EP4200665A1

EP4200665A1 - A zonal diffractive ocular lens

Info

Publication number: EP4200665A1
Application number: EP20797892.5A
Authority: EP
Inventors: Sven Thage Sigvard HOLMSTRÖM
Original assignee: VSY Biyoteknoloji ve Ilac Sanayi AS
Current assignee: VSY Biyoteknoloji ve Ilac Sanayi AS
Priority date: 2020-08-21
Filing date: 2020-08-21
Publication date: 2023-06-28
Also published as: WO2022039683A1

Abstract

An ophthalmic multifocal lens, comprising at least three focal points, said lens having a light transmissive lens body comprising at least a first portion with a first refractive base power that coincides with the central area of said light transmissive lens body and a second portion with a second refractive base power is proposed. Said ophthalmic multifocal lens further comprises a multifocal symmetric grating combined with said second portion, wherein the combination with said second portion is configured such that one diffraction order of said multifocal symmetric grating adds to the base refractive power of said central area coinciding with the first portion for a design wavelength and said diffraction grating has a diffraction efficiency higher than that of the corresponding diffraction grating with equal light distribution.

Description

A ZONAL DIFFRACTIVE OCULAR LENS

Technical Field of the Present Invention

The present disclosure generally relates to ophthalmic lenses and, more specifically, to ophthalmic eyeglasses, ophthalmic contact and intra-ocular multifocal lenses, the multifocality being provided by a combination of diffraction zone(s) and refraction zone(s).

Background of the Present Invention

Diffractive lenses for ophthalmological applications are constructed as hybrid lenses with a diffractive pattern added onto a refractive body. Often one side of the lens is purely refractive, while the other side has a diffractive grating superpositioned over a refractive base line. The refractive baseline can be spherical, or alternatively have an aspherical shape of sorts. The diffractive pattern is added onto the refractive baseline. The diffractive part can in general be applied to any of the two sides of the lens, since when a diffractive pattern is to be combined with a refractive surface with some special feature it generally does not matter if they are added to the same side or if one is added to a first side and the other to a second side of the lens. Concurrently, two diffractive patterns may be combined either by superpositioning on one side, or by adding them on separate sides in an overlapping fashion. The optical power of the lens for a specific diffraction order can be calculated by addition of the refractive base power and the optical power of that diffraction order.

Note that a diffraction grating that functions as a lens has a pitch that in absolute terms varies with the radius. The pitch depends on the refractive index, the design wavelength, and the optical power of the first diffraction order. The pitch is determined so that the optical path difference (OPD) through the lens to the focal point of the first diffraction order has a difference of exactly one wavelength per period. To show the periodicity of a diffraction grating one will often plot the diffractive lens profile versus the square of the radius. When plotted like this the periods (grating pitch) is equidistant, more exactly the period pitch in r²-space is 2A/D, where A is the design wavelength and D the optical diffractive power of the first order in diopters.

The term diffractive lens is sometimes used for the well-known Fresnel lenses. A Fresnel lens consists of concentric zones with vertical steps in the zone junctures. The zones in a Fresnel lens are often of equal width and the optical properties of each zone can be analyzed with refractive theory. However, the diffractive lenses discussed here are lenses which require diffractive analysis.

The most well-researched type of diffraction lens proper is the phase-matched Fresnel lens as taught by Rossi et al. in their 1995 study titled "Refractive and diffractive properties of planar micro-optical elements". This type of lens makes use of a sawtooth diffractive unit cell and a step height corresponding to a phase modulation of exactly 2n. In phase-matched Fresnel element design, planar microlenses that display refractive as well as diffractive behavior in a manner adjustable by phase-matching number Mj for each diffractive zone. An increased Mj increases the height as well as the width of respective zone. All energy in such a lens goes into one order, for the case where each Mj equals 1 all energy goes into the first order, meaning that these are monofocal diffractive lenses. Indeed, the only diffractive shape available with 100% efficiency is a sawtooth profile constructed to have full modulation of a design wavelength, that is that the vertical jump in zone junctures corresponds to an phase retardation that is an integer multiple of the number of wavelengths. It is often desired to provide more than one focal point. For ophthalmic lenses it can often be advantageous to provide e.g. far vision and near vision simultaneously. The most light-efficient lens possible for providing two focal points uses a sawtooth profile similar to the phase matched Fresnel described above, but with a decreased height. The highest possible diffraction efficiency for such a lens is close to 81%. For diffractive lenses optimized for more than two focal points sawtooth patterns are not the most efficient and, as will be discussed below, higher diffraction efficiencies are possible. In recent years it has become more common with lenses providing three distinct focal point, often far, intermediate, and near vision.

The vast majority of ophthalmic diffractive trifocal lenses make use of sawtooth profiles. Combining sawtooth profiles of two bifocal diffractive lenses to achieve trifocality is known in the art. This results in diffractive lenses with the usable orders arranged asymmetrically with respect to the 0^th order, e.g. a trifocal lens might make use of orders 0, +1, and +2 orders or 0, +2, and +3. In US 9320594, a diffractive trifocal lens is disclosed wherein the optical thickness of the surface profile changes monotonically with radius within each zone, while a distinct step in optical thickness at the junction between adjacent zones defines a step height. The step heights for respective zones may differ from one zone to another periodically so as to tailor diffraction order efficiencies of the optical element wherein the step heights may alternate between two values. In EP 2377493, a method of manufacturing an aphakic intraocular lens that is capable of ensuring every multi-focusing effect more securely, while reducing the impact of aperture changes and lens eccentricity is suggested. EP 2503962 discloses an intraocular lens including an anterior surface and a posterior surface and having a substantially antero-posterior optical axis wherein one of these anterior and posterior surfaces includes a first diffractive profile forming at least one first diffractive focal point of order +1 on said optical axis, and a second diffractive profile forming a second diffractive focal point of order +1, said two diffractive focal points are distinct and at least one portion of said second diffractive profile is superposed to at least one portion of the first diffractive profile. US 9223148 proposes a lens with more than two powers, one of which is refractive and one other diffractive in the least. US5017000 suggests a multiple focal point profiled phase plate having a plurality of annular concentric zones spaced according to the formula r(k) = sqrtfconstant x k) where r(k) is the zone radii and k is a zone; in which a repetitive step is incorporated in the profile and has an optical path length greater or less than one-half wavelength.

One of the prior art publications in the technical field of the present invention may be referred to as EP 3435143, teaches an ophthalmic multifocal diffractive lens comprising focal points for near, intermediate and far vision. The lens comprises a light transmissive lens body providing a refractive focal point, and a periodic light transmissive diffraction grating, extending concentrically over at least part of a surface of the lens body and providing a set of diffractive focal points. The diffraction grating is designed to operate as an optical wave splitter, the refractive focal point providing the focal point for intermediate vision and the diffractive focal points providing the focal points for near and far vision. The diffraction grating has a phase profile arranged for varying a phase of incident light at the lens body optimizing overall efficiency of light distribution in the refractive and diffractive focal points. The orders of this lens are arranged symmetrically around the 0^th order and operates in at least the -1, 0, and +1 orders.

Diffractive lenses with sharp transitions in the diffraction profile, including e.g. lenses with sawtooth profiles and binary profiles, give rise to machining difficulties and, for a finished lens, scattering of light, increased incidence of several unwanted optical phenomena such as stray light and glare i.e. the difficulty of seeing in the presence of bright light such as direct or reflected sunlight or artificial light such as car headlamps at night, and halo effects i.e. white or coloured light rings or spots seen at dim light, i.e. under mesopic conditions. Diffractive lenses without sharp transitions are better performing with respect to these issues and also have higher potential diffraction efficiency, at the very least for multifocal lenses with an odd number of focal points. It has also been suggested that sinusoidal or smooth diffractive profiles are more biocompatible compared to sawtooth profiles because of reduction in the debris precipitation effect, as explained in Osipov et al. in their 2015 study "Application of nanoimprinting technique for fabrication of trifocal diffractive lens with sine-like radial profile." as published in Journal of biomedical optics 20, no. 2 (2015): 025008.

According to the teaching of W02019020435, it is known that the light distribution in the focal points of an ophthalmic lens comprising a diffraction grating having a continuous periodic phase profile function and usable orders symmetrically arranged around the 0^th order is tunable over a relatively large intensity range, by modulating one or both of the argument and amplitude of the phase profile function as a function of the radius or radial distance to the optical axis of the lens body. A trifocal lens known in the art is was put forth in the past few years, with respect to teachings of EP 20170183354 and aforementioned WO 2019020435 which comprise a trifocal lens that operates in the -1, 0, and +1 orders. A general approach to construct a lens is also known from the teaching of US5017000. The resulting diffractive lens is a diffractive lens operating in the 0, +1, and +2 orders.

According to the teaching of WO 2019020435 a trifocal lens can be constructed by starting from a linear phase grating optimized for diffraction efficiency and an equal light distribution between usable diffraction orders. Linear phase gratings have been researched and developed with the intent of creating beam splitters. The general theory of optimization of linear phase gratings is taught in Romero and Dickey's 2007 study titled "Theory of optimal beam splitting by phase gratings. I. One-dimensional gratings" in Journal of the Optical Society of America Vol. 24, No. 8 (2007) p. 2280-2295. The existing literature on diffractive phase grating has focussed on finding optimal solution, meaning maximized diffraction efficiency, for the case of equal intensity distribution among a certain number of orders.

