IMPROVEMENTS IN OR RELATING TO DEFORMABLE NON-ROUND MEMBRANE
ASSEMBLIES
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 minima!
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.
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, US 1269422 discloses a lens with spaced opposed
wails 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 wails. The
pressure of the fluid within the envelope is adjustable to change the degree of curvature of
the membrane, thereby adjusting th power of the lens. In some examples, the volume of the
envelope may be adjusted, as in US 1269422 or WO 99/061940 Al. Alternatively the
amount of fluid within the envelope may be adjusted, as in US 2576581, US 3161718 and
US 3614215, In either case, an increase in the fluid pressure within the envelope causes
deformation of the flexible membrane
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 boitom region of the lens, thereby only allowing the user to see close objects in
focus when looking downwardly
A disadvantage of the fluid-filled lenses discfosed 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.
It is desirable therefore to provide an adjustable non-round ens, in which the lens is not
distorted as the optical power of the lens is increased.
US 53 29 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 e!astomeric 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. US 5371629 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 US 5371629 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 US 5371629 is unpractical for various reasons and was never
commercialised. In particular, despite its teachings, US 5371629 fails to disclose a liquidfilled
lens that avoids unwanted distortion when the membrane is distended, and the degree
of distortion encountered in the liquid-filled lens of US 5371629 renders the ens unusable,
WO 95/27912 A Tproposes 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.
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 nonround
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 nonround
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 nonround
drivers for acoustic use.
n one aspect of the present invention therefore there is provided a deformable membrane
assembly as claimed in claim 1 below.
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.
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.
n another aspect of the present invention therefore there is provided a deformable
membrane assembly as claimed in claim 6 below.
Alternatively the means for moving the supporting ring or support for adjusting the volume
of th 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.
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
Suitably the flexible envelope may comprise the one wa l 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 t r adjusting the volume of the
envelope. The rear wall may be rigid or substantially rigid or may b supported stably at
least round a peripheral edge.
The means for moving the supporting ring or support may be configured to act between the
membrane supporting ring a d the rear wall, n 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.
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.
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, n 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 n 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 n 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.
n 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 t 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 ease 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.
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 comers
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.
n 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
ong 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.
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.
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. t will be understood that where th 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 nimum displacement from the planar datum in the opposite
direction.
In embodiments where the ring is non-round and th 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 rin 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 th membrane when distended.
n 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
n
- I -
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.
The membrane may be continuously adjustable between an un-actuated state and fully
distended state. The supporting ring may be planar when un-actuated.
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 t
will be understood that if the actuation point were to be held in a different position by the
ring engaging member at tha 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.
n 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.
n 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.
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.
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.
Advantageously the supporting ring may comprise two or more ring elements, and the
me bra e may be sandwiched between two adjacent ring elements.
According to another aspect of the present invention therefore there is provided a deformable
membrane assembly as claimed in claim 8 below.
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 ead 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.
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 f r 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 suppor to increase or
decrease the pressure of the fluid therein.
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.
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 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 2 (astigmatism) and Z (sphere for distance correction). Opticians
typically prescribe lenses based on these formulae. Higher order surfaces with additional
components a e also possible if additional controi 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 † < j ) may also be possible where the shape of the
membrane edge permits.
Further, various linear superpositions of scaled Zernike polynomials of the form Z m are
possible:
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.
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 controi points. The element
shape may be selected to suit the calculation being performed. The selected parameters to b
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.
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 thai the calculated
shape of the membrane converges with the desired predefined form.
A reinforcing diaphragm may be provided thai 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.
In yet another aspect of the present invention therefore there is provided a deformable
membrane assembly as claimed in claim 28 below.
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.
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.
In some embodiments, in the plane of ring, the membrane may be longer in one dimension
than it is in another dimension. In such eases, 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.
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.
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.
Suitably said rear wall may be shaped to provide a fixed lens.
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.
Thus, in some embodiments, the front cover and/or rear cover may provide a fixed optical
power t r 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.
In some embodiments the envelope may be housed within a retaining ring.
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,
Said article of eyewear may typically comprise a frame with a rim portion; the deformable
membrane assembly may be mounted within the rim portion.
Following is a description by way of example only with reference to the accompanying
drawings of embodiments of the present invention.
