Trifocal intraocular lens with extended range of vision and correction of longitudinal chromatic aberration

Abstract

Disclosed is an intraocular lens (IOL) including an anterior surface, a posterior surface and an optical axis. At least one of the anterior or posterior surfaces has a diffractive profile formed thereon. The diffractive profile has diffractive focal points for far vision, intermediate vision, and near vision. The diffractive profile corresponds to a superposition of a first partial diffractive profile and a second partial diffractive profile, the first partial diffractive profile has a focal point of order +n that coincides with the diffractive focal point for intermediate vision or with the diffractive focal point for near vision, the second partial diffractive profile has a focal point of order +n that coincides with the diffractive focal point for far vision and a focal point of higher order than +n that coincides with the diffractive focal point for near vision.

Claims

1. An intraocular lens, comprising: an anterior surface, a posterior surface and an optical axis, the lens being made of a material that has a refractive index; wherein at least one of the anterior or posterior surfaces has a diffractive profile formed thereon, the diffractive profile having: a diffractive focal point for far vision, a diffractive focal point for intermediate vision, and a diffractive focal point for near vision; wherein the diffractive profile corresponds to a superposition of a first partial diffractive profile and a second partial diffractive profile; the first partial diffractive profile has a focal point of order+n that coincides with either the diffractive focal point for intermediate vision or with the diffractive focal point for near vision; the second partial diffractive profile has: a focal point of order+n that coincides with the diffractive focal point for far vision, a focal point of higher order than +n that coincides with the diffractive focal point for near vision; wherein each of the first and second partial diffractive profiles has a plurality of steps with corresponding step heights; wherein in at least a portion of the diffractive profile, the step heights are selected such that n<a.sub.1+a.sub.2<n+1, wherein: $a_1=\stackrel{\_}{h_1}/\left(\frac{\lambda }{.Math.n_2-n_1.Math.}\right),a_2=\stackrel{\_}{h_2}/\left(\frac{\lambda }{.Math.n_2-n_1.Math.}\right),$ h.sub.1 is the average of the step heights of the first partial diffractive profile in the portion of the diffractive profile, h.sub.2 is the average of the step heights of the second partial diffractive profile in the portion of the diffractive profile, λ=550 nm, n.sub.2 is the refractive index of the lens material, n.sub.1=1.3345, and n=1 or n=2.

2. The intraocular lens of claim 1, wherein n=1, and wherein the second partial diffractive profile has: a focal point of order+2 that coincides with the diffractive focal point for intermediate vision; and a focal point of order+3 that coincides with the diffractive focal point for near vision.

3. The intraocular lens of claim 1, wherein the step heights of the first and second partial diffractive profiles fulfill the following condition in at least the portion of the diffractive profile: a.sub.2>a.sub.1.

4. The intraocular lens of claim 1, wherein n=1, and wherein the step heights of the first and second partial diffractive profiles fulfill the following conditions in at least the portion of the diffractive profile: 0.5<a.sub.1<1, and 0.5<a.sub.2<1.

5. The intraocular lens of claim 1, wherein n=1, and wherein the step heights of the first and second partial diffractive profiles fulfill the following conditions in at least the portion of the diffractive profile: 0.5<a.sub.1<0.7 and 0.6<a.sub.2<0.9.

6. The intraocular lens of one of the preceding claims, wherein n=1, and wherein the step heights of the first and second partial diffractive profiles fulfill the following conditions in at least the portion of the diffractive profile: 0.53<a.sub.1<0.62 and 0.7<a.sub.2<0.8.

7. The intraocular lens of claim 1, wherein n=1 and the step heights a.sub.1 of the first partial diffractive profile are <1, while the step heights a.sub.2 of the second partial diffractive profile are >1.

8. The intraocular lens of claim 7, wherein the step heights of the first and second partial diffractive profiles fulfill the following conditions in at least the portion of the diffractive profile:0.25<a.sub.1<0.45 and 1.20<a.sub.2<1.40.

9. The intraocular lens of claim 7, wherein the step heights of the first and second partial diffractive profiles fulfill the following conditions in at least the portion of the diffractive profile: 0.30<a.sub.1<0.40 and 1.25<a.sub.2<1.35.

10. The intraocular lens of claim 7, wherein the step heights of the first and second partial diffractive profiles fulfill the following conditions in at least the portion of the diffractive profile: 0.33<a.sub.1<0.37 and 1.28<a.sub.2<1.32.

11. The intraocular lens of claim 1, wherein the diffractive focal points for intermediate vision and for far vision are both located on the optical axis at a distance from each other corresponding to between +0.5 and +1.5 dioptres.

12. The intraocular lens of claim 1, wherein the diffractive focal points for near vision and for far vision are both located on the optical axis at a distance from each other corresponding to between +1.5 and +2.5 dioptres.

