Patent Description:
It is known to produce an ophthalmic element comprising a vision base element and a film structure that is adhered to at least one face of this vision base element. To ensure that the ophthalmic element so-obtained enables vision to a wearer, the film structure is at least partially optically transparent.

The vision base element generally has a non-planar optical surface that can be classified according to two principal curvature values along two perpendicular directions for any point in this surface of the vision base element. Where one of the curvature values equals zero, corresponding to the case of ruled surfaces, like a portion of a cylindrical or conical fold as particular examples, the optical surface of the vision base element is said to be monoclastic. Where none of the curvature values equals zero but both have same sign, the optical surface of the vision base element is said to be synclastic. Where both curvature values are non-zero and have opposite signs, the optical surface of the vision base element is said to be anticlastic. Using the Gauss curvature definition where the Gauss curvature value equals the product of both principal curvature values, the monoclastic surfaces correspond to zero Gauss curvature value, the synclastic surfaces correspond to positive Gauss curvature values, and the anticlastic surfaces to negative Gauss curvature values. The film structure has a thickness ranging from one hundred nanometers to several or hundreds of micrometers. Thus, the film structure is flexible and supposed to conform to the optical surface of the vision base element. For cost reasons in the manufacture of the film structure, this film structure most often has initially a planar shape.

However, all film structures used up to now have a positive Poisson ratio, which means that when these film structures are stretched parallel to an axial direction, they shrink parallel to a transverse direction which is perpendicular to the axial direction. This behavior causes wrinkles, cracks and delaminations when the film structure is made to conform to the vision base element, when this latter has a synclastic or anticlastic surface.

A known method for avoiding such wrinkles, cracks and delaminations when adhering the film structure onto a vision base element having a synclastic or anticlastic surface is to perform beforehand a preforming of the film. Nevertheless, because the film structure always has a residual elastic behavior, such preforming generates residual stresses within the film structure, which may cause wrinkles, cracks and delaminations to appear again once the film structure has been applied onto the vision base element or later in the lifetime of the product.

In order to reduce these residual stresses in the film structure, which are due to the elastic component of its deformation, it is also known to apply heat treatments to the film structure adhered to the vision base element, but such heat treatments are time-consuming and expensive. In addition, some heat treatments at high temperature may damage fragile compounds contained either in the film structure or in the vision base element. Moreover, these heat treatments do not sufficiently suppress the residual stresses to ensure a long-term efficiency on product reliability. <CIT> provides a comparative example.

There is thus a need for ophthalmic elements manufactured in such a way as to reduce the manufacturing complexity and cost, while ensuring that stresses produced when adhering the film structure to the vision base element do not cause wrinkles, cracks and delaminations to appear.

For meeting these objects or others, a first aspect of the invention proposes an ophthalmic element comprising a vision base element and a film structure that is adhered to a curved optical surface of the vision base element, so that the film structure conforms to this optical surface, wherein the film structure comprises at least one auxetic film that has a negative Poisson ratio v. The Poisson ratio v is defined as: <MAT> where εtrans is a relative transverse strain of the auxetic film, and εaxial is a relative axial strain of the auxetic film, these relative transverse strain and relative axial strain being positive for increases in transverse dimension and axial dimension, respectively, of the auxetic film, or being negative for decreases in these transverse and axial dimensions.

Since the auxetic film has a negative Poisson ratio, the relative transverse strain εtrans and the relative axial strain εaxial can be both positive or both negative, allowing the film structure to conform to the optical surface of the vision base element when this latter is synclastic, while limiting the stresses produced in the film structure. Wrinkles, cracks and delaminations in the ophthalmic element are suppressed as a result. The mechanical and visual performances of the ophthalmic element are thus improved.

Moreover, the manufacture of the ophthalmic element comprising the auxetic film is simplified and reduced in cost, since no preforming or stress-suppressing post-annealing may be necessary. But the invention may be combined with a thermal curing, in particular if a heat-curable material is used, for instance for attaching the auxetic film to the vision base element.