In the drawings:
FIG. 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;
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;
FIG. 3 is a front elevation of the first lens assembly in accordance with the invention in the
un-actuated state;
FIG, 4 is a cross-section of the first lens assembly along the line I - V of FIG. 3;
FIG. 5 is a cross-section of the first lens assembly along the line V-V of FIG, 3;
FIG. 6 is a cross-section of the first lens assembly along the fine VI-VI of FIG. 3;
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 V -V of FIG. 3;
F!G. 8 is an exploded view of the first lens assembly showing the parts of the assembly;
FIG. 9 is a front elevation of the flexible membrane and membrane supporting rings of the
fsrst 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;
FIG. 0 shows the membrane and rings of FIG. 9 in an actuated state and projected onto a
notional sphere of radius R;
FIG is a cross-section of the first lens assembly which corresponds to FIG. 4 but shows
the assembly in an actuated state;
FIG. 12 is a cross-section of the first lens assembly which corresponds to FIG. 5 but shows
the assembly in an actuated state;
FIG. 3 shows the displacement of the membrane of the first lens assembly in an actuated
state, as calculated by static finite element analysis (FEA);
FIG. 14 shows the uniformity of the optical power of the first lens assembly in an actuated
state, as calculated by FEA;
FIG. 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;
FIG. 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;
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;
FIG. 8 shows the variation in the optical power as calculated by FEA of the first lens
assembly of the invention;
FIGS. 1 A-C show schematically in cross-section the first lens assembly of the invention in
the un-actuated state (FIG. A), an actuated state (FIG. B) and a de-actuated state
(FIG. 19C);
-
FIGS, 20A-C show schematically the front elevation of the first lens assembly of the
invention in the im-actuated sta e (FIG, 20A), a actuated state (FIG. 2GB) and a de-actuated
state (FIG, 20C);
FIGS. A-C show schematically in cross-section a second square lens assembly of the
invention in an un-actuated state (FIG, A), an actuated state (FIG. 2 B) and a de-actuated
state (FIG. 2 C) ;
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) a d a de-actuated
state (FIG. 22C);
FIG, 23 shows how the distance between the optical centre and the membrane supporting
rings varies in the first lens assembly;
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.
FIG, 25 shows schematically in cross-section a flexible membrane and single supporting ring
in accordance with the invention; and
FIG, 26 shows schematically n cross-section the flexible membrane and supporting rings of
the first lens assembly in accordance with the invention.
With reference to FIG. 1, a pair of eyeglasses 90 (UK: spectacles) comprise a frame 92
having two ri 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 fi rst lens
assembly 1 in accordance with the present i ventio One of th 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.
As best seen in F G. 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.
As well as eyeglasses, the lens assembly of the present invention is equally well applicable
to other e s applications, such as goggles, helmets and scientific and optical instruments of
various sorts. n 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 controllab!y
adjustable shape may be required.
The first lens assembly 1 is especially suitable for use in the correction of presbyopia. n
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 refocus
smoothly from a far-away object to a near, reading-distance object.
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,
n FIG. 8, the component parts of the first lens assembly I 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.
At the front of the first lens assembly 1 is a transparent front cover plate 4, made of glass or
a suitable polymeric material n the first lens assembly the front cover plate is about 1.5mm
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 F G. 4 for example, in the
present embodiment, the front cover plate 4 is plano-convex.
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 be low.
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 0.3mm thick, but other suitable
materials may b used and the thickness adjusted accordingly to provide the desired stiffness
as discussed below. Next is a transparent non-porous, eiastie membrane 8. In the first
assembly the membrane 8 is made of Mylar® and is about 50 microns thick, bu 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 0 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 fluidtight
sea with at least the rear supporting ring 0.
The rear surface of the second supporting ring 0 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 , a rear wall 19 and a forward sealing flange 20. In the first
assembly the dish-shaped part 12 is made of transparent DuPont® 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 2 is
sealingly adhered to the rear surface of the diaphragm 24 with a suitable adhesive such, for
example, as Loctite 3555.
A layer of a suitable transparent pressure-sensitive adhesive (PSA) such, for example, as
3M® 821 (not shown) adheres the rear wall 19 of the dish-shaped part 12 to a front face 17
of a transparent rear cover plate having a rear face 14. In the first lens assembly 1
described herein the Saver 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 I is about 1.5mm thick, but again this ay be varied as desired. The rear cover
plate sits as the rearmost layer within the retaining ring 6. As with the front cover plate 4,
in some embodiments, the rear cover plate 6 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.
The retaining ring 6 comprises a forwardly extending side wa l 3 having an inner
surface 23, which side wall 3 terminates in a front edge 5. The front cover plate 4 sits on
and is bonded to the front edge 5 of the retaining ring 6 so that the lens assembly constitutes
a closed unit. As best seen in FIGS. 4, 5, 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.
The dish-shaped part 12, membrane 8, second supporting ring 0 and diaphragm 24 thus
define a sealed interior cavity 2 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 6 form the opposite optical surfaces of an adjustable lens. As described
above the rear cover plate 6 is a meniscus lens. In an un-actuated state, the membrane is
planar so th lens has the fixed optical power afforded by the rear cover plate , with zero
addition from the membrane 8. However, when actuated as described below, the
membrane 8 is inflated to protrude forward y 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 , the lens 1 is biconcave. The greater the curvature of the
membrane 8, the greater the additional optica! power afforded by the membrane 8. For nonoptical
applications th fluid, along with the other parts of the assembly, do not need to be
transparent.
The side wail of the dish-shaped par 2 provides a flexible seal between the rear wall
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, 0 and the rear cover
plate 6 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 1 supporting rings, for example
by Loctite® 55.