13. The intraocular lens of claim 1, wherein the diffractive focal points for intermediate vision and for far vision are both located on the optical axis at a distance from each other corresponding to between +1.5 and +2.0 dioptres.

14. The intraocular lens of claim 1, wherein the diffractive focal points for near vision and for far vision are both located on the optical axis at a distance from each other corresponding to between +3.0 and +4.0 dioptres.

15. The intraocular lens of claim 1, wherein at a pupil size of 4.5 mm and with green light at a wavelength of 543 nm, the modulation transfer function (MTF) at 50 cycles/mm as a function of position on the optical axis displays distinguishable peaks corresponding to the diffractive focal points for far, intermediate, and near vision.

16. The intraocular lens of claim 1, wherein at a pupil size of 4.5 mm, 50 cycles/mm and with green light at a wavelength of 543 nm, either the MTF value corresponding to the diffractive focal point for near vision is greater than the MTF value corresponding to the diffractive focal point for intermediate vision, or the MTF value corresponding to the diffractive focal point for near vision is less than the MTF value corresponding to the diffractive focal point for intermediate vision.

17. The intraocular lens of claim 16, wherein at a pupil size of 4.5 mm, 50 cycles/mm and with green light at a wavelength of 543 nm, the MTF value corresponding to the diffractive focal point for far vision is larger than the MTF value corresponding to the diffractive focal point for near vision.

18. The intraocular lens of claim 1, wherein at a pupil size of 2.0 mm, 50 cycles/mm and with green light at a wavelength of 543 nm, the MTF value corresponding to the diffractive focal point for near vision is larger than the MTF value corresponding to the diffractive focal point for far vision.

19. The intraocular lens of claim 1, wherein at a pupil size of 2.0 mm, 50 cycles/mm and with green light at a wavelength of 543 nm, the MTF as a function of position on the optical axis stays constantly above 0.13 in a range extending from the diffractive focal point for near vision to the diffractive focal point for far vision.

20. The intraocular lens of claim 1, wherein at a pupil size of 2.0 mm, 50 cycles/mm and with green light at a wavelength of 543 nm, the MTF as a function of position on the optical axis stays constantly above 0.2 in a range extending from the diffractive focal point for near vision to the diffractive focal point for far vision.

21. The intraocular lens of claim 1, wherein: a first extended depth of focus is defined as the difference between a focal power of the diffractive focal point for near vision and a focal power of the diffractive focal point for far vision; a second extended depth of focus is defined as the difference between a focal power of the diffractive focal point for intermediate vision and a focal power of the diffractive focal point for far vision; and the first extended depth of focus is an integer multiple of the second extended depth of focus.

22. The intraocular lens of claim 21, wherein the first extended depth of focus is either two or three times the second extended depth of focus.

23. The intraocular lens of claim 1, wherein the diffractive profile has a plurality of non-vertical steps having a width between 4 μm and 100 μm.

24. The intraocular lens of claim 1, wherein the diffractive profile has a plurality of non-vertical steps having a width between 10 μm and 50 μm.

25. The intraocular lens of claim 1, wherein the diffractive profile has rounded edges with a radius of curvature of 0.1 μm or greater.

26. The intraocular lens of claim 1, wherein: the steps of the first partial diffractive profile are centered with respect to the optical axis approximately at radial positions r.sub.n measured from the optical axis, the radial positions being: r.sub.n=√{square root over (2n.Math.λ.Math.F.sub.1)}; the steps of the second partial diffractive profile are centered with respect to the optical axis approximately at radial positions r.sub.n measured from the optical axis, the radial positions being: r.sub.n=√{square root over (2n.Math.λ.Math.F.sub.2)}; wherein: n is the number corresponding to each step in the respective partial diffractive profile counted from the center of the profile, F.sub.1 is the focal length of the diffractive focal point of order+1 of the first partial diffractive profile, F.sub.2 is the focal length of the diffractive focal point of order+1 of the second partial diffractive profile, and F.sub.2 is an integer multiple of F.sub.1.

27. The intraocular lens of claim 26, wherein F.sub.2=2.Math.F.sub.1 or F.sub.2=3.Math.F.sub.1.

28. The intraocular lens of claim 1, wherein: the steps of the first partial diffractive profile are centered with respect to the optical axis within 5% of radial positions r.sub.n measured from the optical axis, the radial positions being: r.sub.n=√{square root over (2n.Math.λ.Math.F.sub.1)}; the steps of the second partial diffractive profile are centered with respect to the optical axis within 5% of radial positions r.sub.n measured from the optical axis, the radial positions being: r.sub.n=√{square root over (2n.Math.λ.Math.F.sub.1)}; wherein: n is the number corresponding to each step in the respective partial diffractive profile counted from the center of the profile, F.sub.1 is the focal length of the diffractive focal point of order+1 of the first partial diffractive profile, F.sub.2 is the focal length of the diffractive focal point of order+1 of the second partial diffractive profile, and F.sub.2 is an integer multiple of F.sub.1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic plan view of an IOL according to an embodiment of the invention,

