Technical Field

The present invention relates to a method for manufacturing a custom optical element from a flexible blank. Furthermore, it relates to an apparatus for manufacturing such a custom optical element as well as to a flexible blank from which such a custom optical element can be manufactured. In addition, the invention discloses a method for correcting at least one aberration of an optical system by means of manufacturing and integrating such a custom optical element into the optical system.

Introduction and Background Art

In technical or biological optical systems, a plurality of optical aberrations such as defocus, astigmatism, coma, spherical aberrations, chromatic aberrations etc. can limit the imaging performance of the optical system. These optical aberrations can be mathematically described by expanding one or more wavefront function (s) traveling through the optical system with circular pupils in a series of Zernike polynomials representing the different types of aberrations introduced by the optical system. Depending on the radial order of the respective Zernike polynomial, aberrations are classified as "low-order" or "high-order". E.g., defocus and astigmatism are usually referred to as "low-order aberrations" because the order of the describing Zernike polynomial is 2 or less. Other aberrations, e.g., coma and spherical aberrations are usually referred to as "high-order aberrations" if the radial order of the describing Zernike polynomial is 3 or more.

In other words, due to optical aberrations in the optical system, light is misdirected by the components of the optical system and thus causes the image to be an imperfect replica of the to-be-imaged object. Therefore, various approaches have been undertaken to provide custom optical elements that correct for at least some of the aberrations of the optical system to improve overall imaging quality.

US 7, 701,641 B2 and US 2010/0265457 Al disclose a method for producing custom wavefront aberrators which are capable of correcting high-order aberrations in an optical system. This is achieved by a specific refractive index profile of the wavefront aberrator which is produced by locally varying a polymerization grade of a layer of polymerizable material positioned between two solid transparent plates. Due to an additional intentional diffusion step for unpolymerized monomers in the polymerizable material and a subsequent curing step, it is claimed that the disclosed wavefront aberrators are more stable against thermal and solar exposure and that greater optical paths differences (OPDs) than in conventional devices can be achieved.

US 2010/0195212 Al relates to a method for optimizing human visual function by proposing a personalized optical element which tries to correct low and high order aberrations of the human eye for eye pupil diameters of 3 mm and less. For pupil diameters above 3 mm it is proposed to maximize the modulation transfer function (MTF) below the Nyquist frequency and to minimize the MTF above the Nyquist frequency of retinal sampling.

US 2008/0252845 Al discloses an apparatus and method for correcting low- and high-order aberrations of the human eye. A proposed custom wavefront aberrator which is adapted to be placed on a human eye relies on a pair of solid transparent plates which are separated by a layer of curable (polymerizable) material. Depending on the extent of curing (polymerization) , a specific ref active index profile can be produced. US 5,833,890 and US 4,382,902 disclose methods for producing elastic lenses comprising specific curable polymer materials.

However, all the proposed methods and optical elements have the disadvantage that their correction abilities remain limited and/ or that their manufacturing is relatively complicated and only possible with expensive manufacturing equipment.

Disclosure of the Invention

Hence, it is a general objective of the present invention to provide an improved method for manufacturing a custom optical element which is suitable for correcting low- and/ or high-order aberration of an optical system. Further objectives are to provide an apparatus for manufacturing such a custom optical element, to provide a blank from which such a custom optical element can be manufactured, and to provide a method for correcting at least one aberration of an optical system by means of such a custom optical element.

Accordingly, a method for manufacturing a custom optical element from a flexible blank comprises the step of applying at least one force to at least one section (i.e., a position determined by coordinates) of the flexible blank. Alternatively or in addition to applying a new force to a section of the flexible blank, at least one existing force, i.e., a force that is already affecting a section of the flexible blank can be changed in this step. The blank from which the optical element is manufactured is flexible (i.e., reversibly deformable) and it comprises a curable material, e.g., an UV-curable fluid such as a liquid or a gel. In response to the above mentioned application and/ or change of the force or forces that is/ are affecting one or more section (s) of the flexible blank, a curvature of the flexible blank changes at least locally, i.e., in the region where the applied force (s) is/ are changed. In other words, the shape of the flexible blank is at least locally altered by exerting pressure on it. As a result of the changed surface curvature of the flexible blank, light is differently refracted by the blank compared to the original state, or - in other words - a refractive power profile (RPP) of the flexible blank is changed. The term "refractive power profile" in this regard is to be understood as a position-dependent function or matrix, wherein said function or matrix is indicative of a local refractive power of the blank, i.e., a refractive power of the blank at the respective position of the blank.

In another step of the manufacturing method, the curable material of the flexible blank is at least locally and at least partially cured, i.e., hardened, e.g., polymerized. Thereby, further changes of the refractive power profile are at least locally prevented or impeded.

