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Another contacts type of multifocal, a trifocal, has an area in-between disposable that corrects for intermediate distances (usually about 28 in). Conventional bifocals and trifocals have visible lines between the areas of different correction; contacts 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. (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 disposable be required to wear safety eyewear over the contact. Some occupations, such as contacts construction or auto repair, may require safety lenses and safety frames. Physicians disposable 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). contacts Prescriptions, if necessary, are then given to patients for glasses. The disposable 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. Plastic frames are durable, can accommodate just about any lens prescription, and are available in a wide range of prices. They are contacts also offered in a variety of plastics (including acrylic, epoxy, cellulose acetate, cellulose propionate, polyamide, and nylon) and in different colors, shapes, and levels of resistance to breakage. Epoxy frames are resilient and return to their original shape after being deformed, so they do not disposable need to be adjusted as frequently contacts as other types. Nylon frames are almost unbreakable. They revert to their original shape after extreme trauma and distortion; disposable because of this property, though, they cannot contacts be readjusted after they are manufactured.Fit The patient should have the distance between the eyes (PD) measured, so that the optical centers of the lenses will be in front of the patient''s pupils. Bifocal heights also have to be measured with the chosen frame in place disposable and adjusted on the patient. Again, this is so the lenses will be positioned correctly. If not positioned correctly, the patient may experience eyestrain or other problems. contacts This can occur with over-the-counter reading glasses disposable.
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