(2) FIG. 2 is a schematic sectional view of the IOL according to FIG. 1, where diffractive focal points for near, intermediate and far vision, as well as a virtual respective focal point are shown,

(3) FIG. 3 is a schematic view of the diffractive profile for an IOL of the invention that can be generated by a superposition of the first and second partial profiles shown in FIGS. 4a and 4b,

(4) FIG. 4a is a schematic view of the first partial profile used in constructing the diffractive profile of FIG. 3,

(5) FIG. 4b is a schematic view of the second partial profile used in constructing the diffractive profile of FIG. 3,

(6) FIG. 4c is a close-up view of the first two steps of the profile of FIG. 3, in which a smoothening using a convolution with a Gaussian mollifier with two exemplary variances is shown,

(7) FIG. 5a shows the MTF at 50 cycles/mm as a function of diffractive power and for different pupil apertures for a trifocal IOL of the invention,

(8) FIG. 5b shows the MTF at 50 cycles/mm as a function of diffractive power and for different pupil apertures for a trifocal IOL according to prior art,

(9) FIG. 6a shows the distribution of light energy among the focal points for far, intermediate and near vision as a function of pupil aperture for the trifocal IOL according to an embodiment of the invention,

(10) FIG. 6b shows the distribution of light energy among the focal points for far, intermediate and near vision as a function of pupil aperture for the trifocal IOL according to prior art,

(11) FIG. 7 shows the longitudinal chromatic aberration (LCA) at the focal points for far, intermediate and near vision for two trifocal IOLs according to prior art and two trifocal IOL's according to the invention, wherein in each case, one of the IOLs is made from PMMA and one IOL is made from the applicants proprietary hydrophobic acrylic material GF as described in WO 2006/063994 A1, and

(12) FIG. 8 is a plan view of an IOL according to an embodiment of the invention, with an asymmetric design having an optical portion that is off-centered by 0.3 mm with regard to the geometrical center of the IOL.

DETAILED DESCRIPTION

(13) For the purposes of promoting an understanding of the principles of the invention, reference will now be made to a preferred embodiment illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated IOL and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates.

(14) The term “near vision” as used herein may e.g. correspond to vision provided when objects at a distance from the subject eye of between about 30 cm to 60 cm are substantially in focus on the retina of the eye.

(15) The term “far vision” may correspond to vision provided when objects at a distance of at least about 180 cm or greater are substantially in focus on the retina of the eye.

(16) The term “intermediate vision” may correspond to vision provided when objects at a distance of about 60 cm to about 150 cm from the subject eye are substantially in focus on the retina of the eye. Note also that predicting the most appropriate IOL power for implantation has limited accuracy, and an inappropriate IOL power can leave patients with what is referred to in the art as “residual refraction” following surgery. Accordingly, it may sometimes be necessary for a patient who has received an IOL implant to also wear spectacles to achieve good far vision.

(17) A general configuration of an intraocular lens 10 according to an embodiment of the invention is illustrated in FIGS. 1 and 2. As may be seen in these figures, the lens includes a central optical body 12 and, in this exemplary configuration, two flexible supports 14, so-called “haptics” (not shown in FIG. 2), on the outer edge of the lens 10 in order to support it in the capsular bag when it is implanted in the eye of a patient. However, other alternative configurations are known to one skilled in the art and applicable in an intraocular lens according to the invention, such as for example a larger number of haptics, loop-shaped haptics, etc.

(18) The intraocular lens 10 according to the illustrated embodiment of the invention is a lens of the diffractive type. The central optical body 12 includes an anterior face 16 and a posterior face 18, and has a substantially anteroposterior axis 20. The anterior and/or posterior faces 16, 18 have curvatures such that the lens 10 would direct a portion of the incident light onto a refractive focal point 22, or of “diffractive order zero”, on the optical axis. In other words, without any diffractive profile on the anterior or posterior surface 16, 18, incoming light beams that propagate parallel to the optical axis 20 from the left in FIG. 2 would be focused at the refractive focal point 22. However, as will be explained in more detail below, with the specific choice of diffractive profiles according to the invention, only very little light is actually directed to the refractive focal point. Graphically speaking, in preferred embodiments of the present invention, the refractive focal point 22 is a “deactivated” or a “virtual focal point”, which is indicated by the hatched lines in FIG. 2.

(19) In the embodiment shown, the lens 10 has an asphericity with an aspherical aberration of −0.11 μm at an aperture or pupils size of 5.0 mm. This asphericity ensures a natural balance between the sensitivity to contrast and the field depth by inducing a moderate positive spherical aberration of the aphakic eye, the average spherical aberration of the human cornea being around +0.28 micrometers. In an alternative embodiment, the asphericity may be higher allowing to compensate for the cornea aberration to a higher degree. This would allow for an even better image quality, albeit at the price of making the optical performance of the lens more sensitive toward lens decentration and tilt within the eye.