The cavity 22 is filled during manufacture with a transparent oil 1 (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 . The oil is also chosen so as to not be harmful
to a wearer's eye in the event of a leakage.
As shown in F GS. 6 and 7, the first lens assembly 1 may b received and seated snugly in a
rear rim par 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. n some
embodiments, the rear rim portion 93b may be formed integrally with the retaining ring 6.
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.
- 1/ -
t 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, 0 and are joinabie to the
diaphragm 24 or rear supporting ring .
Various different materials may suitably be used for the supporting rings 2, 0 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 0,3mm thickness); bein
joinabie to the adjacent components; having low creep (to continue to perform over multiple
uses); and being elasticaliv deformable. Other possibilities are titanium, glass and sapphire.
By "joinabie" is meant by joinabie by adhesive, crimping, laser welding or ultrasonic
welding or any other means that would be apparent and available to those skilled in the art.
Different adhesives may suitably be chosen that are able to join the parts of the assembly
durably, are creep resistant, are of a suitable viscosity to be applied when constructing the
Sens 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.
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).
Th first lens assembly 1 provides an adjustable lens having a focal power that can be
adjusted by controlling pressure of the fluid 1 within the cavity 22 and the shape of the
bendable supporting rings 2, , 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.
As best seen in FIG. 9 the width of the supporting rings 2, 0 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 . 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.
It can be seen from F G. 8 that each of the supporting ribs 3a, 3b, the supporting rings 2 0
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 Whe 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 n the other parts. These holes 28a, 28b define an actuation point ® for
attachment of an actuation device to the lens assembly 1 to cause it to be compressed in use.
Compression of the lens I is described in more detail below. The actuation device may be
housed in the adjacent temple ar 94 of the frame 92, In some embodiments the lens
assembly may be expanded in a similar manner to reduce the pressure of fluid within the
cavity 22,
Adjacent the protruding tab 26 at the one short side 7 of the assembly, the inner edge of each
of the supporting rings 2, SO deviates outwardly as best shown in FIG. 9 to form a generally
semi-circular recess 30, The side wall 8 of the dish-shaped part 2 has a similar,
corresponding recess 30 which aligns with the recesses 30 of the supporting rings 2, 1 when
the lens is assembled. The membrane 8 includes a corresponding semi-circular protruding
portion 3 1 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, a separate hole though the supporting rings 2, may be provided instead of
said semi-circular recess 30
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 th 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 inertia! 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, 0 to bend in use. The reinforcing diaphragm 24, which strengthens the
supporting rings 2, 0 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.
n he first ens 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 d e 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, 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.
n 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.
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 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 i , 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
0.55mm 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 0 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.
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, 0. 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, 0 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.
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 on y a small effect on the bending stiffness in
the z direction (i.e. normal to the rear wail ) . This z-direction effect is accounted for in the
design of the supporting rings 2, 0. Thus the stiffness of the assembly 1 is increased for the
purpose of maintaining tension in the flexible membrane 8, but the supporting ri gs 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, a d 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, 0 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.
n non-optical applications, there is no need for the diaphragm to be transparent.
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 of the
retaining ring 6 and serves to support the diaphragm 24 and front and rear supporting rings 2,
10. The side wall 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.
At said other short side 9 of the first lens assembly 1, the retaining ring 6 defines two hinge
points ©1, © 2 · see FIG. 0 . As shown in FIG. 4, the stacked parts 2. 3b, 8, , 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 ©1, ©2, such that they remain stable when the lens is
actuated as described below.
The supporting rib 3b provides additional stiffness for the supporting rings 2, 10 in the
region of the hinge points ©1, ©2 and between them. In the first lens assembly 1. the hinge
points ©1, © 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 ©1, ©2 are not required to bend much or at all.
The other supporting rib 3a similarly provides additional stiffness for the supporting rings 2,
0 at the aforementioned actuation point ® so that deformation of the membrane 8 is
properly controlled, as explained in more detail below. n some embodiments the supporting
ribs 3a, 3b may be omitted; they are generally useful for regions of the supporting rings 2,
that are not required to deform significantly during actuation of the assembly.
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.
FIG. 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 Zer ike polynomials. For non-optical
applications, different shapes may be required. The lens assembly 1 in an actuated state is
shown in FIGS. and 12.
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 @ and hinge points 1. © 2 are shown. A force F may be applied at the
actuation point ® by means of an actuation device connected via the holes 28a, 28b.
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, 0 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 ® below the ine L, representing the plane of the membrane 8 in the un-actuated state,
but where the long sides 3, 5 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.
In FIG. 0 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 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).
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
tensing 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)(l/R) (I)
Where n is the refractive index, I 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).