Generally, the film structure may comprise one or several additional components in addition to the auxetic film, but these additional components allow the auxetic behavior to be provided to the film structure by the auxetic film.

In possible embodiments of the invention, the optical surface of the vision base element may have two non-zero curvature values at at least one point of this optical surface, along two directions which are tangential to the optical surface and perpendicular to each other. In particular these two non-zero curvature values may be both positive or both negative. The optical surface of the vision base element is therefore a synclastic surface. Thanks to the auxetic behavior of the film structure, the stresses in this film structure are limited so that the film structure can conform to the synclastic surface without wrinkling, cracking or delaminating from the vision base element.

Also in possible embodiments of the invention, the auxetic film may be provided with a cut pattern. Thanks to such cut pattern, any material commonly used in the ophthalmic field, namely optically transparent materials, can acquire an auxetic behavior.

In invention embodiments where the auxetic film is made of an optically transparent material, in particular such polymer-based material, the ophthalmic element may further comprise portions of an index-matching material that fill gaps of the auxetic film due to the cut pattern. The index-matching material then has a refractive index value which is substantially equal to that of the auxetic film. In this way, the cut pattern becomes invisible or almost invisible. For such invention embodiments, the index-matching material may be of one of the following types: an adhesive, in particular a pressure-sensitive adhesive, a hot-melt adhesive, a UV-cured adhesive composition, a thermally-cured adhesive composition, and also a primer, a hard coat, etc. without limitation. The index-matching material can therefore be adapted to different manufacturing conditions.

Again for such invention embodiments, a layer of the index-matching material may extend between the vision base element and the auxetic film, continuously across the optical surface, so as to permanently connect the auxetic film to the vision base element. The index-matching material may therefore be used to adhere the film structure to the optical surface of the vision base element, in addition to making the cut pattern invisible. But also possibly, the index-matching material located between the vision base element and the auxetic film may be combined with a connecting film, this latter producing the attachment of the index-matching material provided onto the auxetic film to the vision base element.

Alternatively, the layer of the index-matching material may extend continuously over the auxetic film on a side thereof that is opposite the vision base element, so as to form an optical surface of the ophthalmic element.

In invention embodiments where respective materials of the vision base element and the auxetic film are transparent and have substantially equal values of refractive index, the material of the vision base element may fill the gaps of the auxetic film that are due to the cut pattern. No additional material is required for suppressing the light scattering.

Generally for the invention, the film structure may be multilayered and/or comprise cells which are juxtaposed next to one another parallel to the optical surface of the vision base element. The film structure can therefore provide the ophthalmic element with a great variety of functions, depending on the type and operation of the film structure. In particular, the film structure may be adapted to produce at least one of the following functions: an antireflecting function, an electrical conduction function, a solar-protection function, a photochromic function, an electrochromic function, a polarizing function, a function based on a holographic optical element, or a dioptric function, either passive or active dioptric function. The ophthalmic element is therefore provided with the one or several functions of concern.

Generally for the invention again, the ophthalmic element may form a spectacle lens, a helmet glass, a skiing mask, a diving mask, a goggle glass, an augmented reality device or a virtual reality device. The ophthalmic element is therefore a solution for a wide range of applications.

A second aspect of the invention proposes a method for manufacturing an ophthalmic element according to the first invention aspect. The method comprises:.

Thanks to the auxetic behavior of the film structure, the occurrence of stresses, wrinkles, cracks and delaminations is limited or suppressed when the film structure is bonded to the optical surface of the vision base element.

When the auxetic film is made of an optically transparent material, and the ophthalmic element further comprises portions of an index-matching material that fill gaps of the auxetic film provided with the cut pattern, the index-matching material having a refractive index value substantially equal to that of the auxetic film, the method may comprise:.

The index-matching material thus bonds the film structure and the vision base element to each other while making the cut pattern invisible. No additional material is therefore needed to ensure that the film structure and the vision base element are bonded to form the ophthalmic element.

The index-matching material creates therefore an optical surface of the ophthalmic element while making the cut pattern invisible by filling the gaps of the auxetic film. Moreover, the mold may be designed to confer a desired shape to the optical surface of the ophthalmic element, as formed by the index-matching material. The method thus allows to obtain easily ophthalmic elements having different shapes of optical surface.