(20) On its anterior face 16, the lens 10 has a relief 24 resembling a diffractive profile, which is only schematically indicated in FIG. 1. The diffractive profile 24 is illustrated in FIG. 3 and formed by the superposition of a first diffractive profile 26, illustrated in FIG. 4a, and a second diffractive profile 28, illustrated in FIG. 4b. In FIGS. 3, 4a and 4b, all units on both axes are in μm. Accordingly, it is seen that in these figures, the height of the profiles is considerably exaggerated with respect to the radial distance r from the optical axis 20.

(21) The first diffractive profile 26 is a profile of the kinoform type, approximately fitting the function:

(22) $\begin{array}{cc}{H}_1\left(r\right)=a_1\left(1-\frac{r^3}{R^3}\right)\frac{\lambda }{2\pi }\left(\frac{1}{n_2-n_1}\right)\left(\mathrm{mod}\left[\left[F_1-\sqrt{r^2+F_1^2}\right)2\frac{\pi }{\lambda },2\pi \right]+\pi \right)& \mathrm{Eq}.1\end{array}$

(23) The term “kinoform profile” is e.g. explained in “Diffractive Optics-Design, Fabrication and Test” by Donald O'Shea et al., SPIE tutorial texts; TT62 (2004), and refers to diffractive optical elements whose phase-controlling surfaces are smoothly varying. This is different from so-called “binary optical elements” with a discrete number of phase-controlling surfaces, e.g. surfaces introducing a zero and a π phase difference on the incident wavefront. In this equation, H.sub.1(r) is the height of the first partial diffractive profile 26, as a function of the radial distance r relatively to the optical axis, R is the radial distance from the outer edge of the lens to the optical axis, λ, is the wavelength at which the eye has greatest sensitivity (normally 550 nm), n.sub.2 and n.sub.1 are refractive indexes of the material of the lens and of its implantation medium, a.sub.1 is an amplitude parameter (0.57 in the illustrated embodiment), and F.sub.1 is the focal length of the focal point of order +1 of this first partial diffractive profile 26 (555 mm for +1.8 diopters in this embodiment).

(24) The second partial diffractive profile 28 is also a profile of the kinoform type, approximately fitting the function:

(25) $\begin{array}{cc}{H}_2\left(r\right)=a_2\left(1-\frac{r^3}{R^3}\right)\frac{\lambda }{2\pi }\left(\frac{1}{n_2-n_1}\right)\left(\mathrm{mod}\left[\left[F_2-\sqrt{r^2+F_2^2}\right)2\frac{\pi }{\lambda },2\pi \right]+\pi \right)& \mathrm{Eq}.2\end{array}$

(26) In this equation H.sub.2(r) is the height of the second diffractive profile 28, as a function of the radial distance r with respect to the optical axis, a.sub.2 is an amplitude parameter (0.74 in the illustrated embodiment) and F.sub.2 is the focal length of the focal point of order +1 of this second partial diffractive profile 28 (1110 mm for +0.9 diopters in this embodiment).

(27) While equations 1 and 2 define first and second partial profiles 26, 28 having vertical steps and sharp edges defined by the modulo function, the edges of the actual profiles will be rounded, and the steps would be inclined rather than vertical. A suitable shape of the first and second partial profiles 26, 28 can be obtained by a convolution of the above profile functions H.sub.1(r) and H.sub.2(r) with a corresponding smoothening function, which is referred to as a “mollifier” in the art. There is a variety of suitable mollifiers that would lead to a desired smoothening or rounding of the sharp edges and inclination of the steps. In fact, as the skilled person will appreciate, any convolution will lead to a rounding of sharp edges and inclination of vertical steps of a step function.

(28) In a preferred embodiment, the mollifier M(r) can be represented by a Gaussian function as follows:

(29) $M\left(r\right)=\frac{1}{\sqrt{2{\mathrm{\pi \sigma }}^2}}\exp \left\{-\frac{r^2}{2{\sigma }^2}\right\}$

(30) The convolution of the profile function H(r) and the mollifier M(r) is defined in the usual manner as:
H*M=∫H(x)M(r−x)dx

(31) FIG. 4c shows the results of the convolution of the combined profile H(r) (see equation 3 below) with the mollifier M(r), where the variance σ.sup.2 is expressed in terms of a convolution parameter “conv”, which has the unit micrometers, as follows:

(32) ${\sigma }^2={\mathrm{conv}}^2.Math.\frac{1}{8.Math.{10}^6.Math..Math.\ln \left(0.5\right).Math.}$

(33) In FIG. 4c, examples of the result of the convolution for three values of conv, namely conv=0 μm, 25 μm and 50 μm, are shown. For conv=0 μm, the mollifier M(r) corresponds to the Dirac delta function, which leaves the original profile H(r) unaffected. For increasing values of cony, the edges of the steps are increasingly rounded, and the inclination of the originally vertical steps increases.