Because of reasons given above it is often advantageous to use multifocal, hybrid lenses utilizing smooth diffractive gratings utilizing both positive and negative diffraction orders. However, such lenses that exist in the prior art have several limitations.

After implantation of an IOL in the human eye, the new focussing properties of the eye as a whole have to be measured. That is, the complete visual system consisting of the new lens and the remainder of the eye of the user is measured integrally, as a first objective indication of the result of the implantation of the IOL. In practice, most physicians rely on a simple measurement by an autorefractometer. For sawtooth type IOLS, for example, the measurement indeed typically returns the far focus. For lenses with sinusoidal diffractive patterns over the full extent of the lens optics some combinations of autorefractometers and lenses can provide the power of the refractive base, which is often not at same optical power as far vision. Other combinations of autorefractometers and sinusoidal diffractive lenses provide a power in between the power of the refractive base and that of one of the diffractive powers. Although protocols exist for measuring all the foci of the IOL correctly, completely applying such protocols is often perceived as too time consuming. Generally in diffractive multifocal lenses with asymmetric gratings the height of the grating can be reduced to increase the intensity of the refractive focal point. With sawtooth diffractive gratings this can be used to increase one of the outermost diffraction orders relative to all other orders, which can sometimes be advantageous.

An often desired feature in multifocal lenses is to provide a relatively even intensity distribution for mesopic conditions while providing a much stronger relative intensity for far vision for the larger pupils available in scotopic conditions. With sawtooth multifocal diffractive lenses this is often provided with the help of apodization, which in this context refers to a diffractive grating with decreasing height with increasing radius, as taught in Davison, J. A., & Simpson, M. J. (2006). History and development of the apodized diffractive intraocular lens. Journal of Cataract & Refractive Surgery, 32(5), 849-858. For a lens using a smooth sinusoidal grating this simple method can't be used for this purpose.

Aforementioned W02019020435 discloses lenses derived from a linear phase grating optimized for equal intensity between three orders. The intensity distribution is then fine-tuned by laterally shifting the diffraction grating along the radial direction. However, the current state of the art does not allow for the use of lenses using smooth sinusoidal gratings with higher diffraction efficiency than this. One underlying reason for this is because gratings with very high diffraction efficiencies require undesired intensity distributions when used in well-known configurations.

After implanting a multifocal intraocular lens, there is always an adaptation time before the advantages of the lens are fully appreciated by a user. This is due to adaptation processes in the eye and also in the brain of a user. Clinical observations show that after implantation users adopt first to the far focus, and, for a multifocal lens, eventually in a few days or weeks to the two additional focal points, i.e. near and intermediate focus. With a lens center fully optimized for multifocality for all pupil sizes, however, the adaptation time also for the far focus will be increased. This can be unpleasant and uncomfortable for some users.

Accordingly, there is a need for an improved ophthalmic lens that makes use of smooth sinusoidal diffractive gratings, but solves the limitations of such lenses in current state-of-the-art and make possible lens designs that provide larger freedom in tuning and controlling the relative intensities of three or more diffraction orders or focal points, in particular to provide markedly different intensity ratios for different aperture sizes, providing a possibility to measure a diffractive focal point easily, improving the adaptation time of a user or patient, and improving the ability to take advantage of diffraction gratings with very high diffraction efficiency.

Objects of the Present Invention

Primary object of the present invention is to provide an ophthalmic multifocal lens, at least comprising three focal points, one of them providing far vision.

Another object of the present invention is to provide an ophthalmic multifocal lens comprising at least a first and a second portion, providing a first and a second refractive optical power respectively, the portions being arranged concentrically. A further object of the present invention is to provide an ophthalmic multifocal lens comprising a symmetric diffractive grating providing at least three focal points combined with the first portion, the 0^th order of said diffractive grating adds, for a design wavelength, to the optical power of the first portion.

A still further object of the present invention is to provide an ophthalmic multifocal lens comprising a symmetric diffractive grating such that one of the diffraction orders other than the 0^th order of the diffractive grating adds to the refractive optical power of the second portion.

A still further object of the present invention is to provide an ophthalmic multifocal lens wherein the refractive base powers of the first and second portions are different, and at least one of the two have a sawtooth diffractive grating with arbitrary height.

A still further object of the present invention is to provide an ophthalmic multifocal lens with optimized multifocality wherein the diffractive efficiency is greatly improved.

Brief Description of the Present Invention

In a first aspect, there is provided an ophthalmic multifocal lens, at least comprising a focal point for far vision. The lens having a light transmissive lens body comprising a symmetric (i.e. optical powers are symmetrically aligned around 0^th order) diffraction grating extending concentrically in radial direction from an optical axis of the lens body across a part of a surface of the lens body. The lens comprises at least a first and a second portion, each portion having a different refractive base line and each portion providing a refractive optical power that is different than that of any other portion, the portions being arranged concentrically, and a symmetric diffractive grating on the first portion, arranged so that, for a design wavelength, at least two of the focal points of the diffraction grating coincide with and are contributed to by the refractive power of the two portions.

Assume a lens designed for providing a target focal point for near vision at diffraction order +1, a target focal point for far vision at diffraction order -1, and a target intermediate refractive focal point, also indicated as 0 (zero) order. When the focal point of the monofocal peripheral portion coincides with the target focal point for far vision, for example, this provides to ophthalmologists a portion of the lens that can be measured unambiguously.

The present invention allows for a very different intensity distributions for different pupil sizes without the need of any diffraction patterns containing sharp edges, such as those in sawtooth or binary diffraction profiles. On the contrary, lenses according to the invention can provide three or more foci for photopic and mesopic conditions with a higher diffraction efficiency than what is possible using asymmetric diffraction gratings combined with e.g. a very strong far vision provided in scotopic conditions.

A typical autorefractometer will measure at the perimeter of the pupil of the patient. However, an ophthalmologist often will measure in light conditions that render the pupil to be approximately 3 mm in diameter. If, for example, the multifocal central portion of a lens according to the present invention is 2.3 mm in diameter a 3 mm pupil would with the type of autorefractometer mentioned return the refractive power provided by the peripheral portion

The lens body, in accordance with the present disclosure, comprises a first portion that comprises a monofocal central zone providing far vision, extending over a distance in radial direction from the optical axis of the lens body across part of the surface of the lens body and a second portion with a symmetric diffraction grating with high diffraction efficiency.

The present disclosure is based on the insights that by providing a monofocal center of the ophthalmic lens and a multifocal periphery with high diffraction efficiency, having a focal point coinciding with one of the diffractive focal points provided by the diffraction grating, improves the ease of accurate in-vivo measurements of the far vision of the lens, shortens adaptation times for users to the far focus, and allows for useful and practical designs with very diffraction efficiencies of the underlying linear diffraction grating, as the strong far vision of the central portion can balance the weaker diffractive far vision of a highly efficient grating.

Assume a lens designed for providing a target focal point for near vision at diffraction order +1, a target focal point for far vision at diffraction order -1, and a target intermediate refractive focal point, also indicated as 0 (zero) order. When the focal point of the monofocal central portion coincides with the target focal point for far vision, for example, adaptation of the visual system of a patient, i.e. the combination of lens and eye for the targeted far vision can be measured for pupil sizes in the range of the size of the monofocal central zone.

A typical autorefractometer will measure at the perimeter of the pupil of the patient. However, an ophthalmologist often will measure in light conditions that render the pupil to be approximately 3 mm or less in diameter. A typical size of the pupil when measuring a lens manufactured according to the present disclosure has a diameter of about 1-2 mm. Hence, with such dimensions, a person performing the measurement knows that the measurement returns a result based on the focal point of the monofocal central zone. Further, with the present disclosure, a strong far or near focus provided by the monofocal central zone is available for a large range of pupil sizes. In outside ambient and/or day-time conditions, the far or near focus will dominate. This leads to a faster adaptation time for the focus provided by the monofocal central zone and a more comfortable experience until all foci are accepted by the visual system of the user, compared to multifocal lenses known in the art.

As mentioned above, from the absence of concentric rings or zones having sharp edges, lenses having a continuous and smooth profile without any sharp edges are less susceptible to glare or scattering due to non-uniformities in the path that incident light travels through the lens, and also to produce less halos, while being easier to manufacture according to a calculated profile compared to sawtooth type or binary type gratings or reliefs, for example. A higher diffraction efficiency in any case leads to less stray light. For manufacturing technologies based on diamond turning or similar forms of machining a smooth profile will be more reliable as well as faster and cheaper to fabricate than profiles with sharp edges such as sawtooth or binary profiles.

An important step in the manufacturing of ophthalmic lenses by micromachining or diamond turning, for example, is mechanical polishing to get rid of cutting traces. It is necessary to get rid of all visible cutting traces to comply with quality requirements and medical regulations for intraocular lenses. Obtaining extremely low levels of cutting traces, however, requires expensive machinery as well as slow cutting. If lenses are polished post-cutting, the machine may be allowed to work faster. Sharp angles, corners or edges in the height profile of diffractive lenses complicate the process of mechanical polishing. If mechanical polishing is not possible in view of the height profile of the lens, one needs to either utilize chemical polishing, which requires hazardous chemicals, or manufacture lenses without the requirement of polishing. The latter leads to much increased manufacturing costs because of one or both of lower yields and more expensive machinery.