When both the vision base element and the auxetic film are made of optically transparent materials that have substantially equal refractive indices, the method may comprise:.

In such latter implementations, no index-matching material different from the material of the vision base element is needed to render the cut pattern invisible. The ophthalmic element can be manufactured in this way more quickly as the shaping of the vision base element, its adhesion to the auxetic film and the suppression of the light scattering take place at the same time.

In improved embodiments of the invention, the method may further comprise the following step performed before arranging the film structure within the mold:.

In this way, stresses that may be generated in the auxetic film when it is provided with the desired final shape are further reduced, in addition to the reduction already provided by the auxetic behavior of the film structure.

In particular, the auxetic film may be designed so that a shape of the film structure changes to a progressive surface upon heating and/or applying differential pressure to the film structure. The invention can thus be further improved in case the ophthalmic element has a progressive surface.

In all implementations of the invention that use a mold, this mold may be an injection mold or a casting mold, including molds provided with improvements such as vacuum assisted molds or UV-transparent molds.

In addition, other processes may be implemented alternatively for applying the index-matching material onto one surface. For instance, the index-matching material may be applied onto the vision base element surface or onto the auxetic structure by spin-coating, spray-coating, inkjet, lamination on one or the other of the surfaces, etc. Still another process which may be implemented is making the index-matching material to diffuse through capillarity into the gaps of the auxetic film or structure, possibly using a vacuum-assisted diffusion process.

Generally, the cut pattern of the auxetic film may be formed by implementing one of the following processes:.

The cut pattern that confers the auxetic behavior to the film structure can thus be produced using a simple and inexpensive technique. The ophthalmic element can therefore be low-cost and easy to manufacture.

Other features and advantages of the invention disclosed herein will become apparent from the following description of non-limiting embodiments, with reference to the appended drawings, in which:.

In these figures, elements represented do not correspond to actual dimensions or dimension ratios. In addition, same references indicated in several ones of the figures denote same elements.

According to <FIG>, an ophthalmic element <NUM> comprises a vision base element <NUM> and a film structure <NUM>. Optionally, the ophthalmic element <NUM> may further comprise a portion of an index-matching material <NUM> (<FIG>) that will be described later. The ophthalmic element <NUM> forms for example a spectacle lens, a helmet glass, a skiing mask, a diving mask or a goggle glass depending on the design and shape of the vision base element <NUM>. The case of a spectacle lens will be used hereafter for illustration purpose. Also for illustrative purpose, the film structure is applied onto a convex surface of the vision base element in the detailed implementations of the invention which are described in detail hereafter, but it may be alternatively applied onto a concave surface of the vision base element, or two auxetic films applied on one convex surface and one concave surface of the vision base element, respectively.

The vision base element <NUM> comprises a front face <NUM> and a rear face <NUM>, which are usually convex and concave respectively. The actual shapes of the front face <NUM> and rear face <NUM> depend on an optical power and astigmatism values that are to be produced by the ophthalmic element <NUM>. The vision base element <NUM> may be a semi-finished eyeglass with only the front face <NUM> being a final optical surface, or an eyeglass with both the front face <NUM> and rear face <NUM> being final optical surfaces. In addition, a peripheral edge <NUM> of the vision base element <NUM> connects the front face <NUM> and rear face <NUM> to each other. The peripheral edge <NUM> may be circular for a semi-finished eyeglass, or may conform in size and shape to a spectacle frame in which the ophthalmic element <NUM> being a spectacle lens is intended to be mounted. For clarity sake, the following description will be limited to invention embodiments where the film structure <NUM> is arranged on the convex front face <NUM>, but other configurations may be contemplated as well. The front face <NUM> then constitutes the optical surface of the vision base element as introduced in the general part of this description. As shown in <FIG>, the front face <NUM> of the vision base element <NUM> has two curvatures C<NUM> and C<NUM> at any point P in this face. These curvatures C<NUM> and C<NUM> are defined respectively with respect to two directions D<NUM> and D<NUM> which are tangential to the front face <NUM> at point P and perpendicular to each other. In most cases, both curvatures C<NUM> and C<NUM> are non-zero and of one and same sign, i.e. corresponding to one and same curvature orientation. In such cases, the front face <NUM> is said to be a synclastic surface. However, using an auxetic film structure <NUM> as described below may also be implemented with a monoclastic or anticlastic surface of the vision base element <NUM>.