(34) Note that the rounding of the sharp profile steps by means of a convolution is already described in the aforementioned previous application WO'169, where the inclined steps and round edges can also be seen in FIGS. 3, 4a and 4b.

(35) The relief or “profile” 24 resulting from the superposition of both of these partial profiles 26, 28 therefore approximately fits the formula: Eq. 3:
H(r)=H.sub.1(r)+H.sub.2(r),
as illustrated in FIG. 3. In this embodiment F.sub.2=2.Math.F.sub.1, which means that every second step position of the first partial profile 26 coincides with a step of the second partial profile 28, or, in other words, that the second diffractive profile has an average spatial frequency half of the one of the first diffractive profile. The combined profile 24 therefore has large steps, resulting from the addition of a step of the first partial profile 26 with a step of the second partial profile 28, alternating with small steps, corresponding to one step out of two of the first partial profile 26.

(36) Note that in the case where the profiles are not apodized, the factor (1−r.sup.3/R.sup.3) in equations 1 and 2 is simply 1, as is the case in the embodiment shown herein.

(37) Further, in this way the focal point of order +2 of the second partial profile 28 coincides on the optical axis 20 with the focal point of order +1 of the first partial profile 26.

(38) In the embodiment shown in FIGS. 3, 4a and 4b, a.sub.1 is 0.57, and a.sub.2 is 0.74. This is very different from the embodiment shown e.g. in WO'169, where a.sub.1=0.44 and a.sub.2=0.27. This different choice of amplitudes leads to an entirely different optical behavior. In fact, it is seen that the IOL 10 has a focal point for far vision 30 (see FIG. 2) that coincides with the focal point of order +1 of the second partial diffractive profile 28, a focal point 32 for intermediate vision that coincides with the focal point of order +2 of the second partial diffractive profile 28, and also with the focal point of order +1 of the first partial diffractive profile 26, and a focal point for near vision 34 that coincides with the focal point of order +3 of the second partial diffractive profile 28.

(39) In an alternative embodiment, the steps of the second partial profile 28 could coincide with every third step of the first partial profile 26, in which case the diffractive focal point of order +1 of the first partial profile 26 would coincide with and contribute to the focal point for near vision 34.

(40) In the embodiment shown, only a negligible amount of light is focused on a position on the optical axis 20 that would correspond to the refractive focal point 22, or, in other words, the diffractive focal point of order 0.

(41) It should be appreciated that the first and second partial profiles 26, 28 are in a sense only virtual or “auxiliary” profiles that mainly serve to construct the “total profile” 24. In particular, it is not per se clear that a given focal point of a partial profile will also be present in the diffraction pattern of the total, combined profile. However, it is seen that if the coefficients a.sub.1 and a.sub.2 are properly chosen, the total profile 24 does exhibit diffractive focal points that can in fact be attributed to the diffractive focal points of the individual partial profiles 26, 28. Further, by properly choosing the factors a.sub.1 and a.sub.2, a distribution of energy between the different focal points of the total profile 24 can be partitioned in a very useful way, as will be demonstrated below.

(42) The inventors have found out that in embodiments of the present invention, the percentage of light directed to the focal point 34 for near vision depends in good approximation on the sum of a.sub.1 and a.sub.2, while the ratio of the percentage of light directed to the intermediate vision focal point 32 over the percentage of light directed to the far vision focal point 30 is essentially governed by the ratio a.sub.1/a.sub.2. Further, the inventors could derive empiric equations for estimating the light partition between the three focal points for near, intermediate and far vision as follows:
% Near=20*[(a.sub.1+a.sub.2)EXP(2*(a.sub.1+a.sub.2)/1.5)]  Eq. 4
% Inter/% Far=1*[(a.sub.1/a.sub.2)EXP(2*(a.sub.1/a.sub.2))]  Eq. 5
% Far=[100−Eq4]/[1+Eq5]  Eq. 6
% Inter=100−Eq6−Eq4  Eq. 7

(43) Herein, “% Near”, “% Inter” and “% Far” indicate the percentage of light energy directed to the respective focal point 34, 32, 30 for the near, intermediate and far vision, where the three percentages are chosen such as to add up to 100%. In other words, these equations only reflect the distribution of light between the respective focal points, but not the distribution of the light around the respective focal points.

(44) The above equations 4-7 are found to give fairly good predictions of the actual distribution of light, provided that 1<a.sub.1+a.sub.2<2 and 0.5<a.sub.1<1 and 0.5<a.sub.2<1.

