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(Lenses made from polycarbonate, the same type of plastic used for the space shuttle windshield, are about 50 times stronger than other lens materials.) Also, people at risk of receiving electric shock should avoid metal frames.People employed in certain occupations may be prohibited from wearing contact, or may be required to wear safety eyewear over the contact. Some occupations, such as construction or auto repair, may require safety lenses and safety frames. Physicians and employers should be consulted for recommendations. Description Eyes are examined by optometrists (O.D.) or by ophthalmologists (M.D. or D.O.--doctor of osteopathy). Prescriptions, if necessary, are then given to patients for glasses. The glasses are generally made by an optician. A separate lenses-fitting exam is necessary if the patient wants contact, because an lenses prescription is not the same as a lenses prescription.

Over 32 million people in the United States wear these small lenses that fit on top of the cornea. They provide a field of view unobstructed by lenses frames; they do not fog-up or get splattered, so it is possible to see well while walking in the rain; and they are less noticeable than any lenses style. On the other hand, they take time to get accustomed to; require more measurements for fitting; require many follow-up visits to the eye doctor; can lead to complications such as infections and corneal damage; and may not correct astigmatism as well as lenseses, especially if the astigmatism is severe.

contact The concept of corrective lenses that sit directly on the eyeball was developed in the ninth century, but it was not until the late 20th century that manufacturing and grinding techniques for contact were perfected. Originally contact were made of glass, then later of hard plastic, and still later of flexible, highly oxygen-permeable soft plastic. Today, the majority of lenses worn are of two basic types: rigid gas permeable (so-called "hard" lenses) and lenses. best lenses are smaller and more durable, but require a longer period of adjustment for the wearer. Hard contact can correct some vision problems, like astigmatism, that the lenses cannot. The lenses are slightly larger than best lenses. They are more comfortable and less expensive, but can be easily torn. The latest generations of contact are extended and disposable lenses. extended lenses are intended to be worn for several weeks at a time, while disposable lenses are thrown away every day or every few weeks, or in the newest versions, every day.