In the lower half of FIG. 10, the edge of the membrane 8 is maximally displaced at the
actuation point ® in the direction of application of the force F. The hinge points © I , 2
coincide with points on the edge of the membrane 8 (as defined by the supporting rings 2, 10
in the first lens assembly I) that involve substantially no displacement upon deformation of
the membrane 8. t 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. 0) . In order to control optimally the deformation of the membrane 8,
the hinge points (8)1, © 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 ®1, ® 2 from the desired spherical (or other) shape,
resulting in unwanted distortion of the membrane. Suitably the hinge points © 1, ©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 ©1, © 2 and the optical centre OC can be tolerated, notwithstanding the resulting
distortion that wiii occur in the vicinity of one or both hinge points ©1, ©2. In FIG. 10, it
can be seen that one hinge point © 1 is situated further from the centre OC than the other
hinge point ©2, leading to some distortion of the membrane in the corners of the lens
adjacent the hinge points ©1. ©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.
t wi l be appreciated that maximal displacement of the membrane 8 occurs at the actuation
point © , which should always lie on th 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®, 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 shouid 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 arc about the optical centre,
additional actuation point(s) (active or passive) may be required to maintain the surface
fidelity, t will be seen from FIG. 10 thai in the first lens assembly 1, the points furthest
away from the centre OC are in the corners of the membrane 8, between the on g sides 3, 5
and the one short side 7 - identified as positions ® and © in FIG. 10. However the
actuation point ® is proximate to these points and the stiffening rib 3a serves to distribute
the load applied a the actuation point © along the one short side 7 of the membrane 8 with
an acceptable degree of distortion of the membrane shape.
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, 0 at the
actuation point ® rearwardly towards the rear cover plate 6, resulting in greater fluid
pressure in the cavity and greater forwards distension of the membrane.
The way that this variable deformation is achieved for the first lens assembly 1 according to
the invention is described in greater detail below.
FIGS. 3-5 show the first lens assembly 1 is its un-actuated state, and FIGS. 1 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. 1 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-direetion 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 © and ® 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. described
above.
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, 0 at the point ® on the one short
side 7 of the assembly to move the supporting rings 2, , and the membrane 8 damped
between them, rearward!y towards the rear cover plate 6. 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
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, 0 are hinged at the two points ©1, © 2 on the other
short side 9 of the assembly 1, The hinge points ar 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 2
attached thereto, the diaphragm 24 and the supporting rings 2, 0 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 ©1, © 2 The side wall 18 of the dish-shaped
part allows a small amount of movement, so that the support rings 2, 10 can ove closer
towards the bottom wall 19 of the dish-shaped part 8 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 nonround,
it is ab e to adopt the desired spherical (or other shape) form by virtue of the
construction of the supporting rings 2, 10.
The force applied to the one short side 7 of the supporting rings 2, 0 at the actuation
point ® , 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. 1 shows the supporting
rings 2, 0 exhibiting a degree of bending upon application of the actuating force F The
supporting rings 2, 10 remain substantially stationary at the hinge points ©1, © 2 (although
there is a degree of local tilting of the rings 2, 0 at these points). However, towards the
middles of the long sides 3,5 of the assembly including points © and © , the rings flex
forwards as described above, in an opposite direction to the force F, so that the supporting
rings 2, 1 adopt a profile that would conform to the surface of a sphere (or other form)
having same shape as the membrane 8. f 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, 0 implies that they cannot remain flat when the membrane is distended,
The ability of the supporting rings 2, 0 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, 0 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, 0 produces a corresponding variation in
cross-sectional area of the support rings 2, 0 and thus a corresponding predetermined
variation of the second moment of area of the support rings n 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.
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 ® , the liquid 11, being
essentially incompressible, is forced to occupy a more central region of the cavity 22, owing
to the elastic ity 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 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.
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 on y 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 95/27 , 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, 0 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.
Upon actuation, when the flexible membrane 8 bulges forwards as shown in FIGS. 0 and
, the amount of fluid 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 ens. The displacement of
the oil causes the membrane to adopt the actuated shape, thus increasing the power of the
lens. The fluid is sealed within the cavity 22 by the membrane 8. the diaphragm 24 and
the dish-shaped part 12.
It wi l 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 i FIGS. 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 ®1, © 2 on the
supporting rings 2, 0 remain essentially stationary and at least a major portion or portions of
the supporting rings 2,10, including the hinge points ®1, ©2, adopt a spherical (or other
form) profile.
The actual variation in width of the support rings 2, 0 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
wil 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, 0 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,
0, 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-tensson 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 ® .
An example FEA analysis output for a supporting ring is shown in FIG. 13. The grayscale
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 ®, with circular
contours proving essentially spherical deformation. This figure shows the deformation in 2-
dimensions; it wil 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. 3 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.
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 . 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 Zernike polynomials can be
found in Principles of Optics1
A priority for ophthalmic applications, for instance, is to be ab e to achieve vision correction
with a linear superposition of Z 2 (astigmatism) and Z° (sphere for distance correction),
Opticians typically prescribe lenses based on these formulae. Higher order surfaces with
' "Principles of Optics" M. Born and E. Wo f, 7* Ed, C.U.P, (1999). ISBN 0-521-64222-1
additional components z†Jar 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 sim ilar magnitude to the number of control points. Higher order surfaces
with components Z k < j ) may also be possible where the shape of the membrane edge
permits.