(45) A way of estimating the optical priority of an intraocular lens comprises determining experimentally its modulation transfer function (MTF). The MTF of an optical system can e.g. be measured according to annex C of ISO 11979-2 and reflects the proportion of the contrast which is transmitted through the optical system for a determined spatial frequency of a test pattern, which frequency is defined as “cycles/mm” or “lp/mm”, “lp” denotes “line pairs”. Generally, the contrast decreases with an increase in spatial frequency. As a first approximation, the percentage of light (E f %) directed to a given focal point is obtained from the MTF peak values at 50 cycles/mm according to the following equation:
% Ef=MTF peak/(MTF far+MTF inter+MTF near)*100,  Eq. 8
with f denoting one of the far, the intermediate or the near focal point.

(46) In FIG. 5a, MTF curves of the trifocal lens 10 according to an embodiment of the invention versus the focal power in dioptres are shown for different pupil apertures in an eye model according to the ISO 1 standard, at 50 cycles/mm and with monofocal green light (543 nm). The dotted curve corresponds to a pupil size of 4.5 mm and shows three peaks corresponding to the focal point for far vision at 18.25 dpt, to the focal point for intermediate vision at 19.15 dpt and to the focal point for near vision at 20.05 dpt, respectively. The spacing in dioptres (dpt) between two consecutives MTF peaks is 0.9 dpt, thus corresponding to two powers additions of +0.9 dpt and +1.8 dpt with respect to the far focus, respectively. For this lens at 4.5 mm aperture, the distribution of the light between the three focal points is 46.67% for far vision, 33.33% for near vision and 20% for intermediate vision.

(47) This is in good agreement with the distribution of light according to equations 4, 6 and 7 above, which would yield a distribution of 45.06% for far vision, 22.89% for intermediate vision and 32.05% for near vision. Accordingly, it is seen that the empiric equations 4 to 7 capture the distribution of light among the focal points quite well.

(48) It is further seen in FIG. 5a that for a pupil aperture of 4.5 mm, rather little light is directed elsewhere than on these three focal points, and in particular that little light is directed to the position at 17.35 dpt corresponding to the refractive or “zero order” focal point, which is indicated in FIG. 5a for illustration purposes only. It is therefore seen that the zero order focal point is only a “virtual focal point” or “deactivated”.

(49) FIG. 5a further shows the MTF curve at 50 cycles/mm for a pupil aperture of 3.75 mm in the chain-dotted line, for a pupil aperture of 3.0 mm in the solid line and for a pupil aperture of 2.0 mm in the dashed line. As can be seen from FIG. 5a, by decreasing the pupil aperture from 4.5 mm to 3.0 mm, the MTF peaks for near and intermediate vision merge into a broader single peak, so that at these small pupil apertures, the IOL essentially becomes bifocal. By further constricting the pupil aperture to 2.0 mm, the two residual MTF peaks give rise to a single very broad and very high peak. This can be attributed at least partly to the well-known “pinhole” diffraction, which becomes more significant at small apertures, wherein the light wavefront is then affected to a larger degree by the edges of the hole.

(50) It is worth noting that this pin-hole diffraction contributes to an extended depth of focus, i.e. for smaller pupil apertures, the MTF drops increasingly less between the focal points. At a pupil aperture as low as 2.0 mm, the pin-hole effect is maximized, and the MTF stays above 0.2 in the entire range between 18 dpt and 20.5 dpt, i.e. throughout the entire range from near to far vision. It is further seen that at small pupil sizes such as 2.0 mm, the MTF at 18.25 dpt (far vision) drops considerably, while the MTF at near and intermediate vision (20.05 dpt and 19.15 dpt) dramatically increase. This is also seen in FIG. 6a, where the percentage of light directed to any given focal point of the IOL according to the embodiment of the invention is shown as a function of pupil aperture, where the percentage of light is related with the MTF in the way defined in equation 8 above.

(51) As is seen in FIG. 6a, for large pupil apertures (4.5 mm), the fraction of light directed to the focal point for far vision exceeds the fractions for near and intermediate vision, while with decreasing aperture, the fraction of light directed to the focal points for near and intermediate vision increases, while the fraction of light directed to the focal point for far vision decreases, and in fact drops below that of the other two focal points. This behavior is unusual for trifocal IOLs, but in fact highly advantageous, because far vision is often needed under poor light conditions, where the pupil size tends to be large due to the natural pupil accommodation reflex, while near and intermediate vision are typically needed under good light conditions, for example when reading a book or working on a computer. The IOL according to preferred embodiments of the invention hence meets both demands extremely well. In particular, providing more light at the focal point for far vision than for near and intermediate vision under poor light conditions, should improve the image quality by limiting photic phenomena, such as halos, under large pupil apertures and mesopic conditions, the out of focus and closer images being less intense.

(52) The behavior of the IOL of the invention shall be compared with that of the trifocal IOL of WO'169, where the MTF is shown for comparison in FIG. 5b, and the distribution of light energy among the respective focal points is shown in FIG. 6b. Note that in the embodiment according to WO'169, apodisation was used. As can be seen from FIG. 5b, similar to the IOL of the invention, the peaks corresponding to the focal points for near and intermediate vision merge when the pupil size decreases from 4.5 mm to 3.0 mm, and the depth of focus increases. However, unlike the IOL of the invention, in the prior art trifocal lens of WO'169, without apodisation the relative distribution of light among the three focal points is approximately independent of the pupil size (see FIG. 6b). Accordingly, this prior art IOL without apodisation does not allow the far vision being dominant at low light conditions (large pupil apertures) and the near vision being dominant at good light conditions (small pupil apertures) in the same lens.

