Optipedia • SPIE Press books opened for your reference.
Excerpt from Field Guide to Visual and Ophthalmic Optics
Contact lenses are artificial lenses temporarily placed on the cornea. Since the cornea absorbs oxygen from the air, oxygen permeability is crucial for contact lenses. Early “hard” contact lenses had poor oxygen permeability since they were made of polymethylmethacrylate (PMMA). Alternative materials are used in newer lenses to markedly increase the transmission of oxygen and promote corneal health. Rigid gas permeable materials have replaced hard lenses. Hydrogel materials are used for “soft” lenses.
Rigid gas permeables (RGPs)
Advantages: RGPs do not flex or conform to the shape of the cornea. When properly fit, the space between the lens and the cornea fills with tears, effectively index matching with the cornea. This property masks corneal toricity that introduces astigmatism and irregularities such as keratoconus. RGPs tend to scatter less light and have a more predictable surface shape, providing high-quality optics.
Disadvantages: RGPs can be difficult and time consuming to fit. Lens motion on the cornea is needed to promote tear circulation and corneal health. This motion can cause intermittent correction of vision. RGPs also cannot mask astigmatism in the crystalline lens. Sophisticated toric designs are required to reduce residual astigmatism. Finally, rigid materials can cause a foreign body sensation, and an adjustment period is usually necessary.
Hydrogels (soft lenses)
Advantages: A soft lens is flexible and conforms to the shape of the cornea, promoting ocular comfort. These lenses are also easy to fit since matching the back surface of the lens to corneal shape is not as crucial. Soft lenses are stable on the eye, allowing a constant level of visual correction.
Disadvantages: These lenses cannot mask corneal toricity or irregularities since they conform to the corneal surface, causing a transferral of the underlying shape. Toric lenses for astigmatism have had only minor success. Hydrogel materials tend to have a decreased clarity compared to RGPs. Soft lenses are typically made in a dehydrated state. Small surface irregularities are magnified upon hydration.