It is to be distinctly understood that the step of applying and/ or changing (a) force (s) affecting the refractive power profile of the blank can be performed multiple times during the manufacturing process of the custom optical element. After the first local and/ or partial curing of the curable liquid of the flexible blank, the step of applying and/ or changing a force (s) affecting the refractive power profile of the flexible blank can be repeated one or more times thereby leading to another change in curvature and thus refractive power profile of incompletely cured sections of the flexible blank. Then, another curing step can be performed and so on, up to a point where the curable liquid in the flexible blank is fully cured and the custom optical element is fully manufactured from the flexible blank. This iterative approach has the advantage that almost any desired surface shape and therefore refractive power profile of a custom optical element (describable by, e.g., a series of Zernike polynomials) can be produced from a flexible blank.

In an advantageous embodiment, the manufacturing method comprises a further step of deriving a profile dataset, i.e., a function or matrix which is indicative of the current refractive power profile of the flexible blank. This can, e.g., be achieved by an interfer- ometric measurement or by a wavefront analysis of an originally plane-wave light beam traveling through the flexible blank. This profile dataset is then compared to a desired or "ultimate" refractive power profile for the custom optical element. Thus, deviations between the current refractive power profile of the flexible blank as described by the profile dataset and the ultimately desired refractive power profile of the custom optical element can be detected.

Advantageously, depending on the result of this comparison, an adjustment function A (e.g., again a position-dependent function or matrix) can be derived. In other words, this adjustment function A comprises a description or "recipe" on how the current refractive power profile of the flexible blank as described by the profile dataset needs to be changed to match the ultimate refractive power profile for the custom optical element. Then, the application/ change of the force (s) to the flexible blank depend on this adjustment function A and the curvature and therefore the refractive power profile of the flexible blank approaches the ultimate refractive power profile for the custom optical element.

Because the current refractive power profile may not only depend on the curvature of the blank/ optical element, but also on a refractive index of the blank- /optical element-material itself, a change of said refractive power profile as induced by the curing step(s) can advantageously be anticipated (at least if the characteristics of the curable material are known) and taken into account in the derivation of the proper adjustment function A. As an advantage, the refractive power profile of the custom optical element (i.e., the flexible blank after its full curing) can thus be optimized to match the ultimate refractive power profile as closely as possible.

Advantageously, a magnitude of the adjustment function A is indicative of the deviation between the measured profile dataset (which is indicative of the current refractive power profile of the flexible blank) and the ultimate refractive power profile for the custom optical element. As an example, the magnitude of the adjustment function A can, e.g., be defined as follows: From the plurality of position-dependent values of A, at least 1 value can be derived which can, e.g., be a maximum absolute value of A (independent of the position or coordinates over the flexible blank) , or an integral over A2 over all coordinate values over the flexible blank can be computed. Alternatively, the absolute- values of the original position-dependent values of A can be considered as position dependent magnitudes of A. Then, the curable liquid in the flexible blank is fully cured (i.e., the custom optical element is finished) after the magnitude (s) of A decreases below a threshold, or - in other words - after the current refractive index profile of the flexible blank converges towards the ultimate refractive power profile for the custom optical element. With such a thresholded approach, the true refractive power profile of the optical element (i.e., after full curing of the flexible blank) can be optimized to match the ultimate refractive power profile as closely as possible.

In another advantageous embodiment of the manufacturing method, the flexible blank comprises a central portion (i.e., the area extending from an optical axis z of the flexible blank radially outwards to a first radius) and an outer portion (the area extending radially outwards from said first radius to an outer edge of the flexible blank) . Then, the curable material in the central portion is cured prior to curing the curable material in the outer portion. As an alternative, a 3-fold-, 4-fold-, ... , n-fold-subdivision of the flexible blank with uniform or unequal areas together with a suitable 3- , 4-, n-fold-curing procedure is possible as well.

Thus, the curvatures of the two or more portions of the flexible blank can be independently adjusted. The number of curing steps does not have to match the number of subdivisions.

This can advantageously be achieved by performing a step of applying and/ or changing (a) force (s) affecting the refractive power profile of uncured regions of the flexible blank between curing the central portion and curing the outer portion of the flexible blank. A respective approach is suitable for an n-fold-subdivision as discussed above. Then, in the 2-step example, the central portion is already cured when the refractive power profile of the outer portion is adjusted.

Furthermore, an apparatus for manufacturing a custom optical element from a flexible blank by means of a method as discussed above is disclosed. This apparatus comprises an actuator for applying at least one of said forces to at least one of said sections of said flexible blank thereby changing the refractive power profile of the flexible blank. The actuator can alternatively or additionally also change at least one of said forces that are applied to at least one of said sections of said flexible blank thereby changing the refractive power profile of the flexible blank.