Variants of the first ens assembly 1 of the invention are able to produce static membrane
shapes corresponding to any such polynomial for whichj-k. Various complex surfaces are
known to b 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 are
possible:
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.
In some embodiments of the lens assembly of the present invention, a spherical Zernike
function may be used, but higher spherical order functions can a so be used if desired, bycreating
a shape that is the sum of a number of Zernike polynomials.
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. I 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, parameters perform t is
possible to determine which regions of the supporting rings 2, need to be tuned (or which
other parameters should be adjusted) to improve the correlation of the first and second
outputs.
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,
0 (and the force F applied) can be designed. This iterative process has been carried out to
achieve a number of working embodiments of t e supporting rings 2, in accordance with
the invention. Thus the supporting rings 2, 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 ©1, © 2 a d actuation point ® relative to th desired optical
centre QC
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™ optical software available from Radiant Zemax,
LLC of Redmond, Washington) using the calculated membrane shape. By way of example,
FIG. 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.
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 optica! centre. The lens assembly is thus usable for
viewing a near object without the need to adjust head position or the direction of gaze.
FIGS, 5 and 6 show sample FEA outputs from designing the membrane reinforcing
diaphragm 24. FIG, 5 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 unactuated.
The grayscale 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,
FIG, 6 shows the corresponding FEA output for the first lens assembly 1 which includes the
diaphragm 24. n 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.
FIGS, and 8 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 greyscaie shows
much greater uniformity.
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, 0 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,
FIGS. 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, , the
retaining ring 6, the diaphragm 24 and other detailed features are omitted for clarity.
FIGS. 1 A and 20A show the lens assembly 1 n its un-actuated state, in this condition, the
membrane 8 is flat.
in FIGS. 19B and 2GB, the lens assembly 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, 0 at the actuation
point ® in a direction to urge the supporting rings 2, 0 towards the rear wall of the dishshaped
part 12. The rear wall 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, B). This
causes the one side 7 of the supporting rings 2, 10 to move closer to the rear wa l 1 of the
dish-shaped part 12. The other short side 9 of the supporting rings 2, 0 is anchored at the
hinge points ©1, © 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 . This
tilting movement which is exaggerated in FIG. B, is accommodated by the flexible seal
formed by the side wall 1 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.
In F GS. C 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 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.
The iens assembly 1 hereinbefore described operates by tilting the rings 2, 0 towards the
rear wall 9 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 rirtg(s) are tilted or otherwise moved
away from the rear wall to increase the volume of the cavity and thereby reduce th 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.
FIGS. 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.
The construction of the second lens assembly 101 is similar to that of ens assembly 1; parts
of the second lens assembly 0 1 that are the same as or similar to those of the first lens
assembly 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 10 0 ,
The second iens assembly 10 has a square shape. While the first lens assembly 1 uses
"angled compression" of the fluid cavity 22 for actuation, the second lens assembly 0 1 uses
"cushion" (or uniform) compression as described below.
FIGS. 2 and 22A show the un-actuated state of the second lens assembly 101 in
accordance with the invention.
In FIGS. 2 B and 22B, the second lens assembly 0 1 is show 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 2 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, of the second
ens 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 wail 9 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 08, bu 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 , an
actuation point © is situated at each corner 121 of the membrane 108, and each actuation
point ® is displaced by the same amount as the assembly 0 1 is actuated as the other points-
Intermediate the corners 12 1 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 03, 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 forward ly between the corners 1 , and may even be displaced forwards relative
to the un-actuated position towards the centre of each side at points © , © , © and © to
produce the required spherical profile.
In an alternative embodiment, the rings 102, 0 could be held stationary at the corners 12 ,
e.g. by formations of the kind used in the first lens assembly 1 for the hinge points © ,
2, and an actuating force F applied uniformly to the rear cover plate 1 6 in the z-direction,
as shown n FIG. 21B, A reaction force would then be applied to the rings at the substitute
hinge points © in the corners 1 where the rings are held.
Upon actuating the second lens assembly 1 as described above, the flexible side wall 18
of the dish-shaped part 2 is compressed uniformly, increasing the pressure of the fluid
within the cavity 22. This causes the membrane 08 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 02, 0 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. and 22, the rings 102, 0 are thicker at the corners 121 than they are between the
corners, allowing the rings intermediate the comers to flex forwardly relative to the corners
in the manner described above.
In view of the even movement of the supporting rings 102, 0 towards the rear covesplate
, a smaller total displacement of the supporting rings 02, 0 may be required to
inflate the membrane 08 fully as compared with a similarly dimensioned "angled
compression" assembly. Thus the thickness of the second lens assembly 0 1 may be
minimised.
