actuating force F is applied, directly or
indirectly, to the supporting rings 2, 10 at the point {circle
around (A)} on the one short side 7 of the assembly to move the
supporting rings 2, 10, and the membrane 8 clamped between them,
rearwardly towards the rear cover plate 16. The force is applied
about half-way along the one short side 7 and the actuating device
should be arranged to react against the retaining ring 6 which is
held within the rim 93 of the frame 92 which thus serves as a
support.
[0130] There are various means by which the actuating force may be
applied that will be apparent to those skilled in the art; some
embodiments are disclosed below. The force should be applied in a
direction that is substantially normal to the plane of the
supporting rings 2, 10. As described above, the supporting rings 2,
10 are hinged at the two points {circle around (H)}1, {circle
around (H)}2 on the other short side 9 of the assembly 1. The hinge
points are designed to remain stable during actuation of the lens
assembly 1 by means of the formations 39 within the retaining ring
6: when assembling the lens assembly 1, the rear cover plate 16,
with the dish-shaped part 12 attached thereto, the diaphragm 24 and
the supporting rings 2, 10 with the membrane 8 held between them
are pre-assembled as a stack and then inserted into the retaining
ring 6 and slid under the formations 39 at the hinge points {circle
around (H)}1, {circle around (H)}2. The side wall 18 of the
dish-shaped part 12 allows a small amount of movement, so that the
support rings 2, 10 can move closer towards the bottom wall 19 of
the dish-shaped part 18 to increase the pressure of the fluid
within the cavity, which in turn causes the membrane 8 to distend
forwardly towards the front cover plate 4, adopting a spherical (or
other) shape as shown in FIG. 12, thereby to increase the focal
power of the lens, as described above. Even though the membrane is
non-round, it is able to adopt the desired spherical (or other
shape) form by virtue of the construction of the supporting rings
2, 10.
[0131] The force applied to the one short side 7 of the supporting
rings 2, 10 at the actuation point {circle around (A)}, in
combination with the hydrostatic pressure applied to the membrane
by the fluid within the cavity, causes the supporting rings 2, 10
to bend. FIG. 11 shows the supporting rings 2, 10 exhibiting a
degree of bending upon application of the actuating force F. The
supporting rings 2, 10 remain substantially stationary at the hinge
points {circle around (H)}1, {circle around (H)}2 (although there
is a degree of local tilting of the rings 2, 10 at these points).
However, towards the middles of the long sides 3,5 of the assembly
including points {circle around (D)} and {circle around (E)}, the
rings flex forwards as described above, in an opposite direction to
the force F, so that the supporting rings 2, 10 adopt a profile
that would conform to the surface of a sphere (or other form)
having same shape as the membrane 8. If the supporting rings 2, 10
were circular, they would remain planar when the membrane deforms
spherically, but the non-round shape of the rings 2, 10 implies
that they cannot remain flat when the membrane is distended,
[0132] The ability of the supporting rings 2, 10 to flex in this
manner and thus control the deformation of the membrane 8 to avoid
unwanted distortions of the spherical or other shape is made
possible by the predetermined variation in width of the supporting
rings 2, 10 round their extent, and in particular in view of the
fact that they are made narrower at the points where they are
required to bend the most to adopt the desired profile. The
predetermined variation in the width of the supporting rings 2, 10
produces a corresponding variation in cross-sectional area of the
support rings 2, 10 and thus a corresponding predetermined
variation of the second moment of area of the support rings. In
particular the width of the supporting rings 2, 10 is continuously
adjusted around the rings and reaches a minimum towards the middles
of the long sides 3, 5 where the bending is thus greatest. In the
absence of significant variation in other parameters, a difference
in the second moment of area results in a difference in the bending
stiffness.
[0133] As shown in FIGS. 10-12, the flexible membrane 8 is caused
to bulge forwards in an opposite direction to that of the actuating
force F. When the supporting rings 2, 10 are moved closer to the
rear of the cavity at the actuation point {circle around (A)}, the
liquid 11, being essentially incompressible, is forced to occupy a
more central region of the cavity 22, owing to the elasticity of
the membrane 8, thus increasing the curvature of the optical
surface defined by the membrane 8 and the optical thickness of the
cavity between the membrane 8 and the rear supporting plate 16 at
the optical centre OC of the assembly, thus producing a higher
power lens. Specifically, the deformation of the flexible membrane
8 is centred on the point OC as shown in FIG. 10 which thus forms
the vertex of the lens.
