The name for normal eyesight. The first “20” refers to the distance in feet between the eye and the eye chart. The second number changes according to how clearly the eye can see the small letters on the chart. A high second number indicates impaired vision.
For example, when the second number is “200” it indicates that a “normal” eye can see at 200 feet what the eye being tested can only read when that person stands 20 feet from the chart. That eye being tested is therefore very myopic (nearsighted). If the second number is “40”, myopia is only mild, as the eye being tested can see from 20 feet what a “normal” eye sees from only 40 feet away.
Surgical term for “removal”, done by cutting, scraping or vaporizing.
The American Board of Ophthalmology, which certifies eye doctors after they complete its education program and pass the examination.
Ability of the eye’s lens to change its curvature according to the distance being focused on. It becomes steeper for near vision and flatter for far vision.
Visual sharpness or clarity. Tested by the Snellen eye chart and expressed as “20/20” when found to be “normal”.
Age-Related Macular Degeneration (ARMD)
Degeneration of the eye’s macula, the small area of the retina which gives central vision. Peripheral vision is unaffected.
The anatomical term for a front part of a structure, as opposed to a back (posterior) part.
The fluid-filled front chamber of the eye, behind the cornea and in front of the lens.
Absence of the eye’s lens, as happens in cataract surgery, when the lens is removed and replaced with an intraocular lens.
The fluid which fills the eye’s anterior chamber between the cornea and the lens.
A refractive error in the eye where the cornea has an oval shape instead of a round shape. This creates two curvatures which refract light differently and prevent a clear image from forming on the retina. One of the lower order aberrations corrected by LASIK.
Best Corrected Visual Acuity
Sharpest vision as corrected by glasses or contact lenses.
Glasses with two areas of focus: one for distance vision and a smaller area at the lower edge for near vision
Adjective referring to the use of both eyes together: binocular vision.
Inflammation of the eyelids.
The small area on the retina where the optic nerve connects to the eye. There are no light-sensitive cells here which means no vision is possible at this spot.
In the context of ophthalmology, approval of an eye doctor by the American Board of Ophthalmology, which offers an education program followed by an examination, and certifies those who pass as being fully qualified to practice ophthalmology.
An error in LASIK flap creation, where the microkeratome cuts the flap incompletely with a hole in it.
A tiny opacity in the eye’s lens which blocks some of the incoming light. Cataracts tend to be age-related and are progressive. If left untreated, more will form and they will enlarge until no light can pass through the lens and blindness occurs. Cataract surgery replaces the lens with an intraocular lens.
Central Ablation Zone
The central corneal area treated by the LASIK laser, which ablates (removes) small pieces of tissue to correct the corneal shape. There is a transitional zone around it blending into the natural corneal surface.
A tiny area of the LASIK treatment area which is wrongly not treated and therefore remains microscopically higher than the surrounding corneal surface. It will cause double vision.
The muscles which control the lens curvature. With age, they become less able to steepen the lens curvature so that near vision gradually becomes more blurry – a condition known as presbyopia.
Clear Lens Exchange
Another name for cataract surgery where the eye’s lens is replaced with an artificial lens.
One of the higher order aberrations where light sources have a streak next to them like a comet tail. It is sometimes a side effect of Traditional LASIK, but is diagnosed and treated by Custom LASIK.
A lens with an inward curvature.
Infection or inflammation of the lining over the sclera (white part of the eye) and on the inside of the eyelids. It is contagious and is treated with eyedrops.
A method of correcting vision through use of a curved lens placed directly on the cornea. There are many types of contact lenses, some designed to treat eye conditions other than poor vision.
A lens with an outward curvature.
The clear front covering over the iris and pupil. It allows light into the eye and refracts (bends) it on the way in. LASIK corrects vision by reshaping the corneal curvature, to change the angles at which it refracts incoming light. That enables the light to focus clearly and give sharp vision at all distances.
A thin piece of surface tissue cut on the cornea at the beginning of a LASIK surgery. it remains connected with a hinge and the eye surgeon folds it back to create access to the corneal tissue below, where the laser treatment is done.
A surgery which replaces the eye’s cornea with a donor cornea. It can treat severe eye infections or injuries, and some eye diseases such as Keratoconus and Fuch’s Dystrophy which affect the cornea.