Smooth diffractive geometries in accordance with the present disclosure allow for polishing and therefore lead to a significant increase in yield, compared to lenses having sharp transitions in their height profile.

Present invention aims to teach combinations of certain types of gratings and zones with differing refractive base power, that are enabling technical outcomes related to the improvement of the shortcomings noted in the prior art. Considering the ambiguities of measurements conducted on the lenses, teaching of the present invention puts forth lenses having multifocal portions with orders symmetrically arranged around the 0^th order with monofocal portions, arranged concentrically with respect to one another. Teaching of the present invention also provides multifocal lenses marked with a wider set of intensity distribution profiles, most notably those with a wider difference in intensity distribution between different apertures. Addressing this, lenses with strong multifocality for small apertures (e.g. photopic and mesopic condition), but with a dominant far vision aspect for larger pupils, i.e. scotopic vision are proposed. Another proposition by the present invention are types of lenses with monofocal central portions and peripheral portions comprising highly efficient diffraction grating.

The vast majority of ophthalmic diffractive trifocal lenses make use of sawtooth profiles. Combining sawtooth profiles of two bifocal diffractive lenses to achieve trifocality is known in the art. This results in diffractive lenses with the usable orders arranged asymmetrically with respect to the 0^th order, e.g. a trifocal lens might make use of orders 0, +1, and +2 orders or 0, +2, and +3. Such diffraction gratings are henceforth referenced as asymmetric gratings.

The highest possible diffraction efficiency for most useful intensity distribution for diffractive trifocal lenses is provided by smooth sinusoidal surfaces with usable orders symmetrically arranged around the 0^th order.

The orders of such a lens are arranged symmetrically around the 0^th order and operates in at least the -1, 0, and +1 orders. Gratings with that arrangement of usable orders will henceforth be referenced as symmetric gratings.

When comparing diffractive surfaces, an important factor is the diffractive efficiency. Diffraction efficiency is a measure of how much of the optical power is directed into the desired diffraction orders, or, when talking about diffractive lenses in particular, how much of the optical power is directed into the desired focal points. For bifocal lenses, where the surface of the lens body is optimized to provide an as good vision as possible at two distinct distances, the highest possible diffraction efficiency is reached by using the principles of a phase- matched Fresnel lens, which makes use of a sawtooth or jagged type diffraction pattern. Reference is made to the publication "Refractive and diffractive properties of planar micro-optical elements", by M. Rossi et al., in Applied Optics Vol. 34, No. 26 (1995) p. 5996-6007, which publication is herein incorporated by reference.

Because of the sharp edges of a sawtooth or jagged type diffraction pattern, as a consequence of the discontinuities in the diffraction profile, Fresnel lenses have all the drawbacks described above, in particular with respect to glare and halos, while it is also difficult to fabricate such structures with high precision. However, for a trifocal lens, lenses designed to provide an as good vision as possible to three distinct focal points, the optimal grating is one without any sharp edges.

It is often advantageous to first consider linear phase grating, since that field has a well-developed theory and can be utilized for diffractive lenses. For the special case of a trifocal linear grating with an equal intensity distribution to each order, it is shown specifically that the optimal solution is a structure without sharp edges in the publication "Analytical derivation of the optimum triplicator", by F. Gori et al., in Optics Communication 157 (1998), p. 13-16, which publication is herein incorporated by reference.

The publication "Theory of optimal beam splitting by phase gratings. I. Onedimensional gratings", by L. A. Romero and F. M. Dickey, in Journal of the Optical Society of America Vol. 24, No. 8 (2007) p. 2280-2295, which publication is herein incorporated by reference, discloses this more generally, proving that at the very least that optimal gratings for equal splitting into odd number of orders have continuous profiles. This latter paper provides the mathematical tools to find the optimal linear phase grating for any given set of target orders and any given intensity distribution among those target orders. The optimal grating is defined as the linear diffraction grating with the highest diffraction efficiency for the specified intensity distribution. It is noted that the publications by Gori et al. and Romero et al. discuss linear phase gratings only with the intent of creating beam splitters.

According to the present invention, a lens such as an intraocular lens, more remarkably a multi-zonal multifocal intraocular lens that is capable of correcting optical aberrations for a variety of human eyes with different corneas under a wide range of lighting conditions and that may perform in different orientations is proposed.

The lens according to the present invention is an intraocular lens comprising at least a first and a second portion, wherein each portion having a different refractive baseline and each said portion provides an optical power that is unidentical to that of any other portion. In an embodiment, said at least a first and a second portion are arranged concentrically; the first portion comprising a symmetric diffractive grating, one of the diffraction orders (other than the 0^th order) thereof substantially coinciding with the optical power of the second portion.

Figure 1 shows, in a simplified manner, the anatomy of the human eye 10, for the purpose of illustrating the present disclosure. The front part of the eye 10 is formed by the cornea 11, a spherical clear tissue that covers the pupil 12. The pupil 12 is the adaptable light receiving part of the eye 10 that controls the amount of light received in the eye 10. Light rays passing the pupil 12 are received at the natural crystalline lens 13, a small clear and flexible disk inside the eye 10, that focuses light rays onto the retina 14 at the rear part of the eye 10. The retina 14 serves the image forming by the eye 10. The posterior cavity 15, i.e. the space between the retina 14 and the lens 13, is filled with vitreous humour, a clear, jelly-like substance. The anterior and posterior chambers 16, i.e. the space between the lens 13 and the cornea 11, is filled with aqueous humour, a clear, watery liquid. Reference numeral 20 indicates the optical axis of the eye 10.

For a sharp and clear far field view by the eye 10, the lens 13 should be relatively flat, while for a sharp and clear near field view the lens 13 should be relatively curved. The curvature of the lens 13 is controlled by the ciliary muscles (not shown) that are in turn controlled from the human brain. A healthy eye 10 is able to accommodate, i.e. to control the lens 13, in a manner for providing a clear and sharp view of images at any distance in front of the cornea 11, between far field and near field.

Ophthalmic or artificial lenses are applied to correct vision by the eye 10 in combination with the lens 13, in which cases the ophthalmic lens is positioned in front of the cornea 11, or to replace the lens 13. In the latter case also indicated as aphakic ophthalmic lenses.

Multifocal ophthalmic lenses are used to enhance or correct vision by the eye 10 for various distances. In the case of trifocal ophthalmic lenses, for example, the ophthalmic lens is arranged for sharp and clear vision at three more or less discrete distances or focal points, often including far, intermediate, and near vision, in Figure 1 indicated by reference numerals 17, 18 and 19, respectively. Far vision is in optical terms when the incoming light rays are parallel or close to parallel. Light rays emanating from objects arranged at or near these distances or focal points 17, 18 and 19 are correctly focused at the retina 14, i.e. such that clear and sharp images of these objects are projected. The focal points 17, 18 and 19, in practice, may correspond to focal distances ranging from a few meters, to tens of centimeters, to centimeters, respectively. Usually ophthalmologists choose lenses for the patients so that the far focus allows the patient to focus on parallel light, in the common optical terminology it is that the far is focused on infinity. Ophthalmologists will, when testing patients, commonly measure near vision as 40 cm distance from the eyes and intermediate vision at a distance of 66 cm, but other values can be used.

The amount of correction that an ophthalmic lens provides is called the optical power, OP, and is expressed in Diopter, D. The optical power OP is calculated as the inverse of a focal distance f measured in meters. That is, OP = 1/f, wherein f is a respective focal distance from the lens to a respective focal point for far 17, intermediate 18 or near vision 19. The optical power of a cascade of lenses is found by adding the optical powers of the constituting lenses, for example. The optical power of a healthy human lens 13 is about 20 D.

Figure 2a shows a top view of a typical ophthalmic multifocal aphakic intraocular lens 30, and Figure 2b shows a side view of the lens 30. The lens 30 comprises a light transmissive circular disk-shaped lens body 31 and a pair of haptics 32, that extend outwardly from the lens body 31, for supporting the lens 30 in the human eye. Note that this is one example of a haptic, there are many known haptic designs. The lens body 31 has a biconvex shape, comprising a center part 33, a front or anterior surface 34 and a rear or posterior surface 35. The lens body 31 further comprises an optical axis 29 extending transverse to front and rear surfaces 34, 35 and through the center of the center part 33. Those skilled in the art will appreciate that the optical axis 29 is a virtual axis, for the purpose of referring the optical properties of the lens 30. The convex lens body 31, in a practical embodiment, provides a refractive optical power of about 20 D.

