Optics and Refraction
Babies have poor vision at birth but can see faces at close range, even in the newborn nursery. At about six weeks a baby's eyes should follow objects and by four months should work together. Over the first year or two, vision develops rapidly. A two-year-old usually sees around 20/30, nearly the same as an adult.
Parents should be aware of signals of poor vision. If one eye turns or crosses, that eye may not see as well as the other eye. If the child is uninterested in faces or age-appropriate toys, or if the eyes rove around or jiggle (nystagmus), poor vision should be suspected. Other signs to watch for are tilting the head and squinting. Babies and toddlers compensate for poor vision rather than complain about it.
Should a baby need glasses, the prescription can be determined fairly accurately by dilating the pupil and analyzing the light reflected through the pupil from the back of the eye.
A baby's vision can also be tested in a research laboratory where brain waves are recorded as the child looks at stripes or checks on a TV screen. The test is called Visual Evoked Potential (VEP). Another test called preferential looking or Teller Acuity Cards uses simple striped cards to attract the child's attention. In both tests, as the stripes grow smaller, the child eventually does not respond (with brain waves or by looking at the stripes).
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Childhood Reading Problems
When children have difficulty reading, parents often think poor vision is the problem. If a visit to an ophthalmologist rules out any medical or vision problems, it may be a learning disability.
A learning disability is a disparity between a person's ability and performance in a certain area. It has nothing to do with intelligence or IQ. A learning disability can make it difficult to succeed in school and, if untreated, gets worse, causing a child to lose self-confidence and interest in school.
Identifying the learning disability is the first step in treating it. Dyslexia, a reading disability that may involve reversing letters and words, is one of the many learning disorders that can affect reading.
Exercises have been used to improve the coordination or focusing of the eyes. Since poor reading is not usually an eye problem, these exercises rarely prove helpful. Colored lenses, special diets or vitamins, jumping on trampolines, or walking on balance beams have also been prescribed without much success. Over time, these methods have tended to fall out of favor.
Children with learning disabilities benefit from various educational programs, in or out of school. Parents also play a vital role. They can support their children by reading with them at home. Children with learning disabilities need to be encouraged to develop strengths and interests so they can fully develop their unique talents and abilities.
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Children and Vision
People are often confused about the importance of glasses for children. Some believe that if children wear glasses when they are young, they won't need them later. Others think wearing glasses as a child makes one dependent on them later. Neither is true. Children need glasses because they are genetically nearsighted, farsighted, or astigmatic. These conditions do not go away nor do they get worse because they are not corrected. Glasses or contacts are necessary throughout life for good vision.
Nearsightedness (distant objects appear blurry) typically begins between the ages of eight and fifteen but can start earlier. Farsightedness is actually normal in young children and not a problem as long as it is mild. If a child is too farsighted, vision is blurry or the eyes cross when looking closely at things. This is usually apparent around the age of two. Almost everyone has some amount of astigmatism (oval instead of round cornea). Glasses are required only if the astigmatism is strong.
Unlike adults, children who need glasses may develop a second problem, called amblyopia or lazy eye. Amblyopia means even with the right prescription, one eye (or sometimes both eyes) does not see normally. Amblyopia is more likely to occur if the prescription needed to correct one eye is stronger than the other. Wearing glasses can prevent amblyopia from developing in the more out-of-focus eye.
Children (and adults) who do not see well with one eye because of amblyopia, or because of any other medical problem that cannot be corrected, should wear safety glasses to protect the normal eye.
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Children's Eye Safety
Accidents resulting in serious eye injuries can happen to anyone, but are particularly common in children and young adults. More than 90% of all eye injuries can be prevented with appropriate supervision and protective eyewear.
Goggles and face protection can prevent injuries in sports like baseball, basketball, racket sports, and hockey. It is more difficult to protect against injuries in boxing, though thumbless gloves help.
People who must rely on only one good eye should wear polycarbonate safety glasses all the time and should wear safety goggles for sports and other dangerous activities. Choose frames and lenses that meet the American National Standards Institute standard for safety (Z87.1).
Appropriate adult supervision is key in preventing all eye injuries. Children should never be allowed to play with fireworks or BB guns. Sharp and fast-moving objects, such as darts, arrows, scissors, knives, and even pencils or pens can be dangerous. Special care should be taken when working around lawn mowers, which can throw rocks and debris, and when banging two pieces of metal together, which can dislodge small shards of metal. Chemicals such as toilet cleaners and drain openers are especially hazardous.