The vision base element <NUM> may be of any optically transparent material that is commonly used in the ophthalmic field, in particular a polymer-based material.

The film structure <NUM> has an initial planar shape and is flexible. The initial planar shape of the film structure <NUM> is a major issue for its manufacturing cost. Its thickness usually ranges from one hundred nanometers to several tens or hundreds of micrometers. However, such film structure <NUM> exhibits important stresses when forced to conform to a synclastic or anticlastic surface. In <FIG>, the film structure <NUM> is shown as a disk-shaped patch tangential to the front face <NUM> at point P. But it is then distant along the sag direction from the front face <NUM> near the peripheral edge <NUM>. The front face <NUM> being curved and the film structure <NUM> being initially planar, hash signs that represent light reflections in <FIG> are drawn with curved dashed lines when they relate to the front face <NUM> of the vision base element <NUM> in backplane position, and with straight continuous lines when they relate to the film structure <NUM> in foreground position.

The film structure <NUM> is at least partially transparent for vision applications, at least within an optically useful area of the ophthalmic element <NUM>.

The film structure <NUM> may comprise a single layer, but it may alternatively be multilayered as represented in <FIG>. In such latter case, the film structure <NUM> comprises at least one support film <NUM> and at least one layer <NUM>. For example, a plurality of layers <NUM> that are superposed on each other may have been deposited on the support film <NUM> using at least one thin film coating process. In particular, such multilayer configuration for the film structure <NUM> is appropriate for providing an antireflecting function or a solar protection function.

Alternatively, the film structure <NUM> may have a cellular structure as denoted by reference number <NUM> in <FIG>. Then, it comprises again the support film <NUM>, but this latter is provided with cell-separating walls <NUM> and a capping layer <NUM>. The capping layer <NUM> seals the cells <NUM> at the top edges 34E of the cell-separating walls <NUM>. In this way, the cells <NUM> are appropriate to contain permanently functionalization material portions. Such cellular film structure has been widely described in the literature, and suits for providing the ophthalmic element <NUM> with photochromic or electrochromic functions, for example.

The film structure <NUM> may also or alternatively comprise a refractive index gradient or any variation of refractive index.

When such film structure <NUM> in accordance with <FIG> is made to conform to the synclastic front face <NUM> of the vision base element <NUM>, the support film <NUM> forms cracks or wrinkles which result into delaminations in the final ophthalmic element <NUM>. The present invention overcomes this technical problem by using a support film <NUM> that is auxetic. For this reason, the support film <NUM> has been called directly auxetic film in the general part of the present description and below. Unlike most of films, the auxetic film <NUM> can expand simultaneously along two directions which are perpendicular to each other and both tangential to the film, at at least one location in the film or any location therein. In the same way, it can also shrink simultaneously along both directions. This behavior is commonly expressed using the Poisson ratio v, which quantifies a response of the film along a transverse direction when a change in length is applied to the film along an axial direction. Both axial and transverse directions are perpendicular to each other and tangential to the film. The relative axial strain εaxial is εaxial = <MAT>, where A<NUM> and Af are length values along the axial direction of an elementary film portion at point P, respectively when no stress is applied to this film portion and when a non-zero stress is applied. Similarly, the relative transverse strain εtrans is <MAT>, where T<NUM> and Tf are length values along the transverse direction of the same elementary film portion, respectively when no stress is applied to the film portion and when the non-zero stress is applied along the axial direction. The Poisson ratio v is then <MAT>. The films which were applied on vision base elements before the present invention have positive values for the Poisson ratio. Unlike such prior implementations, the support film <NUM> used according to the invention has a negative value for the Poisson ratio, namely it is auxetic. In this way the auxetic film <NUM> can expand at any point P where it has the auxetic behavior simultaneously along two directions which are perpendicular to each other and parallel to the film. Similarly, it can shrink simultaneously along these two directions. Thus, auxetic films suit especially for conforming to synclastic surfaces, without generating much internal stresses. For this reason, when arranged on a synclastic surface of the ophthalmic element <NUM>, the auxetic film <NUM> does not tend to generate wrinkles, cracks and delaminations over time.

