Abstract:
A multifocal intraocular lens is provided having two external refractive surfaces and a longitudinal axis, and a diffractive structure superimposed on one of the surfaces in the form of a Fresnel zone. To reduce eye traumatism during surgery, to shorten post-surgical time period, to increase visual acuity, and to ensure a constant quality image at any distance from an object, a multifocal intraocular lens is proposed in which at least one additional refractive surface is inserted between the two external refractive surfaces, wherein the at least one additional refractive surface divides the lens volume into zones made of materials having different refraction coefficients.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to medicine or, more specifically, to ophthalmology, and is intended for eyesight correction by implantation in an mammal eye, especially human eye. 
         [0002]    It is known that there exist hybrid type multifocal lenses, which have both a refractive and a diffractive part (See U.S. Pat. No. 4,637,697 Multifocal Contact Lenses Utilizing Diffraction and Refraction; International Patent Application Publication No. WO/2004/113959 Bifocal Multiorder Diffractive Lenses For Vision Correction; U.S. Pat. No. 5,089,023 Diffractive/Refractive Lens Implant; U.S. Pat. No. 7,025,456 Diffractive Lenses For Vision Correction; Russian Patent RU 2,303,961 Multifocal Intraocular Lens; and U.S. Pat. No. 5,344,447 Diffractive Trifocal Intraocular Lens Design (prototype)). 
         [0003]    The drawback of this invention (as well as of all diffractive-refractive intraocular lenses (IOL), irrespective of their design features) lies in the fact that the thickness of the hybrid IOLs of this type does not differ from and therefore cannot be smaller than the thickness of the ordinary monofocal refractive IOLs, both planoconvex and biconvex refractive lenses, on one of which there is a diffractive microstructure in the form of rings, the radii of which coincide with the radii of the Fresnel zones. Then, in the case of such an IOL type, the refractive component of the lens has the optical power D calculated for distance vision, that is in the same way as for ordinary monofocal IOL. The additional optical power of the diffractive component is made to be not more than ΔD=3-4 diopters, in order to provide for the accommodation depth necessary for near vision at a distance of 25-35 cm. In the case of diffractive-refractive IOLs the light flux is directed into two focuses, for distance vision (the optical power equals D) and for near vision (the optical power equals D+ΔD). 
         [0004]    A reduction in traumatism of the eye in the process of cataract extraction with IOL implantation, in actual fact, can be efficiently achieved only by way of using smaller and smaller incisions—less than 1.5 mm. At the present time there are phacoemulsificators that make it possible to do cataract extraction through such small incisions, but there are no (and there cannot be any) refractive IOLs that could be implanted through the small size incisions, including manufacture of diffractive-refractive IOLs under all the above mentioned patents. In fact, this creates a ban (a barrier) for utilizing this principle to create multifocal (pseudo-accomodating) IOLs designed for implantation through super small incisions. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention accomplishes the objects to reduce eye traumatism during the surgery, to shorten post-surgical time period, to increase visual acuity, and to ensure a constant quality image at any distance from an object. 
         [0006]    These objects are solved by a multifocal intraocular lens (IOL) which comprises two external refractive surfaces and a longitudinal axis, with a diffractive structure superimposed on one of them in the form of the Fresnel zone, wherein between its external refractive surfaces at least one additional refractive surface is inserted, which divides the lens volume into zones made from materials having different refraction coefficients. 
         [0007]    Unlike in known solutions, the diffractive component of such IOL is much bigger than the known ones have, and together with the refractive part it ensures light entry into one focus—for distance vision. For example, if the optical power of the refractive part of the IOL equals 10 diopters, and the diffractive part equals 10 diopters as well, then the optical power of that IOL equals 20 diopters. Such an IOL will have a thickness that is half as large as an ordinary refractive lens with the same optical power. Separation of light energy between focuses for distance vision and near vision is carried out on account of creation of inside refractive surfaces, which divide the optical part of the IOL into zones made of different materials with different refraction coefficients. 