The apparatus further comprises an analyzer for deriving said profile dataset which is indicative of the current refractive power profile of the flexible blank. Thus, the current refractive power profile of the flexible blank in its current shape can be obtained. The apparatus further comprises a curing device for at least locally and at least partially curing the curable material of the flexible blank. Thereby, further changes of the refractive power profile are at least locally prevented or impeded.

With such an apparatus the production of a custom optical element can be simplified because no pre- produced injection molding tools are necessary. Ideally, such a custom optical element can even be produced on- the-spot from a flexible blank, e.g., in the same laboratory where the custom optical element will later be integrated into the optical system. Thus, the need for expensive stock-keeping of non-custom optical elements is no longer existent or at least reduced and custom optical elements can be manufactured "on-demand".

Advantageously, . the apparatus further comprises a movable holder which is adapted to position the flexible blank with respect to the actuator and/ or the analyzer and/ or the curing device. Positioning with six degrees-of-freedom may be possible (three translations, three rotations) . Thus, positioning possibilities of the flexible blank and the ways of how the apparatus can interact with the flexible, blank are extended and more refractive power profiles become possible.

In another advantageous embodiment of the apparatus, the analyzer comprises a wavefront sensor and/ or an interferometer to derive the profile dataset indicative of the current refractive power profile of the flexible blank. Alternatively or additionally, the actuator of the apparatus comprises at least one piezoelectric element and/ or at least one force transmission element, e.g., a force transmission pole, for applying and/ or transferring said force (s) to the flexible blank. Alternatively or additionally, the curing device comprises at least one of the group of a laser, a digital light projector, a beam scanner, an LED, an LED-array, a tunable aperture, and a tunable beam expander for at least locally and/ or at least partially curing the curable material in the flexible blank. Thus, an easier and more precise manufacturing of the custom optical element becomes possible .

In another advantageous embodiment of the apparatus, the apparatus further comprises at least one lens shaper, i.e., an element that is connected to the actuator of the apparatus in a removable or firm manner. This lens shaper transfers at least one of the forces that change the refractive power profile of the flexible blank from the actuator to the flexible blank. Thus, a reproducible mechanical connection between the apparatus and any flexible blank that is inserted into the apparatus is simplified.

Advantageously, a module of elasticity of such a lens shaper is higher than an average module of elasticity of said flexible blank (at least with an un- cured curable material) . Furthermore, the lens shaper advantageously comprises a plurality of pressure points at which the lens shaper is connected to the actuator of the apparatus in a removable or firm manner. These pressure points are adapted to transfer at least one of the forces from the actuator of the apparatus to the lens shaper and thus to the flexible blank where the forces change the refractive power profile of the flexible blank. Thus, a more reproducible mechanical connection between the apparatus and any flexible blank that is inserted into the apparatus is simplified.

In an advantageous embodiment of the apparatus, the lens shaper is ring shaped. This simplifies a reproducible mechanical connection between the apparatus and the flexible blank, in particular for flexible blanks that are rotationally symmetric about an optical axis.

In another advantageous embodiment of the apparatus, the apparatus further comprises a control unit which is adapted to control the units of the apparatus to perform the steps of a manufacturing method for a custom optical element as discussed above. Furthermore, the control unit receives input from the analyzer to gather information about a current refractive power profile of the flexible blank. Thus, an automatic or semi-automatic manufacturing of the custom optical element can be achieved when, e.g., an ultimate refractive power profile for the custom optical element is known.

Furthermore, a flexible blank which comprises a curable material (e.g., a UV-curable fluid such as a liquid or a gel) is disclosed. This blank is adapted to be used to manufacture a custom optical element as discussed above with a manufacturing method as discussed above by means of an apparatus as discussed above. Such a flexible blank is reversibly insertable into the apparatus, a curvature of the flexible blank is changeable by a force (F) that is exerted . onto -the flexible blank by, e.g., the actuator of the apparatus, and the curable material of the flexible blank is curable by the curing device of the apparatus. Thus, the flexible blank serves as a base product to manufacture a custom optical element with a custom refractive power profile that is customized for a specific use in an optical system, e.g., to correct for low- and/ or high-order aberration of an optical system (see below) . Therefore, by using such a flexible blank, a custom optical element is easier to produce and the need for expensive stock-keeping of non-custom, standard optical elements is reduced.

