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Fraction commonly used to indicate normal vision. A bottom number larger than twenty indicates diminished vision. For example, a person with 20/400 vision would see a line of text that is twenty feet away with the same readability that a person with "normal" vision would see that line of text from 400 feet away. A


Abbreviation for American Academy of Ophthalmology or the American Academy of Optometry.


Abbreviation for American Board of Eye Surgeons.


Abbreviation for Anterior Basement Membrane Dystrophy.


Abbreviation for American Board of Ophthalmology.


Abbreviation for American Society for Cataract and Refractive Surgery.

Ablation Removal

                In the case of the excimer laser, the frequency of energy causes the molecules of the cornea to loose the "glue" that holds them together.  They simply fall away from each other and the remaining cornea.

Ablation Zone

The area of the cornea including the fully corrected optical ablation zone and the transition zone.


The ability of the eye to change focus from distant objects to objects closer than optical infinity, approximately 20 feet (6 meters).  Like when driving and you switch from looking down the road to reading the speedometer.  Accommodation is achieved when the lens shape is changed by small muscles around the lens pushing and pulling.

Accommodative Insufficiency

Loss of or less than expected amount of accommodation.


Abbreviation for American College of Eye Surgeons


Clearness, as in visual acuity. The most common measure of visual acuity is the Snellen acuity chart. Normal acuity being 20/20 (6/6 metric version).


The accessory structures of the eye including the eyelids, lacrimal apparatus, etc.


Ophthalmic equipment company.     .

Age Related Macular Degeneration

Destruction and loss of the photoreceptors in the macula region of the retina resulting in decreased central vision and, in advanced cases, blindness.


Abbreviation for Astigmatic Keratotomy.

ALK Laser

Automated lamellar keratoplasty (removal of a part of the central corneal epithelium) combined with excimer laser PRK to effect a change on the refractive error.


Ophthalmic equipment and drug company.    .


Often called "lazy eye" it is a unilateral (occasionally bilateral) condition in which, in the absence of any obvious structural anomalies or ocular disease, the best corrected vision is less than 20/20 (6/6).  Amblyopia is often occurs in an eye that did not have adequate use during early childhood.  

Most often amblyopia results from either a misalignment of a child's eyes, such as crossed eyes, or a difference in image quality between the two eyes (one eye focusing better than the other.) In both cases, one eye becomes stronger, suppressing the image of the other eye. If this condition persists, the weaker eye may becomes useless.


With early diagnosis and treatment, the sight in the "lazy eye" can be restored.


Abbreviation for Age-related Macular Degeneration.

American Academy of Ophthalmology

Ophthalmologist membership organization.

American Academy of Optometry

Optomotrist membership organization. 

American Board of Eye Surgeons

Ophthalmologist certification organization.  Affiliated with the American College of Eye Surgeons.  

American Board of Ophthalmology

A medical specialty board that administers a 1-1/2 year long education and examination process for ophthalmologists.  After an ophthalmologist has passed the examination, (s)he is "Board Certified". 

American College of Eye Surgeons

Ophthalmologist membership organization.  Affiliated with the American Board of Eye Surgeons.

American College of Surgeons

Medical membership organization.  Members are called Fellows.  Physicians who are members often use the acronym FACS after their name.

American Optometric Association

Optometric membership organization.     .

American Society of Cataract and Refractive Surgery

Ophthalmologist membership organization.     .


Any imperfection in refractive state of the eye. Examples would be hyperopia, myopia, or astigmatism.

Amsler Grid

A hand held chart featuring horizontal and vertical lines, usually white on black background, used to test for central visual field defects.

Angioid streaks

Crack-like irregularities that appear in Bruch's membrane.


Congenital absence of eyes.


Absence of eyes.


Front or forward portion.

Anterior Basement Membrane Dystrophy

Also called Epithelium Basement Membrane Dystrophy, is a disease that disrupts the ability of the epithelium to attach to the lower layers of the cornea.  The epithelium will often grow unevenly or will detach from the cornea.

Anterior Chamber

The space in front of the iris and behind the cornea.

Anterior Ocular Segment

The part of the eye anterior to the crystalline lens, including the cornea, anterior chamber, iris and ciliary body


Micro-nutrients that destroy or neutralize free radicals.