         [0008]    The technical result of using the invention enables making multifocal (pseudo-accommodating) IOLs with the thicknesses approximately two times smaller than those of ordinary (refractive) monofocal IOLs and of all hybrid, diffractive-refractive IOLs. 
         [0009]    According to an embodiment of the invention, the proposed multifocal intraocular lens comprises two external refractive surfaces, on one of which there is a diffractive structure in the form of rings, and between the external refractive surfaces additional refractive surfaces are inserted dividing the lens volume into zones made of materials with different refraction coefficients. Each zone may have a refraction coefficient being different from any other refraction coefficient of any other zone. 
         [0010]    The zones may be arranged one after the other generating at least substantially parallel layers made of different materials, which differ in refraction coefficient in such a manner that centers of all surfaces coincide with the optical axis of the lens. 
         [0011]    One of the two external refractive surfaces of the lens may be a sphere, preferably a radial sphere, and the other one may be a plane with a diffractive structure in the form of rings superimposed all over its surface, the radii of the rings coinciding with the radii of the Fresnel zones. Between the two external refractive surfaces there is exactly one surface dividing the volume of the lens into two zones made of two different materials with different refraction coefficients, according to one embodiment. 
         [0012]    According to another preferred embodiment, there are two surfaces dividing the volume of the lens into three zones made of three different materials with different refraction coefficients, wherein adjacent zones may have different refraction coefficients or each zone may have a refraction coefficient being different from the refraction coefficient of any other zone. In principle, there may be a number m of surfaces dividing the volume of the lens into a number of m+1 zones, with m≧2. 
         [0013]    The difference in the refraction coefficients of the different materials used to make the lens should not be ≦0.02. At the same time, all the materials must have a refraction coefficient that is not less than 0.02 bigger than refraction coefficient of the ocular fluid. That is, the upper limit of a material&#39;s refraction coefficient should not be less than 1.336+0.02=1.356 (1.336 is the refraction coefficient of ocular liquid according to reference data). In theory, the maximum difference may comprise up to 0.64 (if the refraction coefficient of the material equals 2.0, then 2.0−1.356=0.64; meanwhile, materials with a maximum refraction coefficient of 1.55 are used; that is, the actual maximum difference comprises 0.194). Herein, it is better not to use specific figures but to specify an interval in terms of dependence on the refraction coefficient of material used. So, making definitions: n is a refraction coefficient of ocular fluid, n1 is a refraction coefficient of an external refractive surface (refractive-diffractive), n3 is a refraction coefficient of the second internal additional refractive surface. Then, the refraction coefficient of the material of the first internal additional refractive surface lies in the interval n+0.02&lt;n2&lt;n1−0.02, the material refraction coefficient of the second internal additional refractive surface lies in the interval n1+0.04&lt;n3&lt;n2−0.02. 
         [0014]    The diffractive structure in the form of rings, the radii of which coincide with the radii of the Fresnel zones, is preferably manufactured in such a way, preferably based on appropriate calculation, that the additional optical power of this structure is preferably ensured within the interval of 8 to 12 diopters. 
         [0015]    The statistical average for the human eye refraction is 20-22 D (diopters). The additional optical power of the diffractive part being from 8 to 12 D makes it possible to decrease the thickness of the lens significantly, e.g., by half or in a corresponding different magnitude depending on the additional optical power. 
         [0016]    The microrelief of the diffractive structure may be in the form of at least substantially right-angled profile grooves for every only even or only odd Fresnel zones with the depth 
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         [0000]    micrometers, with fractional accuracy of deviation to be not more than 5% 
         [0017]    Alternatively, the microrelief of the diffractive structure may be at least substantially in the form of triangle profile grooves uniting every two neighboring Fresnel zones with the height of the triangle 
         [0000]    
       
         
           
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         [0000]    micrometers with fractional accuracy of deviation to be not more than 5%. Where n1 in either case is the refraction coefficient of the lens zone, which has the diffractive structure on it, n is the index of refraction of ocular fluid. 
