(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 sfx 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 theatrical lenses-fitting exam is necessary if the patient wants contact, because an lenses prescription is not the same as a lenses prescription.

(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 sfx 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 theatrical lenses-fitting exam is necessary if the patient wants contact, because an lenses prescription is not the same as a lenses prescription.

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 contacts 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 sfx 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.