Contact lenses are used primarily to correct vision problems such as nearsightedness, farsightedness and astigmatism. However, there are also lenses which are made for specialized uses. It is to the benefit of the millions of contact lens users all over the world that due to research and the development of manufacturing technology, there are contact lenses available to suit different types of eye health needs. The types of contact lenses are grouped into several categories, depending on the material used, wearing periods, frequency of disposal and design.
There are three types of materials used to manufacture contact lenses. The hard type of lens is made from a substance called polymethyl methacylate, commonly referred to as PMMA or Perspex/Plexiglas. This was an improvement from those which were made of glass and caused eye irritation. With PMMA, lenses were more wearable but this type is rarely used today. Currently, the most popular type is the soft lens, made from a material resembling a gel. Many opt to use this kind because of its high hydration level in addition to allowing more oxygen to pass through to the eye. There is also the RGP type or oxygen permeable lens, manufactured from a rigid plastic and commonly prescribed for people with severe astigmatism and presbyopia.
In terms of wearing time, contact lenses are divided into daily and extended wear. Daily wear lens have to be removed every night and the extended type enables the user to wear it for a number of days without removal.
The frequency of disposal of contact lenses varies. This difference only applies to the soft type of lens since the gas permeable kind has to be removed every night. Some need to be replaced everyday and there are those that can be used for up to two weeks before discarded. These are the disposable types, which can be daily, weekly or bimonthly. Continuous wear are replaced every month but are worn for 30 days. Planned replacement lenses are those that can be changed less frequently, most commonly a year. Replacement is necessitated by the buildup of proteins and lipids in soft contact lenses. Although cleaning products might slow the process of protein accumulation, eventually the lenses have to be replaced.
The design of contact lenses is dictated by its use. The most typical is the spherical, which are used for nearsightedness and farsightedness. For astigmatism, toric lenses are the most suitable. A counterpart to bifocal spectacles is the bifocal lens, which contains different zones for both near and far visions. Other designs also exist, although these are less common and are made for uncommon eye conditions.