contact The concept of corrective lenses that sit directly on the eyeball was developed in the ninth century, but it was not until the late 20th century that manufacturing dry and grinding techniques for contact were perfected. Originally contact eyes were made of glass, then later of hard plastic, and still later of flexible, highly oxygen-permeable soft plastic. Today, the majority of lenses worn are of two basic types: rigid gas permeable contact (so-called "hard" lenses) and lenses. best lenses are smaller lens and dry more durable, but require a longer period of adjustment for the wearer. Hard contact can eyes correct some vision problems, like astigmatism, contact that the lenses cannot. The lenses are slightly larger than best lenses. They are more comfortable and less expensive, but can be easily torn. The latest generations lens of contact are extended and disposable lenses. extended lenses are intended to be worn for several weeks at a time, while disposable lenses are thrown away every day or every few weeks, dry or in the newest versions, every day.

They are flatter and lighter than conventional eyes lenses.There are many lenses and lens-coating options for individual contact needs, including coatings that block the ultraviolet (UV) light or UV and blue light which have been found to be harmful to the eyes. Such lens coatings are not needed on polycarbonate lenses, which already have UV protection. UV coatings are particularly important on sunglasses and ski goggles. Sunglasses, when nonprescription, should be labeled with an indication that they block out 99-100% of both UV-A and UV-B dry rays. There are anti-scratch coatings that increase the surface hardness of lenses (an important feature when using plastic lenses) and anti-reflective (AR) coatings that eliminate eyes almost all glare and allow other people to see the eyes of the contact wearer. AR coatings may be particularly helpful to people who use computers or who drive at night. Mirror coatings that prevent other people from seeing the wearer''s eyes are also available. There lens and dry is a whole spectrum of tints, from light tints to darker tints, used in sunglasses.

Simultaneous lenses bring distant and near objects into focus at the same time. The center of the lens corrects distance vision; the outer part corrects near vision. * Segmented lenses correct distance vision in the top half and near vision in the bottom half.Monovision An alternative to bifocal contacts, monovision lenses correct distance vision in one eye and near vision in the other. Modified monovision allows wearers to put a bifocal lens in one eye and a single-vision lens in the other.Implants Designed for mild nearsightedness, KeraVision Intacs are placed in the periphery of the cornea by an ophthalmic surgeon. Made of polymer, eyes a type of contact hard plastic, lens they require no maintenance and can be surgically removed. The outpatient procedure has a 97-percent success rate of 20/40 vision or better.