A primary care physician or an emergency room can treat minor injuries, such as a foreign body or an abrasion (scratch) on the cornea. Any foreign material must be removed from the eye. An antibiotic drop or ointment may be applied, perhaps with an eye patch for comfort.
More serious injuries, like blood inside the eye (hyphema), a laceration (cut), or rupture of the eye, require examination by an ophthalmologist. Both surgery and hospitalization may be necessary.
Chemicals that burn should be rinsed from the eye immediately. The ultimate outcome depends on the severity of the injury, which cannot always be identified in the initial examination.
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Color blindness (color vision deficiency) is a condition in which certain colors cannot be detected. There are two types of color vision difficulties: inherited (congenital) problems that you have at birth, and problems that develop later in life.
People born with color vision problems are unaware what they see is different from what others see unless it is pointed out to them. People with acquired color vision problems are aware that something has gone wrong with their color perception.
Congenital color vision defects usually pass from mother to son. These defects are due to partial or complete lack of the light-sensitive photoreceptors (cones) in the retina, the layer of light-sensitive nerve cells lining the back of the eye. Cones distinguish the colors red, green and blue through visual pigment present in the normal human eye. Problems with color vision occur when the amount of pigment per cone is reduced or one or more of the three cone systems are absent. This limits the ability to distinguish between greens and reds, and occasionally blues. It involves both eyes equally and remains stable throughout life.
There are different degrees of color blindness. Some people with mild color deficiencies can see colors normally in good light but have difficulty in dim light. Others can't distinguish certain colors in any light. In the most severe form of color blindness everything is seen in shades of gray.
Except in the most severe form, color blindness does not affect the sharpness of vision at all. It does not correlate with low intelligence or learning disabilities.
Most color vision problems that occur later in life are a result of disease, trauma, toxic effects from drugs, metabolic disease, or vascular disease. Color vision defects from disease are less understood than congenital color vision problems. There is often uneven involvement of the eyes and the color vision defect will usually be progressive. Acquired color vision loss can be the result of damage to the retina or optic nerve.
There is no treatment for color blindness. It usually does not cause any significant disability. It can, however, prevent employment in an increasing number of occupations.
Change in color vision can signify a more serious condition. Anyone who experiences a significant change in color perception should see an ophthalmologist.
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Over 24 million people choose contact lenses to correct vision. When used with care and proper supervision, contacts are a safe and effective alternative to eyeglasses. And with today's new lens technology, many people who wear eyeglasses can also successfully wear contacts.
Contacts are thin, clear discs that float on the tear film that coats the cornea, the curved front surface of the eye. Contacts correct the same refractive conditions eyeglasses correct: myopia (nearsightedness), hyperopia (farsightedness) and astigmatism (an oval- rather than round-shaped cornea).
Contact lenses can be made from a number of different plastics. The main distinction among them is whether they are hard or soft. Most contact lens wearers in the United States wear soft lenses. These may be daily wear soft lenses, extended wear lenses or disposable lenses. Toric soft lenses provide a soft lens alternative for people with slight to moderate astigmatism.
Hard lenses are usually not as comfortable as soft lenses and are not as widely used. However, rigid gas permeable lenses provide sharper vision for people with higher refractive errors or larger degrees of astigmatism.
The majority of people can tolerate contact lenses, but there are some exceptions. Conditions that might prevent an individual from successfully wearing contact lenses include dry eye, severe allergies, frequent eye infections, or a dusty and dirty work environment.
Individuals who wear any type of contact lens overnight have a greater chance of developing infections in the cornea. These infections are often due to poor cleaning and lens care.
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Contacts and Cosmetics
Contact lens wearers who wear cosmetics on a daily basis may be especially vulnerable to eye problems. Misuse of products and adverse reactions to ingredients used in cosmetic formulas cause lens deposits, eye irritation, allergy, dryness, injury and infection. Knowing which products to use and how to use them is important for long-term, problem-free contact lens wear.
Before handling lenses, wash your hands with a mild soap such as Neutrogena, Ivory or a clear glycerin soap. Or, use one of the specialty soaps for contact lens wearers such as AOSoap or Optisoap. Avoid soaps containing cream, deodorant, antiseptics, or heavy fragrances.
