Patent FR-2344042-A1 (FR)
Title: Multifocal ophthalmic lens for presbyopia and aphalia - has geometrical and optically continuous variable convex front surface and coacting conicoid back surface
IPC: G02C7/02
Filed: 1976-03-10
Abstract: Proposed is a multifocal ophthalmic lens of homogeneous transparent optical material, useful for the correction of the refractive error and the accommodative insufficiency or absence of accommodation in presbyopia and aphakia, having a geometrically and optically continuous variable convex front surface at least a portion of which changes continuously and regularly in refractive power, and a coacting conicoid back surface of eccentricity zero or greater or toric back surface. The variable front surface is characterised by having a single pair of orthogonal principal planes each of the planes of the pair intersecting the variable convex front surface normally at all points. The first of the principal planes, generally horizontal in use, intersect the front surfaces normally in a conic greater arc of eccentrically zero or greater. the derivative of curvature of the variable front surface vanishes at the great arc at least in sections by all planes orthogonal to it. The arc provides a unique tangential junction between an upper and a lower portion of the variable front surface. There is geometrical and optical continuity and a continuous and regular change in curvature and refractive power in crossing the conic great arc without localised distortion in the field fo vision through the lens. The second of the principal planes is a generally vertical plane of symmetry of the variable front surface, intersecting orthogonally the great arc and the first principal plane and forming the axis of the variable front surface. The axis intersects the centre of curvature of the great arc when its eccentricity is zero and coincides with the major axis and intersect both foci and the centre of curvature of the apex of the great arc when its eccentricity is greater than zero, and intersects the variable front surface normally in a principal curve, at the point where the axis intersects the variable front surface, namely, the axial umbilical point, the curvature of the great arc and the principal curve are equal. The derivative of curvature and the principal curve and of the variable front surface in all meridian sections containing the axis, vanish or are zero at the axial umbilical point. The principal curve at least below the great arc, increases in curvature and refractive power downwardly in a continuous and regular manner and at an accelerated rate with increasing distance from the axial umbilical point of the variable front surface. All transverse sections of the variable front surface below the great arc, by planes orthogonal to the principal curve, are conics of eccentricity greater than zero who se axes containing the foci of the conics lies in he vertical principal plane and intersect normally the principal curve. The curvature of the conic transverse section at the principal curve below the great arc increases in a continuous and regular manner at a rate substantially equal to the accelerated rate of increase in curvature of the principal curve itself. Every transverse section of the variable front surface above the great arc by planes orthogonal to the principal curve are conics of eccentricity zero or greater. The lines of intersection of the orthogonal planes and the vertical principal planes all intersect at the common centre of curvature of the conics when their eccentrically is zero. The axes of the conics which contain their foci when their eccentricities ar greater than the zero coincide with their respective lines of intersection of the orthogonal planes and the vertical principal plane. The eccentricities of the conic transverse section below the great arc increase continuously and regularly with distance from he great arc. All transverse sections by planes orthogonal to the principal curve and conics of eccentricity greater than zero when the great arc is a conic of eccentricity greater than zero, the derivative of curvature of all of the conic transverse sections of eccentricities greater than zero vanish at the principal curve, there being sufficient thickness of the lens to allow for the generating, grinding and polishing of a coacting conicoid back surface of eccentricity zero or greater or toric back surface to incorporate an ophthalmic prescription into the lens.