|
|
(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 contactsbargain contact. Some occupations, such as construction or auto repair, may require safety lenses and safety frames. Physicians and employers should be consulted for recommendations. contactsbargain Description Eyes are examined contactsbargain by optometrists (O.D.) or by ophthalmologists (M.D. contactsbargain 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. Glass was the first material to be used for lenses lenses, and was used for several hundred years before plastic was introduced. The crown glass used for lenses lenses has an index of refraction of 1.52.Optical-quality acrylic 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 can also choose between polycarbonate, which is the contactsbargain 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 contactsbargain lenses, 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 prescriptions.
|