Contact lenses should be inserted before any cosmetics are applied to prevent contaminating the lens by makeup and disrupting makeup by tears. Mascara should be used sparingly and only on the outer half of the lashes. Besides being a potential irritant, mascara is frequently a source of infection. Even with the best of care, mascara and eyeliner should be replaced every three months. Use a light touch with eyeliners and shadows, as they may cause blepharitis, an infection of the eyelid that can lead to styes and chalazion. Don't use eye liner pencils inside the lower eyelid. Color pigments can cause irritation, damage contact lenses, or lodge underneath the contact lens and scratch the cornea.
Prevent contamination of your makeup by keeping it dry and avoiding contact with fingers. Keep applicators clean and replace them after approximately three months. Hair spray, deodorant, cologne, mousse, nail polish and nail polish remover should be used before inserting your lenses. If one of these products gets into your eye it can cause permanent damage to the contact lens surface. If you must use hair spray while wearing contacts, close your eyes tightly while spraying and then leave the area quickly. Aerosol mist lingers in the air for some time after spraying.
Never wear contacts when using hair dyes, permanent wave lotions, or medicated shampoos.
Use cosmetics labeled "hypoallergenic," "for contact lens wearers," or "for sensitive eyes." Approximately one in ten women have either a respiratory or skin allergy to perfume. Hypoallergenic brands are designed be free of irritants such as perfumes and lanolin. Lanolin may be used in cosmetics and soaps and is one of the most common allergens, causing redness, itching, and blotchy skin spots.
Wash your hands and remove contact lenses before removing make-up. Your fingers are less likely to be contaminated by pigments, creams and oils from cosmetic products when the lenses are removed first.
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Extended-Wear Contact Lenses
Some people do not consider wearing contact lenses because they think the required cleaning, disinfecting, storing, and inserting are too much trouble. They may also want the option of occasionally napping or sleeping with their contacts in their eyes.
Extended-wear contacts are designed to appeal to these people. They require less maintenance than daily wear lenses and because they are thinner and allow more oxygen to reach the eyes, they may be left in the eye overnight.
To use extended-wear contact lenses, you must be free of external eye disease, have normal tear function, and be motivated to take care of them.
Infection is the most significant complication of extended-wear contact lens use. They must be removed at least once a week and thoroughly cleaned and disinfected. Many studies show the cornea is put at increased risk of infection by wearing contact lenses overnight. The risk of developing an infection in the cornea is 10-15 times greater for those who wear extended-wear contacts overnight than for those who use daily wear soft lenses. This risk increases with the number of consecutive days the contacts are worn overnight. Infections may range from simple conjunctivitis to blinding endophthalmitis, which is a serious infection that travels through all layers of the eye.
The decision to accept the risks and benefits of extended-wear contacts requires a process of evaluation between you and your doctor. Once you are carefully fit for your contact lenses, follow-up exams with your ophthalmologist to ensure continuing eye health is important. As with any contact lens, extended-wear contacts should be removed at the first sign of redness or discomfort.
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Eyeglasses for Infants and Children
Prescriptions for glasses can be measured in even the youngest and most uncooperative children by using a special instrument called a retinoscope to analyze light reflected through the pupil from the back of the eye.
Most lenses today, especially for children, are made of plastic, which is stronger and lighter than glass. It is a good idea to get a scratch-resistant coating on plastic lenses. Children can be rough with glasses and plastic lenses scratch easily.
Color tints or tints that respond to changes in light can be incorporated into lenses. For children, the tint should not be so dark that the child has trouble seeing indoors.
Frames come in all shapes and sizes. Choose one that fits comfortably but securely. There are devices available to keep glasses in place, a good idea for active children and young children with flat nasal bridges. Cable temples, which wrap around the back of the ears, are good for toddlers. Infants may require a strap across the top and back of the head instead of earpieces. Flexible hinges hold glasses in position, allow the glasses to "grow" with the child, and prevent the side arms from being broken.
Children often do not like their glasses although the prescription is correct. Distraction, positive reinforcement, and bribery help children get in the habit of wearing glasses. If all else fails, your ophthalmologist can prescribe an eye drop that blurs vision when the glasses are not in place. This often overcomes the child's initial resistance to wearing glasses.
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Sixty percent of the 161 million Americans who wear prescription eyewear choose eyeglasses. Wearing eyeglasses is one of the simplest ways to correct vision problems.