[0134] In prior art fluid-filled lenses, in order to ensure
spherical bulging of the membrane, the membrane is held by a
supporting structure that is stiff and circular, so that only a
circular portion of the membrane is unconstrained and can bulge
forwards upon increasing the pressure of fluid. In some lenses (see
e.g. GB 2353606 A) this is achieved by making the entire lens
assembly circular in shape. In other lenses such for example as the
one disclosed in WO 95/27912, the supporting structure comprises a
stiff border around a circular central aperture where the membrane
can bulge forwards. In WO 95/27912 the border is wide and bulky in
places, which is aesthetically undesirable. By contrast in the
present invention, whilst the short sides 7, 9 of the supporting
rings 2, 10 are somewhat wider than the long sides 3, 5, as can be
seen from FIG. 9, they are still relatively narrow in comparison
with the area of the lens. Thus from an aesthetic point of view,
spherical (or other) deformation of the membrane 8 is achieved
without any significant adverse impact on the appearance of the
lens assembly 1, which has a non-circular shape and relatively thin
edges.
[0135] Upon actuation, when the flexible membrane 8 bulges forwards
as shown in FIGS. 10 and 11, the amount of fluid 11 held in the
cavity 22 remains constant, but because the membrane 8 changes in
shape from a relatively flat profile to the distended profile
shown, some of the transparent oil is displaced into the central
area of the lens. The displacement of the oil causes the membrane
to adopt the actuated shape, thus increasing the power of the lens.
The fluid 11 is sealed within the cavity 22 by the membrane 8, the
diaphragm 24 and the dish-shaped part 12.
[0136] It will be understood by those skilled in the art that the
spherical deformation of the supporting rings 2, 10 and of the
flexible membrane 8 that is depicted in FIG. 10 is provided by way
of example only to illustrate the change in shape of the various
parts of the assembly 1, and that the deformation provided by the
assembly of the invention may vary from that shown. In particular
for any given lens assembly 1, the membrane 8 is continuously
deformable between its un-actuated position, in which it is planar,
and a fully distended position, as determined by the actual
configuration and properties of the materials used for the assembly
1. In each position between the un-actuated position which provides
no optical power and the fully distended position, the hinge points
{circle around (H)}1, {circle around (H)}2 on the supporting rings
2, 10 remain essentially stationary and at least a major portion or
portions of the supporting rings 2,10, including the hinge points
{circle around (H)}1, {circle around (H)}2, adopt a spherical (or
other form) profile.
[0137] The actual variation in width of the support rings 2, 10
that is required to obtain the predetermined variation in bending
moment round the rings, as described above, may be calculated by
Finite Element Analysis (FEA). For quasi-static or low frequency
optical and other applications, static FEA should be employed
adequately. However, where the surface is intended for acoustic
applications, dynamic FEA is appropriate. As those skilled in the
art will be aware, FEA--whether static or dynamic--involves
numerous iterations performed using a computer with the input of
selected parameters to calculate the membrane shape that would
result in practice with an increasing force F applied at the
actuation point(s). The element shape is selected to suit the
calculation being performed. For the design of the rings 2, 10 of
the present invention, a tetrahedral element shape has been found
to be suitable. The selected parameters to be input include the
geometry of the supporting rings 2, 10, the geometry of the
membrane 8, the modulus of the membrane 8, the modulus of the rings
2, 10, including how the modulus of the rings varies round the
rings (which may be defined empirically or by means of a suitable
formula), the amount of pre-tension in any of the parts, the
temperature and other environmental factors. The FEA programme
defines how the pressure applied to the membrane 8 increases as
load is applied to the rings at the actuation point {circle around
(A)}.
[0138] An example FEA analysis output for a supporting ring is
shown in FIG. 13. The greyscale shows the degree of displacement of
the membrane 8 away from its planar un-actuated configuration;
contours of displacement are superimposed on the greyscale. The
membrane shows maximal forwards deformation in its central region
and maximal rearwards deformation (in the direction of the applied
force F) at the actuation point {circle around (A)}, with circular
contours proving essentially spherical deformation. This figure
shows the deformation in 2-dimensions; it will be understood
however that this corresponds to 3D spherical deformation in
practice. The first lens assembly 1 of the invention achieves a
substantially undistorted spherical lens, centred on the point OC.
It can be seen from FIG. 13 that the point OC is different from the
observed geometric centre of the lens 1, which is shown by the
point where the vertical and horizontal lines cross. This FEA
output is referred to as the "first FEA output" below.
[0139] In order to design precisely the rings 2, 10 for optical
use, the output of the FEA analysis may be approximated to the
desired shape of the membrane as defined by a polynomial function.
In general terms, the shape of an optical surface may described by
one or more Zernike polynomial functions. These have the general
formula Z.sub.n.sup..+-.m. Various shapes, as defined by Zernike
functions or combinations of more than one such function, are
possible using the present invention. An explanation of the various
Zemike polynomials can be found in Principles of Optics.sup.1
"Principles of Optics" M. Born and E. Wolf, 7.degree. Ed, C.U.P.,
(1999). ISBN 0-521-64222-1
[0140] A priority for ophthalmic applications, for instance, is to
be able to achieve vision correction with a linear superposition of
Z.sub.2.sup..+-.2 (astigmatism) and Z.sub.2.sup.0 (sphere for
distance correction). Opticians typically prescribe lenses based on
these formulae. Higher order surfaces with additional components
Z.sub.j.sup..+-.j are also possible in accordance with the present
invention if additional control points (as described below) are
provided on the edge of the membrane, where j scales in similar
magnitude to the number of control points. Higher order surfaces
with components Z.sub.j.sup..+-.k (k.ltoreq.j) may also be possible
where the shape of the membrane edge permits.
