Ophthalmic lens of changing power

Claim 1

An optical lens of transparent optical material, useful for the correction of presbyopia, having a convex aspheric front surface which is a nonaxial portion of a surface of revolution having an apical cusp, all meridian sections of said convex lens surface being identical elliptical arcs with the least curved portion of the elliptical arc in the upper portion of the lens, all sections of said convex lens surface other than those sections normal to the axis of revolution being noncircular, the axis of revolution of said convex surface coinciding with a straight portion of a modified evolute of said elliptical arc determined by first determining a series of modified evolutes by adding for each modified evolute to be determined a different fixed increment of curvature units to the value of the curvature at each point along said ellipse and using the reciprocals of said sums at each point as distances measured along normals from each of said points of said ellipse toward said modified evolute as the locus of each said modified evolute, then taking a base curve point on said ellipse corresponding to the vertical elliptical meridian in the upper part of the lens, for far vision, and selecting an add point in the lower part of the lens, for reading, corresponding to the added positive spherical power for the correction of a wearer''s presbyopia at a chord distance of predetermined length across said vertical elliptical meridian from said base curve point, then drawing normals from said Base curve point and from said add point through all of said modified evolutes, and finally selecting that modified evolute which is substantially a straight line between said two last named normals, so that said surface is characterized by having a substantially constant difference in principal curvatures at all points along all meridian sections providing a substantially constant astigmatism at all points on said surface, with the transmeridional power being greater than the meridional power, while both principal curvatures along any elliptical arc meridian section change continuously and regularly and by substantially equal amounts to provide an accelerating surface, said lens having a negatively curved back surface optically coacting with said front surface, there being a common normal to both surfaces through the geometrical center of the front aspheric surface and through the midpoint of said chord, said back surface at least neutralizing said constant astigmatism of the front surface at the common normal, and the difference in dioptric powers between said base curve and add points being the added power required for the correction of the wearer''s presbyopia.