In an advantageous embodiment of the flexible blank, the flexible blank further comprises at least one lens shaper to receive at least one of the forces that change the curvature of the flexible blank from the actuator of the apparatus. This lens shaper can be used instead of or in addition to the lens shaper that can be present in the apparatus. The lens shaper can have vari- ous forms, e.g., it can be ring-shaped for a rotationally symmetric flexible blank and it can comprise a plurality of pressure points that receive the force (s) from the actuator of the apparatus. This has the advantage that a reproducible mechanical connection between the interchangeable flexible blank and the actuator of the apparatus is easier to establish.

In an advantageous embodiment, of the flexible blank, the flexible blank comprises a solid container. The term "solid container" refers to a container with a module of elasticity that is higher than an average module of elasticity of the curable material in its uncured form. The container can enclose at least a part of the fluid. This has the advantage that, e.g., with a round, rotationally symmetric flexible blank two surface sections (i.e., a bottom surface and a side surface) of the flexible blank are formed by the solid container and thus have a defined shape or surface whereas the third surface (the top surface in this example) has a changeable curvature dependent on the applied forces from the actuator that interact with different sections of the curable material of the flexible blank.

Advantageously, in such an embodiment, the container comprises a locally varying refractive power profile itself. E.g., in the above example, the locally varying refractive power profile of the container itself could act as an aspheric or as a spherical lens and/ or as a cylindrical lens. Thus, the locally varying refractive power profile of the container of the flexible blank could be used to correct for low-order aberrations such as defocus and/ or astigmatism whereas the custom surface of the curable material of the flexible blank could be used to correct for high-order aberrations such as spherical aberrations.

Advantageously, such a locally varying refractive power profile is achieved by a diffractive ele- ment, in particular by a Fresnel lens that is part of the container. Alternatively or additionally, the container can comprise at least one curved optical surface, in particular an optically transparent curved optical surface. Thus, a locally varying refractive power profile of the container becomes easier to implement.

In another advantageous embodiment of the flexible blank, the flexible blank further comprises a flexible membrane which is adapted to transfer at least one of the forces from the actuator of the apparatus to the curable material of the flexible blank. Thus, the curable material of the flexible blank is not in direct contact with the actuator from the apparatus and a risk for contamination of the curable material is reduced.

Advantageously, the container of the flexible blank is more rigid than the membrane of the flexible blank and the curable material is fully enclosed by the container and the membrane. Advantageously, the membrane is suspended in the container and thus seals an inner volume of the container where the curable material is arranged. Thus, the curable material can be fully sealed and the risk for contamination of the curable material is reduced .

Advantageously, the flexible blank further comprises a flexible sacrificial layer arranged between the curable material and the membrane. Thus, the membrane can be easier detached from the curable material when the sacrificial layer is modified, e.g., disintegrated.

More advantageously, such a sacrificial layer comprises a material which is dissolvable by a solvent. Furthermore, the curable material is - at least in its cured form - a biocompatible material and it is not dissolved by said solvent. In particular, such a solvent can be water. As an advantage, after the curable material has been cured, the optical element can be immersed in said solvent thus dissolving the sacrificial layer and detach- ing the membrane from the cured curable material body. Thus, manufacturing of a biocompatible custom optical element is simplified.

. Furthermore, a method for correcting at least one high- or low-order aberration of an optical system in at least a part of a field-of-view of the optical system is disclosed. This is achieved by means of a custom additional optical element as discussed above. This additional custom optical element is not yet manufactured or part of the optical system. The method comprises the step of deriving an aberration dataset indicative of at least one optical aberration of the optical system. This can, e.g., be achieved by sensing a wavefront of a beam of light that travels through the optical system by means of, e.g., a wavefront sensor such as a Hartmann-Shack sensor .

In another step of the method, an ultimate refractive power profile for a custom additional optical element is derived based on said aberration dataset. The ultimate refractive power profile is derived such that a future integration of said - not yet manufactured - additional optical element into said optical system corrects for said aberration (s) of said optical system at least in said field-of-view .

In another step of the method, the additional custom optical element with said ultimate refractive power profile is manufactured from a flexible blank as disclosed above with a manufacturing method as discussed above by means of an apparatus as discussed above.

In another step of the method, the additional custom optical element is then integrated into said optical system thus correction for said aberration ( s ) at least in said field-of-view of said optical system. Thus, the imaging performance of the optical system can be improved . In an advantageous embodiment of the method, the method comprises the correction of at least one low- order aberration and at least one high-order aberration of said optical system by means of said additional custom optical element. Thus, the imaging performance of the optical system can be improved.

In an advantageous embodiment of the method, the optical system is a human eye. Thus, human vision can be improved. ^'

In an advantageous embodiment of the method, the additional optical element is foldable. The method then comprises the further steps of folding the additional custom optical element and unfolding the additional custom optical element. Thus, integration of the additional custom optical element into the optical system is simplified.

