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    Changing the shape of the eye's lens to focus on near and far objects.

    The capacity to discriminate fine details of objects.

    See: lazy eye

    angle-closure glaucoma
    Type of glaucoma in which the aqueous is trapped by the iris bending too close to the cornea. If this occurs suddenly, it may be a medical emergency.

    antireflective coatings
    Chemical coatings applied to lenses to reduce glare inside and outside eyeglasses. Reduced glare improves both day and night vision and reaction time, thereby enhancing safety. Coatings also make glasses more attractive to look at by reducing their "shop window" appearance.

    The diameter (mm) of the opening of an optical system that determines the size of the shaft of light that travels through the instrument; the diameter of an objective lens of a telescope or microscope.

    aqueous humor
    A watery liquid between the lens and cornea, which bathes and feeds the cornea, lens and iris.

    A lens with a front surface that is not curved like a sphere and that has a relatively flatter outside edge. Useful for correction of high refractive errors.

    A common vision defect, caused by the irregular shape of the cornea, which blurs and distorts eyesight.

    background retinopathy
    The first stage of diabetic retinopathy. Although vision is rarely affected at this stage, there is blood-vessel damage.

    bifocal lenses
    Glasses with two vision-correction zones: one for seeing near objects and one for distance vision. Bifocal lenses are really two lenses, with an obvious line separating the two viewing zones.

    binocular vision
    The brain's ability to combine the images received in both eyes to form a single, sharp image. Convergence and divergence of both eyes are necessary to achieve binocular vision at all distances.

    blind spot
    The connection point of the optic nerve from the retina to the brain. There are no rods or cones around this area of the retina. Therefore, this is the one part of the eye that does not sense light, or "see."

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    Cloudiness of the eye’s crystalline lens, which may prevent a clear image from forming on the retina.

    central vision
    The central area of the field of view in which vision is strongest and sharpest in humans.

    A harmless bump caused by a blocked oil-gland duct in the upper or lower eyelid.

    The choroid — the middle membrane of the eye — is part of the uveal tunic, between the retina and outer coating. It supplies blood to the eye. See: uvea.

    chronic angle-closure glaucoma
    The form of angle-closure glaucoma in which the pressure build-up of liquid in the front part of the eye acts over time and not suddenly.

    ciliary muscle
    Also called the ciliary body. A tiny muscle that changes the thickness of the lens to achieve close and distant vision.

    ciliary process
    Secretes the liquid from behind the iris that feeds and bathes the middle and front eye structures.

    color blindness
    A vision defect in which colors are not seen normally. A term covering a broad range of conditions in which color-sensing cone cells of the eye are either absent or do not function properly.

    computer vision syndrome (CVS)
    Defined by the American Academy of Optometry as "the complex of eye and vision problems related to near work, which are experienced during or related to computer use."

    Cone-shaped photoreceptors located in the retina that sense color and detail in bright lighting conditions. Three pigments of cones sense the three colors of light (red, green and blue) to provide color vision.

    The clear, protective membrane that lines the eyelids and covers exposed areas of the sclera.

    The ability of both eyes to converge, or move closer together, in order to focus on near objects.

    The eye's outer window. Sensitive, protective and transparent, the cornea's unchanging, curved surface provides 75 percent of the eye's focusing ability, or refractive index.

    corneal topography
    A relief map that reveals corneal contours and shows any variations in corneal curvature.

    corneal transplantation
    An operation that removes a defective cornea and replaces it with healthy corneal tissue from a donor. When used for keratoconus, it has a 90 percent success rate.

    cortical cataract
    With this type of cataract, wedge-shaped spokes extend from the outer rim of the lens to the central core (nucleus). The spokes block light, causing glare and loss of contrast.

    crypts of Henle
    Microscopic pockets, located in the loose sections of conjunctiva around the eyeball, that secrete mucin into tears.

    diabetic retinopathy
    A retinal blood-vessel disease that affects people with diabetes.

    Sensitivity to two colors rather than the three needed to perceive all the colors in the spectrum by combining primary colors.

    The ability of both eyes to diverge, or move further apart, in order to focus on distant objects.

    dry eye
    Dryness of the eye’s surface caused by too little tear production, too much tear drainage, changes in tear quality and a host of other problems. Symptoms include red, burning eyes and foreign body sensations.

