Fix Errors in Vision with Contact Lenses

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Dr. Bryan Dempsey is in private practice and well known for his contribution in health and fitness.

Contribution from Bryan Dempsey

By Bryan Dempsey

Fix Errors In Vision
Safe Contact Lances

Contacts lens are devices thatcorrect refractive errors in vision. Eyeglass lenses are mounted in frames wornon the face, sitting mostly on the ears and nose, so that the lenses are positioned in front of the eyes. Contact lenses appear to be worn in directcontact with the cornea, but they actually float on a layer of tears thatseparates them from the cornea.

Polycarbonate Lens
Contact lenses purpose is to correct or improve the vision of people with nearsightedness (myopia), farsightedness (hyperopia), presbyopia, and astigmatism. People allergic to certain plastics should not wear contact lenses or eyeglass frames or lenses manufactured from that type of plastic. People allergic to nickel should not wear Flexon frames. People at risk of being in accidents that might shatter glass lenses should wear plastic lenses, preferably polycarbonate.

Safety Eyewear
Contact lenses may be prohibited for People employed in certain occupations, or may be required to wear safety eyewear over the contact lenses. Some occupations, such as construction or auto repair, may require safety lenses and safety frames. Physicians and employers should be consulted for recommendations. Eyes are examined by optometrists (O.D.) or by ophthalmologists (M.D. or D.O.--doctor of osteopathy).

Opticians Make Them
Prescriptions, if necessary, are then given to patients for glasses. The glasses are generally made by an optician. A separate contact lens-fitting exam is necessary if the patient wants contact lenses, because an eyeglass prescription is not the same as a contact lens 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.

Greater Lens
Lenses work by changing the direction of light so that images come into focus on the retina. The greater the index of refraction of the lens material and the greater the difference in the curvature between the two surfaces of the lens, the greater the change in direction of light that passes through it, and the greater the correction.

The Focals
Lenses can be unifocal, with one correction for all distances, or they can be correct for more than one distance (multifocal). One type of multifocal, the bifocal, has an area of the lens (usually at the bottom) that corrects for nearby objects (about 14 in from the eyes); the remainder of the lens corrects for distant objects (about 20 ft from the eyes). Another type of multifocal, a trifocal, has an area in-between that corrects for intermediate distances (usually about 28 in).

Conventional Focals
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.

Must Be Transparent
To be suitable for eyeglass 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.

Optical-Quality Acrylic
Optical-quality acrylic was introduced for eyeglass 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.

Polycarbonate Or Polyurethane
Today, eyeglass wearers can also choose between polycarbonate, which is the 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 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. They are flatter and lighter than conventional lenses.

Rigid Gas-Permeable
Rigid gas-permeable (RGP) daily-wear lenses are made of plastic that does not absorb water but allows oxygen to get from the atmosphere to the cornea. (This is important because the cornea has no blood supply and needs to get its oxygen from the atmosphere through the film of tears that moves beneath the lens.) They must be removed and cleaned each night. Rigid gas-permeable (RGP) extended-wear lenses are made from plastic that also does not absorb water but is more permeable to oxygen than the plastic used for daily-wear lenses. They can be worn up to a week.

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