         [0018]    In certain cases, the microrelief structure may be in the form of or may comprise grooves having a cross section of intricate profile, for example along the line of a sinusoidal function. 
         [0019]    The triangle profile lens may have two diffraction maximums—the zeroth-order diffraction maximum (0) and the plus first-order diffraction maximum (+1). The right-angled profile lens may have three diffraction maximums—the zeroth-order diffraction maximum (0), the plus first-order diffraction maximum (+1) and the minus first-order diffraction maximum (−1), Varying the depth of the microrelief makes it possible to alter power distribution among the diffraction foci of the lens. According to this invention, the depths of the grooves are selected in such a manner that in the case with the triangle profile, all the energy would be concentrated only in the first order (+1) maximum, and with the right-angled profile it would be concentrated in the plus first-order (+1) and minus first-order (−1) diffraction maximums. In the case with the right-angled profile with image formation on the retina, only the plus first-order maximum (+1) plays the main part; the minus first-order maximum (−1) does not influence the image quality due to a long distance from the plus first-order maximum (+1). 
         [0020]    The Fresnel zone radii, calculated without taking into account the spherical aberration of the optical system of the eye, are directly proportional to the square roots of the integers designating the Fresnel zone&#39;s index number rk=r1 √k, where k=1,2,3 . . . , r1 is the radius of the first Fresnel zone calculated in such a way that the prescribed optical power of the diffractive structure is ensured within the interval of 8 to 12 diopters. When calculating the radii of the Fresnel zones according to the formula rk=r1 √k, where k=1,2,3 . . . , the spherical aberration of the optical system of the eye affects the image quality insignificantly, only when the diffractive structure is superimposed not all over the entire external plane surface of the lens, but only in its central part. The radii of the Fresnel zones superimposed all over the external plane surface of the lens should be calculated taking into account the spherical aberration of the entire optical system of the eye. 
         [0021]    This kind of calculation can only be done with the help of known procedures of computer modelling of the entire optical system of the eye, which makes it possible to reduce or to minimize the spherical aberration of the optical system of the eye, including the cornea and all the refractive surfaces of the crystalline lens. 
         [0022]    The IOL may contain either one additional refractive surface in its central part, which is represented by a spherical segment with the diameter d 1  within the range of 1.6 to 2.6 mm, or 1.7 to 2.5 mm, preferably 1.8 to 2.4 mm or 2.0 to 2.2 mm, further changing radially outwardly into a plane. Such construction of the additional refractive surface avoids undesirable optical effects, such as a ‘halo’ with any intensity of the light on account of ‘superfluous’ light that appears on the boundary line of two zones with different refractive coefficients and goes beyond the circumference of the lens. Alternatively, two additional refractive surfaces may be provided, the first of which, counting from the external surface of the lens with the diffractive structure, is located in its central part and is represented by a spherical segment with the diameter d 2  within the range of 1.4 to 1.8 mm, or 1.5 to 1.7 mm, further changing into a plane, and the second additional surface in its central part is represented by a spherical segment with the diameter d 3  within the range of 2.1 to 2.6 mm or 2.2 to 2.5 mm or 2.3 to 2.4 mm into a plane. 
         [0023]    The thickness of the planes hpl ( FIG. 6 ) mentioned above may be in the region of 25 to 150 micrometers or 50 to 100 micrometers. The thickness of the planes hpl may be the thickness of the flat end of the internal lens. The thickness of the planes does not influence the image quality. In the optical part the thickness may be variable, determined by the refraction needed for each patient. 
         [0024]    The total thickness of the end face of the lens h 0  may be 200-250 micrometers ( FIG. 6 ), in some instances, less preferred, 100-500 micrometers or 150-350 micrometers. Thickness h 0  may refer to the overall thickness of the flat end of the lens without taking into consideration hMAX, i.e., the thickness of the diffractive microrelief. Smaller thickness does not allow placing all the planes of additional refractive surfaces inside the lens, generated by two external reflective surfaces, and a greater thickness of the end face of the lens leads to the complication of incurvation and implantation into the eye of a patient. 