[0141] Variants of the first lens assembly 1 of the invention are
able to produce static membrane shapes corresponding to any such
polynomial for which j=k. Various complex surfaces are known to be
possible and useful for certain applications. For example, laser
vision correction surgery often works to certain higher order
functions, and thus alternative embodiments of the lens assembly of
the invention might be used as an alternative to laser surgery.
Various linear superpositions of scaled Zernike polynomials of the
form Z.sub.n.sup..+-.m are possible: [0142] Z.sub.2.sup..+-.2,
Z.sub.2.sup.0, Z.sub.j.sup..+-.j, Z.sub.j.sup..+-.k
(k.ltoreq.j)
[0143] In general, except at their periphery, surfaces achievable
by deforming a membrane with pressure may have one or more local
maxima or one or more local minima, but not both, in addition to
saddle points. The shapes that are achievable are necessarily
limited by the shape of the periphery, which is stable in use.
[0144] In some embodiments of the lens assembly of the present
invention, a spherical Zernike function may be used, but higher
spherical order functions can also be used if desired, by creating
a shape that is the sum of a number of Zernike polynomials.
[0145] The first FEA output is then correlated with the desired
Zernike function across the membrane ("second polynomial output")
to see how well the first FEA output approximates to the desired
shaped as defined by the chosen Zernike function. Depending how
well the first FEA and second polynomial outputs correlate with one
another, the relevant parameters of the lens can be adjusted to
achieve a better fit on the next iteration. In other words, by
seeing how well the simulated deformation of the membrane 8, as
calculated by FEA, approximates to the desired surface shape as
described by the selected Zernike polynomial function, one can see
how well the chosen supporting ring 2, 10 parameters perform. It is
possible to determine which regions of the supporting rings 2, 10
need to be tuned (or which other parameters should be adjusted) to
improve the correlation of the first and second outputs.
[0146] The above-described iterative process is carried out over a
number of different lens powers so that a lens whose power varies
continuously with deformation of the supporting rings 2, 10 (and
the force F applied) can be designed. This iterative process has
been carried out to achieve a number of working embodiments of the
supporting rings 2, 10 in accordance with the invention. Thus the
supporting rings 2, 10 are designed to bend variably round their
extent and with respect to the adjustment in lens power required.
The variation in width of the supporting rings 2, 10 in the x-y
plane, perpendicular to the optical z-axis of the lens, round their
extent can also be adjusted for different lens shapes, taking into
account the locations of the hinge points {circle around (H)}1,
{circle around (H)}2 and actuation point {circle around (A)}
relative to the desired optical centre OC.
[0147] Once the shape of the membrane 8 has been calculated by FEA
as described above, the optical properties of the membrane as an
optical lens surface may be determined by suitable optical ray
tracing software (e.g. Zemax.TM. optical software available from
Radiant Zemax, LLC of Redmond, Wash.) using the calculated membrane
shape. By way of example, FIG. 14 shows how the spherical lens
power varies across the membrane 8 of the first lens assembly 1
when distended, the distended shape being calculated by static FEA.
The darkest areas show the greatest lens power, and as can be seen
from FIG. 14, the inflated membrane 8 produces a lens surface which
has a satisfactorily uniform spherical lens power.
[0148] In view of the fact that the degree of deformation of the
flexible membrane 8 can be adjusted smoothly through a range, the
lens assembly of the invention represents a significant improvement
over conventional bifocal lenses, where the wearer needs to glance
downwards to look through the near-vision lens. By using the lens
assembly 1 of the present invention, the lens power can be adjusted
on demand for near vision and occurs in an optimal region of the
lens, namely in the region of the optical centre. The lens assembly
is thus usable for viewing a near object without the need to adjust
head position or the direction of gaze.
[0149] FIGS. 15 and 16 show sample FEA outputs from designing the
membrane reinforcing diaphragm 24. FIG. 15 shows the pre-tension
across the flexible membrane calculated by FEA in a lens assembly
in accordance with the invention that is similar to the first lens
assembly 1 described above, but which omits the diaphragm 24, with
the membrane un-actuated. The greyscale reveals significant
variation in the pre-tension in the membrane, with several regions
of relatively greater tension and several regions of relatively
lower tension; the tension in the membrane is noticeably
uneven.
[0150] FIG. 16 shows the corresponding FEA output for the first
lens assembly 1 which includes the diaphragm 24. In this assembly 1
the membrane 8 exhibits significantly less variation in pre-tension
when un-actuated than the one of FIG. 15. Over its area, the
membrane of FIG. 15 displays a 30% variation in pre-tension while
the membrane of FIG. 16 has only an 8% variation.
