worn for 5 to 10 years, but had several drawbacks. Lenses with different replacement schedules can be made of the same material. Although the materials are alike, differences in the manufacturing processes determine if the resulting lens will be a "daily disposable" or one recommended for two or four week replacement. However, sometimes manufacturers use absolutely identical lenses and just repackage them with different labels. Manufacturing Typically, soft contact lenses are mass-produced, while rigids are custom-made to exact specifications for the individual patient. Spin-cast lenses – A soft lens manufactured by whirling liquid silicone in a revolving mold at high speed. Diamond turning – This type is cut and polished on a CNC lathe. The lens starts out as a cylindrical disk held in the jaws of the lathe that is equipped with an industrial-grade diamond as the cutting tool. The CNC lathe may turn at nearly 6000 RPM as the cutter removes the desired amount of material from the inside of the lens. The concave (inner) surface of the lens is then polished with some fine abrasive paste, oil, and a small polyester cotton ball turned at high speeds. To hold the delicate lens in reverse manner, wax is used as an adhesive. The lens' convex (outer) surface is thus cut and polished by the same process. This method can be used to shape rigid as well as soft lenses. In the case of softs, the lens is cut from a dehydrated polymer that is rigid until water is reintroduced. Molded – Molding is used to manufacture some brands of soft contact lenses. Rotating molds are used and the molten material is added and shaped by centripetal forces. Injection molding and computer control are also used to create nearly perfect lenses. The lens is kept moist throughout the entire molding process and is never dried and rehydrated. Many companies make contact lenses. In the United States, there are five major manufacturers: Johnson & Johnson; maker of Acuvue lenses The Cooper Companies: through its CooperVision Alcon Bausch Health: through its Bausch & Lomb subsidiary X-Cel Specialty Contacts; maker of Westcon lenses. Prescriptions The parameters specified in a contact lens prescription may include: Material / Brand name Base curve radius (BC, BCR) Diameter (D, OAD) Power in diopters Center thickness (CT) Prescriptions for contact lenses and glasses may be similar, but are not interchangeable. Prescribing of contact lenses is usually restricted