In the embodiment shown, at the front surface 34 of the lens body 31 a periodic light transmissive diffraction grating or relief 36 is arranged, comprised of rings or zones extending concentrically with respect to the optical axis 29 through the center part 33 over at least part of the front surface 34 of the lens body 31. The diffraction grating or relief 36 provides a set of diffractive focal points. Although not shown, the diffraction grating or relief 36 may also be arranged at the rear surface 35 of the lens body 31, or at both surfaces 34, 35. In practice, the diffraction grating 36 is not limited to concentric circular or annular ring-shaped zones, but includes concentric elliptic or oval shaped zones, for example, or more in general any type of concentric rotational zone shapes. In practice the optic diameter 37 of the lens body 31 is about 5 - 7 mm, while the total outer diameter 38 of the lens 30 including the haptics 31 is about 12- 14 mm. The lens 30 may have a center thickness 39 of about 1 mm. In the case of ophthalmic multifocal contact lenses and spectacle or eye glass lenses, the haptics 32 at the lens body 31 are not provided, while the lens body 31 may have a plano-convex, a biconcave or plano-concave shape, or combinations of convex and concave shapes. The lens body may comprise any of Hydrophobic Acrylic, Hydrophilic Acrylic, Silicone materials, or any other suitable light transmissive material for use in the human eye in case of an aphakic ophthalmic lens.

Figure 3 schematically illustrates, the optical operation of a known periodic light transmissive diffraction grating or relief 42 of a lens 40 comprising a biconvex light transmissive circular disk-shaped lens body 41. This type of lens, combining refractive and diffractive power is also referred to as a hybrid lens. The lens 40 is shown in a cross-sectional view in radial direction of the lens body. The diffraction grating or relief 42 comprises a plurality of repetitive, contiguously arranged, prism shaped transparent diffractive optical elements, DOEs, 43. The DOEs 43 extend in concentric zones around the center part 45 of the lens body 41, in a manner similar to the rings or zones of the grating or relief 36 shown in Figure 2a. For illustrative purposes, the DOEs 43 of the diffraction grating 42 are shown as well-known jagged or saw-tooth type elements, comprising a continuous, sloping light receiving surface 44, such as a linear or curved sloping light receiving surface 44. Gratings or reliefs in which the DOEs 43 alternate between two heights, spaced apart in radial direction of the lens body 41, are called binary type reliefs (not shown). The repetition period or pitch of the DOEs 43 monotonically decreases in radial direction from the center or optical axis of the lens and varies with the square of the radial distance.

Pitch depends on the refractive index, the design wavelength, and the optical power of the first diffraction order. Pitch is determined so that the optical path difference (OPD) through the lens to the focal point of the first order has a difference of exactly one wavelength per period. To visualise the periodicity of a diffraction grating, one would often plot the diffractive lens profile versus the square of the radius. When plotted as such, the periods (grating pitch) are equidistant, more exactly the period pitch in r² is |2λf|, where λ is the design wavelength and f the inverse of the optical power of the first diffractive order.

In the art, one side of the lens is purely refractive, while the other side has a diffractive grating super positioned over a refractive base line. The refractive baseline can be e.g. spherical or having some sort of aspherical shape. The diffractive pattern, which is added onto the refractive baseline, may in general be applied to any of the two sides of the lens. Therefore; if a diffractive pattern is to be combined with a refractive surface with some special feature, it generally bears little importance if they are added to the same side or, if one is added to a first side and the other to a second side of the lens. Concurrently, two diffractive patterns may be combined either by superpositioning on one side, or by adding them in an overlapping fashion on separate sides. In the disclosures pertaining to the present invention, combining of two lens structures shall always be understood as allowing for both possibilities. The optical power of the lens for a specific diffraction order are calculable by addition of the refractive base power and the optical power of that diffractive order.

An incident or primary light beam 46 that passes the grating 42 and the lens body 41 is, respectively, diffracted and refracted and results in an output or secondary light beam 47. Secondary light beams, namely the refracted and diffracted light waves 47 form a plurality of focal points at the optical axis 48 of the lens 40, due to constructive interference of the secondary light beams 47. Constructive interference occurs when the optical path difference between secondary light beams 47 arriving from the lens body 41, at a particular focal point, is an integer multiple of their wavelength, i.e. the light waves are in- phase, such that their amplitudes add-up in a reinforcing manner. When the difference in optical path length travelled by interfering light waves 47 from the lens body 41 is an odd multiple of half of the wavelength, such that a crest of one wave meets a trough of another wave, the light waves (secondary light beams 47) partly or completely extinguish each other, i.e. the light waves are out of phase, not resulting in focal points at the optical axis 48 of the lens body 41.

The points of constructive interference at various distances from the lens body 41 are generally designated diffraction orders. The focal point that corresponds to the focal point that originates due to refractive operation of the curvature of the lens 40 is indicated by order zero, 0. The other focal points are designated by orders -i-m and -m, wherein m is a positive integer value. That is, m = +1, +2, +3, etc. if the respective focal point occurs at the left-hand side of the zero order when viewed in the plane of the drawing, i.e. at a distance in the direction towards the lens body 41, and designated by orders m = -1, -2, -3, etc. if the respective focal point occurs at the right-hand side of the zero order when viewed in the plane of the drawing, i.e. at a distance in the direction away from the lens body 41. Such is illustrated in Figure 3.

It is noted that the above allocation of the positive and negative diffraction orders in some publications and handbooks may be reversed with respect to their position relative to the zero order. This, for example, becomes the case when the theory in the publication by Romero et al. is applied directly as has been done here. If not otherwise indicated, the present description adheres to the convention as shown in Figure 3.

The diffraction relief 42 can be designed to provide focal points at different distances from the lens body 41. The periodic spacing or pitch of the DOEs 43 substantially determines where the points of destructive and constructive interference occur at the optical axis 48 of the lens, i.e. the position of the diffractive orders at the optical axis 48. By the shape and height of the DOEs 43 the amount of incident light that is provided at a point of constructive interference, i.e. at or in a particular diffraction order, is controlled.

In case of a diffraction grating or relief 42 providing diffraction orders that are regularly spaced at both sides of the zero order, the grating or relief is called a symmetric wave splitter or diffractive grating, as the incident light beam 46 is diffracted or split into orders that are symmetrically arranged with respect to the zero order. A grating or relief producing a non-regular spacing of diffractive orders, such as +1, +2, -3, -5 is called an asymmetric diffractive grating. The common cases of diffraction gratings producing usable orders at 0^th order and +1 or 0^th, +1, and +2 are also asymmetric diffractive gratings.

The light energy in secondary light beams 47 that are focussed or diffracted in focal points or orders that do not contribute to image forming at the retina 14 of the human eye 10 is lost and reduces the overall efficiency of the lens 40, and hence the quality of images perceived by a human being using such lens. In practice, for optimally designing a lens, it is advantageous if the focal points for providing or correcting far, intermediate and near vision to the human eye, such as illustrated in Figure 1, for example, can be set beforehand, and a diffraction grating 42 is provided that maximizes the overall efficiency of the light energy received from the incident light beam 46 in these pre-set focal points is optimal.

In scientific literature, a diffraction grating optimizing overall efficiency of the light distribution in pre-set or target diffraction orders is found from determining a linear phase-only function or phase profile that generates the target diffraction orders with a maximum overall efficiency q or figure of merit defined as the sum of the normalized light energies of all these target orders. These diffractive gratings can then be shaped into lenses by adjusting the argument so that they have equidistant periods in the r² space.

Those skilled in the art will appreciate that the lens body 41 may comprise a plano-convex, a biconcave or plano-concave shape, and combinations of convex and concave shapes or curvatures (not shown).

Figure 4a shows a top view of an ophthalmic multifocal aphakic intraocular lens 50, working in accordance with the present invention, and Figure 4b shows a side view of the lens 50. The difference over the prior art, exemplified in Figure 2 are in the optics of the lens. The lens body 56 has a biconvex shape, comprising a front or anterior surface 54 and a rear or posterior surface 55. The skilled person would know that for some embodiments one or both of the anterior surface 54 and the posterior surface 55 might be concave or planar, depending on the refractive baseline needed for a specific application. In this application of the invention the lens body, in accordance with the present disclosure, comprises a central lens portion 51 and a peripheral lens portion 53, that is combined with a symmetric multifocal diffraction grating 52. The central lens portion 51 and the peripheral lens portion 53 have different refractive powers. The lens is constructed such that, for a design wavelength, the one of the diffractive orders of the symmetric multifocal diffraction grating 52 contributes to the refractive focal point of the central lens portion 51. Figure 4 shows a lens where one side of the lens is purely refractive, while the other side has a diffractive grating super positioned over a refractive base line. As explained above in relation to Figure 3 this is only one configuration. It is possible, for example, to distribute the diffractive grating over both sides, or superposition the diffractive grating to either side of a plano-convex or planoconcave lens.

The shape or height profile of the refractive base line for any of the portions of the lens may be selected among a plurality of continuous refraction profiles known from monofocal lenses, such as spherical, or based on monofocal diffractive surface, or aspherical surfaces, which are among the most general known shapes of monofocal lenses known in the art. Monofocal diffractive surfaces refers to the phase-matched Fresnel lenses discussed earlier. By adjusting the phase matching number an arbitrarily wide unbroken monofocal zone can be created through diffractive optics. It is possible combine different types of refractive surfaces in one lens, so that the central portion and the peripheral portion consist of different types of refractive surfaces. The manufacturing of refractive of diffractive surfaces can be carried out by any of laser micro machining, diamond turning, 3D printing, or any other machining or lithographic surface processing technique, for example.