[0151] FIGS. 17 and 18 show the calculated spherical lens powers
for the first lens assembly 1 and for the similar lens assembly in
which the diaphragm 24 is omitted. Again, it can be seen that the
variation in optical spherical power is much less in FIG. 18; the
greyscale shows much greater uniformity.
[0152] The reinforcing diaphragm 24 thus provides significant
benefits in improving the uniformity of the pre-tension in the
membrane when un-actuated and the optical spherical power of the
membrane when distended, i.e. actuated, that are independent of the
shape of the membrane. Effectively the diaphragm 24 increases the
stiffness of the supporting rings 2, 10 in the x-y plane defined by
them without significantly affecting the stiffness of the rings
transverse to the plane in the z-axis. As noted above, the
reinforcing diaphragm 24 of the invention may be advantageously
used for this purpose in any fluid filled assembly with a
pre-tensioned flexible membrane of a controllable shape forming a
wall of the cavity, such as an optical surface of a fluid-filled
lens, regardless of the outline shape of the membrane. The
diaphragm 24 may therefore also be used in round fluid-filled lens,
for example.
[0153] FIGS. 19 and 20 show schematically the mode of actuation of
the first lens assembly 1. The lens assembly 1 is actuated by
"angled compression". The front and rear plates 4, 16, the
retaining ring 6, the diaphragm 24 and other detailed features are
omitted for clarity.
[0154] FIGS. 19A and 20A show the lens assembly 1 in its
un-actuated state. In this condition, the membrane 8 is flat.
[0155] In FIGS. 19B and 20B, the lens assembly 1 is actuated to
increase its optical power by the application of a force F applied
to the one side 7 of the supporting rings 2, 10 at the actuation
point {circle around (A)} in a direction to urge the supporting
rings 2, 10 towards the rear wall 19 of the dish-shaped part 12.
The rear wall 19 of the dish-shaped part is held stationary and
thus supported by the rear cover plate 16 and retaining ring 6 (not
shown in FIG. 19B). This causes the one side 7 of the supporting
rings 2, 10 to move closer to the rear wall 19 of the dish-shaped
part 12. The other short side 9 of the supporting rings 2, 10 is
anchored at the hinge points {circle around (H)}1, {circle around
(H)}2 by the formations 39. The supporting rings 2, 10 thus tilt
rearwardly under the influence of the force F to subtend an acute
angle with the rear wall 19. This tilting movement, which is
exaggerated in FIG. 19B, is accommodated by the flexible seal
formed by the side wall 18 of the dish-shaped part 12. As a result
of this squeezing together of the supporting rings 2, 10 and the
rear wall 19 of the part 12, the hydrostatic pressure within the
cavity increases, causing the membrane 8 to become distended,
flexing convexly outwardly as shown.
[0156] In FIGS. 19C and 20C, the actuation force is removed which
allows the supporting rings 2, 10 to return to their un-actuated,
relaxed state as a result of their intrinsic resilience. The side
wall 18 of the dish-shaped part 12 is thus caused or allowed to
uncompress, relieving the hydrostatic pressure within the cavity.
In turn, the membrane 8 is allowed to return to its un-distended
un-actuated position.
[0157] The lens assembly 1 hereinbefore described operates by
tilting the rings 2, 10 towards the rear wall 19 of the dish-shaped
member 12 to reduce the volume of the cavity 22 and thereby to
increase the pressure of the fluid 11, causing the membrane 8 to
distend outwardly. However those skilled in the art will appreciate
that the same principles may be applied to a membrane assembly in
which the membrane supporting ring(s) are tilted or otherwise moved
away from the rear wall to increase the volume of the cavity and
thereby reduce the pressure of the fluid, resulting in the membrane
caving inwardly. The shape of such a concave membrane may be
controlled in an analogous manner by providing a ring or rings
having a variable second moment of area such that upon deformation
of the membrane the ring or rings adopt the profile needed to
produce the desired predefined form in the membrane.
[0158] FIGS. 21 and 22 show a second lens assembly 101 according to
the invention. Each of FIGS. 21A-C shows a cross-sectional view of
the second lens assembly 101 at a different state of actuation, and
FIGS. 22A-C show corresponding front elevations.
[0159] The construction of the second lens assembly 101 is similar
to that of lens assembly 1; parts of the second lens assembly 101
that are the same as or similar to those of the first lens assembly
1 are not described again below, but are referred to by reference
numerals that are the same as the reference numerals for the
corresponding parts of the first lens assembly 1 but increased by
100.
[0160] The second lens assembly 101 has a square shape. While the
first lens assembly 1 uses "angled compression" of the fluid cavity
22 for actuation, the second lens assembly 101 uses "cushion" (or
uniform) compression as described below.
[0161] FIGS. 21A and 22A show the un-actuated state of the second
lens assembly 101 in accordance with the invention.
