Method for producing bifocal contact lenses

A method of producing a plastic bifocal contact lens blank, for use in the preparation of plastic bifocal contact lenses, having a concave base curve on its posterior side, a second, convex and a third, convex surface on its anterior side and a circumferential edge from a button having first, posterior and second, anterior surfaces, a circumferential edge wall, a geometric center at the intersection of horizontal and vertical axes parallel to the planes of the anterior and posterior surfaces, said method consisting of, in combination, the steps of:

• a. generating a first, concave surface on the first side of said button to form a lens blank wherein the center of curvature of said lens blank surface lies on an axis of rotation offset from the geometrical center axis of said lens blank by a prism offset distance;
• b. generating a second, convex surface on the second side of said lens blank wherein the center of curvature of said convex surface lies on an axis through the geometrical center axis of the lens blank;
• c. forming a third, convex surface in said lens blank second side wherein the center of curvature of the third, convex curve lies on the axis of rotation which passes through the center of curvature of the second, convex surface;
• and
• d. trimming the first, concave surface to form a fourth, concave surface on the posterior side of said button the curvature of said fourth surface corresponding to the base curve.