Progressive power lens

Claim 1

A progressively variable focal power lens comprising a progressive power lens surface having a distance portion, a reading portion, a progressive power portion, and peripheral zones, at least a portion of the surface can be defined in parametric form with reference to a three-dimensional coordinate system in which an origin of the coordinate system is at a vertex of the lens surface, the z axis is perpendicular to the lens surface at the vertex, the x axis is vertical relative to the lens surface, and the y axis is horizontal relative to the lens surface, wherein each cross-section of the portion parallel to the y-z plane is defined as a two-dimensional curve having a central portion defined by a parametric function Z m (x,y) and peripheral zones defined by a function Z n (x,y), and wherein: ##EQU6## where: Z no (x) is a function that describes the geometry of an umbilical line of the lens surface; B.sub.(x) is a conic constant for a conic component of the function; ρ.sub.(x) is an y-z component of local radius of curvature R m (x) of the umbilical line defined by the function Z mo (x) ; C.sub.(x) and N.sub.(x) are dependent functions that provide a smooth tangent point of intersection between Z m (x,y) and Z n (x,y) ; Z no (x) is a function that is similar to Z mo (x) in the bottom half of the lens surface but includes constants that control the intersection of Z m (x,y) and Z n (x,y) and, therefore, the width of the reading portion, in the top half of the lens, ##EQU7## where R B is the base curve radius of the surface; R.sub.(x) is a distance between a point x 1 y 1 and a line parallel to the x axis and intersecting a center of a distance portion; A.sub.(x) Y L .sbsp.(x) is a function that reduces the astigmatism in the lens periphery just below center and to reduce the maximum astigmatism; A.sub.(x) is a function that is small in order that suppresses the contribution of the A.sub.(x) Y Y .sbsp.(x) component in the central portion of the curve; L.sub.(x) is selected to minimize and balance astigmatism in the lens, in the areas where Y equals approximately 18 to 23 millimeters, Z n (x,y) becomes a circle of the same radius and slope as the central curve at that point.