[0162] In FIGS. 21B and 22B, the second lens assembly 101 is shown
in an actuated state to increase its optical power. However,
instead of tilting the supporting rings relative to the rear wall
of the dish-shaped part 112 by applying a force to one side of the
assembly to tilt the rings about hinge points on an opposite side,
the supporting rings 102, 110 of the second lens assembly 101 are
pushed at a plurality of actuation points (that are spaced round
the rings, so that at each actuation point the rings are displaced
relative to the support afforded by the frame 92 towards the rear
wall 119 by a predetermined distance according to the desired
membrane shape. That is, at each actuation point, the rings 102,
110 are displaced according to the desired locus of displacement of
the rings at those points to achieve the desired membrane shape.
The precise location of the actuation points and the amount of
their displacement will depend on the outline shape of the membrane
108, but in general according to the invention an actuation point
should be situated at each point on the rings where the
displacement is a local maximum. Thus in the second lens assembly
101, an actuation point {circle around (A)} is situated at each
corner 121 of the membrane 108, and each actuation point {circle
around (A)} is displaced by the same amount as the assembly 101 is
actuated as the other points.
[0163] Intermediate the corners 121 of the membrane 108, the square
outline shape of the membrane means that it deviates inwardly from
a round configuration. This means that when the membrane is
distended spherically, the sides 103, 105, 107, 109 of the membrane
should be displaced in the z-direction by a smaller amount than the
corners 121, so that the sides arch forwardly between the corners
121, and may even be displaced forwards relative to the un-actuated
position towards the centre of each side at points {circle around
(C)}, {circle around (D)}, {circle around (E)} and {circle around
(F)} to produce the required spherical profile.
[0164] In an alternative embodiment, the rings 102, 110 could be
held stationary at the corners 121, e.g. by formations of the kind
used in the first lens assembly 1 for the hinge points {circle
around (H)}1, {circle around (H)}2, and an actuating force F
applied uniformly to the rear cover plate 116 in the z-direction,
as shown in FIG. 21B. A reaction force would then be applied to the
rings at the substitute hinge points {circle around (H)} in the
corners 121 where the rings are held.
[0165] Upon actuating the second lens assembly 101 as described
above, the flexible side wall 118 of the dish-shaped part 112 is
compressed uniformly, increasing the pressure of the fluid 11l
within the cavity 122. This causes the membrane 108 to inflate and
bulge outwardly in a convex manner. In spite of the square shape of
the membrane, the width and thus bending modulus of the rings 102,
110 is varied round the membrane such that they deform in a
controlled, predetermined manner, as calculated by FEA for
instance, to maintain a spherical (or other preselected) profile,
such that the membrane is caused to deform spherically (or
according to the other preselected profile). Specifically, in the
embodiment shown in FIGS. 21 and 22, the rings 102, 110 are thicker
at the corners 121 than they are between the corners, allowing the
rings intermediate the corners to flex forwardly relative to the
corners in the manner described above.
[0166] In view of the even movement of the supporting rings 102,
110 towards the rear cover plate 116, a smaller total displacement
of the supporting rings 102, 110 may be required to inflate the
membrane 108 fully as compared with a similarly dimensioned "angled
compression" assembly. Thus the thickness of the second lens
assembly 101 may be minimised.
[0167] In order to return the second lens assembly 101 to the
un-actuated state, the actuating force is removed from the
actuation points {circle around (A)} (or from the rear cover plate
as applicable) and the rings are allowed to return to the
un-actuated starting position as shown in FIGS. 21C and 22C. In
some embodiments, the resilience of the dish-shaped part 112 may be
sufficient to restore the rings to the un-actuated state when the
actuating force is removed. However, in a variant, the assembly may
be actively returned to the un-actuated position by driving the
rings 102.110 at the actuating points in the opposite direction or
by holding the rings 102, 110 and apply a reverse force -F (see
FIG. 21C) to the rear cover plate 116 to pull the plate away from
the rings. The pressure of the fluid 111 within the cavity 122 is
thus relieved, allowing the membrane and the rings to return to
their planar configuration.
[0168] The first and second lens assemblies 1, 101 are similar to
one another in that they both require application of a force to
compress the assembly. The difference between them resides
primarily in the number of actuation points {circle around (A)} and
hinge points {circle around (H)}. In the first lens assembly 1
there is one actuation point {circle around (A)} on one short side
7 of the assembly and two hinge points {circle around (H)}1,
{circle around (H)}2 on the other short side 9 that define a
tilting axis. The long sides 3, 5 are unconstrained and are free to
bow forwards as the cavity 22 is compressed. In the second lens
assembly 101, there are no hinge points, but actuation points
{circle around (A)} are provided at each corner 121 where maximal
compression of the cavity 122 is required to achieve the desired
membrane shape.