In possible embodiments of the invention, the auxetic film <NUM> is made of an optically transparent polymer-based material. In particular, the auxetic film <NUM> may be made of cellulose triacetate, also known as TAC, or polyethylene terephtalate, also known as PET, or cyclo-olefin polymers or copolymers, known also as COP or COC, or any polymer-based material commonly used in the ophthalmic field. Its auxetic behavior may be provided by a cut pattern which extends across the thickness of the film and may extend throughout the entire film area that is to conform to the curved optical surface of the vision base element. Several such cut patterns are well known, and they can be produced at low cost in the support film <NUM> using one of the following processes: mechanical cutting including matrix stamping, laser cutting and ultrasonic cutting. <FIG> and <FIG> show two examples of cut patterns which may be implemented to provide the support film <NUM> with the auxetic behavior. In these figures, P denotes a base pattern which is repeated across the film <NUM> for obtaining distributed auxetic behavior. In <FIG> and <FIG>, reference <NUM> denotes gaps in the auxetic film <NUM> caused by the cut pattern.

These gaps <NUM> would scatter light, be unaesthetic and even render vision through the ophthalmic element <NUM> blurry without implementing the invention improvement now described. For avoiding light scattering due to the gaps <NUM>, the ophthalmic element <NUM> also comprises the portion of index-matching material <NUM>. This portion <NUM> fills the gaps <NUM> in the auxetic film <NUM> so as to suppress light scattering which may be caused by these gaps otherwise. The index-matching material is selected to have a refractive index value which equals that of the auxetic film <NUM>. The cut pattern is then invisible and the ophthalmic element <NUM> provides excellent vision comfort and is aesthetic.

In the configuration of the ophthalmic element <NUM> shown in <FIG>, the portion of index-matching material <NUM> fills the gap <NUM> and further forms a continuous layer <NUM> between the auxetic film <NUM> and the vision base element <NUM>. If the film structure <NUM> comprises layers <NUM> and/or the cellular structure <NUM>, these are supported by the auxetic film <NUM> on its side which faces away from the vision base element <NUM>. The ophthalmic element <NUM> may then be manufactured in the following way. First, the auxetic film <NUM> and the vision base element <NUM> are both arranged within a mold <NUM> with a gap therebetween as shown in <FIG>. Then, the index-matching material is injected into the mold <NUM> between the auxetic film <NUM> and the vision base element <NUM> as shown by arrow F(<NUM>), for filling the gaps <NUM> and forming simultaneously the continuous layer <NUM> between the auxetic film <NUM> and the vision base element <NUM>. For the index-matching material to penetrate more easily into the gaps <NUM> and filling them completely, the injection process may be combined with pumping of gas from inside of the mold <NUM>. The layer <NUM> ensures permanent connection of the auxetic film <NUM> to the vision base element <NUM>. The index-matching material implemented in this way may be a pressure-sensitive adhesive, a hot-melt adhesive, a UV-cured adhesive composition or a thermally-cured adhesive composition. For the configuration of the ophthalmic element <NUM> so-obtained, the film structure <NUM> forms the front face of the final ophthalmic element <NUM>. The injection of the index-matching material between the base vision element <NUM> and the auxetic film <NUM> forces this latter to conform to an internal face S<NUM> of the mold <NUM>, thereby defining the final shape of the front face of the ophthalmic element <NUM>.