(53) A further advantage of the trifocal IOL 10 of the invention is that it allows to diminish or correct longitudinal chromatic aberration (LCA). FIG. 7 shows the longitudinal chromatic aberration (LCA) at the focal points for far, intermediate and near vision for two IOLs according to the invention and two IOLs according to WO'169. Herein, “LCAf” denotes the longitudinal chromatic aberration at a given focal point (f), where “f” represents a respective one of the focal points (i.e. far, intermediate or near vision). Each of these focal points corresponds to an additional optical power as compared to the focal point for far vision in diopters, which are indicated on the horizontal axis of FIG. 7. Accordingly, in the exemplary embodiment, focal points for far vision correspond to 0 dpt, focal points for intermediate vision correspond to 0.9 dpt and focal points for near vision correspond to 1.8 dpt on the horizontal axis of FIG. 7.

(54) The numeric value of LCAf is obtained by the shift of the MTF-peak measured on an optical bench at 50 cycles/mm and a pupil aperture of 4.5 mm, expressed in diopters, when the light changes from monochromatic red (650 nm) to monochromatic blue (480 nm). This shift can be measured for each of the three MTF-peaks corresponding to the three focal points, and the results are shown in FIG. 7.

(55) In FIG. 7, the solid lines indicate the values of LCAf for two IOLs according to WO'169 made from different materials, namely PMMA (cross symbol) and GF (dots), where GF is a proprietary hydrophobic acrylic material of the present applicant as disclosed in WO2006/063994 A1. The Abbe numbers of PMMA and GF are 53.23 and 42.99, respectively. The Abbe number is a measure of the material's dispersion, i.e. the variation of its refractive index with wavelength, where high values indicate low dispersion. In the trifocal lenses of WO'169, the focal point for far vision (0 dpt in FIG. 7) is a purely refractive focal point. At 0 dpt, both of the prior art trifocal lenses show a positive value for LCAf, amounting to 0.3 dpt in case of PMMA and 0.65 dpt in case of GF. A positive value of LCAf is expected, because for these materials, the index of refraction increases with decreasing wavelength, so that the refractive optical power for blue light is larger than the refractive optical power for red light. Moreover, a higher value for LCAf is found for the GF-lens as compared to the PMMA-lens, due to its smaller Abbe number.

(56) In the prior art IOLs of WO'169, the focal point for near vision (at 1.8 dpt) corresponds to the diffractive focal point of order +1 of a first partial diffractive profile, to which a contribution of the focal point of order +2 of a second partial diffractive profile is added. The focal point for the intermediate vision (at 0.9 dpt) corresponds to the diffractive focal point of order +1 of the second partial diffractive profile. As was explained in the summary of the invention, the LCA for diffractive focal points is “negative” in the sense that the diffractive optical power increases with increasing wavelength. Accordingly, the negative LCA at the diffractive focal points lowers the total LCAf at the focal points for intermediate vision to 0.05 (PMMA) and 0.40 (GF), and even further lowers the total LCAf at the focal points for near vision to 0.08 (PMMA) and 0.15 (GF).

(57) Further shown in FIG. 7 with broken lines are the values for LCAf for two IOLs according to the invention, where the cross-symbols again represent an embodiment based on PMMA and the dot-symbols represent an embodiment in GF. As can be seen in FIG. 7, for the IOL of invention, the LCAf curves are vertically shifted to lower values as compared to the respective prior art IOL made of the same material. In particular, for the focal points for far vision (0 dpt), the value LCAf for the GF-lens is lowered to 0.4 diopters and the LCAf value for the PMMA lens is lowered to −0.03 dpt, which means that there is practically no longitudinal chromatic aberration for the focal point for far vision in this PMMA-based embodiment of the invention.

(58) The reason why the LCA at the focal point for far vision is reduced as compared to the prior art trifocal lens of WO'169 is that according to the invention, the focal point for far vision is a diffractive focal point, namely a focal point of order +1 of the second partial profile, which therefore provides for a negative LCA, that compensates at least partially the positive LCA due to the refractive power of the lens. It is therefore seen that particularly if the GF material is to be used, the trifocal lens of the invention is clearly favorable with regard to LCA as compared to the prior art trifocal lens of WO'169.