Figure 5 shows a monofocal diffractive lens operating in the first order. Figure 5a of these images shows the diffractive profile as it actually is, while 5b shows the lens plotted versus the square of the radius, clearly showing the periodicity in r²-space. The vertical axes show the profile height H(r). Generally for diffractive lenses operating in the first order the pitch is determined so that the optical path difference (OPD) through the lens to the focal point of the first diffraction order has a difference of exactly one wavelength per period. To show the periodicity of a diffraction grating one will often, as here, plot the diffractive lens profile versus the square of the radius (often referred to as r²- space). When plotted like this the periods (grating pitch) is equidistant, more exactly the period pitch in r²-space is 2A/D, where A is the design wavelength and D the optical diffractive power of the first order in diopters.

The lens in Figure 5 is monofocal lens of the first order, which is accomplished by making use of a sawtooth diffractive unit cell and a step height corresponding to a phase modulation of exactly 2n (two pi, or two times pi). All energy in a lens of such fashion goes into the +1 order, or -1 order, depending on the definition. This is the only type of diffraction grating with 100% diffraction efficiency. By adjusting the phase matching number to m the height of each zone is increased to m²Π and the width of each zone in r²-space is multiplied with m. Such a lens is said to be a monofocal diffractive lens operating in the m^th order. Lenses with m>l are sometimes referred to as a Multi-Order Diffractive (MOD) lens, which is sometimes used to decrease the thickness of a lens and to decrease longitudinal chromatic aberrations. As is the case with other diffractive profiles it is possible to combine this profile with other refractive or diffractive profiles, on the same or opposite sides of a lens (not shown in the figure).

Figure 6 shows a trifocal diffractive lens profile of the sawtooth type and its spectrum. The upper graph of this figure conveys the diffractive profile of said lens, plotting the height versus the radius from the optical center. The lower graph contains the spectrum of said lens, with the intensity, I, as a function of the diopter. The intensity is displayed in arbitrary units. If the height of the monofocal diffractive lens in Figure 5 is decreased so the phase modulation corresponding to the step height is less than 2n the light will be split between the 0^th order and the 1^st order. One way to create a trifocal diffractive lens is to combine two such bifocal diffractive lenses with different first diffractive orders. This idea is known in the art with slight variations, most prominent examples thereof are disclosed in: US9320594, EP2377493, EP2503962B1, US9223148B2. A general approach to construct such a lens is also known from the teaching of US5017000. The resulting diffractive lens is an asymmetric diffractive lens operating in the 0, +1, and +2 orders. There are other, more direct, ways to calculate the ideal asymmetric trifocal lens.

A feature often desired in multifocal lenses is to provide a relatively even intensity distribution for mesopic conditions while providing a much stronger relative intensity for far vision for the larger pupils available in scotopic conditions. With sawtooth multifocal diffractive lenses this is often provided with the help of apodization, which in this context refers to a diffractive grating with decreasing height with increasing radius, as taught in Davison, J. A., & Simpson, M. J. (2006). History and development of the apodized diffractive intraocular lens. Journal of Cataract & Refractive Surgery, 32(5), 849-858. For a lens using a smooth symmetric grating this simple method can't be used for this purpose, which is why an alternative strategy is needed. Such apodization directly applied onto a multifocal lens based on a symmetric grating would lead to a very strong 0^th order for large pupils.

In the specific case of the diffractive lens profile shown in Figure 6, providing far vision, intermediate vision, and near vision. The lens is modelled for a refractive base of 18.5D, which is directly used for far vision. The diffractive powers of the lens are 1.5D (intermediate vision), and 3D (near vision).

The vast majority of diffractive ophthalmological lenses known in the art utilize "asymmetric" diffractive gratings, as demonstrated with reference to Figures 5 and 6. When ascribing symmetric or asymmetric property to multifocal ophthalmic lenses, what is considered is which orders it makes use of, or renders useful. Symmetric diffractive lenses utilize of orders in a way that is symmetric around the 0^th order. Note that symmetric diffraction gratings are defined by which orders they utilize, not by the light distribution in these orders. Some symmetric diffractive lenses may be tuned so that there is a significant difference in light intensity between e.g. +1 and -1 orders. To do this, tuning the diffraction grating's unit cell needs to be manipulated so as to become asymmetric, however this is not what is referred to with symmetric or asymmetric diffraction gratings. A diffraction grating tuned as such would still be considered a symmetric diffraction grating.

Albeit the terms symmetric and asymmetric are not commonly used referring to diffraction gratings, they are nonetheless very suitable for the teaching of the disclosed invention, and are in line with the use of terms in the literature in the way that a diffraction grating is often defined by which orders are rendered useful to the user. In a bifocal lens, more than two orders will have non-zero light intensity, but the intensity difference (in particularly at the design wavelength) tends to be obvious.

A special case of the optimum trifocal grating with equal intensity distribution is demonstrated with respect to Figures 7, 8, and 9 which, as a special case, was first disclosed in Gori 1998. As taught in EP 20170183354, even a diffractive unit cell with equal intensity distribution may be used in lenses with non-equal distribution with some small adjustments in lens design. One of such techniques is the case where the diffractive profile is horizontally (laterally) shifted, which changes the light distribution between orders for finite apertures. The most efficient linear phase grating for three orders has a diffraction efficiency of 92.56. Figure 7 shows again a trifocal lens according to EP20170183354 and W02019020435, this time with the modelled spectrum. This symmetric lens operates in the the -1, 0, and +1 orders. Such symmetric lenses tend to perform in higher diffraction efficiencies than the asymmetric sawtooth lenses. Additionally, comparatively smoother shapes of the gratings are quite desirable due to a greatly limited prevalence of scattered light, glare and halos. The Osipov 2015 study additionally puts forward the idea that, lenses with smooth gratings should be "more biocompatible because of the reduction of the debris precipitation effect".

There are different ways to calculate and tune symmetric diffractive lenses in the art. One early example is the 7-focal lens described in the paper by Golub et al., "Computer generated diffractive multi-focal lens" published in Journal of modern optics 39, no. 6 (1992): 1245-1251. As a continuation of this, additional embodiments in the already mentioned Osipov 2015 study as well as the study published in 2012 by Osipov et al. called "Fabrication of three-focal diffractive lenses by two-photon polymerization technique" published in Applied Physics A 107, no. 3 (2012): 525-529. In these papers trifocal, symmetric lenses made by modifications to a sinus grating are disclosed. These studies however fail to indicate as to the determination of the absolute optimum diffractive lenses, albeit offering good approximations. A different approach is also disclosed in US5760871A, as well as IL104316.

Yet another way to tune and optimize diffractive lenses, whether symmetric or asymmetric is by first finding the optimum linear phase grating for a desired light distribution, i.e. not a lens, but a linear diffraction that splits a light beam into a certain number or new beams. By treating the x-axis of the linear grating as the r² space of a diffractive lens, any such linear phase can be turned into a lens. The best available information on optimizations of linear phase gratings in the art can be found in Romero and Dickey' 2007 study. Using this theory, it is possible to define the orders of interest and the relative intensity distributions of respective order and find the equation for the optimum (most efficient) grating for those input values.

It further shows that at the very least symmetric gratings with a contiguous set of orders have optimum gratings without discontinuities for relatively equal intensity distributions. Some symmetric gratings with non-contiguous order sets also have gratings without discontinuities. In the Romero and Dickey study, only gratings with equal intensity distributions are shown, however using the provided theory gratings with non-equal distributions are also documented.

The commonly investigated case is the case of equal intensity distribution between the chosen orders. The highest attainable diffraction efficiency for equal intensity distribution in a linear grating for three focal points is 92.56%. The phase profile , for such a linear grating was originally defined by Gori et al. as: wherein: With this definition one period is exactly 1 unit long.

Using (1) a lens with a phase profile function Φ(r) built on this grating could be defined as wherein: have a constant value, and

T is the period or pitch of the diffraction grating in r² space, [mm²].

The value of the amplitude modulation function A(r) may be constant over the lens surface, such as between 1.05 - 1.15, for example, in order to take into account a reduction in the height of the diffractive grating by a finishing operation of the lens, such as by polishing. For lens bodies not requiring such a finishing operation, the value of A(r) may be 1. Note that this formula provides the phase modulation. When creating an actual lens, the refractive index of the lens material as well as the surrounding medium have to be taken into account, which is trivial for the skilled person. The lateral shift, S, is a way to express the phase shift of the periodic grating, the choice of which tunes the behaviour of the lens. Since the term phase has multiple meaning in this document the term is elsewhere in the document simply referred to as the lateral shift.

Reference numeral 60 in Figure 7a shows an example of height profile or amplitude profile of a continuous periodic diffraction profile in r² space, expressed in mm², as disclosed by W02019020435, and Figure 7b shows the same height function along a linear scale as function of the radial distance r, based on the phase profile function (Φ(r) according to equation (2). As with the monofocal sawtooth-based lens in Figure 5 or the trifocal sawtooth-based lens in Figure 6 the lens is strictly periodic in r²-space, as expected.

The amplitude of the height profile H(r) 61 is depicted at pm-scale along the vertical axis. The optical axis, running through the center of the lens body, is assumed to be at a radial position = 0, whereas the radial distance /"measured in outward direction from the optical axis is expressed in mm along the horizontal axis.