An accommodative intraocular lens used in cataract surgery to replace the natural lens.
The trade name for a wavefront LASIK system originally manufactured by VISX. In 2006, VISX was acquired by Advanced Medical Optics, Inc. (AMO).
Ability to assess relative distances of objects. It is a result of using two eyes to look at the same object. The dominant eye focuses directly on it and the other eye sees it at a slight angle. The brain learns to judge distance by comparing the two images.
Enlargement of the eye’s pupil, the opening in the center of the iris. It enlarges automatically in dim light so as to allow more light into the eye and give clearer vision. It can also be dilated by special eyedrops.
The unit of measurement used in glasses prescriptions.
The eye which looks at objects directly. Each person has a dominant eye and a non-dominant eye and between them they give us depth perception.
Duplicates of each image. Also called Ghosting.
Dry Eye Syndrome
A chronic condition of insufficient tears to keep the eye adequately moistened. Symptoms are itchiness, burning, a scratchy feeling, and surprisingly, excess tearing. Dry Eyes can be measured with a small strip of special paper attached to the lower eyelid for a few minutes.
An interior layer of cells. In the eye it refers to the layer of cells on the back side of the cornea, bordering the anterior chamber.
A form of LASIK vision correction where the corneal flap is created more thinly than in standard LASIK. It is a way of offering LASIK safely to people with thin corneas.
Any outside layer of cells. In the eye is refers to the surface corneal cells.
The name of the laser used for LASIK vision correction. It is a cool, ultraviolet laser.
Acronym for Fellow of the American College of Surgeons.
Hyperopia. It is a refractive error causing blurry near vision. One of the lower order aberrations.
Little specks or strings that appear in the visual field, especially when looking at a blank background like the sky. They drift away when you try to focus on them. They are shadows on the retina of tiny clumps in the vitreous humor, the fluid in the eye’s posterior chamber (between the lens and the retina.
The degree to which a lens can refract light to focus at a given distance. An eye’s focusing power is a joint action of the cornea and lens, which both refract incoming light to focus on the retina. A myopic eye has too much focusing power given the eyeball’s length front to back. A hyperopic eye has too little focusing power.
Food and Drug Administration (FDA)
The U.S. federal department which evaluates and approves (or doesn’t approve) new medical devices and medications. Approval is specific, given for using that device or medication for specific health needs. A long period of supervised and successful testing is required before approval is given.
The center of the eye’s macula, which is the most light-sensitive area on the retina. The fovea gives us clear visual details but does not work well in dim light. Our night vision is provided by the retinal cells called rods, which cluster around the retinal periphery.
Double vision where there is a faint duplicate of objects.
An eye disease which will cause blindness if not treated. When it is diagnosed early it can be managed with special eyedrops. There are several types of glaucoma but they all involve damage to the optic nerve.
One of the higher order aberrations where light sources appear to have a ring of light around them. It was a side effect of early LASIK, but is diagnosed and treated by Wavefront-guided LASIK.
A clouding condition in the cornea which scatters light. It may have any of several possible causes, such as scar tissue, excess moisture, or inflammation.
Higher Order Aberration
A type of vision error arising from microscopic irregularities on the corneal surface. They are detected by Wavefront diagnosis and treated by Custom LASIK. There are over 60 higher order aberrations, many of which are expressed mathematically rather than with a name. They correspond to specific wavefront patterns as light reflects back from the eye to the diagnostic machine.
Some examples are:
A refractive error in the eye which gives blurry near vision. Also called Farsightedness. It is corrected by LASIK surgery.
Inside the eye.
The pressure exerted by the fluid inside the eye against the eye’s periphery. It is often a factor in Glaucoma.
The colored part of the eye. For more about the eye’s anatomy, please see our Education Page.
Tracking of eye movement during a LASIK surgery. The Wavefront vision correction system stays focused on the iris of the eye being treated. When the eye makes any slight movement, the computerized system moves with it to stay precisely targeted on the treatment area.
Inflammation on the cornea which can cause scarring and vision impairment.
A vision correction surgery first devised in the 1950s where part of the cornea was removed, frozen, resculpted, and stitched back on. It was a precursor to the more accurate and sophisticated procedure we know as LASIK. The name LASIK is an acronym for Laser Assisted in-situ Keratomileusis, referring to the facts that: (a) a laser is used; and (b) the correction is done in place, without removing any part of the cornea entirely.
