referring to fig1 , an intracorneal diffractive lens whose central axis is designated by a has an external diameter d that can be between 5 and 9 mm , and a mean curvature defined by a radius r that can be between 7 and 9 mm . this lens has a convex outer surface s 1 and a concave inner surface s 2 , and its thickness e measured between the two surfaces s 1 and s 2 can be between 0 . 05 mm and 0 . 5 mm . the useful zone of the lens , centered on the axis a , is a circle whose diameter d can be between 3 and 7 mm , depending on the external diameter d of this lens . this useful zone comprises a succession of “ full ” rings 2 made of optically active material and with increasing diameters , all of them being centered on the axis a and being separated from one another by “ empty ” intermediate annular zones 3 . the “ full ” rings 2 and the “ empty ” intermediate zones 3 have a width that decreases uniformly from the central axis a in the direction of the periphery of the lens , the geometry of the “ full ” rings 2 conforming to the principle of the fresnel zone lens . in the embodiment in fig1 , the intracorneal lens further comprises , at its center , a profiled disk 4 which is made of the same optically active material as the “ full ” rings 2 and which is surrounded concentrically or coaxially by these “ full ” rings 2 . the central disk 4 can be likened to a first “ full ” ring with inner radius equal to zero . the “ empty ” intermediate zones 3 are in fact filled by an optically inactive or weakly active material , which is in particular a hydrogel whose percentage of water is equal to or greater than 78 %. this can be a hydrogel of acrylate or methacrylate , acrylamide or methyacrylamide , polyester , vinyl copolymer or similar . this hydrogel is not only present between the “ full ” rings 2 but can also entirely coat these “ full ” rings 2 , and also the central disk 4 , by extending as far as the outer surface s 1 and inner surface s 2 . in all cases , this hydrogel forms a “ cement ” interconnecting all the rings 2 , thereby stabilizing the structure of the lens . the “ full ” rings 2 and the central disk 4 are made of a material having an optical index different than that of the cornea . the material can also be a hydrogel , but one whose percentage of water is less than 78 %, preferably between 50 % and 70 %. the “ full ” rings 2 , which can be between five and thirty in number ( the drawing shows in a simplified way a very small number of rings ), have a permeability less than that of the cornea and provide , together with the disk 4 , the diffraction needed for the desired correction of vision . the “ empty ” intermediate annular zones 3 , filled with hydrogel , provide the connection between the rings 2 while being permeable to the flow of nutrients and to oxygen . the outer surface s 1 and inner surface s 2 can be parallel and thus have no effect on the correction that is obtained , or , by contrast , they can be non - parallel and configured in such a way as to participate in the visual correction by virtue of an additional refractive effect . such an intracorneal diffractive lens combining two materials can be produced by molding and overmolding techniques . in particular , it can be produced by a twin injection procedure . fig2 , in which the elements corresponding to those described above are designated by the same reference signs ( letters or numbers ), shows a variant of this intracorneal diffractive lens . in this variant , the central disk is omitted . the lens thus comprises , at its center , an “ empty ” circular zone 5 , or one that is filled with optically inactive or weakly active but permeable material , such as a suitable hydrogel ; the central circular zone 5 is surrounded concentrically by the first “ full ” annular zone , that is to say by the first ring 2 . in one variant , not illustrated directly in the drawing , of this intracorneal diffractive lens , the rings 2 have a lower optical index than that of the “ cement ” that connects these rings . in this case , the “ cement ” remains in particular a hydrogel whose water content is close to 78 %, while the rings 2 are made of a hydrogel whose water content is greater than that of said “ cement ” and is typically greater than 85 % or are even formed by water . fig3 illustrates a variant , of the lens according to fig1 , in which the “ full ” rings 2 and the central disk 4 are interconnected by a fine membrane 6 made of the same optically active material . the membrane 6 , here embedded in the “ empty ” intermediate annular zones 3 , remains permeable to nutrients because of its very slight thickness , and it thus still allows said “ empty ” annular zones 3 to perform their function . fig4 similarly illustrates a variant , of the lens according to fig2 , in which the “ full ” rings 2 are interconnected by a fine membrane 6 made of the same optically active material . in the absence of a central disk , the membrane 6 is here present in the “ empty ” intermediate annular zones 3 and also in the central circular zone 5 and , as before , it does not form an obstacle to the transfer of nutrients . finally , fig5 shows another embodiment , in which the “ full ” rings 2 of the lens are interconnected by material bridges 7 of radial orientation that are made of the same optically active material as these rings 2 . by virtue of being thin , the material bridges 7 form , between themselves and the rings 2 , wide spaces in the shapes of arcs of a circle that are filled by the inactive but permeable material of the “ empty ” intermediate zones 3 . as will be appreciated , the presence of the membrane 6 or of the material bridges 7 facilitates the production of the lens , without impairing vision and without adversely affecting the permeability of the “ empty ” intermediate annular zones 3 . it would not constitute a departure from the scope of the invention , as defined in the attached claims , if one were to modify : the dimensions of the lens ; the nature of its component materials ; the number of its full rings ; the nature of the sight defect corrected by this lens .
the invention relates to a lens that can be implanted in the cornea for correcting vision defects , such as presbyopia . it consists of a zonal diffractive lens with phase inversion that comprises an alternation of optically active or “ full ” annular areas and of optically inactive or empty annular areas which are all concentric . the empty annular areas are filled with an optically inactive “ cement ” that binds together the “ full ” annular areas in order to ensure the stability thereof . the cement is a hydrogel pervious to nutrients and oxygen having an optical index close to that of the cornea .