Irs order to return the second lens assembly 101 to the un-actuated state, the actuating force is
removed from the actuation points © (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 102.1 0 at the actuating points in the opposite direction or by holding the rings 102,
0 and apply a reverse force - F (see FIG. 2 C) to the rear cover plate 1 6 to pull the plate
away from the rings. The pressure of the fluid 11 within the cavity 122 is thus relieved,
allowing the membrane and the rings to return to their planar configuration,
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 ® a d hi ge points ® . In the first Sens
assembly 1 there is one actuation point ® on one short side 7 of the assembly and two hinge
points ©1, © 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 10 , there are no hinge points, but actuation points © are provided at each
corner 121 where maximal compression of the cavity 122 is required to achieve the desired
membrane shape.
n general, the membrane assembly of the present invention utilises semi-active control of
the shape of the supporting rings 2, 10; 102, 10 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 apprec iate that the region of the
rings 2, 10; 102, 0 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.
There should be at least one control point within each sector of the rings 2, ; 02, . By
a "sector" is meant a region of the rings between two adjacent unsupported minimal points
on the rings 2, 10; 102, 0 where the rings approach locally closest to the defined centre of
the membrane 8; 08. At these minimal points, the displacement of the rings 2, 10; 102, 0
towards the rear wall 1 when actuated is a local minimum. In fact, in the embodiments
described, the rings 2, 10; 02 10 are actually displaced forwards, away from the rear wall
1 when actuated, and so in these embodiments the minimal points are actually points of
local maximum displacement forwardly relative to the assembly.
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, 0; 02, 0 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, 0 should be
unconstrained at points intermediate the control points, where the desired displacement of
the rings 2, 0; 02, 0 towards the rear wall is less than at the neighbouring control
points, so that the edge of the membrane 8; 08 may arch forwardly relative to the positions
i would have adopted if the rings were inflexible, except short lengths of the rings 2, 0;
102, 0 may be supported, e.g. by stiffening ribs such as stiffening ribs 3a, 3b, if the
supported region of the rings 2 10; 102, 0 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
distort ions
F G 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 t 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 - S , S2.
Sectors S and S2 are each defined between two adjacent unsupported minimal points ©
and © which as described above, are disposed approximately midway along the two long
sides 3, 5 of the membrane 8. Sector Si includes said other short side 9 and the maximal
point©!, while sector S2 includes the one short side 7 and the maximal points ® and©.
The actuation point ® is disposed intermediate the two maximal points ® and© In a
perfect membrane assembly according to the invention, an actuation point would be
provided at each of the maximal points ® and©with point ® technically being a local
minimal point, but for convenience and practicality, a single actuation is provided at point ®
between points ® and©. As best seen in FIG 23, the distance from the rings 2, 0 to the
optical centre OC of the membrane is generally constant between the two maximal points ®
and©, and while actuation point ® is technically a minimal point (a local minimum turning
point), the displacement of the rings at point ® is still positi ve (® is further from the opiical
centre than the hinge points © 1 and ©2) and, as a minimal point, it is insignificant in
comparison with the major turning points © and © , and the stiffening rib 3a serves to
support the rings 2, between the adjacent maximal points © and ©across the minimal
point at ® and to distribute the load applied at the actuation point ® along the one short side
7 of the assembly.
Sector S also includes the hinge point ©2, which is not disposed at a maximum or minimal
point, but heips 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 of the invention, a hinge point can be
used at any control point on the membrane supporting rings 2, 0 where the rings do not
move (or do not move substantially) during actuation of the lens. The hinge points ©1, © 2
of the first lens assembly 1 are thus disposed within the same sector and define a tilting axis
T (see FIG. 0) that is bisected substantially perpendicularly by an axis between the tilting
axis T and the actuation point®. 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 © . In some embodiments adjacent hinge points may be situated in
adjacent sectors if there is a minima! point between them.
FIG. 24 shows how the distance between the optical centre OC and the rings 102, 0 varies
in the second lens assembly 101 round the rings 102, 110. As can be seen there are four
unsupported minimal points © , © , © and © , where the rings 102, 0 are disposed locally
closest to the centre OC. The corners 12 of the assembly are furthest away from the centre
OC, and so these comprise maximal points. An actuation point ® is placed at each corner
121, and the sides 103, 105, 107, 109 are left unconstrained. The four minimal points © ,
® , © and © define four sectors S1-S4, and a respective one of the actuation points ® is
disposed within each sector. In the alternative embodiment where an actuating force F
applied uniformly to the rear cover plate 6 in the z-direction, as shown in FIG 2 B, a
hinge point ® may be placed in each corner 121, and this is possible because the effective
displacement of the rings 102, 0 in each corner 121 is the same, so the effective
displacement at each hinge point is the same.
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; 0 1 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. n some embodiments a separate
actuation mechanism for each lens assembly ; 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 ; 101 is desired, the
fluid pressure could be adjusted with a pump.
It will be appreciated that the use of separate front and rear supporting rings 2, 0; 102, 10
is not essential to achieve the basic functionality of the lens assembly ; 101 of the present
invention, and in some variants the membrane 8; 08 may be supported by a single flexible
ring. However, it has been found tha the use of two or more supporting rings is
advantageous for controlling for example the rate of twist in the supporting rings 2, 0, and
particularly during manufacture of the assembly.