The gland which produces tears. It is located below the eyebrow and lacrimal ducts (tear ducts) drain the tears out through small openings at the inner corners of the eyes into the nasal passages.
A bright light man-made for a specific purpose. Laser light rays travel parallel to each other and directionally, which means the laser can be very precisely focused. Each laser is a specific color – wavelength – and some are invisible, being ultraviolet or infrared. Some are immensely hot and other relatively cool and suitable for medical use. Medical lasers can also be cooled by water or some type of filter. The laser used for LASIK is a cool, ultraviolet light. The word “laser” is now used as a word but was originally an acronym for Light Amplification by the Stimulated Emission of Radiation.
Laser Assisted in-situ Keratomileusis. A refractive surgery which uses an excimer laser to vaporize small pieces of the cornea according to a detailed treatment plan, for the purpose of creating 20/20 vision (or better).
Refractive error equal to 20/400 or worse, with correction, in the better eye. That means that a person with “normal” vision can see something clearly from 400 feet away but the legally blind person must be only 20 feet away to see it.
A transparent curved structure which allows light to travel through it. Lenses are convex or concave, depending on whether they are designed to focus light or scatter it. A convex lens focuses light; a concave lens scatters it. The eye’s lens is suspended behind the iris. It changes its curvature according to whether focus is on near objects or far objects.
Limbal Relaxing Incisions (LRI)
A way of modifying the corneal shape to treat astigmatism. The limbus is the border between the cornea and the sclera. Tiny incisions here can create a rounder cornea to correct the oval shape causing the blurriness of astigmatism.
Lower Order Aberration
A term for myopia, hyperopia and astigmatism collectively. These three lower order aberrations are treated by LASIK. There are also over 60 higher order aberrations.
The central part of the eye’s retina which gives us central vision (direct vision), as opposed to peripheral vision.
An age-related disease where the macular cells deteriorate and become progressively unable to give sharp central vision. It can be
Wet -- new blood vessels grow beneath the retina, but do not help as they leak and further impair vision; or
Dry – No new blood vessels form. Dry macular degeneration can lead to Wet macular degeneration.
Swelling of the macula which causes blurriness, light sensitivity, and a pink tinge to one’s vision. It is usually temporary and can be caused by an injury, some diseases, and sometimes eye surgery.
One millionth of a meter.
A surgical cutting instrument used in LASIK to create the corneal flap. It is extremely precise and sharp.
An adjective describing vision using one eye.
A treatment used for presbyopia and at times for other conditions, where one eye is corrected for near vision and the other either left as it is or corrected for far vision. The brain learns to interpret the two images appropriately.
Nearsightedness, also called shortsightedness. – clear close-up vision but blurry distance vision. A myopic eye has too much focusing power for the eye’s length front to back, so images from distant objects focus in front of the retina instead of on it. It is one of the three lower order aberrations of the eye. LASIK surgery reshapes the cornea to correct myopia.
Another name for myopia or shortsightedness.
Involuntary shaking of the eyes. There are about 45 types of nystagmus but just two main types: early onset (associated with vision loss) and adult onset (associated with neurological disorders).
Off Label Use
The term for using a medication or medical device for something not specifically approved by the Food and Drug Administration (FDA). A doctor may make a judgment call to do that.
A doctor who has trained in the medical specialty of treating eye diseases and conditions.
The hand-held device which ophthalmologists use to examine the eye. It has a light that is shone into the person’s eye and the doctor can see inside the eye through any of a variety of magnifying lenses which are part of the ophthalmoscope.
The area of the eye where light travels through between the cornea and the retina. All structures along this zone must be fully transparent to allow the light through unimpeded. Please see our Education pages for more on the eye’s anatomy.
Optical Ablation Zone
The part of the Optical Zone corrected by the LASIK laser – i.e., the stromal layer of the cornea.
The blind spot. It is a circular area in the center of the retina where the optic nerve enters the eye and connects with the retinal nerves. Blood vessels also enter at this place. Because of the presence of these other structures, there are no light-sensitive cells and the retina does not receive images.