         [0025]    The thickness of the convex portion of the lens depends on the radii of its spherical surface, which in turn is calculated in such a way that with target refractive coefficients of internal surfaces to provide partition of light energy between focuses with a difference of 3-4 diopters. 
         [0026]    The multifocal lens according to the invention may be used in other fields than surgery, if appropriate, e.g., in systems wherein the lens is immersed or embedded with one or both external refractive surfaces in a fluid, wherein the fluid preferably may have a refractive coefficient like water (pure water) or aqueous solutions, including solutions having a refractive coefficient about 1.28 to 1.4, especially 1.30 to 1.37, most preferably about or equal 1.336. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0027]    The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
           [0028]      FIG. 1  is a diagram showing the optical part of the lens with the right-angled profile diffractive structure and two additional refractive surfaces dividing the volume of the lens into three zones made of three different materials having different refraction coefficients n1, n2, n3. In the Figures the reference numerals designate:  1 —the external refractive surface of the lens;  2 —the external refractive surface with a diffractive structure;  3 —the microrelief of the right-angled profile diffractive structure;  4 —the first internal refractive surface;  5 —the second internal refractive surface;  6 —the zone with the refraction index of the material n1;  7 —the zone with the refraction index of the material n2,  8 —the zone with the refraction index of the material n3; hmax—the diffractive structure microrelief depth. 
           [0029]      FIG. 2  is a diagram showing the ring-type diffraction zones on the plane surface of the lens, calculated with the help of computer modelling, taking into account the spherical aberration of the optical system of the eye, superimposed all over the external plane surface of the lens. In the Figures the reference numerals designate:  9 —the central ring-zone with a radius r1;  10 —concentric ring-type diffraction zones with radii r2 . . . rk. 
           [0030]      FIG. 3  is a diagram showing the ring-type diffraction zones on the plane surface of the lens, calculated according to the formula rk=r1 √k, where k=1,2,3 . . . , superimposed on the central part of the plane surface of the lens to minimize the spherical aberration. 
           [0031]      FIG. 4  is a graphical representation of the dependency of the diffractive structure rings radii on their numbers: curve  1  represents the Fresnel zones&#39; radii calculated according to the formula rk=r1 √k, where k=1,2,3 . . . ; curve  2  represents the Fresnel zones&#39; radii calculated with the help of computer modelling, taking into account the spherical aberration of the optical system of the eye. 
           [0032]      FIG. 5  is a diagram showing the lens with the right-angled profile diffractive structure and one additional refractive surface ( 4 ) dividing the lens volume into two zones ( 6  and  7 ) made of two different materials having different refraction coefficients n1 and n2, respectively. 
           [0033]      FIG. 6  is a diagram showing the lens with the triangle profile diffractive structure and two additional refractive surfaces ( 4 , 5 ) dividing the lens volume into three zones ( 6 , 7 , 8 ) made of three different materials with different refraction coefficients n1, n2, and n3, respectively. 
           [0034]      FIG. 7  is a graphical representation of the light intensity distribution produced by the optical system of the eye with a bifocal diffractive-refractive lens with the right-angled profile of microrelief. 
           [0035]      FIG. 8  is a diagram showing the additional refractive surface in the form of a spherical segment with the diameter d 1  within the range of preferably 2.0 to 2.2 mm, further changing into a plane. 