[0169] In general, the membrane assembly of the present invention
utilises semi-active control of the shape of the supporting rings
2, 10; 102, 110 by actively controlling the position of the rings
at a plurality of control points at spaced locations round the
rings, which control points may be hinge points or actuation
points, and allowing the rings 2, 10; 102, 110 to flex freely
between the control points. An actuation point is a point at which
the displacement of the rings is either actively controlled to
achieve compression of the cavity 22; 122, or the displacement of
the rings is modified by a passive element, a spring for example. A
hinge point is a point where rings are held in a fixed position,
but the rings are allowed to tilt if required to allow the cavity
to be compressed by `angled compression` such, for example, as in
the first lens assembly 1. Those skilled in the art will appreciate
that the region of the rings 2, 10; 102, 110 that is affected by a
control point should be as small (localised) as possible, and
adjacent control points should not, in general, be rigidly
connected to each other, to allow the rings to flex along the rings
as required to achieve the desired shape. Generally there must be
at least three control points (hinge points or actuation points) in
order to define stably the datum plane of the membrane 8.
[0170] There should be at least one control point within each
sector of the rings 2, 10; 102, 110. By a "sector" is meant a
region of the rings between two adjacent unsupported minimal points
on the rings 2, 10; 102, 110 where the rings approach locally
closest to the defined centre of the membrane 8; 108. At these
minimal points, the displacement of the rings 2, 10; 102, 110
towards the rear wall 19 when actuated is a local minimum. In fact,
in the embodiments described, the rings 2, 10; 102, 110 are
actually displaced forwards, away from the rear wall 19 when
actuated, and so in these embodiments the minimal points are
actually points of local maximum displacement forwardly relative to
the assembly.
[0171] The "centre" is the predefined centre of the desired
distended shape of the membrane. In the case of a lens assembly,
the centre may be the optical centre OC at the vertex of the
inflated membrane. Within each sector, the control point should be
positioned at or close to the maximal point at which the rings 2,
10; 102, 110 are disposed locally furthest away from the centre; in
other words where displacement of the rings 2, 10; 102 rearwards
towards the rear wall 19 is a local maximum in the actuated state.
The rings 2, 10; 102, 110 should be unconstrained at points
intermediate the control points, where the desired displacement of
the rings 2, 10; 102, 110 towards the rear wall 19 is less than at
the neighbouring control points, so that the edge of the membrane
8; 108 may arch forwardly relative to the positions it would have
adopted if the rings were inflexible, except short lengths of the
rings 2, 10; 102, 110 may be supported, e.g. by stiffening ribs
such as stiffening ribs 3a, 3b, if the supported region of the
rings 2, 10; 102, 110 does not significant deviate from a circular
locus relative to the optical centre OC. However, the support for
the rings should still allow some flexing of the rings, including
in the direction along the rings to avoid unwanted distortions.
[0172] FIG. 23 shows how the distance between the optical centre OC
and the rings 2, 10 varies in the first lens assembly 1 round the
rings 2, 10. The units in FIG. 23 are arbitrary. It will be
appreciated that if the membrane were round, then the plot-line
would be flat. As shown in FIG. 10, the membrane 8 of the first
lens assembly 1 defines two main sectors--S1, S2. Sectors S1 and S2
are each defined between two adjacent unsupported minimal points
{circle around (D)} and {circle around (E)} which, as described
above, are disposed approximately midway along the two long sides
3, 5 of the membrane 8. Sector S1 includes said other short side 9
and the maximal point {circle around (H)}1, while sector S2
includes the one short side 7 and the maximal points {circle around
(B)} and {circle around (C)}. The actuation point {circle around
(A)} is disposed intermediate the two maximal points {circle around
(B)} and {circle around (C)}. In a perfect membrane assembly
according to the invention, an actuation point would be provided at
each of the maximal points {circle around (B)} and {circle around
(C)} with point {circle around (A)} technically being a local
minimal point, but for convenience and practicality, a single
actuation is provided at point {circle around (A)} between points
{circle around (B)} and {circle around (C)}. As best seen in FIG.
23, the distance from the rings 2, 10 to the optical centre OC of
the membrane is generally constant between the two maximal points
{circle around (B)}and {circle around (C)}, and while actuation
point {circle around (A)} is technically a minimal point (a local
minimum turning point), the displacement of the rings at point
{circle around (A)} is still positive {circle around (A)} is
further from the optical centre than the hinge points {circle
around (H)}1 and {circle around (H)}2) and, as a minimal point, it
is insignificant in comparison with the major turning points
{circle around (E)} and {circle around (F)}, and the stiffening rib
3a serves to support the rings 2, 10 between the adjacent maximal
points {circle around (B)} and {circle around (C)} across the
minimal point at {circle around (A)} and to distribute the load
applied at the actuation point {circle around (A)} along the one
short side 7 of the assembly.