Brief Description of the Drawings

The invention will be better understood and objectives other than those set forth above will become apparent when consideration is given to the following detailed description of the invention. This description makes reference to the annexed drawings, wherein:

Figures la, lb, and lc show a top view, a perspective view, and a sectional view of a flexible blank 10 with no forces applied to the flexible blank, figures 2a, 2b, and 2c show a top view, a perspective view, and a sectional view of a flexible blank 10 with a plurality of first forces applied to all pressure points 107 of a lens-shaper 17 of the flexible blank 10,

figures 3a, 3b, and 3c show a top view, a perspective view, and a sectional view of a flexible blank 10 with a plurality of second forces applied to all pressure points 107 of the lens-shaper 17 of the flexible blank 10, figures 4a, 4b, and 4c show a top view, a perspective view, and a sectional view of a flexible blank 10 with a plurality of second forces applied to all pressure points 107 of the lens-shaper 17 of the flexible blank 10 as well as a laser 123, a tunable beam expander 125, and a tunable aperture 124 for curing the curable material 11,

figures 5a, 5b, 5c, and 5d show a top view, a perspective view, and two sectional views (along EE in figure 5a for figure 5c and along FF in figure 5a for figure 5d) of a flexible blank 10 with two second forces F3 and with two third forces F4 which are each applied to pressure points 107 of the lens-shaper 17 of the flexible blank 10,

figure 6a shows an exploded perspective view of a flexible blank 10, wherein the flexible blank 10 comprises a lens-shaper 17 with pressure points 107, a membrane 14, a sacrificial layer 18, a curable material 11, and a container 15,

figure 6b shows a perspective view of the curable material 11 of the flexible blank 10 of figure 6a ,

figure 6c shows a perspective view of a cured portion 111 of the curable material 11 of the flexible blank of figures 6a and 6b, wherein the cured portion 111 forms a custom optical element 1,

figure 7 shows a perspective view of an apparatus 100 comprising an actuator 102, an analyzer 103, a movable holder 101, and a curing device 104, and

figure 8 shows an optical system 900 with a custom optical element 1.

Modes for Carrying Out the Invention

Description of the Fig Figure la shows a top view, figure lb shows a perspective view, and figure lc shows a sectional view along AA of figure la of a flexible blank 10 with no external forces (Fl = 0 N) applied to the flexible blank. The flexible blank 10 is adapted to be used to manufacture a custom optical element 1 as discussed above, in particular in an apparatus 100 as discussed above that carries out the steps of a manufacturing method as discussed above. In the shown embodiment of the flexible blank 10, the flexible blank 10 comprises a ring-shaped lens shaper 17 with twelve pressure points 107 (only two of them are referenced for clarity) . These pressure points 107 form sections S of the flexible blank 10 to which (a) force (s) F is/ are applied. The flexible blank 10 furthermore comprises a bowl-shaped container 15 in which an optically transparent curable material 11, specifically a curable fluid 11 is arranged. The top part (along +z, z being the optical axis of the flexible blank

10 and the optical element 1, respectively) of the container 15 is covered by a flexible membrane 14 which is suspended on the container 15. Thus, the curable material

11 is sealed in a volume that is formed by the container 15 on the bottom (along -z) and on the sides (along the lateral directions x and y, which stand perpendicular on the optical axis z) and by the flexible membrane 14 on the top. The lens shaper 17 is arranged on top of said membrane 14. Furthermore, the flexible blank 10 comprises a Fresnel lens 16 at the bottom part (along -z) of the container 15. This Fresnel lens 16 produces a locally varying (i.e., laterally along x and/ or y) refractive power profile RPP on the container 15. In other words, the bottom part of the container 15 acts as an optical lens. This could also be achieved by an optically transparent curved optical surface 151 (dashed) on the bottom part of the container 15. A dissolvable sacrificial layer 18 is arranged between the curable material 11 and the membrane 14. which enables (by dissolving the sacrificial layer 18) the detachment of the membrane 14 from the curable material 11, e.g., after the curable material 11 has been at least partially cured.

Figure 2a shows a top view, figure 2b shows a perspective view, and figure 2c shows a sectional view of a flexible blank 10 with a plurality of first forces F2 > Fl = 0 applied to the flexible blank 10, specifically to all the pressure points 107 of the lens-shaper 17. Here, only one force vector is shown for clarity. The application of the forces F2 to the pressure points 107 pushes down (along -z) the flexible lens-shaper 17. Thus, the pressure points 107 of the lens shaper 17 are used to receive the forces F2 from, e.g., an actuator of an apparatus 100 as discussed above and to transfer these forces to the curable material 11. As it can be seen in the sectional view (along BB of figure 2a) in figure 2c, the application of the forces F2 to the flexible blank 10 causes a change in a curvature C of the flexible blank 10, specifically here of the top surface of the curable material 11 which is covered by the flexible membrane 14. This change in the curvature C leads to a change in a refractive power profile RPP of the flexible blank 10, specifically here of the curable material 11. It should be pointed out that the refractive power profile RPP of the container 15 is in this embodiment not (or at least not considerably) changed by the application of the forces F2. All the other references are the same as in figures la-c.