           [0036]      FIG. 9  is a diagram showing two additional refractive surfaces, the first of which, counting from the external surface of the lens with the diffractive structure, is located in the central part and is represented by a spherical segment with the diameter d 2  within the range of preferably 1.7 to 1.8 mm, further changing into a plane, and the second additional surface in the central part of the lens is represented by a spherical segment with the diameter d 3  within the range of preferably 2.4 to 2.5 mm, further changing into a plane. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    A proposed intraocular lens variant is depicted in  FIG. 1 . The lens has a plano-convex shape formed by two external refractive surfaces, one of which is represented by a sphere ( 1 ), and the other one is represented by a plane ( 2 ) with a diffractive structure microrelief in the form of rings superimposed all over its surface, the radii of these rings coinciding with the radii of the Fresnel zones ( 3 ). Inside the lens there is one ( 4 ) or two refractive surfaces ( 4 ,  5 ) represented by spheres. The external refractive surface, represented by a sphere, creates the main optical power by refraction phenomena. The additional optical power is provided by diffraction on the diffractive structure microrelief ( 3 ) and refraction on one or two internal surfaces ( 4 ,  5 ). 
         [0038]    The microrelief is superimposed on the plane surface of the lens ( 2 ) in such a way that ring-type diffractive zones are formed on its surface ( FIG. 2 ): the central zone ( 9 ) having the radius n and the ring-type concentric zones ( 10 ) with the radii r2, . . . rk. The Fresnel zones&#39; radii depicted in  FIG. 2  have been calculated with the help of computer modelling, taking into account the spherical aberration of the optical system of the eye, in such a way that the prescribed optical power of the diffractive structure is ensured within the interval of 8 to 12 diopters. The statistical average for the human eye refraction is 20-22 D (diopters). The optical power of the diffractive part being 8 to 12 D makes it possible to decrease the thickness of the lens approximately by half. The diffractive structure, similar to the one depicted in  FIG. 2 , provides for additional optical power of 10 diopters on condition that the radius of the first ring-type Fresnel zone r1=0.25 mm. 
         [0039]    The number and positioning of the diffraction zones depend on the needed value of the additional optical power that the lens needs to provide, the diameter of the lens, the light wave length, and the degree of influence on the spherical aberration of the optical system of the eye. The proposed lens variant depicted in  FIG. 2  and the variant depicted in  FIG. 3  differ from each other in the ways of minimizing the influence on the diffraction image of the spherical aberration of the optical system of the eye. The lens proposed in  FIG. 2  has the diffractive structure superimposed on almost its entire plane surface. The elimination of the spherical aberration&#39;s influence is achieved, in this case, by selecting, with the help of computer modelling, a special law regulating the dependency of the diffractive relief rings&#39; radii on the rings&#39; numbers. 
         [0040]    The lens proposed in  FIG. 3  has the diffractive structure superimposed on just the central part of the plane surface of the lens. This kind of the proposed lens design makes it possible to minimize the spherical aberration&#39;s influence on the diffraction image. This is illustrated by  FIG. 4 , which shows the dependencies of the rings&#39; radii on their numbers, calculated both according to the formula rk=r1 √k (curve  1 ) and with the help of computer modelling, taking into account the spherical aberration (curve  2 ). 
         [0041]    In  FIG. 4  it is evident that in the central part of the lens, where the spherical aberration is small, both of the curves almost coincide; if the diffractive relief ( 3 ) is superimposed only on the central part of the plane surface of the lens, then the spherical aberration&#39;s influence on the diffraction image will be insignificant. The design in  FIG. 3  actualizes this very way of minimizing the spherical aberration&#39;s influence on the diffraction image. 
         [0042]    One of the variants of the proposed lens has the right-angled profile of the diffractive structure ( FIG. 5 ). A lens with the right-angled profile of the diffractive structure without any additional refractive surfaces provides three diffraction maximums—the plus first-order diffraction maximum (+1), the zeroth-order diffraction maximum (0), and the minus first-order diffraction maximum (−1). 
         [0043]    Another variant of the proposed lens has the triangle profile of the diffractive structure ( FIG. 6 ). A lens with the triangle profile of the diffractive structure without any additional refractive surfaces provides two diffraction maximums—the plus first-order diffraction maximum (+1) and the zeroth-order diffraction maximum (0). 
