Eyeglasses and contact lenses
Eyeglasses and contact lenses are devices that correct refractive errors in vision. Eyeglass lenses are mounted in frames that are worn on 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 direct contact with the cornea, but they actually float on a layer of tears that separates them from the cornea.
The purpose of eyeglasses and contact lenses is to correct or improve the vision of people with nearsightedness ( myopia ), farsightedness (hyperopia), presbyopia, and astigmatism.
Eyes are examined by optometrists (OD) or by ophthalmologists (MD or DO). Prescriptions, if necessary, are then given to patients for glasses. Eyeglasses are generally made by an optician. A separate contact lens-fitting exam is necessary if an individual wants contact lenses, because an eyeglass prescription can differ from a contact lens prescription.
More than 140 million people in the United States wear eyeglasses. People whose eyes have refractive errors do not see clearly without glasses, because the light emitted from the objects they are observing does not come into focus on their retinas. For people who are farsighted, images come into focus behind the retina; for people who are nearsighted, images come into focus in front of the retina. For both, the result is a blurring of vision.
LENSES 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.
Lenses can be unifocal, with one correction for all distances, or they can 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 [35 cm] from the eyes); the remainder of the lens corrects for distant objects (about 20 ft [6 m] from the eyes). Another type of multifocal, a trifocal, has an area in-between that corrects for intermediate distances (usually about 28 in [71 cm]) Conventional bifocals and trifocals have visible lines between the areas of different correction; however, lenses in which 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 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.
Glass was the first material to be used for eyeglass lenses and was used for several hundred years before plastic was introduced. The crown glass used for eyeglass lenses has an index of refraction of 1.52.
In the early 2000s eyeglass wearers can choose polycarbonate or polyurethane materials for their lenses. Polycarbonate is the most impact-resistant material available for eyewear, and polyurethane 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. Parents whose children have 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.
There are many lenses and lens-coating options for individual needs, including coatings that block the ultraviolet (UV) light and/or blue light which have been found to be harmful to the eyes. Such coatings are not needed on polycarbonate lenses, which already have UV protection. UV coatings are particularly important on sunglasses and ski goggles. Sunglasses, when nonprescription, should be labeled with an indication that they block out 99 to 100 percent of both UV-A and UV-B rays.
There are anti-scratch coatings that increase the surface hardness of lenses (an important feature when using plastic lenses) and anti-reflective (AR) coatings that eliminate almost all glare and allow other people to see the eyes of the wearer. AR coatings may be particularly helpful to people who use computers or who drive at night. Mirror coatings that prevent other people from seeing the wearer's eyes are also available. There is a whole spectrum of tints, from light to dark, used in sunglasses. Tint, however, does not block-out UV rays, so a UV coating is needed. Polaroid lenses that block out much of the reflected light also allow better vision in sunny weather and are helpful for people who enjoy boating. Photosensitive (photochromatic) lenses that darken in the presence of bright light are handy for people who do not want to carry an extra set of glasses. Photochromatic lenses are available in glass and plastic.
FRAMES Frames can be made from metal or plastic, and they can be rimless. There is an almost unlimited variety of shapes, colors, and sizes. The type and degree of refractive correction in the lens determine to some extent the type of frame most suitable. Some lenses are too thick to fit in metal rims, and some large-correction prescriptions are best suited to frames with small-area lenses.
Rimless frames are the least noticeable type, and they are lightweight because the nosepiece and temples are attached directly to the lenses, eliminating the weight of the rims. They tend not to be as sturdy as frames with rims, so they are not a good choice for children, or for people who frequently remove their glasses and put them on again. They are also not very suitable for lenses that correct a high degree of farsightedness, because such lenses are thin at the edges.
Metal frames are less noticeable than plastic, and they are lightweight. They are available in solid gold, gold-filled, anodized aluminum, nickel, silver, stainless steel, and titanium and titanium alloy. Until the late 1980s, when titanium-nickel alloy and titanium frames were introduced, metal frames were, in general, more fragile than plastic frames. Titanium frames, however, are very strong and lightweight. An alloy of titanium and nickel, called Flexon, is strong, lightweight, and returns to its original shape after being twisted or dented. It is not perfect for everyone, though, because some young people are sensitive to its nickel content. Flexon frames are also relatively expensive.
Plastic frames are durable, can accommodate just about any lens prescription, and are available in a wide range of prices. They are 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 need to be adjusted as frequently as other types. Nylon frames are almost unbreakable. They revert to their original shape after extreme trauma and distortion; because of this property, though, they cannot be readjusted after they are manufactured.
FIT An individual should have the distance between the eyes (PD) measured, so that the optical centers of the lenses will be in front of the person's pupils. Bifocal heights also have to be measured with the chosen frame in place and adjusted on the person. Again, this is so that the lenses will be positioned correctly. If not positioned correctly, the individual may experience eyestrain, headache , or other problems.
