Originally, lenses hard contact were made of a material called PMMA. Although still available, the contacts more common types of contact are listed below:Rigid gas-permeable (RGP) daily-wear lenses are made of plastic that does not absorb water vision but allows oxygen to get from the atmosphere to the cornea. (This lenses is important because the contacts and vision cornea has no blood supply and needs to get its oxygen lenses and contacts from the atmosphere through vision the film of tears that moves beneath lenses the lens.) They must be removed and cleaned each night. Rigid gas-permeable contacts (RGP) extended-wear lenses are made from plastic that also does not absorb water but is more vision and lenses permeable to oxygen than the plastic used for daily-wear lenses. Another type of multifocal, a trifocal, has an area in-between that corrects for intermediate distances contacts (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 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 vision from either glass or plastic (hard resin). The advantage of plastic lenses is that it is lightweight and more impact resistant than glass. The advantage of glass contacts is that it is scratch resistant and provides the clearest vision and lenses possible vision. (Lenses made from polycarbonate, the same type of plastic used for the space shuttle windshield, contacts are about 50 times stronger than other lens materials.) Also, people at risk of receiving vision electric shock should avoid metal frames.People employed in certain occupations may be prohibited lenses from wearing contact, or may be required to wear contacts safety eyewear over the contact. Some occupations, such as construction or auto repair, may vision require safety lenses and safety frames. Physicians and employers should be consulted for recommendations. Description Eyes are examined lenses by optometrists (O.D.) or by ophthalmologists (M.D. or D.O.--doctor contacts of osteopathy). Prescriptions, vision if necessary, are then given to patients for glasses. lenses The glasses are generally made by an optician. A separate lenses-fitting exam is necessary if the patient wants contact, because contacts an lenses prescription is not the same as a lenses prescription vision.
|