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Another type of multifocal, a trifocal, has an area in-between that corrects for intermediate distances (usually about 28 in). Conventional bifocals and trifocals have visible lines between the areas of different correction; contactsedgeiiixt however, lenses where the correction gradually changes from one area to the other, without visible lines, have been available since the 1970s. Such lenses are sometimes called progressives or no-line bifocals.To be suitable for lenses lenses, a material must be transparent, without bubbles, and have a high index of refraction. The greater the index of refraction, the thinner the lens can be. Lenses are made from either glass or plastic (hard resin). The advantage of plastic is that it is lightweight and more impact resistant than glass. The advantage of glass is that it is scratch resistant and provides the clearest possible vision. The lenses Council reports that 26 million Americans wear contact. Among lenses wearers, approximately 80% wear the soft type and 18% wear rigid gas-permeable lenses. The Council reports that about 11% of lenses wearers, approximately contactsedgeiiixt 2.8 million people, are under 18.InfantsThere are few instances when corrective lenses--lenseses contactsedgeiiixt or contacts--are prescribed for infants. However, when an infant develops cataracts , a condition known as infantile aphakia, contact may be prescribed following surgery. In 1993, the Journal of the American Medical Association reported contactsedgeiiixt that contact were safe and effective for use with infants following cataract surgery. Routine care of the contactsedgeiiixt lenses was easily learned by the baby''s parents or caregivers. The primary drawback to wearing contact, especially for children, is the requirement that they be cleaned and disinfected daily in order to prevent an infection, which could result in permanent damage to the eye. Careful choice of lenses and consistent lens care will minimize the risk of infection. Wearers of best lenses are the least prone to infection but the lenses are much less comfortable, more expensive, and can pop out of the eye more easily than lenses. Daily wear lenses put the wearer at a slightly higher risk of infection contactsedgeiiixt because they absorb more external contactsedgeiiixt liquids and airborne contaminants contactsedgeiiixt.
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