Abbreviation for American Optometric Association.


The absence of the eye's natural crystalline lens, usually after cataract removal.

Aphakic Spectacles

Thick, plus-powered eyeglasses that were once the standard correction for optical power following extraction of cataract. The glasses were cumbersome and greatly distorted peripheral vision. Today, an intraocular lens (IOL) is implanted in the eye after the cataract is removed.


A procedure used to remove harmful substances from the blood.

Aqueous Fluid

See aqueous humor.

Aqueous Humor

Clear watery fluid that flows between and nourishes the lens and the cornea. It is secreted by the ciliary processes.

Argon Laser

Laser light produced from argon gas. The main wavelengths are 488.0 nm blue and 514.5-nm pea green light, but nine separate wavelengths in the blue-green visible light spectrum are produced.


Abbreviation for Age Related Macular Degeneration.

Astigmatic Keratotomy

A surgical procedure in which microscopic incisions are placed in the peripheral cornea to create a more spherical shape. Similar to Radial Keratotomy.


A condition in which the surface of the cornea is not spherical, but is irregularly shaped like the back of a spoon. An astigmatic cornea causes light images to focus on two separate points in the eye, creating a distorted image. Symptoms range from visual discomfort in mild cases, to severe blurring, ghosting, and distortion similar to a reflection in a fun-house mirror. The amount of astigmatism you have will appear in the second number of most glasses prescriptions. The angle of direction (cylinder meridian or axis) of the astigmatism will be shown in degrees. A prescription of -4.00 - 1.00 x 30�, would indicate that you have 1.00 diopter of astigmatism at 30 degrees axis.


A type of inherited allergic response involving elevated immunoglobulin E. Sometimes called a reagin response, it means that you have hay fever, bronchial asthma, or skin problems like urticaria or eczema. It can also be acquired, sometimes following hepatitis or extended contact with solvents or alcohol.


The absence of refractive error symmetry between both eyes.   If one person's eye has a refractive error that is more than one diopter greater than the other, it is called anisometropia.

Anterior Chamber

The anterior chamber is in the front of the eye behind the cornea and in front of the iris.

Automated Lamellar Keratoplasty

An incisional refractive surgery technique for low to moderate myopia. In the procedure, the refractive surgeon places an instrument called an automated microkeratome on the eye which removes, in a shaving motion, a thin layer of cornea only microns thick. An even thinner layer of cornea underneath this top cap is removed, and the top cap is replaced. The procedure does not require sutures.

Automated Perimeter

A computer-driven device used to plot defects in the visual field. Usually the patient's head is placed into this large hemisphere shell. Various points of lights, sometimes of different sizes, intensities, and colors are projected onto the screen. The patient then indicates whether the light is seen and the response is recorded. The computer then plots the effective visual thresholds within the targeted visual field.

B Scan

Ultrasonic procedure used to check for abnormalities or locate foreign bodies within the eye.

Barraquer Cyro-Lathe

An instrument used in performing refractive surgery to correct extreme myopia or hyperopia.

Basement Membrane

A membrane of tissue beneath the epithelium that helps adhere the epithelium to the Bowman's layer or stromal tissue.


Abbreviation for Best Corrected Visual Acuity.

Best Corrected Visual Acuity

This is a measure of best acuity while wearing corrective lenses like glasses or contact lenses.


Corrective lenses that have two powers of correction.  Typically the majority of the lens is corrected for distance vision while a small are is corrected for near vision.  Bifocals and trifocals are normally prescribed for individuals with presbyopia.


Both eyes.

Binocular Vision

The blending of the separate images seen by each of two eyes into a single image. Normal binocular vision yields a stereoscopic image and parallax-induced depth perception.

Black Box Laser

Common term used for a laser built by its owner or by a firm not approved by the FDA. 

Blank spots

A commonly reported symptom of macular degeneration in which patients report that areas of their view disappear.


A chronic or long term inflammation of the eyelids and eyelashes, affecting people of all ages. Among the most common causes are poor eyelid hygiene; excess oil produced by the glands in the eyelids; a bacterial infection; or an allergic reaction. 

Blind Spot

(1) A small area of the retina where the optic nerve enters the eye; occurs normally in all eyes.

