Glass was the first material to be used for lenses lenses, and was used for several hundred years before nonperscriptioncontacts plastic was introduced. The crown glass used for lenses lenses has an index of refraction of 1.52.Optical-quality acrylic nonperscriptioncontacts was introduced for lenses use in the early 1940s, but because it was easily scratched, brittle, and discolored rapidly, it did not supplant glass as the material of choice. Furthermore, it had a relatively low index of refraction, so it wasn''t suitable for people with large refractive errors. A plastic called CR-39, introduced in the 1960s, was more suitable. Today, lenses wearers nonperscriptioncontacts 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 nonperscriptioncontacts higher than the conventional plastics used for lenses, nonperscriptioncontacts and even higher than glass. Patients with high prescriptions should ask about high index material options for their lenses. Aspheric lenses are also useful for high nonperscriptioncontacts prescriptions. Eye nonperscriptioncontacts glasses and contact lenseses and contact are devices that correct refractive errors in vision. lenses lenses are mounted in frames worn on the face, sitting mostly on the ears and nose, so that the lenses are positioned in front nonperscriptioncontacts of the eyes. contact appear to be worn in direct contact with the cornea, but they actually float on a layer of tears that separates them from the cornea.PurposeThe purpose of lenseses and contact is to correct or improve the vision of people with nearsightedness (myopia), farsightedness (hyperopia), presbyopia, and astigmatism.Precautions People allergic to certain plastics should not wear contact or lenses frames or nonperscriptioncontacts lenses manufactured from that type of plastic. People allergic to nickel should not wear Flexon frames. People at risk of being in accidents that might shatter nonperscriptioncontacts glass lenses should wear plastic lenses, preferably polycarbonate.
|