(59) As regards the prior art IOL based on PMMA, the average value of LCAf is already quite low, with moderately positive values at the focal point for far vision, moderately negative values at the focal point for near vision and almost vanishing longitudinal chromatic aberration at the focal point for intermediate vision. In fact, the LCA of the prior art PMMA trifocal lens is similar to that of the trifocal lens of the invention based on GF. The PMMA-version of the trifocal IOL of the invention has the benefit of vanishing LCA for far vision, although at the price of a more negative LCA of −0.7 dpt at the focal point for near vision. Negative values of LCAf for near vision can even be favorable for correcting the aphakic eye LCA, i.e. cornea LCA.

(60) Trifocal IOLs are supposed to lead to an extended range of vision (EROV), from far vision (e.g. +0 dpt) to near vision (e.g. +1.8 dpt), without a discontinuity or significant gap of vision for the intermediate distance. From the MTF diagrams of FIGS. 5a and 5b, it is seen that such an EROV is indeed obtainable with the trifocal lenses of the invention as well as with the IOLs of WO'169. The EROV performance of a lens can be assessed under vital conditions in a more direct way by capturing the USAF targets by “defocusing” the target, i.e. by displacing the US target along the optical axis of the IOL while recording the object image. The applicant has systematically captured USAF-images for the IOL of the invention as well as the IOL of WO'169, for different wavelengths (green, red and blue) and for different pupil apertures (2.0, 3.0, 3.75 and 4.5 mm). It was confirmed that for monochromatic green light, both, the IOL of the invention as well as the IOL of WO'169 exhibit an EROV from 0 dpt to +2 dpt with constant image quality. In particular, both trifocal IOLs were superior to a commercially available bifocal IOL, which showed a degradation of the image quality between 0.75 dpt. and 1.25 dpt, especially for pupil apertures of more than 2.0 mm.

(61) When the light source was changed from green light to red or blue light, it appeared that a commercially available diffractive bifocal lens with two diffractive focal points becomes essentially monofocal for far and near distance in the red and blue light, respectively, with corresponding image quality degradation at near and far distances, respectively. In contrast to this, the two trifocal IOLs according to WO'169 and according to the invention remain trifocal both in blue and red light, with a fully EROV from 0 dpt to 2.25 dpt, although the image quality is slightly affected at far distances for blue light and near distances for red light, as compared to the performance for green light.

(62) Moreover, when comparing the USAF images of the IOL of the invention with those of the IOL according to WO'169, it is seen that the image quality for the IOL of the invention is superior for far vision at large pupil apertures (such as 4.5 mm), and for near vision at small pupil apertures, as was to be expected from the comparison of FIGS. 5a and 5b, and from the comparison of FIGS. 6a and 6b. Namely, as shown therein, the IOL of the invention favors near vision at small pupil apertures and far vision at large pupil apertures, in contrast to the IOL of WO'169, where the distribution of light among the focal points is largely independent of the pupil size.

(63) While longitudinal chromatic aberration of the eye can be corrected by an optical element with longitudinal chromatic aberration equal and opposite to that of the eye, alignment of such elements is critical, as otherwise an additional transverse chromatic aberration is induced, which is proportional to the decentration (see Zhang X, Bradley A, Thibos L N. Achromatizing the human eye: the problem of chromatic parallax. J Opt Soc Am, 1991; 8:686-91). However, the human pupil center is not located concentrically to the center of the capsular bag and it is not coaxial with the optical and visual axes In the vicinity of the visual axis, which joins the fixation point to the fovea by way of the nodal points, the correction of longitudinal chromatic aberration does not result in the induction of transverse chromatic aberration. In an embodiment, the haptics of the intraocular lens (IOL) optic can be advantageously designed to be asymmetrical, in order to allow the optical center of the IOL to be coincident with the presumed location of the visual axis, or the center of the entrance pupil. FIG. 8 is a schematic plan view of an IOL 10 according to an embodiment of the invention, in which the optical portion, i.e. the diffractive profile 24 is off-centered by 0.3 mm with regard to the outer diameter of the IOL.

(64) Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that modifications and changes may be carried out on these examples without modifying the general scope of the invention as defined by the claims.

(65) For example, in alternative embodiments, an intraocular lens according to the invention may have different diffractive profiles, other than kinoforms, or exhibit different ratios between the periodicities and distances of the steps of the two superposed partial diffractive profiles. The partial diffractive profiles may also be superposed only on a portion of the anterior or posterior surface of the lens. The lens may also have different curvatures on its anterior and/or posterior faces, or no curvature, and these curvatures may, depending on the needs, either be aspherical or not. Moreover, other combinations of diffractive orders can be considered in order to achieve the three focal points, especially orders of 1 unit superior to those of the lens according to the invention described here above. In this particular case, the step height would obey the condition 2<a.sub.1+a.sub.2<3.

(66) Although a preferred exemplary embodiment is shown and specified in detail in the drawings and the preceding specification, these should be viewed as purely exemplary and not as limiting the invention. It is noted in this regard that only the preferred exemplary embodiment is shown and specified, and all variations and modifications should be protected that presently or in the future lie within the scope of protection of the invention as defined in the claims.