- contact lens
one of a pair of small plastic disks that are held in place over the cornea by surface tension and correct vision defects inconspicuously. Cf. hard lens, intraocular lens, soft lens.[1885-90]
* * *Thin artificial lens worn on the surface of the eye to correct refractive defects of vision.Early glass contact lenses, invented in 1887, were uncomfortable and could not be worn long. Plastic-based lenses, made to measurements of corneal curvature taken by optical instruments, were first developed in the mid 20th century. Gas-permeable lenses allow more oxygen to reach the eye, thus increasing comfort and wear time. Contact lenses have advantages over eyeglasses for certain visual defects and may be preferred for the sake of appearance and other reasons.
* * *thin, artificial lens worn on the surface of the eye to correct refractive defects of vision. The first contact lens, made of glass, was developed by Adolf Fick in 1887 to correct (optometry) irregular astigmatism. The early lenses, however, were uncomfortable and could not be worn for long. Until the development of optical instruments that could measure the curvature of the cornea (the transparent surface of the eye that covers the iris and the pupil), the contact lens was made by taking an impression of the eye and fashioning a lens on a mold.Contact lenses most effectively neutralize visual defects arising from irregular curvatures of the cornea. They are the preferred treatment for some varieties of astigmatism and aphakia (absence of the eye's crystalline lens). They also can be functionally and cosmetically appealing substitutes for eyeglasses to treat myopia (nearsightedness) and other visual defects.In the mid-1900s, plastic-based contact lenses were designed that rested on a cushion of tears on the cornea, covering the area over the iris and pupil. These older hard-plastic contact lenses had a limited wearing time because of potential irritation of the cornea, and they required a period of adaptation when first worn. Both front and back surfaces of the hard contact lens are spherically curved, altering refractive properties by changing the shape of the tear film on the eye's surface, which conforms to the curve of the rear surface of the contact lens, and by a difference in curvature between the two surfaces of the lens itself. In the 1970s, gas-permeable rigid contact lenses were developed that allowed much more oxygen to pass through to the corneal surface, thus increasing comfort and wear time.Also in the 1970s, larger “soft” lenses, made from a water-absorbing plastic gel for greater flexibility, were introduced. Soft contact lenses are usually comfortable because they allow oxygen to penetrate to the eye's surface. Their large size makes them more difficult to lose than hard lenses. Their delicacy, however, makes them more subject to damage, and, as with all contact lenses, they require careful maintenance. They are less effective than hard lenses in treating astigmatism, because they reflect the underlying corneal curvature more closely. In 2005 hybrid lenses were developed that are gas-permeable and rigid and surrounded by a soft ring. These lenses provide the comfort of a soft lens with the visual sharpness of a hard lens.Contact lenses have particular advantages in treating certain defects that can be corrected only partially by prescription eyeglasses; for example, contact lenses avoid the distortion of size that occurs with thick corrective lenses. However, most contact lenses cannot be worn overnight, as this significantly increases the risk of serious corneal infections.Contact lenses can also be used in certain situations to protect the corneal surface during healing and to relieve discomfort derived from corneal surface problems.
* * *