[0173] Sector S1 also includes the hinge point {circle around
(H)}2, which is not disposed at a maximum or minimal point, but
helps to define the plane of the membrane for which at least three
control points are needed. In the case of a membrane assembly that
operates in the "angled compression" mode described above, e.g.,
the first lens assembly 1 of the invention, a hinge point can be
used at any control point on the membrane supporting rings 2, 10
where the rings do not move (or do not move substantially) during
actuation of the lens. The hinge points {circle around (H)}1,
{circle around (H)}2 of the first lens assembly 1 are thus disposed
within the same sector and define a tilting axis T (see FIG. 10)
that is bisected substantially perpendicularly by an axis between
the tilting axis T and the actuation point {circle around (A)}. The
tilting axis T is also generally parallel to the short sides 7, 9
of the assembly. The optical centre OC is disposed between the
tilting axis T and the actuation point {circle around (A)}. In some
embodiments adjacent hinge points may be situated in adjacent
sectors if there is a minimal point between them.
[0174] FIG. 24 shows how the distance between the optical centre OC
and the rings 102, 110 varies in the second lens assembly 101 round
the rings 102, 110. As can be seen there are four unsupported
minimal points {circle around (C)}, {circle around (D)}, {circle
around (E)} and {circle around (F)}, where the rings 102, 110 are
disposed locally closest to the centre OC. The corners 121 of the
assembly are furthest away from the centre OC, and so these
comprise maximal points. An actuation point {circle around (A)} is
placed at each corner 121, and the sides 103, 105, 107, 109 are
left unconstrained. The four minimal points {circle around (C)},
{circle around (D)}, {circle around (E)} and {circle around (F)}
define four sectors S1-S4, and a respective one of the actuation
points {circle around (A)} is disposed within each sector. In the
alternative embodiment where an actuating force F applied uniformly
to the rear cover plate 116 in the z-direction, as shown in FIG.
21B, a hinge point {circle around (H)} may be placed in each corner
121, and this is possible because the effective displacement of the
rings 102, 110 in each corner 121 is the same, so the effective
displacement at each hinge point {circle around (H)} is the
same.
[0175] It will be understood that the more control points that are
provided, the more accurately the deformation of the membrane can
be controlled. Furthermore, additional actuation points facilitate
improved control of the membrane surface and a wider set of
possible lens shapes. It will be understood by those skilled in the
art, that if lens assemblies 1; 101 of the type described herein
are used in a pair of eyeglasses, such as eyeglasses 90 of FIGS. 1
and 2, a selectively operable actuation mechanism should be
provided to afford the necessary compression of the cavity 22, 122
and fluid pressure adjustment to operate the lens, either directly
or indirectly. Such an actuation mechanism may be conveniently
provided either in the bridge 94 or one or both of the temple arms
93. In some embodiments a separate actuation mechanism for each
lens assembly 1; 101 may be provided in each arm 93, and the
mechanisms linked electronically to provide simultaneous actuation
of the two assemblies 1; 101. The actuation mechanism is not
described herein, but in general terms may be mechanical,
electronic, magnetic, automatic with eye or head movement, or
involve use of a phase change material, such as shape memory alloy
(SMA), wax, or an electroactive polymer. In the event that some
passive control of the lens assembly 1; 101 is desired, the fluid
pressure could be adjusted with a pump.
[0176] It will be appreciated that the use of separate front and
rear supporting rings 2, 10; 102, 110 is not essential to achieve
the basic functionality of the lens assembly 1; 101 of the present
invention, and in some variants the membrane 8; 108 may be
supported by a single flexible ring. However, it has been found
that the use of two or more supporting rings is advantageous for
controlling for example the rate of twist in the supporting rings
2, 10, and particularly during manufacture of the assembly.
[0177] FIG. 25 illustrates the attachment of a flexible membrane
208 to a single membrane supporting ring 210 using an annular layer
254 of adhesive. It has been found that when a membrane 208 is
attached to a single ring 210 with adhesive in this manner, the
tension that is imparted to the membrane 208 causes the membrane
208 to exert a moment around the support ring 210 and pull on one
face of the support ring 210 thereby tending to tilt the supporting
ring 210 locally towards the centre of the lens, as shown in dotted
lines in exaggerated form. This is undesirable because it means
that the ring 210 does not sit squarely with the other components
of the assembly and makes it more difficult to control bending of
the ring 210. Such unwanted torsion in the ring 210 also gives rise
to edge effects in the lens and the introduction of optical
aberrations as a function of the lens power.
[0178] The present invention provides a solution to this problem by
using two supporting rings 2, 10; 102, 110; 302, 310 (see FIG. 26).
FIG. 26 shows an improved assembly method in which a flexible
membrane 308 is held between the front and rear supporting rings
302, 310. In this improved method, the membrane 308 is
pre-tensioned as before, but as well as applying a layer of
adhesive 354 to a front face of a rear support ring 310, a layer of
adhesive 356 is also applied to a rear face of a front supporting
ring 302. This can be done simultaneously or sequentially. The two
supporting rings 302, 310 are then brought together simultaneously
on either face of the membrane 308 as shown to sandwich the
membrane 308 therebetween. Since the flexible membrane 308 is never
held on just one of the rings, the additional support provided by
both rings 302, 310 at once balances any local torsional forces
that would otherwise occur, therefore providing balanced support.