To see images clearly, light rays must focus directly on the retina, the light-sensitive nerve layer that lines the back of the eye. There are different kinds of focusing problems, called refractive errors, which may require corrective lenses. In the case of myopia or nearsightedness, the eye is too long. Light rays focus before reaching the retina and images appear blurry. In hyperopia or farsightedness, the eye is too short, so light rays have not yet focused when they reach the retina. Astigmatism describes an eye with a cornea that is oval shaped instead of round, causing light rays to hit the retina in more than one place.
Eyeglass lenses compensate for an eye that is too long or too short by adding or subtracting focusing power. The lenses create just the right amount of focusing power so light rays focus directly on the retina.
A plus (+) in front of the first number of the eyeglass prescription means the lens corrects farsightedness. A minus (-) in front of the first number means the lens corrects nearsightedness. If a second and third number are present in the prescription, they indicate astigmatism. The higher the first number in the prescription, the greater the correction in the lens.
Lenses are available in glass, regular and high index plastic, and polycarbonate. Although they scratch less easily, glass lenses tend to be heavier and often slide down the nose. Plastic and polycarbonate lenses are lighter and safer than glass but scratch easily. Scratches cannot be removed but they can be avoided or minimized with appropriate care. Scratch resistant coatings can be applied to plastic and polycarbonate lenses but some of these coatings crack if exposed to extreme heat or cold.
Frames come in many shapes and sizes, so it is important to pick a frame that is best for you. Factors to take into consideration when selecting a frame include facial features, age, activities and the prescription itself. Often a strong prescription requires thicker lenses, which can affect your choice of frames. Ask about the quality and expected lifetime of the frame and if there is a frame guarantee.
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Giant Papillary Conjunctivitis (GPC)
Giant papillary conjunctivitis (GPC) is an inflammation of the inner surface of the eyelids, most frequently associated with contact lens wear. It can develop in people who wear either soft or rigid gas permeable contact lenses and can occur at any time, even if an individual has successfully worn contacts for a number of years. Although not vision threatening, GPC can be inconvenient and may require one to stop wearing contacts temporarily or even permanently.
The typical symptoms of GPC include red, irritated eyes, often with itching and mucus discharge. Blurred vision and light sensitivity can also occur. GPC is not an infection, but a hypersensitivity of the membrane covering the inner lids and the whites of the eyes. The inner lining of the eyelid becomes roughened and inflamed by constant blinking over a contact lens or other foreign body such as an artificial eye. Hard, flat elevations in a cobblestone pattern develop on the undersurface of the upper eyelid. Eventually the entire eye becomes irritated.
In most cases, treatment of GPC involves discontinuing the use of contact lenses to allow the eye to rest. Eyedrops are frequently prescribed to control inflammation. Many people find their symptoms are relieved when contact lens wear is discontinued. Unfortunately, the symptoms can return when lens wear is resumed.
Once GPC is under control, it may be helpful to consider changing to new contacts or disposable contacts. Changing lens care systems and cleansing solutions can also be helpful. After an episode of GPC, limit the amount of time lenses are worn, and increase the time slowly.
Once it develops, GPC may be an ongoing problem. Prolonged GPC may be more difficult to treat.
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How to Care for Contact Lenses
The key to avoiding the irritation and infection sometimes associated with contact lens wear is proper cleaning.
There are two main types of lens care systems: heat and chemical disinfection. The appropriate choice depends on the lens type, duration of lens wear and an individual's own biochemistry. Regardless of the type of disinfection system you choose there are a number of common steps that must be followed.
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Over three million people in the United States do not have normal vision even with corrective lenses. If ordinary eyeglasses do not provide clear vision, one is said to have low vision. This should not be confused with blindness. People with low vision still have useful vision that can often be improved with low-vision devices.
Low vision can result from birth defects, inherited diseases, injuries, diabetes, glaucoma or macular degeneration. Although reduced central or reading vision is most common, a person can have low vision in their side (peripheral) vision, or a loss of color vision or contrast sensitivity.
Low vision devices or aides are available in optical and non-optical types. Optical devices use lenses or combinations of lenses to provide magnification. They should not be confused with standard eyeglasses. There are five main kinds of optical devices: magnifying spectacles, hand magnifiers, stand magnifiers, telescopes and closed-circuit television. Different devices may be needed for different purposes. If possible, try the optical device before purchasing it and be sure you understand how to use it.