FIG 25 illustrates the attachment of a flexible membrane 208 to a single membrane
supporting ring 2 0 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 0 and pull on one face of the support ring 2 0 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.
The present invention provides a solution to this problem by using two supporting rings 2,
10; 102, 0; 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, 3 .
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 1 , a layer of
adhesive 356 s also applied to a rear face of a front supporting ring 302. This can be done
simultaneously or sequentially. The two supporting rings 302, 3 0 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 ri gs 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.
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,
08 can be achieved Optical distortion is minimised and the lens can he 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.
In the first and second lens assemblies 1; 0 1 described above, the variation in stiffness of
the membrane supporting rings 2, 0; 2, 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 n 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 xy
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, n 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.
The first and second lens assemblies 1; 101 may suitably be installed in a pair of
eyeglasses 90 such that the flexible membrane 8, 1 8 bulges forwards away from the
wearer's eyes when actuated. This may be preferred for safety reasons, but it wi l be
appreciated that the ens assemblies 1; 101 could equally well be installed in eyeglasses so
that the membrane bulges towards the user's eyes.
n the first and second ens assemblies 1; 101 the cavity 22; 122 is defined in part by the
dish-shaped part 32; 12, the rear wall 1 ; 9 of which is attached to the rear cover plate
16; 6. n a variant, the dish-shaped part 12; 1 2 may be omitted and replaced by a flexible
sealing ring (not shown) which is similar to the side wall 8; 118 alone of the dish-shaped
part and forms a seal between the rear cover plate 1 ; 1 6 and the rear supporting ring 10;
] 0 (or the reinforcing diaphragm 24 if included).
It should also be noted that a fixed prescription lens (for distance or near vision) could be
included in the lens assembly ; 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; 1 6 Such
a fixed power ens should have an optical centre tha is closely aligned with the optical
centre of the adjustable lens OC when actuated.
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.
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 th 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 n general terms, the above described principles can be applied to any
application in which the geometry of a surface needs to be controllably varied.
CLAIMS
1. A deformable membrane assembly comprising an at least partially flexible fluid-filled
envelope, one wail of which is formed by an elastic membrane tha is he d 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; wherein 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, and the
moving means comprise a plurality of ring-engaging members that are arranged to apply a force
to the ring at spaced control points; characterised in that there are at least three control points,
and there is a control point at or proximate each point on the ring where th 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 a inflection point or a turning point in the opposite
direction
2. A deformable membrane assembly as claimed in claim 1, wherein the moving means
apply a force to the ring at each control point in the same direction,
3. A deformable membrane assembly as claimed n claim 1 or claim 2, wherem the moving
means are configured for compressing the envelope.
4. A deformable membrane assembly as claimed in claim 1 or claim 2, wherein the moving
means are configured for expanding the envelope.
5. A deformable membrane assembly as claimed in claim 3, wherein a control point is
disposed at or proximate each point on the ring where the profile of the ring when actuated
exhibits a local maximum displacement in the inwards direction relative to the envelope
intermediate two adjacent points on the ring where the profile of the ring in the direction exhibits
a local minimum displacement in the inwards direction,
6. A deformable membrane assembly comprising a fluid-filled compressible envelope, one
wall of which is formed by a distensible membrane that is held around its edge by a resiliently
bendable supporting ring, a fixed support for the envelope and selectively operable means for
compressing the envelope i a first direction against the support to increase the pressure of the
fluid therein to cause the membrane to deform outwardly in a second opposite direction; wherein
the bending stiffness of the ring varies round the ring such that upon distension of the membrane
the ring bends variably to control the shape of the membrane to a predefined form, and a
plurality of ring-engaging members are arranged to engage the ring at selected spaced control
points for applying the compressive force between the ring and the support; characterised in that
there are at least three control points, and there is a control point at or proximate each point on
the ring where the displacement of the ring in the first direction is a local maximum intermediate
two adjacent points on the ring where the displacement of the ring in the second opposite
direction is a local maximum.
7. A deformable membrane assembly as claimed in any preceding claim, wherein one or
more of said control points are actuation points, where the ring-engaging members are
configured actively to displace the supporting ring relative to the support
8. A deformable membrane assembly as claimed in claim 7 wherein the membrane is
continuously adjustable between an un-actuated state and fully deformed state, and at each
position between the un-actuated and fully deformed states the supporting ring is displaced at the
or each actuation point by the distance required to achieve the predefined form of the membrane.
9. A deformable membrane assemb!y as claimed in any preceding claim, wherein one or
more of said control points are hinge points, where the ring-engaging members are configured to
hold the supporting ring stationary relative to the support.
10. A deformable membrane assembly as claimed in claim 9, wherein the membrane is
continuously adjustable between an un-actuated state and fully deformed state, and the
supporting ring is required to remain stationary at the or each hinge point to achieve the
predefined form of the membrane at each position between the un-actuated and fully deformed
states.
11. A deformable membrane assembly as claimed in claim 9 or claim 0, wherein two
adjacent hinge points define a tilting axis, and there is at least on 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 for adjusting the volume of the
envelope.