The adhesive is then cured. Thus a substantially sandwich planar
structure which holds the pre-tension in the membrane 308 is
formed. Those skilled in the art will appreciate that more than two
supporting rings can be employed if desired, provided that the
membrane is sandwiched between supporting rings in such a way that
the tension in the membrane is applied evenly to the rings on each
side of the membrane to avoid unwanted torsional forces. Thus, for
instance, two or more supporting rings may be provided on each side
of the membrane.
[0179] Various embodiments and aspects of the present invention are
described above, all of which provide for controlled deformation of
the flexible membrane 8, 108. In particular, described embodiments
show how substantially spherical deformation, or deformation
according to one or more Zernike polynomials or similar surface
expansions, of the elastic membrane 8, 108 can be achieved. Optical
distortion is minimised and the lens can be used to provide a
smooth transition from long-distance to short-distance focus. Such
controlled deformation has not been achieved by any prior non-round
fluid filled lenses. It will be understood by those skilled in the
art that deformation according to a Zernike polynomial is not
essential, and the present invention can be used to control
deformation of an elastic membrane 8, 108 to other desired shapes.
The lens assembly of the invention can be used to correct various
optical aberrations which may arise depending on the application.
This can be achieved by design based on combinations of different
Zernike functions.
[0180] In the first and second lens assemblies 1; 101 described
above, the variation in stiffness of the membrane supporting rings
2, 10; 102, 110 round their extents is achieved by varying the
width and hence the second moment of area of the supporting rings
round the rings, while the depth of the rings in the z-direction
remains substantially constant. This stiffness could be adjusted in
different ways: for instance, instead of varying the width of the
rings in the x-y plane, the depth of the rings in the z-direction
could be adjusted. In another alternative, the ring or rings could
comprise an assembly of multiple ring segments, each part being
formed from a material of selected stiffness and the parts being
joined end to end to form the ring. The use of different materials
for different segments of the ring would thus allow the stiffness
of the ring to be adjusted as desired round the ring. The ring
segments could have the same or different lengths as needed; for
instance shorter ring segments would be used in regions of the ring
where the stiffness was required to vary more with distance. In yet
another alternative, heat or chemical treatment of selected regions
of the ring or rings could be used to alter their material
properties. Yet another alternative would be to use a composite
material for the ring or rings and to vary the properties of the
material at selected locations round the ring(s) by altering the
structure of the material, e.g. by changing the orientation of
reinforcing fibres.
[0181] The first and second lens assemblies 1; 101 may suitably be
installed in a pair of eyeglasses 90 such that the flexible
membrane 8, 108 bulges forwards away from the wearer's eyes when
actuated. This may be preferred for safety reasons, but it will be
appreciated that the lens assemblies 1; 101 could equally well be
installed in eyeglasses so that the membrane bulges towards the
user's eyes.
[0182] In the first and second lens assemblies 1; 101 the cavity
22; 122 is defined in part by the dish-shaped part 12; 112, the
rear wall 19; 119 of which is attached to the rear cover plate 16;
116. In a variant, the dish-shaped part 12; 112 may be omitted and
replaced by a flexible sealing ring (not shown) which is similar to
the side wall 18; 118 alone of the dish-shaped part and forms a
seal between the rear cover plate 16; 116 and the rear supporting
ring 10; 110 (or the reinforcing diaphragm 24 if included).
[0183] It should also be noted that a fixed prescription lens (for
distance or near vision) could be included in the lens assembly 1;
101 of the invention. This could be achieved by using a fixed power
lens as the front cover plate 4; 104 and/or as the rear cover plate
16; 116. Such a fixed power lens should have an optical centre that
is closely aligned with the optical centre of the adjustable lens
OC when actuated.
[0184] The adjustable lens assembly 1; 101 of the present invention
as hereinbefore described is capable of providing a variation in
optical power from -8 to +4 dioptres. If a negative lens power is
required, the flexible membrane 8; 108 should be arranged to flex
inwardly to achieve this.
[0185] The present invention may also be used for controlling the
deformation of a surface in other fields such, for example, as
acoustics. By rapidly oscillating the applied force, F, oscillating
pressure waves will be generated in a fluid placed in contact with
the membrane. Since the deformation of the membrane can be
controlled to be spherical in accordance with the invention, such
pressure waves will appear to have originated from a point source.
This ensures that the waves do not exhibit undesirable interference
patterns, whilst allowing a loudspeaker (for instance)
incorporating the membrane as the transducer to be non-round in
form, thus allowing it to be packaged within a confined space, for
example in a television or mobile phone. In general terms, the
above described principles can be applied to any application in
which the geometry of a surface needs to be controllably
varied.
* * * * *