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    In this operation, an eye surgeon grafts a layer of corneal epithelial cells ("skin" cells from the cornea’s upper layers) around the central cone of someone with keratoconus. The grafted cells flatten the cornea, improving nearsightedness and blurred vision. This operation has a success rate to similar corneal transplantation.

    exit pupil
    The size of a shaft of light transmitted to the eye through a telescope or pair of binocular lenses; an important indicator of low-light performance; usually measured in millimeters (diameter).

    extracapsular cataract surgery
    The surgical removal of a cloudy lens that opens the front but leaves the back of its surrounding capsule intact.

    The organ of sight. The key parts of the eye are as follows: the sclera, the choroid, the cornea, the aqueous humor, the iris, the pupil, the lens, the vitreous humor, the retina, the macula, the fovea and the optic nerve.

    eye relief
    The distance between the eye and an eyepiece of a telescope, set of binoculars or microscope.

    The liquid form of any active medication (in various strengths) that is applied by letting a drop glide under the lower eyelid. Some conditions require taking color-coded types of eye drops at specific moments in a day.

    A common name for hyperopia, a widespread vision defect in which distant objects are seen clearly and close objects are seen less clearly. Farsightedness occurs when the eye is too short and/or the cornea too flat.

    field of view
    The width, measured in degrees, feet or millimeters, of the viewing area visible through a pair of binoculars, a telescope or a microscope at a specified distance.

    Stimulation of the retina and/or optic nerve that creates the illusion that we are seeing "stars" or streaks of light. Continuous flashes persisting for more than 20 minutes are potentially dangerous and require immediate medical attention.

    fluorescein angiography
    A test to detect leaky and abnormal blood vessels in the eye. In hospital, an eye specialist injects a yellow or red dye into an arm vein. Exposed to ultraviolet rays, the dye glows yellow-green. The eye specialist photographs the fluorescent dye, as it travels through blood vessels in the eye.

    A small depression in the center of the macula that contains only cones. The fovea is the area of sharpest vision, or acuity, in the eye.

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    ganglion cells
    A layer of the retina, which transmits signals to the optic nerve.

    glands of Manz
    Microscopic glands within the bulbar conjunctiva, arranged in a ring around the cornea, near the scleral junction. Secrete mucin, a proteinous substance that makes up the inner layer of tears.

    glands of Zeis
    Oil-producing glands that surround the eyelashes. The oil forms the outer layer of tears.

    A harsh, uncomfortably bright light or reflection.

    Original material used for eyeglasses. Made mainly of sand (silicon dioxide) plus various elements according to special needs (titanium dioxide for thinness). Glass lenses are long lasting and naturally scratch-resistant. But like a regular drinking glass, they may break if you drop them.

    Condition with moderate to severe vision loss due to areas of failing optic nerves. Excess pressure exerted by the liquid in the front part of the eye (the aqueous) damages the nerves.

    goblet cells
    Large glands in the conjunctiva that secrete mucin, which forms the inner layer of tear film. The mucin layer enables tears to glide across the eye’s surface. Missing or damaged in people with dry eye.

    high-index lenses
    Advanced plastic lenses that are thinner and flatter than conventional plastic or glass lenses.

    high-index plastic
    The third wave of plastic materials for those with moderate to severe correction needs. Effectively hides the level of correction.

    See: farsightedness.

    intracapsular cataract surgery
    The surgical removal of a cloudy lens and its surrounding capsule.

    intraocular pressure (IOP)
    A measure of how much the liquid contained in front of and behind the iris pushes in all directions (particularly affecting the retina in the back of the eye).

    See: intraocular pressure.

    The colored or pigmented eye tissue behind the cornea. The iris acts like a muscular diaphragm, regulating the amount of light that enters the eye through the pupil

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    A condition in which the center of the cornea thins and develops a cone-shaped bulge that blurs and distorts vision.

    A mechanical means of measuring the curvature of the cornea. A series of concentric circles is projected on a 3-mm wide spot in the central cornea by a keratometer. If wavy lines appear in the circles, the person is nearsighted. By using these lines as a guide, the eye doctor can measure the amount of astigmatism (blurred vision). Can also be used to detect and monitor keratoconus.

    Krause glands
    Accessory tear glands located under the eyelids where the upper and lower conjunctiva meet.

    lacrimal gland
    An almond-shaped gland that produces tears. It is located above the eyeball, under a bone near the eyebrow.

    Laser in-situ keratomileusis, a surgical procedure to reshape the cornea using an excimer laser to correct myopia, hyperopia and astigmatism.

    lazy eye
    A common name for amblyopia, which is the loss or lack of development of vision (usually in one eye) during childhood.