12. A deformable membrane assembly as claimed in claim 9, claim 10 or claim 1, wherein
said predefined for has a centre and there are a plurality of hinge points that are substantially
equidistant from the centre of the predefined form.
3 . A deformable membrane assembly as claimed in claim 1 or claim 2, wherein the
supporting ring is generally rectangular, having two short sides and two long sides; the at least
one actuation point is located on one of the short sides, the two adjacent hinge points are located
on or proximate to the other short side.
14. A deformable membrane assembly as claimed in claim 13, wherein the predefined form
has a centre, the one short side generally follows the arc of a circle that is centred on the centre,
and the at least one actuation point is located substantially centrally on said one short side.
15. A deformable membrane assembly as claimed in any preceding claim, wherein the
supporting ring is free to bend passively relative to the support between the control points,
16. A deformable membrane assembly as claimed in any preceding claim, wherein stiffening
elements are provided for stiffening one or more regions of the supporting ring,
17. A deformable membrane assembly as claimed in any preceding claim, wherein the
supporting ring comprises two or more ring elements, and the membrane is sandwiched between
two adjacent ring elements.
. 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 selectively operable means for adjusting the pressure
of fluid in the envelope to cause the membrane to deform: wherein 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; characterised in that the ring comprises
a plurality of ring elements, and the membrane is sandwiched between two adjacent ring
elements.
. A deformable membrane assembly as claimed in any preceding claim, wherein the
supporting ring is made from a substantially uniform and homogeneous material and has a
variable second moment of area to control the bending stiffness round the ring.
20. A deformable membrane assembly as claimed in claim 19, wherein the supporting ring has a
substantially uniform depth and a variable width to control the second moment of area round the
ring.
21. A deformable membrane assembly as claimed in claim 20. wherein the supporting ring is
narrowest where it is required to bend the most to achieve the predefined form when the
membrane is deformed.
22. A deformable membrane assembly as claimed in any preceding claim, wherein the
predefined membrane shape is spherical or a form defined by one or more Zernike
polynomials Z k < f )
23. A deformable membrane assembly as claimed in any of claims 7, 8 or , 3 or 4,
wherein the supporting ring is formed with a protruding tab at the or at least one of the actuation
points for engaging the ring with the ring engaging element.
24. A deformab!e membrane assembly as claimed in any preceding claim wherein the
supporting ring is planar when un-actuated and the membrane is pre-tensioned on th ring.
25. A deformable membrane assembly as claimed in claim 24, wherein a reinforcing
diaphragm is 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.
26. A deformable membrane assembly as claimed in claim 25, wherein the reinforcing
diaphragm is fastened to the supporting ring uniformly round the ring so tha the tension in the
membrane is transmitted uniformly to the diaphragm.
27. A deformable membrane assembly as claimed in claim 25 or claim 26, wherein within
the plane of 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.
28. A deformable membrane assembly comprising an at least part ia y 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, and selectively operable means for adjusting the pressure of
th fluid within the envelope to cause the membrane to deform: wherein the ring is planar when
un-actuated and has a bending stiffness tha 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; characterised in that the membrane is pre-tensioned on the supporting ring, and a
reinforcing diaphragm is provided that is bonded to the supporting ring and has a greater
sti ffness in the plane of the ring than in the direction of bending of the ring
29. A deformable membrane assembly as claimed in claim 28, wherein the reinforcing
diaphragm is fastened to the supporting ring uniformly round the ring so that the tension in the
membrane is transmitted uniformly to the diaphragm.
30. A deformable membrane assembly as claimed in claim 28 or claim 29, wherein within
the plane of 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.
31. A deformable membrane assembly as claimed in any preceding claim, wherein said fluidfilled
envelope comprises an inflexible rear wall that is spaced from the membrane and a flexible
side wall between the membrane and the rear wall.
32. A deformable membrane assembly as claimed in claim , wherein the membrane, rear
wall and fluid are transparent such that the membrane and rear wa form an adjustable optical
lens
33. A deformable membrane assembly as claimed in claim 32, wherein the rear wall is
shaped to provide a fixed lens.
34. A deformable membrane assembly as claimed in claim 32 or claim 33 further comprising
a transparent rigid front cover over the membrane, which front cover is optionally shaped to
provide a fixed ens
35. A deformable membrane assembly as claimed in any of claims 32-34, wherein the
envelope is housed within a retaining ring.
36. An article of eyewear comprising a deformable membrane assembly as claimed in any of
claims 32-35,
37. An article of eyewear as claimed in claim 36 comprising a frame with a rim portion;
wherein the deformable membrane assembly is mounted within the rim portion.
38. A deformable membrane assembly substantially as hereinbefore described with reference
to and as illustrated in FIGS. 3-12 or FIGS. 19-20 or FIGS. 21-22 of the drawings.
39. Eyeglasses substantially as hereinbefore described with reference to and as illustrated in
FIGS.2-12 of the drawings.