Figure 3a shows a top view, figure 3b shows a perspective view, and figure 3c shows a sectional view of a flexible blank 10 with a plurality of second forces F3 > 0, F3 < F2 applied to the flexible blank 10. The second forces F3 are smaller than the first forces F2 from figures 2a-c and they are as well applied to all the pressure points 107 of the lens-shaper 17. Again, only one force vector F3 is shown for clarity. As it can be seen in the sectional view (along CC of figure 3a) in figure 3c, the change in applied forces from F2 to F3 causes the curvature C of the flexible blank 10 to change as well, specifically the curvature C decreases due to the decreasing forces. Here, this is due to the elastic properties of the flexible blank 10, but also pulling forces (i.e., forces pulling on the pressure points 107 of the lens-shaper 17) are imaginable. Changing the forces from F2 to F3 causes the curvature C and therefore the refractive power profile RPP of the flexible blank 10, specifically of the curable material 11, to change. All the other references are the same as in figures la-c and figures 2a-c .

Figure 4a shows a top view, figure 4b shows a perspective view, and figure 4c shows a sectional view of a flexible blank 10 with a plurality of second forces F3 (like in figures 3a-c) applied to all pressure points 107 of the lens-shaper Π of the flexible blank 10. In figure 4b all force vectors F3 are shown for completeness. So far, the flexible blank 10 of figures 4a-4c therefore corresponds to that of figures 3a-c. However, as soon as the curvature C and therefore the refractive power profile RPP of the flexible blank 10 approaches an ultimate refractive power profile URPP for the fully manufactured custom optical element 1 (or - in other words - as soon as a magnitude of the adjustment function A decreases below a threshold) at least in a region of the flexible blank 10, the curable material 11 is cured at least in this region. Here, the refractive power profile RPP matches the desired URPP in the central portion 12 of the flexible blank 10 which surrounds the optical axis z. Therefore, the curable material 11 in this central portion of the flexible blank 10 which is marked by a rectangular area in figure 4c is cured by UV-irradiation from a laser 123. The laser beam is adjusted by a tunable aperture 124 and a tunable beam expander 125 before hitting the curable material 11. The curable material 11 in the outer portion 13 of the flexible blank 10 is cured later. Alternative or additional curing devices include a digital light projector 126, a beam scanner 127, an LED 130, or an LED array 128, each including suitable imaging optics 129 (see figure 7) . Thus, customized curing patterns for the curable material 11 become possible. All the other references are the same as in figures la-c, 2a- c, and 3a-c.

Figure 5a shows a top view, figure 5b shows a perspective view, figure 5c shows a first sectional view (along EE in figure 5a) , and figure 5d shows a second sectional view (along FF in figure 5a) of a flexible blank 10 with two second forces F3 and with two third forces F4 which are each applied to one pressure point 107 of the lens-shaper 17 of the flexible blank 10. The third force F4 is smaller than the second force F3, both forces are nonzero. All four force vectors are shown for completeness. The four involved sections S which are defined by the pressure points 107 of the lens shaper 17 receive the forces F3 and F4 and the flexible properties of the lens shaper 17 transfer it in a distributed manner to the flexible membrane 14 and the curable material 11. As it can be seen in the sectional view along EE of figure 5a in figure 5c, the flexible blank now has a curved surface with a first curvature CI which is primarily caused by applying the second forces F3 to the flexible blank 10. As it can be seen in the sectional view along FF of figure 5a in figure 5d, the flexible blank 10 has a differently curved surface with a second curvature C2 which is primarily caused by applying the third forces F4<F3 to the flexible blank 10. In other words, the shape and therefore the refractive power profile RPP of the flexible blank 10 is no longer rotationally symmetric about the optical axis z. Thus, a refractive power pro- file RPP of the flexible blank 10 acts not only as a spherical lens as in the flexible blanks 10 of figures 2a-c, 3a-c, and 4a-c, but also as a cylindrical lens because the curvature CI along EE is stronger than the curvature C2 along FF. This can be used for astigmatism correction. All the other references are the same as in the previous figures.

