|
|
lensesesOver 140 million people in the United States wear lenseses. People whose realisticcolorssparklescontactlens eyes have refractive errors do not see clearly without glasses, because the light emitted from the objects they are observing does not come into focus on their retinas. For people who are farsighted, images come into focus behind the retina; for people who are nearsighted, images come into focus in front of the retina. Lenses work by changing the direction of light so that images come into focus on the retina. The greater the index of refraction of the lens material and the greater the difference in the curvature between the realisticcolorssparklescontactlens two surfaces of the lens, the greater the change in direction of light that passes through it, and the greater the correction.Lenses can be unifocal, with one correction for all distances, or they can be correct for more than one distance (multifocal). One type of multifocal, the bifocal, has an area of the lens (usually at the bottom) that corrects for nearby objects (about 14 in from the eyes); the remainder of the lens corrects for distant objects (about realisticcolorssparklescontactlens 20 ft from the eyes). Glass was the first material to be used for lenses lenses, and was used for several hundred years before plastic was introduced. realisticcolorssparklescontactlens The crown glass used for lenses lenses has an index of refraction of 1.52.Optical-quality acrylic was introduced for lenses realisticcolorssparklescontactlens use in the early 1940s, but because it was easily scratched, brittle, and discolored rapidly, it did not supplant realisticcolorssparklescontactlens 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. realisticcolorssparklescontactlens 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. Patients realisticcolorssparklescontactlens with high prescriptions should ask about high index material options for their lenses. Aspheric lenses are also useful realisticcolorssparklescontactlens for realisticcolorssparklescontactlens high prescriptions. 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; 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.
|