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 and grinding techniques for contact were perfected. Originally contact 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 (so-called disco "hard" lenses) and lenses. best lenses are smaller contact and more durable, lens but require a longer period of adjustment for the wearer. Hard contact disco can correct some vision problems, like astigmatism, that the lenses contact cannot. The lenses are slightly larger than best lenses. They are more comfortable lens and less expensive, but can be easily disco torn. The latest generations of contact are extended and disposable lenses. extended lenses are intended to be worn contact for several weeks at a time, while disposable lenses are thrown away every day or every few weeks, or in the newest versions, every day.

They are flatter and lighter than conventional lenses.There are many lenses and lens-coating options for individual 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 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 rays. There are anti-scratch lens coatings that increase the surface hardness of lenses (an important feature when using plastic lenses) and anti-reflective (AR) coatings that eliminate almost all glare and allow other people to disco see the eyes of the wearer. AR coatings may be particularly helpful to people who use computers or who drive at contact night. Mirror coatings that prevent other people lens from seeing the wearer''s eyes are also available. There is a whole spectrum of tints, from light tints to darker tints, used in sunglasses.

Over 32 million people in the United States wear these small lenses that fit on top of the cornea. They provide a field of view unobstructed disco by lenses frames; they do not fog-up or get splattered, so it is possible to see contact well while walking in the rain; and they are less noticeable than any lenses style. On the other hand, they take time to get accustomed to; require more measurements for fitting; require many follow-up visits to the eye doctor; can lead to complications such as infections and lens corneal damage; and may not correct astigmatism as well as lenseses, especially if the astigmatism is severe.