A trained technician who fits eyeglasses, placing them into frames and verifying their accuracy. He or she may also grind them from raw materials. In some states opticians also fit contact lenses. Opticians are qualified and licensed by state authorities, not federal.
The large nerve which carries image information from the retina to the brain. It connects with the eye at the optic disc (blind spot) and gathers data from the retinal nerves. It is by damaging the optic nerve that untreated glaucoma causes blindness.
A doctor of optometry (OD, not MD, medical doctor). Optometrists can diagnose vision problems and eye diseases, and prescribe drugs to treat them, as well as contact lenses and glasses. The education is four years post-graduate.
A potential risk of refractive surgery where the eye’s refractive error is corrected too far, creating the opposite error. An example would be overcorrecting myopia to create hyperopia. In most cases, the overcorrection will adjust itself within a few months, and if not, it can be adjusted by an enhancement surgery.
Testing of corneal thickness using a pachymeter. This is done as part of qualifying potential LASIK candidates, as the cornea must have a minimal thickness for safety’s sake.
Side vision, as opposed to central vision which is sharp direct vision. Peripheral vision is gradually destroyed by glaucoma, whereas central vision is gradually destroyed by Macular Degeneration.
The part of cataract surgery which breaks up the clouded lens with ultrasound. This makes it easier to remove. The ultrasound is administered by a small probe through an incision about 3 mm long. The same incision is then used for inserting the intraocular lens to restore vision when cataracts or presbyopia have been impairing it.
Adjective for an eye which has its natural lens, as opposed to Aphakic, which describes an eye without its lens.
Phakic Intraocular Lens
An artificial lens placed in an eye which still has its natural lens. It is positioned in front of the natural lens to increase focusing ability, and can later be removed if necessary.
The floor-standing instrument used to test vision. The person being tested looks through an aperture at an eye chart. The doctor flips between lenses asking you to read the smallest line of letters clearly visible. The results provide the basis for your glasses or contact lens prescription. Early LASIK procedures were based on this same diagnosis, but Custom LASIK uses a different diagnostic method, more subtle and accurate.
Laser removal of corneal tissue done during refractive surgery.
Photorefractive Keratectomy (PRK)
A type of refractive surgery done for people whose corneas are too thin or steep for LASIK. No corneal flap is created. Instead the eye surgeon removes a thin layer of surface cells from the treatment area and discards them. After treatment, the cornea grows new cells to replace those lost. The vision correction itself is done on the stromal layer in the same way as LASIK.
The large area between the lens and the retina. It is filled with vitreous fluid, also called vitreous humor, or just “the vitreous”.
An age-related blurriness of near vision. It is progressive and gradually affects the intermediate and then far vision. The causes are not fully understood. As we age, the lens is less able to steepen but different theories exist as to why. Most treatments are based on the idea that the lens becomes stiffer with age and perhaps the muscles controlling it become weaker. It is therefore replaced with an intraocular lens (IOL). Newer treatments are not yet approved by the FDA.
Small pieces of plastic, silicone, or collagen which are inserted into the puncta, tear drains at the inner corners of the eyes. They can be used to help with dry eyes by slowing the tear drainage.
Singular of puncta – apertures in the inner corners of the eyes which allow tears to drain out into the nasal passages.
The black round opening in the center of the iris, through which light enters the eye. It opens in dim lighting conditions to admit more light, and contracts in bright light to block some of the light.
Glasses designed for near vision. They are necessary when presbyopia sets in around the age of 40 or so.
Changing the angle of light which enters the eye (bending it). The cornea refracts incoming light first, then the lens does so again. Between them in a 20/20 eye they focus the light on the retina. All transparent curved structures refract light.
Too much or too little bending of the light that enters the eye. Too much refraction focuses it before it gets to the retina (myopia). Too little allows it to arrive at the retina unfocused (hyperopia) and if the light could travel through the retina it would then focus behind it. LASIK corrects an eye’s refractive error to provide clear vision
Surgery which changes the refractive power (focusing ability) of an eye. LASIK is an example, as is use of an intraocular lens.
The back surface in the eye which is filled with light-sensitive cells called rods and cones. Cones are mostly in the retinal center and give us clarity of vision and perception of color. Rods tend to be towards the periphery and function more in dim light, giving us our night vision. Images received by the retinal cells are transmitted to the optic nerve and carried to the brain’s vision center at the back of the brain. For more on eye anatomy please see our Education pages.