In this embodiment, the design wavelength of the lens is assumed at 550 nm, the index of refraction of the lens body is set to 1.4618, and the index of refraction of the medium surrounding the lens body is assumed to be 1.336. The amplitude modulation function A(r) is a constant at 1.07, the period T = 0.733 mm² in r² space, and the lateral shift S = 0.

Reference numeral 60 refers to the outer circumference or baseline curvature of the front surface 34 of the lens body 30 having a diffraction grating or relief 36 comprising the height profile H(r) 61 (see Figures 2a and 2b). The skilled person would know that a diffractive profile can be located on either or both of the front and back surfaces of a lens.

The amount of light diffracted by the lens having the height profile H(r) 61 is shown by computer simulated light intensity distributions in Figure 7c. Reference numeral 64 refers to diffraction order 0, providing a focal point for intermediate vision, reference numeral 62 refers to diffraction order -1, providing a focal point for far vision, and reference numeral 63 refers to the +1 diffraction order, providing a focal point for near vision. In the intensity profiles, the intensity /of the diffracted light is depicted in arbitrary units along the vertical axis as a function of the optical power in diopter, D, depicted along the horizontal axis.

The computer-simulated light intensity distributions assume a 20 biconvex lens body 31 of an ophthalmic lens 30 of the type shown in Figures 2a, 2b, designed for targeting a zero order focal point at 20 diopter, D, and first order focal points at 21.5 D and 18.5 D, symmetrically positioned with respect to the zero order. That is, providing a focal point for intermediate vision at 20 D for the zeroth order focal point, providing a focal point for far vision at 18.5 D by diffraction order -1, and 25 providing a focal point for near vision at 21.5 D by the +1 diffraction order. Those skilled in the art will appreciate that these optical powers or focal points may differ for actual lenses, dependent on the target focal points. The examples are calculated using MATLAB™ based simulation software, and assuming a pupil size of 6 mm diameter.

As can be seen from Figure 7c, different from the lens phase profile calculated for the linear optimal triplicator by Gori et al., the amount of light incident at the curved lens body is not distributed equally in the target focal points. This, because the optimum triplicator periodic phase profile function by Gori et al. is calculated for a linear or planar phase grating for which the distances between the periods show a linear dependency, while by transforming same into a lens, the distances between the periods of the phase profile function comprise a square root dependency.

Figure 8a shows a height profile 71 as a function of the radial distance rof a diffraction grating in an embodiment of a trifocal intraocular ophthalmic lens. The design wavelength λ, the index of refraction n of the lens body, the index of refraction n_m of the medium surrounding the lens body, the amplitude modulation function A(r), and the period Tin r² space, for this embodiment, are identical to the parameters of the embodiment illustrated by Figures 7a - 7c. Different from the embodiment of Figures 7a - 7c, diffractive profile is laterally shifted, as illustrated in Figure 8a is modulated by the lateral shift S having a fixed non-zero value. Reference numeral 71 refers to the outer circumference or baseline curvature of the front surface 34 of the lens body 30 having a diffraction grating or relief 36, comprising the height profile H(r) 71, extending from the optical axis. Figures 8b, 8c and 8d show computer simulated light intensity distributions for the lens of Figure 8a for varying pupil sizes. Along the vertical axis of the graphs in Figures 8b, 8c and 8d, the relative intensity, , rel. I, of the refracted and diffracted light with respect to the maximum intensity in one of the focal point is depicted as a function of the optical power in diopter, D, depicted along the horizontal axis. The examples are again calculated using MATLAB™ based simulation software. The computer simulated light intensity distributions assume a biconvex lens body designed for targeting a zeroth order focal point at 20 diopter, D, and first order focal points at 21.5 D and 18.5 D, symmetrically positioned with respect to the zeroth order. That is, providing a focal point for intermediate vision at 20 D for the 30 zeroth order focal point, providing a focal point for far vision at 18.5 D by diffraction order -1, and providing a focal point for near vision at 21.5 D by the +1 diffraction order.

Figure 8b shows the light intensity distribution 72 for a pupil size having a diameter of 1 mm. As can be seen from Figure 8b, almost all the light incident on the lens is concentrated in the focal for intermediate vision at 20 D. That is, when measuring the optical system of a user comprising intraocular lenses according to the embodiment of Figure 8a using an autorefractometer and a light intensity such that the pupil size of the user is about 1 mm in diameter, the focal point actually measured with the autorefractometer is not one of the diffractive focal points but the intermediate or refractive focal point.

Figure 8c shows the light intensity distribution for a pupil size having a diameter of 3 mm. A pupil of such size covers a larger part of the diffractive profile and of the convex surface of the lens as for the 1 mm pupil size shown in Figure 8b. Reference numeral 73 refers to diffraction order 0, providing the focal point for intermediate vision. Reference numeral 74 refers to the -1 diffraction order, providing a focal point for far vision, and reference numeral 75 refers to the +1 diffraction order, providing a focal point for near vision. As can be seen from the intensity profile of Figure 8b, a greater part of the incident light is distributed in the focal point for near vision 75, compared to the amount of light distributed in the focal points for intermediate 73 and far vision 74.

Figure 8d shows the light intensity distribution for a pupil size having a diameter of 6 mm. A pupil of such size generally covers the whole optical system of an ophthalmic lens. Reference numeral 73 again refers to diffraction order 0, providing the focal point for intermediate vision, reference numeral 74 refers to diffraction order -1, providing the focal point for far vision, and reference numeral 75 refers to the +1 diffraction order, providing the focal point for near vision. The impact of applying a lateral shift to the diffractive profile can be seen by comparing Figure 8d with Figure 7c. It can be seen that in this case, shifting the first peak further out from the lens center, as in Figure 8, decreases the relative intensity provided to diffraction order 0, reference numeral 73, providing the focal point for intermediate vision. It is thus seen that in designing a multifocal lens we can on the one hand choose the target intensity distribution of the underlying linear diffraction grating, but we can also fine tune the actual distribution of the lens by lateral shift of the diffraction pattern. The choice of lateral shift to use can also affect the manufacturability of a lens, as different geometries have very different sensitivity to small perturbations.

Figure 9 Shows one more lens using the optimal diffractive unit cell for equal distribution over three orders ([1 1 1]). Figure 9a shows the unit cell for and a histogram showing the resulting order distribution. The diffraction efficiency of the optimal diffraction grating for an equal split over three orders is 92.56%. Figure 9b show the diffractive profile of a lens based on the unit cell above. Inf Figure 9c the resulting energy distribution of the lens in Figure 9b can be seen. The spectrum is modelled at an aperture of 3mm and 20 D base refractive power is assumed.

In equation (1) the optimal linear phase grating for a trifocal grating with equal intensity distribution is shown. It is often advantageous to design a specific optical grating with the required properties. In the already mentioned paper by Romero et al., a methodology is disclosed to find an optimal linear phase grating for a desired set of target focal points and a specified intensity distribution among these. For the case of a trifocal linear grating the complete, non-simplified formula of the linear phase grating based on Romero et al. is: wherein:

With this definition one period is exactly 1 unit long.

The grating in equation (3) can be used for a trifocal part of a lens by substituting x with the square of the lens radius r. More precisely, to arrive at the equivalent of equation (2) x should be replaced . Note that the complete apparatus to find the optimum grating is in not included in the present document, as this is available in the referenced literature. The formula in (3) can also be arbitrarily extended to describe other configurations. Of special interest are lenses with four, five, and seven focal points.

A lens equation equivalent to equation (2) above can now be formed from the linear grating in equation (3). Using the phase profile as defined in (3) one arrives at: wherein:

It is noted that, because of the way the theory from Romero et al. is applied here, the focal points for far and near vision correspond to the positive and negative diffraction orders, respectively. That is just the opposite as used otherwise in the description of the present application. From a theoretical point of view this reversal of the orders and focal points is irrelevant.

If the mathematics by Romero et al. and (3) are used to find the optimal trifocal grating with an equal split over the orders (-1, 0, +1) we arrive at the following equation:

This definition is identical to equation (1) above, except for a 90-degree (0.25 * T) shift. This shift needs to be accounted for when making the lens by appropriately changing S to take this into account. Any one of the formulations in (1) or (5) can be chosen arbitrarily.

If instead of an equal intensity distribution a diffraction grating is to be provided having a (near, intermediate, far) split of [1.2, 1, 1], for example, a way to express an optimal diffractive grating fulfilling these requirements is by applying the teachings of Romero et al. in terms of equation (3), having a diffraction efficiency and the constants set as follows:

One important aspect of the general formulation of optimized trifocal linear gratings in (3) is that, while the equal intensity split, [1 1 1] has an optimal solution with a diffraction efficiency of 92.56%, many other intensity distributions have solutions with significantly higher diffraction efficiencies.

Figure 10 is analogous to Figure 9, but based on the diffractive unit cell optimized for the intensity distribution [1.2 1 1]. This distribution has a diffraction efficiency of 91.26, that is somewhat lower than that for the equal distribution. The way the it is done here the orders in the lens are related to the intensities as [Near, Intermediate, Far]. The chosen convention is that strongest power (corresponding to the focal point closest to the user) is listed first, and the weakest power last. The diffractive lens profile in Figure 10c is identical to the diffractive lens profile in Figure 9c except for the unit cell chosen, with identical refractive indices and lateral shift. The spectrum is modelled at an aperture of 3 mm and 20D base refractive power is assumed. It can be seen in Figure 10c that the near intensity is increased compared to Figure 9c.

