a proposed intraocular lens variant is depicted in fig1 . 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 ). 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 ( fig2 ): 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 fig2 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 fig2 , provides for additional optical power of 10 diopters on condition that the radius of the first ring - type fresnel zone r1 = 0 . 25 mm . 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 fig2 and the variant depicted in fig3 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 fig2 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 . the lens proposed in fig3 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 fig4 , 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 ). in fig4 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 fig3 actualizes this very way of minimizing the spherical aberration &# 39 ; s influence on the diffraction image . one of the variants of the proposed lens has the right - angled profile of the diffractive structure ( fig5 ). 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 ). another variant of the proposed lens has the triangle profile of the diffractive structure ( fig6 ). 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 ). the power distribution among the diffraction maximums may vary . the power distribution is influenced by the depth of the diffractive structure microrelief hmax ( fig6 ). 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 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 . the depth of the triangle profile of the diffractive structure microrelief for the proposed lens is calculated according to the formula 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 ) ( fig6 ). 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 . in one of its variants ( fig5 ) 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 . 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 fig7 . in particular , in fig7 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 fig7 . further , in fig7 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 ). in the other variant ( fig1 ) 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 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 fig5 , 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 . in the other variant , the proposed lens ( fig6 ) 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 . in general , independent from the embodiment of fig6 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 fig6 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 . 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 ( fig8 ). 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 . in the other variant ( fig9 ) 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 ( fig9 ). the method of manufacture of the proposed multifocal intraocular lens ( fig6 ) 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 . 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 . 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 . 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 . 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 . 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 ). this method makes it possible to produce thin multifocal lenses that provide high visual function . 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 .
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 .