    A transparent lentil-shaped body behind the iris that is controlled by the ciliary muscle. The lens provides 25 percent of the eye's focusing power. To focus on close-up objects, the ciliary muscle squeezes the lens to make it thicker. For far-away objects, it flattens the lens to make it thinner.

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    A Latin word, meaning spot, which is used to describe the yellow central area of the retina, where vision is sharpest.

    macular degeneration
    Deterioration of the macula (central spot) of the retina that eventually causes the permanent loss of central vision. There are two types of macular degeneration: dry and wet. The most common cause of vision loss in people older than 60 years of age.

    macular edema
    Swelling of the central spot (macula) in the retina, often caused by blood vessel leakage in certain eye diseases, such as diabetic retinopathy.

    meibomian glands
    Oil-producing glands located within the eyelid. The oil, which forms the outer layer of tears, drains through ducts that open on the eyelid margins.

    mid-index plastic
    A new generation of plastic materials that reduces lens thickness while providing the same optical performance as earlier plastics. Recommended for light to moderate correction.

    misaligned eyes
    The medical term for misaligned eyes. Eye misalignment is a common vision defect in which one or both eyes are turned inward, outward, upward or downward. One or both eyes may move irregularly.

    Vision therapy in which a contact lens with close-up correction is worn in one eye, and (if necessary) a contact for distance correction is worn in the other eye.

    movement parallax
    The perceived motion of near and far objects when either the object or the observer moves.

    See: nearsightedness.

    A common name for myopia, a widespread vision defect in which close objects are seen clearly and distant objects are seen less clearly. Nearsightedness generally occurs because the eyeball is too long or the cornea too curved. Correctable with single-vision lenses.

    nuclear cataract
    This type of cataract clouds the central part (nucleus) of the eye’s lens.

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    objective lens
    A lens or system of lenses that creates an image of an object.

    open-angle glaucoma
    The most common type of glaucoma. Caused by high intraocular pressure resulting from clogging of the filter mechanism that returns liquid to the bloodstream from the front chambers of the eye.

    A medical doctor who specializes in the eye and has completed medical school. Licensed to examine eyes, treat eye diseases and perform eye surgery.

    optic nerve
    The bundle of nerve fibers that transmits light-generated electrical impulses from the retina to the brain.

    optic nerve head
    Also called the optic disk. The area where the optic nerve is attached to the retina and the location of the blind spot.

    Doctors of optometry are independent primary healthcare providers who examine, diagnose, treat and manage diseases and disorders of the visual system, the eye and associated structures, as well as diagnose related systemic conditions.

    Vision therapy that can strengthen, coordinate and improve the functions of both eyes, especially in the early years of life.

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    A technique for removing a cloudy lens during cataract surgery. Sound waves are used to shatter the cataract. The broken pieces are vacuumed from the lens capsule using a long, narrow tube.

    photochromic lenses
    Lenses that darken in bright sunlight and clear in dim light.

    Cells at the back of the retina that contain light-sensitive pigments. When light hits these pigments, they trigger nerve impulses, resulting in vision (see: cones; rods).

    A manufacturing process that adds certain materials to optical lenses which separate light into beams that vibrate in one plane only instead of all planes (like normal light); cuts glare.

    Plastic lenses designed for impact resistance and UV protection. Increasingly popular for sport/sunglasses and prescription lenses.

    The degree to which an object is enlarged. Also called magnification.

    A common vision defect associated with the aging eye’s diminishing ability to focus on close objects. Correctable with bifocals or progressive addition lenses.

    Surgical procedure to reshape the cornea with excimer laser; approved for use in correcting myopia; also known as laser vision correction.

    progressive-addition lenses
    Lenses that have progressively more reading power from top to bottom. They are used for correcting presbyopia.

    proliferative retinopathy
    Severe retinal blood-vessel disease that occurs in people with diabetes. Fragile, new blood vessels and fibrous tissues grow on the retina and extend into the vitreous. These blood vessels may leak, leading to macular edema, retinal detachment and vision loss.

    One entrance to the eye’s drainpipe. The eye has two drains or puncta — in the upper and lower corners of the eyelid margins near the nose. Pulling back the eyelid will reveal these tiny holes.

    The space in the center of the iris where light enters the inner eye. The widening, or dilation, of the pupil is controlled by the iris.