Children may sometimes need a few days to adjust to a new prescription. However, problems should be reported, because the glasses may need to be rechecked.
Over 32 million people in the United States wear these small lenses that fit on top of the cornea. They provide a field of view unobstructed by eyeglass frames. They do not fog up or get splattered, so it is possible to see well while walking in the rain. They are less noticeable than any style of eyeglass. On the other hand, they take time to get accustomed to; require more measurements for fitting; require many follow-up visits to an eye doctor; can lead to complications such as infections and corneal damage; and may not correct astigmatism as well as eyeglasses, especially if the astigmatism is severe.
Originally, hard contact lenses were made of a material called PMMA. The more common types of contact lenses are listed below:
- 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.) These lenses 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.
- Daily wear soft lenses are made of plastic that is permeable to oxygen and absorbs water; therefore, they are soft and flexible. These lenses must be removed and cleaned each night, and they do not correct all vision problems. Soft lenses are easier to get used to than rigid lenses but are more prone to tears and do not last as long.
- Extended-wear soft lenses are highly permeable to oxygen, are flexible by virtue of their ability to absorb water, and can usually be worn for up to one week. They do not correct all vision problems. There is more risk of infection with extended-wear lenses than with daily-wear lenses.
- Extended-wear disposable lenses are soft lenses worn continually for up to six days and then discarded, with no need for cleaning.
- Planned-replacement soft lenses are daily-wear lenses that are replaced on a regular schedule, which is usually every two weeks, monthly, or quarterly. They must also be cleaned.
Soft contact lenses come in a variety of materials. There are also different kinds of RGP and soft multifocal contact lenses available. Monovision, where one contact lens corrects for distance vision while the other corrects for near vision, may be an option for persons with presbyopia. Monovision, however, may affect depth perception and may not be appropriate for everyone. Contact lenses also come in a variety of tints. Soft contacts are available that can change the color of dark-colored eyes. Even though such lenses have no prescription, they must be fitted and checked to make sure that an eye infection does not occur. People should never wear someone else's contact lenses. Doing so can lead to infection or damage to the eye.
Young 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. Children and teens at risk of being in accidents that might shatter glass lenses should wear plastic lenses, preferably polycarbonate. (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, young people whose work places them at risk of receiving electric shock should avoid metal frames.
People employed in certain occupations may be prohibited from wearing contact lenses 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.
Astigmatism —An eye condition in which the cornea doesn't focus light properly on the retina, resulting in a blurred image.
Cornea —The clear, dome-shaped outer covering of the eye that lies in front of the iris and pupil. The cornea lets light into the eye.
Index of refraction —A constant number for any material and any given color of light that is an indicator of the degree of bending of the light caused by that material.
Lens —The transparent, elastic, curved structure behind the iris (colored part of the eye) that helps focus light on the retina. Also refers to any device that bends light waves.
Permeable —A condition in which fluid or certain other substances are allowed to pass through.
Polycarbonate —A very strong type of plastic often used in safety glasses, sport glasses, and children's eyeglasses. Polycarbonate lenses have approximately 50 times the impact resistance of glass lenses.
Presbyopia —A condition affecting people over the age of 40 where the system of accommodation that allows the eyes to focus on near objects fails to work because of age-related hardening of the lens of the eye.
Retina —The inner, light-sensitive layer of the eye containing rods and cones. The retina transforms the image it receives into electrical signals that are sent to the brain via the optic nerve.
Ultraviolet (UV) radiation —A portion of the light spectrum with a wavelength just below that of visible light. UV radiation is damaging to DNA and can destroy microorganisms. It may be responsible for sunburms, skins cancers, and cataracts in humans. Two bands of the UV spectrum, UVA and UVB, are used to treat psoriasis and other skin diseases.
Contact lens wearers must be examined periodically by their eye doctors to make sure that the lenses fit properly
To avoid infection, it is important for contact lens wearers to exactly follow their instructions for lens insertion and removal, as well as cleaning. Soft contact lens wearers should never use tap water to rinse their lenses or to make-up solutions. All contact lens wearers should also always have a pair of glasses and a carrying case for their contacts with them, in case the contacts have to be removed due to eye irritation. Wearing contact lenses increases the risk of corneal damage and eye infections.
Improved vision is the primary result of corrective lenses. The normal expectation is that people will achieve 20/20 vision while wearing corrective lenses. Contact lenses may contribute to improved cosmetic appearance for some users.
Parents of young children requiring corrective lenses should be prepared for broken or lost glasses. Parents of adolescents who wear contact lenses should help their children to maintain a regular cleaning schedule. Glasses normally last one to two years. Growing children may require changes of prescription more frequently than people in other age groups.
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