Figure 6a shows an exploded perspective view of a flexible blank 10, with a lens-shaper 17 with twelve pressure points 107 (only one is referenced for clarity) , a membrane 14, a sacrificial layer 18, a curable material 11, and a container 15. The single components are arranged above one another and are shown in an explosion view for clarity. Other than that, the components act in the same way as described above with regard to the figures la-c, 2a-c, 3a-c, 4a-c, and 5a-d.

Figure 6b shows a perspective view of the curable material 11 of the flexible blank 10 of figure 6a. All other components have been detached from the curable material 11, e.g., by immersing the flexible blank 10 in a solvent after at least a part of the curable material 11 has been cured by UV-radiation . This immersion dissolves the sacrificial layer 18 and enables the disconnection of the lens-shaper 17 and the membrane 14 from the curable material 11. The curable material 11 is a biocompatible material in this embodiment.

Figure 6c shows a perspective view of a cured central portion 111 of the curable material 11 of the flexible blank of figure 6b, wherein the cured portion 111 forms a custom optical element 1. The outer portion of the curable material 11 of figure 6b has not been cured and is removed after detaching the cured curable material 11 from other components of the flexible blank 10. The custom optical element 1 in this embodiment is foldable and thus it is easier insertable into an optical system with to-be-corrected low- and/or high-order aber- rations. Such an optical system can, e.g., be a human eye and the custom optical element 1 can be used to correct defocus, astigmatism, and/or spherical aberrations. Thus, human vision can be improved.

Figure 7 shows a perspective view of an apparatus 100 for manufacturing a custom optical element 1 by means of a method as discussed above. The apparatus 100 comprises an actuator 102 consisting of twelve piezoelectric elements 102a-l with connected force transmission poles 122a-l. Only three piezoelectric elements 102a, 102d, 102h and force transmission poles 122a, 122d, 122h are referenced for clarity. The piezoelectric elements 102a-l and force transmission poles 122a-l are used to create and/or change the force (s) F that are applied to at least one of said sections S of the flexible blank 10.

Furthermore, the apparatus 100 comprises an analyzer 103 for deriving a profile dataset PD which is indicative of a refractive power profile RPP of the flexible blank 10.

Furthermore, the apparatus comprises a curing device 104 for at least locally and at least partially curing said curable material 11 of said flexible blank 10. In this embodiment of the apparatus 100, the curing device comprises a UV-LED 130 which is adjusted by a tunable aperture.124 and a tunable beam expander 125. Alternatives include a laser 123, a digital light projector 126, a beam scanner 127, or a LED array 128, each including suitable imaging optics 129 (dashed). Thus, customized curing patterns for the curable material 11 become possible .

Furthermore, the apparatus 100 comprises a movable holder 101 which positions the flexible blank 10 with respect to the other components of the apparatus 100, i.e., the actuator 102, the analyzer 103, and the curing device 104. Six axes of freedom (three rotational, three translational) are possible. Thus, customized curing patterns for the curable material 11 become possible.

Furthermore, the apparatus 100 comprises a flexible ring-shaped lens-shaper 105 which is adapted to transfer at least one of the forces F from the actuator 102 to the flexible blank 10.

Furthermore, the apparatus 100 comprises a control unit 106 and a user interface Ul/computer interface UI which is adapted to carry out the steps of a method to manufacture a custom optical element 1 from a flexible blank 10 as discussed above. The control unit 106 communicates with the user interface UI and it receives input from the analyzer 103 to gather the profile dataset PD, i.e., information about the current refractive power profile RPP of the flexible blank 10. Then, the control unit 106 compares the profile dataset PD with an ultimate refractive power profile URPP for the custom optical element 1. It then derives an adjustment function A which describes on how the current refractive power profile RPP of the flexible blank 10 needs to be changed to approach the ultimate refractive power profile URPP. Depends on this adjustment function A the operations of the actuator 102 (only one connection is shown for clarity) , the movable holder 101, and the curing device 104 are controlled.

Figure 8 shows an optical system 900 with an object plane Q, two optical lenses LI and L2, and an image plane I. The optical system 900 suffers from optical aberrations which are described by an aberration dataset AD. An ultimate refractive power profile URPP is derived from the aberration dataset AD such that an integration of a custom optical element 1 with URPP into the optical system 900 corrects for at least some of the optical aberration in at least a part of the field-of-view of the optical system 900. Then, such a custom optical element 1 is manufactured as discussed above and integrated into the optical system 900 and thus, the aberration are at least in part corrected.

Definitions :

The term "custom optical element" relates to an optical element such as a lens that is relatively easy to customize (i.e., it is relatively easy to change its refractive power profile RPP) during its manufacturing process. Specifically, it does not relate to an optical element that is manufactured by means of an injection mold, because the basic shape of such an optical element is determined by the shape of the injection molding tool.

