An astigmatism is a defect of the eye in which light from a point source produces a line image on the retina. This condition arises when the cornea or the crystalline lens is not perfectly spherical.
It has been known to correct astigmatism with lenses having different curvatures in two mutually perpendicular directions. Such a lens would have meridians of least and greatest curvature located at right angles to one another on the same surface. This is known as a toroidal surface, and a lens having this geometry is referred to as "toric."
The correction of astigmatism has typically involved using lenses, spectacle or contact, which have a toric surface. A variety of prior art toric lens configurations are known.
U.S. Pat. No. 5,220,359 to Roffman describes a lens for correcting myopia, presbyopia, astigmatism and other focusing problems, in which the lens has a rotationally symmetric, aspheric convex front surface and a concave, spheric back surface. The front surface is defined according to a specific Fourier Transform function.
U.S. Pat. No. 5,300,262 to Glick describes a process for spincasting a compound contact lens having toric surfaces. The lens uses a compound anterior surface to correct vision.
U.S. Pat. No. 5,100,225 to Rothe describes a contact lens having compound anterior surfaces and a smooth, concave posterior surface.
Other patents showing various forms of contact lenses include the following: U.S. Pat. Nos. 5,066,431; 4,884,482; 4,666,249; 4,580,882; 4,211,476; 4,318,595; and 4,555,372.
In spite of the above-noted disclosures, a continuing need exists for a contact lens capable of correcting astigmatism.