Separation of the retinal nerve tissue (light-sensitive cells) from the underlying layer containing blood supply. The cause of retinal detachment is usually an injury to the eye or head, and detachment may happen weeks after the injury itself.
The white part of the eye. It joins to the cornea in front and continues around the eyeball sides behind the retina to the sheath over the optic nerve.
A device used by ophthalmologists to look closely at the eye’s interior. It is a microscope with an array of magnification settings and a strong light. It gives more detail than an ophthalmoscope.
The chart with lines of differently-sized letters used to measure central vision. Named after Herman Snellen, a Dutch ophthalmologist who introduced it in 1862. There have been several versions of it developed since then.
One of the higher order aberrations. It makes light sources look scattered out or spiked out from their center. In the early days of LASIK it was a possible complication after treatment, but Custom LASIK diagnoses the eye’s aberrations so precisely that it will treat any irregularities that might cause starburst.
Misalignment of the two eyes. One eye turns out, in, up or down. It may be just intermittent. It reduces depth perception because the two eyes are not focused on the same point. Also called Turned Eye or Crossed Eyes.
The cornea’s middle layer, thicker than the other layers. This is where the LASIK laser does corneal reshaping, and it is to gain access to the stroma that the corneal flap is made at the start of a LASIK surgery.
A group of hereditary conditions affecting the stroma and causing deterioration of the tissue. A disqualifying condition for LASIK surgery.
A passage within the eye which carries tears. Tears are produced by the lacrimal gland above each eye and they flow in ducts to be distributed by blinking over the eye’s whole surface. They eventually drain out through the puncta, little openings in the inner corners of the eyes.
Toric Contact Lens
A type of contact lens which corrects astigmatism as well as either myopia or hyperopia. Toric contact lenses can be soft or Rigid Gas Permeable (RPG), disposable and colored. For more details on glasses and contacts please see our Education Pages.
The area between where the LASIK laser corrects corneal curvature and where no correction is done. The vaporization of corneal tissue gradually tapers off to avoid creating any sudden changes in contour.
Loss of peripheral vision, so that the eye sees only directly ahead. It can be caused by glaucoma.
Too little correction of refractive error. It is a possible risk of LASIK and can happen if the cornea heals more quickly and strongly than is typical, undoing some of the corrective work done by creating too many new corneal cells.
The middle layer around the eyeball. It includes the iris and lies between the retina inside and the sclera outside. It can become infected in a condition called uveitis.
The trade name of an implantable lens made by Advanced Medical Optics, Inc. It is a phakic intraocular lens, meaning that the eye’s natural lens is left intact and the Verisyse is placed next to it to correct myopia.
Sharpness of vision. Also called Central Vision.
The entirety of what can be seen when the eye looks straight ahead.
A company that manufactured one of the most popular Custom LASIK systems. It was acquired by Advanced Medical Optics, Inc. (AMO) in 2006. Its technology is now combined where possible with that of AMO. An example is the LASIK procedure now called iLASIK, which uses AMO’s Intralase technique and the VISX hardware.
Separation from the retina of the vitreous fluid in the eye’s posterior chamber. Sometimes the vitreous fluid pulls on the retinal surface, leading to retinal detachment, a vision emergency. It is more likely to occur in older people and diabetics.
The gel-like fluid which fills the posterior (vitreous) chamber of the eye, between the lens and the retina
A term used of light waves, referring to the distance between the top of one wave and the top of the next one. Each wavelength creates a specific color, although some are invisible to us, being ultraviolet or infrared. The LASIK laser is ultraviolet.
The name of technology used in Custom LASIK. It diagnoses both the lower and higher order aberrations. That makes Custom LASIK an extraordinarily effective vision correction surgery.
A laser sometimes used in a follow-up surgery to cataract surgery. When the eye’s lens is removed, its membrane pocket is left intact to hold the intraocular lens. Sometimes it becomes cloudy and impairs vision. That can be treated with a YAG laser vaporizing the clumping cells causing the cloudiness. YAG stands for Yttrium-Aluminum-Garnet, material used to generate this laser.