         [0044]    The power distribution among the diffraction maximums may vary. The power distribution is influenced by the depth of the diffractive structure microrelief hmax ( FIG. 6 ). 
         [0045]    The depth of the right-angled diffractive structure microrelief is determined with the help of computer modelling in such a way that the intensity of the plus first-order (+1) diffraction maximum and of the minus first-order (−1) diffraction maximum be at their maximum levels, and the intensity of the zeroth-order (0) diffraction maximum be equal to zero. With the depth of the right-angled profile microrelief calculated according to the formula 
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         [0000]    micrometers (n1 is the refraction index of the lens zone that has the diffractive structure on it, n is the refraction index of ocular fluid equal to 1.336), the minus first-order (−1) diffraction maximum is located beyond the retina and is not involved in the image formation, the intensity of the zeroth-order (0) diffraction maximum with the calculated microrelief depth equals zero, so this maximum does not influence the quality of the image formed by the lens either, only the plus first-order (+1) diffraction maximum participates in forming the image on the retina. For the proposed lens the microrelief depth constitutes 1.65 micrometers. 
         [0046]    The depth of the triangle profile of the diffractive structure microrelief for the proposed lens is calculated according to the formula 
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         [0000]    micrometers {n1 the refraction index of the lens zone, which has the diffractive structure on it, n is the refraction index of ocular fluid equal to 1.336) ( FIG. 6 ). With the calculated microrelief depth, the intensity of the zeroth-order (0) diffraction maximum equals zero, and this maximum does not influence the quality of the image formed by the lens; practically all of the power is concentrated within the plus first-order (+1) diffraction maximum actually forming the image on the retina. For the proposed lens the triangle profile microrelief depth constitutes 3.3 micrometers. 
         [0047]    In one of its variants ( FIG. 5 ) the proposed lens has one additional internal refractive surface ( 4 ), which divides the lens volume into two zones ( 6 ) and ( 7 ) made of materials with different refraction coefficients m, n2, with a right-angled profile microrelief on the plane surface of the lens. In this case each diffraction maximum bifurcates due to the additional refractive surface in the central part of the lens. One part of the light flux going though the central part of the lens goes through two spherical refractive surfaces and forms diffraction maximums in one set of places on the longitudinal axis L. The other part of the light flux, going through the peripheral part of the lens, encounters on its way only one external spherical refractive surface and forms diffraction maximums in another set of places on the longitudinal axis L. Thus, this lens variant provides bifocal vision by using the plus first-order (+1) bifurcated diffraction maximum. 
         [0048]    For example, the axial light intensity distribution for this variant, received with the help of computer modelling of the optical system of the eye, is depicted in  FIG. 7 . In particular, in  FIG. 7  one can see that on the retina (the retina coordinate is 23.5 mm as related to the frontal surface of the cornea) there is one of the two diffraction maximums of the plus first-order (+1) that provides a sharp image of distant objects. The zeroth-order maximum is completely suppressed by the microrelief depth that has been selected. The two minus first-order (−1) maximums are far beyond the retina and beyond the drawing in  FIG. 7 . Further, in  FIG. 7  one can see that the plus first-order diffraction maximum is divided into two approximately equal intensity maximums. This division is conditioned by the influence of the additional internal refractive surface ( 4 ), because of which both the central and the peripheral parts of this lens focus light in two different points on the optical axis. The second maximum provides the near vision (at a 30-33 cm distance). 