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    The bending of light rays as they pass through a substance. In the eyes, light is refracted by the cornea and lens, and focused on each retina.

    refractive index
    A measure of how much light rays are bent when they pass from one medium to another. Water, air, glass and plastic each have a specific refraction index.

    The inner layer at the back of the eye, where light-sensitive rods and cones are located. Chemical changes in the retina transmit electrical signals through the optic nerve to the brain to produce sight.

    retinal detachment
    Separation of the light-sensitive retina from the back of the eye, which may cause vision loss. Laser surgery is required as soon as possible after detachment is detected to secure the retina to the inner eye.

    retinitis pigmentosa (RP)
    A group of rare, inherited degenerative diseases that affect the retina and can lead to various degrees of vision loss or blindness.

    Also called visual purple, a light-sensitive pigment in the rods. It bleaches in light and regenerates in the dark using vitamin A compounds.

    rigid gas-permeable (RGP) lenses
    Hard plastic contact lenses that allow oxygen and carbon dioxide to pass.

    risk factors
    Identifiable characteristics or life history for which there is an observable link to a problem, disease or predisposition toward a health condition.

    The cylindrical photoreceptors located on the retina that do not sense color but enhance peripheral vision and vision in dim light.

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    The tough white part of the eye, or outermost shell, that contains the eyeball.

    The over-production of sebum, the oily secretion from the sebaceous gland. Characterized by oily skin and flaky crust along the edges of the eyelids. If left untreated, seborrhea can cause inflammation or infection.

    scratch-resistant coatings
    Hardened coatings that protect plastic lenses from being scratched.

    single-vision lenses
    Eyeglass lenses that correct a single refractive need, such as myopia or hyperopia.

    Sjogren’s syndrome
    An immune disorder that causes mild to extreme dryness in the eyes and mouth. It dries mucous membranes and affects the lacrimal glands, which make tears. About 3 million Americans suffer from this disorder, which may be related to arthritis.

    slit lamp
    A biomicroscope that helps eye doctors examine the various parts of the eye under high magnification. This special microscope has a chin and headrest. Colored filters can enhance the doctor’s view.

    A lens with a smooth, sphere-shaped or rounded surface that bends light rays equally in all directions.

    spots and floaters
    Pieces of embryonic blood vessels, specks of pigment on the muscle fibers attached to the iris, particles caught in the tear layer in front of the eye, or clumps of collagen in the vitreous humor. Usually harmless, these tiny objects drift freely in the eye and cast shadows across our line of vision.

    standard plastic
    A hard synthetic resin that is light and impact-resistant, but thick and damage-prone (scratches) unless coated with protective varnish.

    stereoscopic vision
    The ability to judge depth accurately or see in three dimensions. The brain achieves stereoscopic vision by merging the images it receives from the slightly different angles provided by each eye.

    Infections of the glands around the eyelashes and under the upper and lower eyelids that may be associated with stress or other eye conditions.

    See: misaligned eyes

    subcapsular cataract
    This type of cataract affects the back of the eye’s lens, causing blurriness and glare.

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    topical eye ointment
    Thick paste of any active medication (in various strengths) that is applied by placing it along the lower eyelid and gently applying it around the tear duct.

    A lens with a cylindrical as well as rounded shape; used to correct astigmatism.

    trabecular meshwork
    Tissue that filters the liquid aqueous after it has circulated toward the outer front edge of the iris. The trabecular meshwork drains back into the bloodstream.

    Eyeglasses with three different viewing areas to provide clear vision at close distances, mid-range and far away. Trifocals have two well-defined visible lines separating the viewing zones.

    ultraviolet rays
    Wavelengths of light below the color blue on the color spectrum. Invisible radiation from sunlight, sunlamps and video terminals. Ultraviolet rays are potentially harmful to the eyes and are strongly linked to the development of cataracts and macular degeneration.

    The middle layer of the eye, which contains the iris, the ciliary body and blood vessels that supply the retina.

    Infection or inflammation of the uvea.

    visual cortex
    The part of the cerebral cortex of the brain that is primarily responsible for interpreting signals from the eye.

    visual field
    What you see, particularly the area surrounding the center of vision. Each eye has a visual field.

    The replacement of clouded vitreous humor (the gel-like substance that fills the eye) with clear saline solution.

    vitreous humor
    A thick, transparent, gel-like substance which fills the eye, located between the lens and retina

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    The distance between two similar points of a given wave. UV light has a shorter wavelength than the light the eye can see.

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