In the configuration of the ophthalmic element <NUM> shown in <FIG>, the portion of index-matching material <NUM> fills again the gaps <NUM> but now forms a continuous layer <NUM> that is located on a side of the auxetic film <NUM> that is opposite the vision base element <NUM>. In case layers <NUM> and/or the cellular structure <NUM> are supported by the auxetic film <NUM>, these are located on the side of the auxetic film <NUM> which faces the vision base element <NUM>. The ophthalmic element <NUM> may then be manufactured in the following way. First, the auxetic film <NUM> is adhered to the front face <NUM> of the vision base element <NUM> using any bonding process known in the art, for example using a pressure-sensitive adhesive. Then, the vision base element <NUM> with the auxetic film <NUM> adhered thereto is introduced into the mold <NUM> as shown in <FIG>, and the index-matching material is injected between the internal face S<NUM> of the mold <NUM> and the auxetic film <NUM> (see arrow F(<NUM>) in <FIG>). For such other configuration of the ophthalmic element <NUM>, its final front face is formed by the continuous layer <NUM> of the index-matching material, this latter being preferably a UV-cured adhesive composition or a thermally-cured adhesive composition. However, it may be advantageous to provide an additional film in the mold <NUM> along its internal face S<NUM> such as a UV- or heat-curable hard coat, for avoiding that the continuous layer <NUM> sticks too much to the mold or that the exposed surface after demolding is sticky.

<FIG> illustrates still another possible configuration for the ophthalmic element <NUM> while implementing again the invention. It applies when the material of the vision base element <NUM> has a refractive index value which equals that of the auxetic film <NUM>. Then, the vision base element <NUM> can replace the portion of index-matching material in addition to its initial functions of supporting element and optical power production. To this end, the material of the vision base element has to fill the gaps <NUM> of the auxetic film <NUM>. As shown in <FIG>, the auxetic film <NUM> is arranged within the mold <NUM> with the layers <NUM> or the cellular structure <NUM> arranged against the internal face S<NUM> of the mold. Then, the material of the vision base element is injected into the mold <NUM> (see arrow F(<NUM>) in <FIG>) so as to fill it completely, thereby forming the vision base element <NUM> and filling the gaps <NUM> of the auxetic film <NUM>. This injection forces the auxetic film <NUM> to conform to the internal face S<NUM> of the mold, thereby defining the final shape of the front face of the ophthalmic element <NUM>. This latter process suits when the respective materials of the vision base element <NUM> and the auxetic film <NUM> adhere to each other, otherwise an intermediate adhesion material is to be deposited on the auxetic film <NUM> and into its gaps <NUM> before it is arranged within the mold <NUM>.

For the manufacturing methods just described with reference to <FIG>, it is possible to further reduce or suppress residual stresses that may exist within the auxetic film <NUM> in the final product. To this end, the auxetic film <NUM> may be designed so that its shape spontaneously changes during the injection process so as to become similar to that of the internal face S<NUM> of the mold <NUM>. This may be achieved by appropriately selecting the cut pattern and variations thereof across the front face <NUM>. Other possibilities for obtaining such shape changes of the auxetic film <NUM> may consist in applying a mechanical and/or thermal treatment to the auxetic film <NUM> before using it in the injection process. The parameters that trigger such shape variations for the auxetic film <NUM> may be the temperature increase and/or differential pressure involved in the injection process.

For some applications of the invention, and in particular when the ophthalmic element <NUM> is a spectacle lens, its front face may be of progressive surface type. In such case, the internal face S<NUM> of the mold <NUM> has a shape that corresponds to this progressive surface.

Claim 1:
An ophthalmic element (<NUM>) comprising a vision base element (<NUM>) and a film structure (<NUM>) that is adhered to a curved optical surface (<NUM>) of the vision base element, so that the film structure conforms to said optical surface, wherein the film structure comprises at least one auxetic film (<NUM>) that has a negative Poisson ratio v, said Poisson ratio v being defined as: <MAT> wherein εtrans is a relative transverse strain of the auxetic film (<NUM>), and εaxial is a relative axial strain of the auxetic film, said relative transverse strain and relative axial strain being positive for increases in transverse dimension and axial dimension, respectively, of the auxetic film, or being negative for decreases in said transverse dimension and axial dimension, respectively, of the auxetic film.