(2) Any gap in the visual field corresponding to an area of the retina where no visual cells are present. Associated with eye disease.

Blue light

                A portion of the light spectrum of macular degeneration that is suspected of being harmful to the retina.


A commonly reported symptom of macular degeneration in which patients report that lines or edges of objects lose their sharpness.

Board Certified

See American Board of Ophthalmology.

Bowman's Layer

A layer of cells of the cornea between the epithelium and stroma.  The Bowman's layer appears to be without specific purpose.


A system of raised letters. People who are legally blind read by touching the letters with their fingers.


This is a method of applying excimer laser energy.  A beam of energy is applied across the entire ablation zone at one time.  See also variable beam and flying spot.

Buttonhole Flap

A complication of LASIK caused when the microkeratome breaks through the top of the cornea. C


See Custom Contoured Ablation Pattern


Demarcation between the sclera and nasal corner of the eye.


Opacity or clouding of the natural crystalline lens that may prevent a clear image from forming on the retina. Cataracts occur naturally and can be induced by trauma.  The cataractous lens may require surgical removal if visual loss becomes significant, with lost optical power replaced with an intraocular lens.

Central Ablation Zone

See Optical Ablation Zone.

Central atrophy

A thinning of the retina that occurs as part of macular degeneration.

Central Island

A complication of photorefractive keratectomy where the laser fails to remove a portion of cornea.  If one views the concave area of the ablation like a lake one can imagine an island sticking up in the center- the visual symptoms would be monocular double vision or distortion.

Central Retinal Artery

The blood vessel that carries blood into eye; supplies nutrition to the retina.

Central Retinal Vein

The blood vessel that carries blood from the retina.

Central serous retinopathy

A malfunction of the retinal pigment epithelium that allows fluid to leak under the retina, causing a limited retinal detachment.

Central Vision

See visual acuity.


Ophthalmic equipment company.     .


The layer filled with blood vessels that nourishes the retina; part of the uvea.

Choroidal neovascularization

A type of set macular degeneration characterized by rapid leaking of fluids under the retina and rapidly appearing visual problems.

Ciliary Muscles

The muscles that relax the zonules to enable the crystalline lens to change shape for focusing.

Ciliary Processes

The extensions or projections of the ciliary body that secrete aqueous humor.


See Conductive Keratoplasty


See Contact Lens Assisted Pharmacologically Induced Kerato Steepening

Clear Lens Replacement

Clear Lens Replacement (CLR) is essentially cataract surgery without the cataract.  The natural lens is removed from the eye and replaced with a clear plastic lens.  Cataract surgery is done because the lens has become cloudy.  CLR is done for purely refractive purposes.

Clinical Trial

An investigative process under the jurisdiction of the FDA whereby device or medicine manufacturers sponsor experiments on people for the purpose of proving or disproving that the device or medicine performs as expected.


Abbreviation for Clear Lens Replacement.


See Complete Ophthalmic Analysis System


Color ring around the pupil.

Color Vision

The ability to perceive differences in color, including hue, saturation and brightness.


When two or more doctors provide care to a patient.  In refractive surgery it is common for an optometrist to provide preoperative testing and postoperative care with an ophthalmologist providing only the surgery.

Concave Lens

A lens with a hollow shape like the inside surface of a ball.   Concave lenses are minus power lenses and are used to correct myopia or nearsightedness.

Conductive Keratoplasty

A refractive surgery procedure for hyperopia and astigmatism that uses a probe to apply high frequency radio waves into the corneal tissue, causing shrinkage.  This controlled shrinkage will reshape the cornea to accommodate refractive error.

Cones, Cone Cells

One type of specialized light sensitive cells (photoreceptors) in the retina that provides sharp central vision and color vision. Also, see rods.


Plastic or silicone shell usually inserted after eye removal to help form the socket and support the eyelids.


The thin transparent mucous membrane that lines the inner surface of the eyelid and covers the white surface of the eye.


Inflammation of the conjunctiva. May be caused by bacteria, virus, allergens, or chemical or ultraviolet light exposure.

Contact Lens

Lens made of glass or plastic designed to maintain contact on the surface of the cornea. Usually used to correct refractive error.