         [0049]    In the other variant ( FIG. 1 ) the proposed lens has two additional internal refractive surfaces ( 4 ,  5 ) that divide the lens volume into three zones ( 6 ,  7 ,  8 ) made of materials with different refraction coefficients n2, n3, with the right-angled profile microrelief on the plane surface of the lens. This variant of the lens provides trifocal vision due to the fact that the plus first-order (+1) diffraction maximum is divided into three approximately equal intensity maximums. This division is conditioned by the influence of the two additional internal refractive surfaces ( 4 ,  5 ), because of which both the middle and the peripheral parts of this lens focus light in three different points on the optical axis 
         [0050]    Bifocal and trifocal vision can also be provided by the proposed lens variants with the triangle profile of the diffraction relief. A lens, similar to the one in  FIG. 5 , but with a triangle relief of the diffraction profile, provides bifocal vision by the bifurcated plus first-order (+1) diffraction maximum, too. This bifurcation is conditioned by the influence of the additional internal refractive surface ( 4 ), because of which both the central and the peripheral parts of this lens focus light in two different points on the optical axis. The zeroth-order diffraction maximum is completely suppressed in this case, due to the selected depth of the triangle profile diffraction relief grooves. 
         [0051]    In the other variant, the proposed lens ( FIG. 6 ) has two additional internal refractive surfaces ( 4 ,  5 ) that divide the lens volume into three zones ( 6 ,  7 ,  8 ) made of materials with different refraction coefficients n, n2, n3, with the triangle profile microrelief on the plane surface of the lens. This variant of the lens provides trifocal vision due to the fact that the plus first-order (+1) diffraction maximum is divided into three approximately equal intensity maximums. This division is conditioned by the influence of the two additional internal refractive surfaces ( 4 ,  5 ), because of which both the middle and the peripheral parts of this lens focus light in three different points on the optical axis. The zeroth-order diffraction maximum is completely suppressed in this case, due to the selected depth of the diffraction profile microrelief. 
         [0052]    In general, independent from the embodiment of  FIG. 6  and independent from the microrelief structure, the curvature c 1  (i.e., radius of curvature) of the first internal refractive surface  5  may be larger than the curvature c 2  (i.e., radius of curvature) of the second internal refractive surface  4 . That is, in either case the curvature in the plane of the drawing of  FIG. 6  as shown (paper plane) is perpendicular to the external front surface of the lens  2 . This may hold especially in a region at or close to the optical axis (i.e., longitudinal axis L) of the lens or at height of central zone  9 . In some instances, depending on the desired optical properties of the lens, the curvature c 2  of an internal refractive surface may be smaller than the curvature c 1 . In general, this relation may be given referring to each pair of surfaces being adjacent in the longitudinal axis, if the lens comprises more than one internal zone. 
         [0053]    The proposed lens contains one additional refractive surface in the central part of the lens, which is represented by the spherical segment with the diameter d 1  within the range of 2.0 to 2.2 mm, further changing into a plane ( FIG. 8 ). In humans the pupil diameter depends on the intensity of light entering the eye—the higher the intensity of light, the smaller is the diameter of the pupil. In a healthy human eye the minimum diameter of the pupil is approximately 3.0 mm, the maximum diameter is approximately 6.0 mm. If d 1 &gt;3.0 mm, then in bright light (minimum pupil diameter) the human being will not be able to see objects clearly either at long or at short distances, depending on the implanted IOL type. The solution in the proposed lens lies in the fact that d 1  is within the range of 2.0 to 2.2 mm. With d 1 ≈2.0 mm and in bright light (pupil diameter ˜3 mm) the light energy entering the eye is approximately equally divided between the two foci. 
         [0054]    In the other variant ( FIG. 9 ) the proposed lens contains two additional refractive surfaces, the first of which, counting from the lens&#39; external surface with the diffractive structure, is located in the central part and is represented by the spherical segment with the diameter d 2  within the range of 1.7 to 1.8 mm, further changing into—a plane, and the second additional surface in the central part is represented by the spherical segment with the diameter d 3  within the range of 2.4 to 2.5 mm, further changing into a plane ( FIG. 9 ). 
