scope:

This American National Standard applies to contact lenses which are devices worn over the front surface of the eye in contact with the preocular tear film. The standard covers rigid (hard) intracorneal and haptic (scleral) contact lenses, as well as flexible paralimbal contact lenses. Rigid lenses maintain their own shape unsupported and are made of transparent optical-grade plastics, such as polymethylmethacrylate (PMMA), cellulose acetate butyrate (CAB), polyacrylate/siloxane copolymers, rigid polysiloxanes (silicone resins), butylstyrenes, fluoropolymers, and fluorosiloxanes, etc. Flexible lenses are easily deformable and may require support for proper shape. A very large subset of flexible lenses are so-called "soft" (hydrophilic) contact lenses, which are made of transparent hydrogels containing water in concentrations greater than or equal to 10%. Flexible lenses can also be made of non-hydrogel materials, e.g., flexible polysiloxanes (silicone elastomers).