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They are flatter and lighter than conventional occular 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 science needed on polycarbonate lenses, which already occular 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 coatings that increase the surface hardness of lenses (an important feature when using plastic lenses) science and anti-reflective occular (AR) coatings that eliminate almost all glare and allow other people to science see the eyes of the 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 is a whole spectrum of tints, from light tints to darker tints, used in sunglasses. They can be worn up to a week. Daily wear lenses are made of plastic that is permeable to oxygen and absorbs water; therefore, they are soft and occular flexible. These lenses must be removed and cleaned each night, and they do not correct all vision problems. lenses are easier to get used to than rigid lenses, but are more prone to tears and do not last as long. Extended-wear lenses are highly permeable to oxygen, are flexible by virtue of their ability science to absorb water, and can usually occular be worn for up to one week. They do not correct all vision problems. There is more of a risk of infection with extended-wear lenses than with daily-wear lenses. Extended-wear disposable lenses are lenses worn continually for up to six days and then discarded, with no need for cleaning. Glass was the first material to be used for lenses science lenses, and was occular used for several hundred years before plastic was introduced. The crown glass used for lenses lenses has an index of refraction of 1.52.Optical-quality acrylic was introduced for lenses use in the early 1940s, but because it was easily science scratched, brittle, and discolored rapidly, it did not supplant glass as the material of choice. Furthermore, it had a relatively occular low index of refraction, so it wasn''t suitable for people with large refractive errors. A plastic called science CR-39, introduced in the 1960s, was more suitable. Today, lenses wearers can also choose between polycarbonate, which is the most impact-resistant material available for eyewear, and polyurethane, which has exceptional optical qualities and an index of refraction of up to 1.66, much higher than the conventional plastics used for lenses, and even higher than glass. occular Patients with high prescriptions should ask about high index material options science for their lenses. Aspheric lenses are also useful for high prescriptions.
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