Figure 11 is analogous to Figure 9 and Figure 10, but based on the diffractive unit cell optimized for the intensity distribution [1.1 1.2 1]. This distribution has a diffraction efficiency of 93.88, that is somewhat higher than that for the equal distribution in Figure 9 as well as higher than the unit cell used in Figure 10. It is notable that the diffraction efficiency of this unit cell is higher than that for the qual intensity split over three orders, which is usually referred to as the optimal split in the literature. For the specific case of trifocal gratings very high diffraction efficiencies are reached for high 0^th order intensities, this is however not a general rule, as is discussed further down. Specifically, this is not the case for diffraction gratings optimized for five focal points.

The diffractive lens profile in Figure 11c is identical to the diffractive lens profile in Figure 9c and in Figure 10c, except for the unit cell chosen, with identical refractive indices and lateral shift. The spectrum is modelled at an aperture of 3mm and 20D base refractive power is assumed. It can be seen in Figure 11c that the relative Far intensity is decreased compared to Figure 9c as well as Figure 10c. The spectrum is modelled at an aperture of 3mm and 20D base refractive power is assumed.

Figure 12 illustrates how selection of diffractive gratings combined with choice of the lateral shift can be used to design a suitable multifocal lens to show the relationship between the diffraction efficiency of the grating and the behaviour of the resulting lens. In the literature on optical beam splitting, partly taught in the present invention, the term "optimal" is typically used for the most efficient grating for a certain number of orders with equal intensity distribution over those orders. However, this is by no means necessarily the case for lenses: For certain lens configurations, a higher total diffraction efficiency can be utilized very well. As can be understood from the demonstrations in Figures 9, 10, and 11, the highest possible diffraction efficiency is dependent on the required intensity distribution.

For a given phase grating the intensity distribution can be tuned (within some limits) by lateral shift of the grating. This is explained in detail in EP20170183354 (See Figures 6 and 7). A shift with a full period will provide the original lens. Figures 12a, 12b, and 12c. show for three different gratings how the simulated intensities for Far, Near, and Intermediate change at a 3 mm aperture for different amount of lateral shift when constructing the lens. Each lateral shift is set as a portion of the period. The three gratings for gratings 1, 2, and 3 are, respectively, optimized for light distributions [1 1 1], [1 1.2 1], and [1.1 0.8 1.2]. These gratings have the respective diffractions efficiencies of 92.56%, 94.51%, and 88.62%. The lenses are configured to provide far vision, intermediate vision (at the 0^th order), and near vision. For each diffraction unit cell these three graphs show the intensity for each focal point as a function of the lateral shift, S, in equation (4). The behaviour in figures 9a through 9c is the expected, where a higher chosen 0^th order leads to a higher intermediate. The same being true, mutatis mutandis, for the other orders. It should, however, be noted that an order that is stronger in the underlying linear phase grating is not necessarily stronger for each possible S. On the contrary, it is obvious from these graphs that choosing the correct value for S is important to get the desired light distribution. Figure 12d plots the sum intensity, in arbitrary units, for each grating. A high sum indicates an efficient diffractive lens. The graph in Figure 12d compares the summation of the three intensities for each chosen diffractive grating. The summation of peaks is very good indication of effective diffractive efficiency. It can be seen that change in this parameter overall corresponds closely to the change in diffraction efficiency between gratings. Note that for a given lateral shift a grating with higher diffraction efficiency provides in general a more efficient lens, but a badly chosen lateral shift with an efficient grating will perform worse than a less efficient grating for a more suitable lateral shift. It is often the case that the choice of lateral shift (i.e. choice of value for the S- parameter) is limited by manufacturability and behaviour for different apertures. An example of such a region, using the calculations in the modelling in Figures 12a through d is 0.45 < S * T < 0.7. It is additionally often advantageous to choose a grating with high diffraction efficiency. Specifically, high diffraction efficiency lenses will of course provide more usable light to the eye, but they also reduce the amount of light going to undesired diffraction orders and light going to undesired effects such as halo and glare. However, trifocal gratings with very high diffraction efficiency used over the full optics of the lens will lead to undesired intensity distributions. For trifocal, symmetrical lenses a very high diffraction efficiency tends to lead to a very strong zeroth order.

Figure 13a shows a pentafocal (having five focal points) diffractive unit cell optimized for the intensity distribution [0.8 1.2 0.80 1.20.80]. This linear phase grating yields a very high diffraction efficiency of 98.98%. This is significantly higher than the optimized diffractive grating for the pentafocal grating with an equal intensity split, [1 1 1 1 1], for which the intensity distribution is 92.13%. Figure 13b show the diffractive lens based on the diffractive unit cell in Figure 13a. This specific implementation has the pitch of the diffractive grating arranged to provide 2.13 D between the strongest and the weakest focus assuming a design wavelength of the lens of 550 nm, the index of refraction of the lens body is for this embodiment set to 1.492, and the index of refraction of the medium surrounding the lens body is assumed to be 1.336. This specific example would thus to a user provide five focal points mostly distributed over and in between far and intermediate vision. By rearranging the pitch of the diffractive grating it is of course possible to have a pentafocal lens providing near vision in addition for far and intermediate vision. In Figure 13c is the modelled spectrum corresponding to the diffractive lens profile in Figure 13b, shown as relative intensities, with the highest peak set equal to 1. The spectrum is modelled at an aperture of 6 mm and 20D base refractive power is assumed. For pentafocal, symmetrical, lenses a very high diffraction efficiency tends to lead to very strong first orders, with the orders being relatively weaker.

Figure 14 shows one way to construct a lens according to the invention by combining a trifocal area with a different refractive base than the central portion of the lens. Figure 14a shows a completely trifocal lens constructed from a linear phase grating optimized for the light distribution of [0.85 1.15 1.11] with a diffraction efficiency of 93.29%, higher than a grating optimized for equal intensity splitting. The pitch is arranged to provide a first order diffractive power of 1.675D. S, as defined by (3) and (4) is set to 0.48 * T. Figure 14b shows the profile of a lens made according to the present invention, less the refractive base power of 20D. The lens has a purely refractive central portion inserted into the diffraction grating from Figure 14a, in this case diameter of the monofocal central zone is 1,03 mm. The monofocal central zone has a power of -1.675D, so that the focal point provided by it lines up with the diffractive far power of the diffractive grating. The refractive base power (not shown) of the lens lines up with the intermediate power. It should be understood that this is only an example, any type of refractive surface can be used, such a portion of a monofocal diffractive grating, a spherical surface or any form of aspherical surface. For many applications a properly designed aspherical refractive surface is advantageous. The modelled intensity distributions in Figure 14c show how this particular lens works. The intensity distribution is relatively even over different apertures. The lens makes use of the highly efficient diffractive unit cell by having no significant peaks other than the three intended ones. A lens according made like this could function as a so called EDOF (Enhanced depth of focus) lens, as it has a strong far vision, but still enough light distributed to near and intermediate to allow for a user to engage in most activities without other ophthalmological means.

The combination of a multifocal grating with a monofocal central zone enables the use of diffractive gratings with very high grating efficiencies, as the strong far vision of the central zone can be used to balance constraints of such gratings.

This type of lens geometry enables multifocal lenses with a wider set of intensity distribution profiles, especially with a wider difference in intensity distribution between different apertures.

Further, this lens configuration provides for an ophthalmologist a way to carry out in-vivo measurements from a monofocal portion of the lens, which is sometimes advantageous. In the specific case of the lens in Figure 14 this monofocal portion coincide with the far vision, which is most of the times the preferred focal point to measure.

Figure 15 shows another lens according to the present invention. In Figure 15a is a pentafocal diffractive unit cell optimized for the intensity distribution [0.87 1.2 0.80 1.2 0.90]. This linear phase grating yields a diffraction efficiency of 98.54%, significantly above that of the grating with the equal intensity split. In Figure 15b a lens profile is shown, less the refractive base of 20D. The unit in Figure 15a is used to create a simple pentafocal grating (not shown) with a first order optical power of 0.84D and an S, as defined by (3) and (4), set to 0.8 * T. The lens has a purely refractive central portion inserted into the diffraction grating from Figure 14a, in this case diameter of the monofocal central zone is 1,14 mm. The monofocal central zone has a power of -1.675D, so that the focal point provided by it lines up with the diffractive far power of the diffractive grating. The refractive base power (not shown) of the lens lines up with the intermediate power. It should be understood that this is only an example, any type of refractive surface can be used, such a portion of a monofocal diffractive grating, a spherical surface or any form of aspherical surface.

The modelling for different aperture sizes in Figure 15c shows that this specific lens provides a strong far vision for all apertures and a well-developed multifocality for larger apertures. Note that a pentafocal lens has a more distributed spectrum than most conventional multifocal lenses. Often one would refer to the two second order foci as far and near and the 0^th order as the intermediate vision. Here there are also two additional foci. Together they could provide virtually seamless vision for a user.