The simplest non-optical technique is to bring the object of interest closer. Non-optical low vision devices include large print books, check writing guides, enlarged phone dials, talking appliances (timers, clocks, computers), and machines that scan print and read out loud.
Government and private agencies have social services available for people with low vision. For more information, contact the following resources:
Veterans may contact the Visual Impairment Services coordinator at their local VA facility.
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Myopic degeneration is an uncommon condition characterized by progressive stretching of the eye that damages the retina, the layer of light-sensitive cells that lines the back of the eye. People with severe nearsightedness (high myopia) are at greater risk for myopic degeneration.
Myopic degeneration commonly occurs during young adulthood with a gradual decrease in central vision. Vision can decrease more abruptly, but typically vision loss is gradual. Although central vision may be lost, side (peripheral) vision usually remains unaffected. Remaining sight can still be very useful and with the help of low-vision optical devices, people can continue many of their normal activities.
The causes of myopic degeneration are not clearly understood but may include biomechanical abnormalities or hereditary factors. The biomechanical theory assumes that the retina, in a myopic eye, is stretched over a larger than normal area because the eye is longer than usual. Over time, the outer coat of the eye, known as the sclera, also stretches in response to forces like internal eye pressure. This stretching of the sclera is thought to lead to retinal degeneration. In the hereditary theory, the retinal changes are thought to be an unavoidable, inherited process.
The only treatment for myopic degeneration is surgery to reinforce the scleral wall. This has been performed with varying degrees of success.
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To see clearly, light rays must be bent or refracted to focus on the retina, the light- sensitive nerve layer that lines the back of the eye. The cornea and lens of the eye work together to bend or refract light rays and bring them together on the retina. If a refractive error is present, the light is not focused directly on the retina, so images appear blurry.
Myopia (nearsightedness): Distance vision is impaired when the eye is too long in relation to the curvature of the cornea. This causes light to focus before it reaches the retina. Close objects look clear but distant objects appear blurry.
Hyperopia (farsightedness): Close vision is impaired, with some impairment of distance vision, as well. The eye is too short in relation to the curvature of the cornea. Light rays are not yet in focus when they reach the retina, so images appear blurry.
Astigmatism (the cornea is oval shaped instead of round): The irregular curvature of the cornea causes light to focus on more than one point on the retina. Uncorrected astigmatism impairs both distance and near vision.
Presbyopia (aging eyes): When young, the lens of the eye is soft and flexible, allowing people to see objects both close and far away. After the age of 40, the lens of the eye becomes more rigid, making it more difficult for the lens to change its shape, or accommodate, to do close work such as reading. This condition is known as presbyopia and is the reason reading glasses or bifocals are necessary at some point after age forty.
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Strabismus refers to misaligned eyes. If the eyes turn inward (crossed), it is called esotropia. If the eyes turn outward (wall-eyed), it is called exotropia. Or, one eye can be higher than the other which is called hypertropia (for the higher eye) or hypotropia (for the lower eye). Strabismus can be subtle or obvious, intermittent (occurring occasionally), or constant. It can affect one eye only or shift between the eyes.
Strabismus usually begins in infancy or childhood. Some toddlers have accommodative esotropia. Their eyes cross because they need glasses for farsightedness. But most cases of strabismus do not have a well-understood cause. It seems to develop because the eye muscles are uncoordinated and do not move the eyes together. Acquired strabismus can occasionally occur because of a problem in the brain, an injury to the eye socket, or thyroid eye disease.
When young children develop strabismus, they typically have mild symptoms. They may hold their heads to one side if they can use their eyes together in that position. Or, they may close or cover one eye when it deviates, especially at first. Adults, on the other hand, have more symptoms when they develop strabismus. They have double vision (see a second image) and may lose depth perception. At all ages, strabismus is disturbing. Studies show school children with significant strabismus have self-image problems.
Amblyopia, or lazy eye, is closely related to strabismus. Children learn to suppress double vision so effectively that the deviating eye gradually loses vision. It may be necessary to patch the good eye and wear glasses before treating the strabismus. Amblyopia does not occur when alternate eyes deviate, and adults do not develop amblyopia.
