Although there are 3 types of contact lenses, Therapeutic, Cosmetic and Corrective only corrective will be discussed at length in this article, please see below for a brief description of the functional attributes of cosmetic and therapeutic lenses.
In order for a person to enjoy perfect or near perfect vision two things need to happen, firstly the eye must be capable of gathering all the information required to build a useful image of the outside world, this information includes colour, brightness, vector and distance. Secondly the brain must be able to interpret this information correctly (some readers may be interested to note that the "image" relayed to the brain from the eye is in fact upside down and must be rotated by the visual cortex).
In myopia (short) or hypermetropia (long sightedness) the ratio of the length of the eyeball to the refractive power of the lens has been changed, giving rise to a "refractive error" or in simple terms the focal point of the lens of the eye is shifted to an area either in front of or behind the retina giving rise to blurred vision. This mismatch is usually brought about by the cilial muscles which grip and manipulate the shape of the lens either squeezing too tightly or not tightly enough, the former leading to myopia and the latter to hypermetropia. The function of a contact lens is to correct this mismatch, a function accomplished by varying the refractive index of the contact lens.
Corrective contact lenses are commonly available to correct myopia, hypermetopia, astigmatism (incorrectly shaped cornea) and presbyopia (diminishing ability to focus of the eye itself) and vary based on wear pattern and on the material used in their construction.
Soft lenses have been available since the 1960s and are referred to as such because they are manufactured from polymers and hydrogels. Lenses manufactured from these material are commonly designed for daily wear and should be removed overnight typically each pair is used only once and then discarded. More recently (since 1999) new materials called Silicon hydrogels have become available which, due to their increased permeability to oxygen have allowed the production of extended wear lenses which are designed to be used continously (including at night) for periods of up to 30 consecutive days or nights.
Rigid lenses in contrast have been available for almost 120 years. Originally amde from glass and therefore not practical for regular use rigid lens technology has advanced significantly first from the advent of PMMA (acrylic) and to the later introduction of rigid-gas-permeable materials. Despite the obivous drawbacks of increased irritability as compared to daily disposable lenses rigid lenses are able to perform functions not suited to softer materials; among these are correcting disorders caused by a malformed cornea by providing in effect an entirely new refractive surface. Rigid lenses are usually reused daily for long periods of time (up to a month or more) and as such good hygiene is essential (read more on this in the section on health concerns).
Disclaimer: The author is not a medical doctor, optician, ophthalmologist or any other medical professional, this article has been compiled using a variety of internet reference sources and while every effort has been made to ensure accuracy this cannot be guaranteed. No preference for product or brand is inferred or intended and the contents of this article are not to be used in whole or in part to inform a decision regarding any aspect of contact lens use.