The examples in Figures 14b and 15b both describe lenses according to the invention where the optical powers of the lens are arranged to coincide with, in addition to the far vision, near and intermediate vision. However this is not by any means the only configuration of interest. The concept of enhanced depth of focus (EDOF) has been increasingly discussed in recent year. Figure 15b describes a lens where between the foci for far, intermediate, and near there are two additional focal points to provide continuous vision for a user.

Yet another configuration of interest is where the focal points of the symmetric diffractive lens according to the patent are arranged for far vision, intermediate vision, and one or more diffractive order forming focal point(s) at optical power(s) in between the far and intermediate visions. Existing EDOF lenses on the market often target only far and intermediate powers, but don't offer a continuous vision between these distances.

Linear phase gratings have in this document been shown as one possible starting point to create multifocal lenses. The existing theory and understanding of such gratings can also be used to analyze lenses, if the underlying linear phase grating can be extracted from the lens. The simplified flow diagram 160 in Figure 16 illustrates steps of a method of measuring the profile of an ophthalmic multifocal lens and to determine the diffraction efficiency of the underlying diffraction grating. The direction of the flow is from the top to the bottom of the drawing.

In a first step, block 161, a region of the lens is selected and measured, preferably along a line normal to the optical axis.

In a second step, block 162, the curvature of the the base refractive power of the selected region is removed from the measured profile. Sometimes a part of the refractive base is actually a monofocal phase matched Fresnel lens or an MOD lens, when this is the case also this monofocal diffractive structure is to be subtracted from the measured profile. In a third step, block 163, the resulting lens profile is plotted versus the square root of the distance from the optical center.

In a fourth step the underlying linear phase is obtained by converting the height profile in the previous step to a phase profile using the design wavelength, refractive index of said ophthalmic lens and the refractive index of the designated environment in the eye.

Finally, in a fifth step, block 164, the diffraction efficiency for the usable orders and the linear grating is calculated. Once the phase profile of the underlying linear phase grating is known, it is possible to calculate the diffraction efficiency. If the phase grating is Φ(x) then the transmission function can be written as

The efficiency of each diffraction order or of a combination of diffraction orders can be found by study of the Fourier coefficients of the transmission function. If the length of the diffractive unit cell is 1, the Fourier coefficients can be written as

For phase-only gratings it is the case that

The Fourier coefficients denoted by k correspond to the respective diffraction orders, the diffraction efficiency, 7, of which can be written as For example, for a trifocal lens working in the orders k = -1, 0, 1 the total diffraction efficiency can be written as

It is trivial to extending these analytical results numerical implementations, such as calculating the diffraction efficiency of a measured diffraction grating.

Other variations to the disclosed examples and embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope thereof. Same reference signs refer to equal or equivalent elements or operations.

According to an embodiment of the present invention, an ophthalmic multifocal lens, comprising at least three focal points, said lens having a light transmissive lens body comprising at least a first portion with a first refractive base power that coincides with the central area of said light transmissive lens body and a second portion with a second refractive base power is proposed.

In another embodiment of the present invention, said ophthalmic multifocal lens further comprises a multifocal symmetric grating combined with said second portion, wherein the combination with said second portion is configured such that one diffraction order of said multifocal symmetric grating adds to the base refractive power of said central area coinciding with the first portion for a design wavelength and said diffraction grating has a diffraction efficiency higher than that of the corresponding diffraction grating with equal light distribution.

In another embodiment of the present invention, the first refractive base of the said first portion and said a second refractive base of said second portion ophthalmic multifocal lens are unequal.

In another embodiment of the present invention, said at least one first portion, said at least one second portion, or both said portions comprise sawtooth diffractive grating.

In another embodiment of the present invention, said sawtooth diffractive grating is monofocal.

In another embodiment of the present invention, said multifocal lens is a trifocal lens and as such said multifocal symmetric grating provides three focal points.

In another embodiment of the present invention, said multifocal lens is a pentafocal lens and as such said multifocal symmetric grating provides five focal points.

In another embodiment of the present invention, said multifocal symmetric grating provides a number of focal points selected from a group including, but not limited to, four, seven, nine focal points.

In another embodiment of the present invention, said multifocal symmetric grating of said at least one first portion and said multifocal symmetric grating of said at least one second portion provide different numbers of focal points.

In another embodiment of the present invention, said multiple focal points provided by said multifocal symmetric gratings are configured for far, intermediate and near vision.

In another embodiment of the present invention, said multiple focal points provided by said multifocal symmetric gratings are arranged so that the power difference between the focal point having the highest optical power and the focal point having the lowest optical power does not exceed 2 Diopter.

In another embodiment of the present invention, said ophthalmic multifocal lens further comprises a transition zone between said at least one first portion and said at least one second portion, whereby a refractive base power or a range of refractive base powers between the refractive powers of said at least one first and at least one second portion is provided.

According to an embodiment of the present invention, a method of measuring the profile of an ophthalmic multifocal lens, preferably from the optical center of said ophthalmic lens and outwards is proposed.

According to another embodiment of the present invention, said method comprises the step of selecting the region of the measured profile with a diffractive pattern.

According to another embodiment of the present invention, said method comprises the step of removal of the curvature of the base refractive power of the selected region. According to another embodiment of the present invention, said method comprises the step of removal of any diffractive periodic grating(s) with a maximum phase modulation higher than the design wavelength of said ophthalmic lens.

According to another embodiment of the present invention, said method comprises the step of plotting the resulting lens profile versus the square root of the distance from the optical center.

According to another embodiment of the present invention, said method comprises the step of obtaining linear phase grating via converting the height profile in the previous step to a phase profile using the refractive index of said ophthalmic lens and the refractive index of the designated environment in the eye.

According to another embodiment of the present invention, said method comprises the step of computing the diffraction efficiency for the usable orders and the design wavelength of the linear grating.

Claims

1) An ophthalmic multifocal lens, comprising at least three focal points, said lens having a light transmissive lens body comprising at least a first portion with a first refractive base power that coincides with the central area of said light transmissive lens body and a second portion with a second refractive base power characterized in that said ophthalmic multifocal lens further comprises a multifocal symmetric grating combined with said second portion, wherein the combination with said second portion is configured such that one diffraction order of said multifocal symmetric grating adds to the base refractive power of said central area coinciding with the first portion for a design wavelength and said diffraction grating has a diffraction efficiency higher than that of the corresponding diffraction grating with equal light distribution.

2) An ophthalmic multifocal lens as set forth in Claim 1, characterized in that the first refractive base of the said first portion and said a second refractive base of said second portion ophthalmic multifocal lens are unequal.

3) An ophthalmic multifocal lens as set forth in any preceding Claim, characterized in that said at least one first portion, said at least one second portion, or both said portions comprise sawtooth diffractive grating.

4) An ophthalmic multifocal lens as set forth in Claim 3, characterized in that said sawtooth diffractive grating is monofocal.

5) An ophthalmic multifocal lens as set forth in Claims 1 to 4, characterized in that said multifocal lens is a trifocal lens and as such said multifocal symmetric grating provides three focal points.

6) An ophthalmic multifocal lens as set forth in Claims 1 to 4, characterized in that said multifocal lens is a pentafocal lens and as such said multifocal symmetric grating provides five focal points.

7) An ophthalmic multifocal lens as set forth in Claims 1 to 4, characterized in that said multifocal symmetric grating provides a number of focal points selected from a group including, but not limited to, four, seven, nine focal points.

8) An ophthalmic multifocal lens as set forth in any preceding Claim, characterized in that said multifocal symmetric grating of said at least one first portion and said multifocal symmetric grating of said at least one second portion provide different numbers of focal points.

9) An ophthalmic multifocal lens as set forth in any preceding Claim, characterized in that said multiple focal points provided by said multifocal symmetric gratings are configured for far, intermediate and near vision.

10) An ophthalmic multifocal lens as set forth in any preceding Claim, characterized in that said multiple focal points provided by said multifocal symmetric gratings are arranged so that the power difference between the focal point having the highest optical power and the focal point having the lowest optical power does not exceed 2 Diopter.

11) An ophthalmic multifocal lens as set forth in any preceding Claim, characterized in that said ophthalmic multifocal lens further comprises a transition zone between said at least one first portion and said at least one second portion, whereby a refractive base power or a range of refractive base powers between the refractive powers of said at least one first and at least one second portion is provided.

12) An ophthalmic multifocal lens as set forth in any preceding Claim, characterized in that said multifocal diffractive grating comprises at least at least one significant order that is arranged at optical powers between far and intermediate vision such that enhanced depth of focus is achieved.

13) An ophthalmic multifocal lens as set forth in any preceding Claim, characterized in that said multifocal diffractive grating comprises at least four significant orders, two of which arranged between far and intermediate vision and the other two of which arranged between intermediate and near vision, configured to provide enhanced continuity of vision between said vision types.

14) A method of measuring the profile of an ophthalmic multifocal lens according to any preceding Claim, preferably from the optical center of said ophthalmic lens and outwards, comprising steps of:

I) selecting the region of the measured profile with a diffractive pattern,

II) removal of the curvature of the base refractive power of the selected region,

III) plotting the resulting lens profile versus the square root of the distance from the optical center,

IV) obtaining linear phase grating via converting the height profile in the previous step to a phase profile using the design wavelength, the refractive index of said ophthalmic lens and the refractive index of the designated environment in the eye, V) computing the diffraction efficiency for the usable orders of the linear grating.