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| Type of lenses |
| Contact Lens Materials |
| Classified by material, there are three types of contact lenses: |
- Hard lenses are made from PMMA — also known as Plexiglas or Lucite. These lenses are virtually obsolete and rarely used.
- Soft lenses are made from gel-like, water-containing plastics, and are most common. They're a bit larger in size than your iris.
- GP lenses, also known as RGP or "oxygen permeable" lenses, are made from rigid, waterless plastics and are especially good for presbyopia and high astigmatism. These lenses are usually about eight millimeters in diameter, which is smaller than your iris.
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| Contact Lens Wearing Time |
| Two types of lenses are classified by wearing time: |
- Daily wear — must be removed nightly
- Extended wear — can be worn overnight, usually for seven days consecutively without removal "Continuous wear" is a term that's sometimes used to describe 30 consecutive nights of lens wear — the maximum wearing time approved by the FDA for certain brands of extended wear lenses.
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Disposal Intervals for Contact Lenses
Even with proper care, contact lenses (especially soft contacts) should be replaced frequently to prevent the build-up of lens deposits and contamination that increase the risk of eye infections. |
| Soft lenses have these general classifications, based on how frequently they should be discarded: |
- Daily disposable — Discard after a single day of wear
- Disposable (used for daytime wear) — Discard after two weeks
- Disposable (used for overnight wear) — Discard after one week
- Continuous wear (used for 30-day wear) — Discard monthly
- Planned replacement — Discard monthly or less frequently
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| Gas permeable contact lenses are more resistant to lens deposits and don't need to be discarded as frequently as soft lenses. Often, GP lenses can last a year or longer before they need to be replaced. |
| Contact Lens Designs |
| Many lens designs are available to correct various types of vision problems: |
- Spherical contact lenses are the typical, rounded design of contact lenses, which can correct myopia (nearsightedness) or hyperopia (farsightedness).
- Bifocal contact lenses contain different zones for near and far vision to correct presbyopia [more about bifocal contacts].
- Orthokeratology lenses are specially designed to reshape the cornea during sleep, providing lens-free daytime wear [more about ortho-k].
- Toric contact lenses correct for astigmatism, as well as for myopia and hyperopia
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