         [0055]    The method of manufacture of the proposed multifocal intraocular lens ( FIG. 6 ) with two external refractive surfaces, on one of which there has been superimposed a diffractive structure in the form of rings, the radii of which coincide with the radii of the Fresnel zones, and between its external refractive surfaces additional refractive surfaces have been inserted, that divide the lens volume into zones manufactured from materials having different refraction coefficients, comprises formation of the optical part by using different photocurable materials with refraction indices n, n2, n3, their casting, UV treatment, and removal of the uncured material, all of this being done consecutively in several stages using quartz casting mold assemblies. The quartz casting molds comprise interchangeable halves, on the work surface of one of which there is a relief presetting the external refractive surface, and on the other off which there is a diffractive structure in the form of rings, the radii of which coincide with the radii of the Fresnel zones. The other halves have work surfaces, on which the internal refractive surfaces of the lens are formed that have spherical holes with the diameter either d 1  or d 2  or d 3  further changing into planes. Additionally, on the work surface of the form half there is a pattern corresponding to the haptic part of the lens. 
         [0056]    The first stage is the formation of the lens component representing the lens zone ( 8 ) restricted by the external refractive surface ( 1 ) and the first internal refractive surface ( 5 ) made of a photocurable material with the refraction index n3. The casting mold is assembled from two halves, the first of which presets the form of the external refractive surface of the lens ( 1 ), and the second one of which presets the form of the first internal refractive surface of the lens ( 5 ). The material is photocured by UV light, the two halves of the casting mold are divided in such a way that the resultant component stays on that half, which forms the external refractive surface of the lens ( 1 ), the uncured material is removed from the surface ( 5 ) of the resultant component with the help of an appropriate solvent—isopropyl alcohol, for instance, and the component is dried until the solvent is gone. 
         [0057]    The second stage is the formation of the lens component representing the lens zone ( 7 ) restricted by the first internal refractive surface ( 5 ) and the second internal refractive surface ( 4 ), made of a photocurable material with the refraction index n2. The manufacturer takes the half of the casting mold with the lens zone formed on it during the first stage ( 8 ), casts the photocurable material with the refraction index n2 and closes it with the other half that presets the form of the second internal refractive surface of the lens ( 4 ). The material is photocured by UV light, the two halves of the casting mold are divided in such a way that the resultant component—zone ( 7 )—stays on that half of the mold on which a zone has already been formed ( 8 ), the uncured material is removed from the surface ( 4 ) of the resultant component with the help of an appropriate solvent—isopropyl alcohol, for instance,—and is dried until the solvent is gone. 
         [0058]    The third stage is the formation of the lens component representing the lens zone (. 6 ); restricted by the external refractive surface with the diffractive structure in the form of rings, the radii of which coincide with the radii of the Fresnel zones ( 2 ). The manufacturer takes the half of the casting mold with the lens zone formed on it during the first stage ( 8 ) and the lens zone formed on it during the second stage ( 7 ), casts the photocurable material with the refraction index n3 and closes it with the half of the form that contains the diffractive structure in the form of rings, the radii of which coincide with the radii of the Fresnel zones. The material is photocured by UV light, the two halves of the casting mold are divided in such away that all the resultant components—zone ( 8 ), zone ( 7 ), zone ( 6 )—stay on that half of the mold, which was used during the first stage, the uncured material is removed from the surface ( 2 ) of the resultant lens with the help of an appropriate solvent—isopropyl alcohol, for instance,—and is dried until the solvent is gone. 
         [0059]    After that the resultant lens goes through additional UV treatment, then the resultant lens is placed into a closed container with isopropyl alcohol at a temperature of no lower than −20° C. and is held there for no longer than 24 hours, then it goes through thermal vacuum drying at a temperature no higher than 70° C. for no longer than 6 hours. 
         [0060]    The elements of the lens support can be formed during any one of the three stages of making the lens, either from the corresponding zone material ( 6 ,  7 ,  8 ) with the refraction index n1, n2, n3, respectively (as a monolith), or from different-materials (for example, polymethylmethacrylate or polypropylene). 
         [0061]    This method makes it possible to produce thin multifocal lenses that provide high visual function. 
         [0062]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.