Notes :

Materials as disclosed in the following hold for all embodiments as discussed above.

The material for the curable material 11 and/ or the lens shaper 17 can be transparent, serai- transparent, absorbing or reflecting and, e.g., comprise or consist of:

- Oils

- Solvents

- Ionic liquids

- Liquid metals

- Dispersions

- PMMA

- Siliconse

- Polymers

- Elastomers

- Epoxys

- Silicones

- Plastics

- Acrylics The material for the flexible membrane 14 can, e.g., comprise or consist of:

- Gels (Optical Gel OG-1001 by Liteway™)

- Elastomers (TPE, LCE, Silicones e.g. PDMS Sylgard 186, Acrylics, Urethanes)

- Thermoplast (ABS, PA, PC, PMMA, PET, PE, PP, PS, PVC,...)

The material for the sacrificial layer 18 can, e.g., comprise or consist of:

- sugar and its compounds such as dextrane

- sugar solutions

- all soluble polymers, e.g., (poly (acrylic acid) (PAA), dextran, Poly (methacrylic acid),

Poly ( acrylamide ) , Poly (ethylene oxide)

- all kinds of photo-resist

- Polyvinyl-alcohols , e.g., owiols

- Butyrals such as Mo ital

- in general all materials which can act as an intermediate layer for affixing the membrane to the curable material and that can be modified in such a manner that the adherence between the membrane and the curable material is at least substantially lost. Such modification (s) can, e.g., be carried out under chemical, mechanical, or physical influence or a combination thereof.

Advantageously, the sacrificial layer 18 and the flexible membrane 14 are permeable or semi-permeable for gases, in particular for the gas used as environmental gas during manufacturing, such that bubbles of gas enclosed inside a sealed volume can easily diffuse through the sacrificial layer 18 and the flexible membrane 14. Additionally, it can be advantageous when the sacrificial layer 18 and the flexible membrane 14 are pre-tensioned, i.e., pre-stretched. The material for the container 15 and/ or the lens shaper 17 can, e.g., comprise or consist of:

- PMMA

- Glass

- PS

- Plastic

- Polymer

- Crystalline material, in particular single crystal material

- Metals.

Optionally, the container 15 and/ or the flexible membrane 14 can comprise optical elements with suitable shapes, e.g.:

- Spherical lenses (convex and concave)

- Fresnel lenses

- Cylindrical lenses

- Aspherical lenses (convex and concave)

- Flat

- Mirrors

- Squares, triangles, lines or pyramids

Any micro- (e.g. micro lens array, diffraction grating, hologram) or nano- (e.g. antireflection coating) structure can be integrated into the container 15 and/or the flexible membrane 14. When an anti- reflective layer is applied to at least one surface of the membrane 14, it is advantageously formed by fine structures having a size smaller than the wavelength of the transmitted light. Typically, this size may be smaller than 5 μηι for infrared applications, smaller than 1 μπι for near-infrared applications, and smaller than 200 nm for applications using visible light.

Any of the following methods can e.g. be used to form an anti-reflection coating: - Casting, in particular injection molding / mold processing

- Nano-imprinting, e.g., by hot embossing nanometer-sized structures

- Etching (e.g. chemical or plasma)

- Sputtering

- Hot embossing

- Soft lithography (i.e. casting a polymer onto a pre-shaped substrate)

- Chemical self-assembly (see e.g. "Surface tension-powered self-assembly of microstructures - the state-of-the-art", R.R.A. Syms, E. . Yeatman, V.M.

Bright, G.M. Whitesides, Journal of Microelectromechani- cal Systems 12(4), 2003, pp. 387 - 417)

- Electro-magnetic field guided pattern forming (see e.g. "Electro-magnetic field guided pattern forming", L. Seemann, A. Stemmer, and N. Naujoks, Nano Lett., 7 (10), 3007 - 3012, 2007. 10.1021/nl0713373 ) .

Any of the following methods can, e.g., be applied for forming and structuring the container 15 and/ or the flexible membrane 14:

- Grinding

- Injection molding

- Milling

- Casting

- Coating.

The custom optical element 1 can be used in a large variety of applications, such as:

- Projection devices, e.g. for applications in the optical part of projectors for macro- and micro- projectors in beamers and hand-held devices

- Displays

- Microscopes

- Cameras - Surveillance cameras

- Vision systems, having any kind of camera

- In research applications

- Phoropters

- Eye

- Lens assemblies

- Lighting applications such as illumination for shops, retail, museums or home applications

- Telecommunication applications (e.g., amplitude modulation) .

. hile there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited to these embodiments but may be otherwise variously embodied and practiced within the scope of the following claims.