A contact lens, or simply contact, is a thin lens placed directly on the surface of the eye. Contact lenses are considered medical devices and can be worn to correct vision, or forcosmetic or therapeutic reasons. In 2004, it was estimated that 125 million people (2%) use contact lenses worldwide, including 28 to 38 million in the United States. In 2010, worldwide contact lens market was estimated at $6.1 billion, while the U.S. soft lens market is estimated at $2.1 billion. Some have estimated that the global market will reach $11.7 billion by 2015. As of 2010, the average age of contact lens wearers globally was 31 years old and two thirds of wearers were female. The term "sphere" comes from the concept that the basic geometry of a corrective lens is a series of curved surfaces that are sections of spheres. These surfaces can be modified as needed to generate more complex corrections by using more complex geometry. The "cylinder" and "axis" components specify how a particular lens is different from a lens composed of purely spherical surfaces.