Strabismus is often treated by surgically adjusting the tension on the eye muscles. The goal of surgery is to get the eyes close enough to perfectly straight that it is hard to see any residual deviation. Surgery usually improves the conditions though the results are rarely perfect. Results are usuallybetter in young children. Surgery can be done with local anesthesia in some adults, but requires general anesthesia in children, usually as an outpatient. Prisms and Botox injections of the eye muscles are alternatives to surgery in some cases. Eye exercises are rarely effective.
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Sunglasses are popular for comfort and fashion, but now there is medical evidence supporting the use of sunglasses to protect the long-term health of the eyes.
More than a dozen studies have shown that spending hours in the sun without proper eye protection can increase the chances of developing age-related eye diseases like cataracts and macular degeneration. Ophthalmologists now recommend wearing UV-absorbent sunglasses and brimmed hats when in the sun long enough to get a suntan or sunburn.
People mistakenly confuse the ability of sunglasses to block UV light with the color and darkness of the lenses. In truth, UV protection comes from a chemical coating applied to the surface of the lens. Shop for sunglasses that absorb 99 or 100% of all ultraviolet (UV) light. Some lens manufacturers' labels say "UV absorption up to 400 nm." This is the same thing as 100% UV absorption.
In addition to UV light, sunlight also has low levels of infrared rays. Infrared wavelengths are invisible and produce heat. The eye seems to tolerate infrared well. Research has not shown a connection between eye disease and infrared light ray exposure.
Polarized lenses cut reflected glare, like sunlight bouncing off water, pavement, or snow. Sunglasses with polarized lenses are popular and useful for fishing, driving, and skiing. Polarization has nothing to do with UV light absorption, but many polarized lenses are now made with a UV-blocking substance.
Wraparound glasses are shaped to keep light from shining around the frames and into the eyes. Studies have shown that enough UV rays enter around ordinary eyeglass frames to reduce the benefits of protective lenses. Large-framed, close-fitting wraparound sunglasses protect the eyes from all angles. Wraparound sunglasses should be considered by commercial fishermen, mountain climbers, skiers, or anyone who spends time at high altitudes or on the water.
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Many types of tinted contact lenses are available. They can enhance and even change the color of one's eyes for cosmetic purposes, for costumes, or provide special effects for the movie industry.
Tinted contacts can make light eyes more blue, green or hazel. They can alter the color of the eyes, such as making brown eyes blue.
Tinted lenses have been used in the movies since 1939. In the movie "Ghostbusters," actors playing gargoyles wore red contact lenses. Reptile lenses were crafted for the commander in "Star Trek" and white contact lenses were used for the Hulk in "The Incredible Hulk." Recently, these costume lenses have become available to the general public.
Tinted contacts may also be used to disguise or improve the appearance of an abnormal eye. They can be used to conceal corneal scars, irregular pupils and to hide shrunken, unsightly eyes. Sometimes tinting a lens can make the lens easier for a person with poor vision to handle. These tints are more subtle handling tints.
Contact lenses for the general public, including those with no correction, are considered medical devices. They must undergo clearance for safety by the Food and Drug Administration (FDA). Color additives used by the manufacturers of costume contact lenses must also be approved for use. Additives in unapproved lenses may be toxic.
Purchase only tinted contacts prescribed by an ophthalmologist, and never share lenses with someone else.
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Visual Field Test
The visual field is the entire area one can see. It includes central and peripheral (side) vision. A visual field test can detect problems with vision in any part of the visual field. Changes in the visual field may be difficult to notice since both eyes are generally used at the same time. One eye can sometimes compensate for some vision loss in the other. A problem may not be detected until each eye is tested separately.
The visual field test provides information that no other test can. It is used to detect many diseases, such as glaucoma or retinitis pigmentosa, which affect the eye, optic nerve, and brain. It can also help diagnose brain tumors, strokes, and other conditions. Visual field testing helps diagnose the disease and can follow the progress of the disease and its treatment.
During a visual field test, one eye is temporarily patched while the other eye is being tested. You are asked to look straight ahead at a fixed spot and watch for targets to appear in your field of vision.
There are two kinds of visual field tests. One method uses moving targets. Targets are moved from outside the visual field (where you can't see them) toward the center of your vision. When you see them, you press a button. The test can be done using a dark screen on a wall (called tangent screen testing) or using a large bowl-shaped instrument (called Goldmann testing).
The other testing method uses small fixed targets that appear briefly as bright or dim lights (called computerized static perimetry). You sit in a chair facing either a bowl-shaped instrument or a computer screen and indicate when you see the targets appear.
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