Contact Lens Assisted Pharmacologically Induced Kerato Steepening

A process of accommodating hyperopic overcorrection following LASIK, PRK and RK, by molding the cornea with a tight fitting contact lens and nonsteroidal anti-inflammatory drugs. 

Contact Lens Disinfectant

An agent that kills surface bacteria and microorganisms on contact lenses.

Contact Lens, Daily Wear

Contact lenses designed to be worn only during waking hours. Frequent & planned replacement contact lenses general term used to refer to contact lens regimens in which lenses are replaced on a planned schedule, either every two weeks, monthly or quarterly.

Contact Lens, Disposable

Contact lenses defined by the U.S. Food and Drug Administration as a contact lens that is used one time and discarded. These can be worn either for a single day or up to seven days, depending on wear schedule prescribed by the eye care professional. Any lens that is intended to be removed from the eye, cleaned, rinsed, disinfected, and reinserted does not qualify for inclusion in this category.

Contact Lens, Extended Wear

Contact lenses designed to be worn around-the-clock for intervals of one to seven days.

Contact Lens, Therapeutic

Contact lenses designed to aid in protecting and helping a sick eye to heal. These unique lenses are frequently combined with precise medication delivery schedules to heal the eye.

Contact Lens, Toric

Contact lenses designed to correct astigmatic refractive errors. Toric lenses are weighted to maintain a specific axis across the cornea.

Contrast Sensitivity

The ability to perceive differences between an object and its background.


The turning of the eyes inward/outward so that they are both "aimed" toward the object being viewed.

Convex Lens

A lens with a bulging surface like the outer surface of a ball. Convex lenses are plus power lenses and are used to correct hyperopia or farsightedness. Also used for reading glasses as required for presbyopia.


The outer, transparent, dome-like structure that covers the iris, pupil, and anterior chamber. The cornea provides approximately two-thirds of the optical power of the eye. Light passes into the eye through the cornea allowing vision. Light also passes out of the eye allowing the iris and pupil of the eye to be seen. The cornea can be considered to have five layers:

1. The corneal epithelium

2. Bowman's layer

3. Corneal stroma

4. Descemet's layer

5. The corneal endothelium.

Corneal Abrasion

A scratch or similar trauma to the outer surface of the cornea. 

Corneal Haze

When the cornea becomes cloudy with opaque white cells, creating vision that is similar to looking through fog.

Corneal Remodeling

The healing of a wound of the cornea.

Corneal Topographical Map

A map of the corneal topography that shows the surface profile of the cornea.

Corneal Topography

A process of mapping the surface details of the cornea with a unique camera/computer combination.

Council for Refractive Surgery Quality Assurance

A nonprofit consumer/patient organization that certifies refractive surgeons based upon refractive surgery outcomes and educates the public about refractive surgery.


In the healthcare industry credentials means the education, licensure, and professional history of a healthcare provider.


Abbreviation for Council for Refractive Surgery Quality Assurance.  Pronounced "SURS-kah".

Crystalline Lens

The natural lens of the eye, located behind the iris, which helps bring rays of light to focus on the retina. The original state of the lens is transparent, but the lens becomes cloudy with age (cataract). The lens has the ability to vary its power to accommodatively focus light from objects closer than optical infinity.

Custom Contoured Ablation Pattern

The customization of laser vision correction treatments for decentered ablations and similar corneal aberrations. The Food and Drug Administration has approved the use of Custom Contoured Ablation Pattern (C-CAP) method under a Humanitarian Device Exemption.


Physicians who use the device can perform a treatment precisely controlled by size, depth and location, making it possible to reshape the cornea for optimum correction, officials said.  Physicians are able to address decentrations and similar problems resulting from a previous treatment on most types of refractive surgery.

Custom Cornea

The trade name for the use of a wavefront guided custom ablation with the LADARVision excimer laser built by Alcon.

Cyclopegic Refraction

An examination of the eye to determine refractive error while the natural crystalline lens of the eye is paralyzed and unable to accommodate.


See Cyclotorsion.


When eyes rotate clockwise or counterclockwise.  This often occurs when a person changes from vertical (standing or sitting up) to horizontal (lying down) position.

Day Blindness

Common term for Hemeralopia.


A complication of refractive surgery. In perfect centration, the center of the corneal ablation exactly coincides with the center of the visual axis and/or pupil. This is like looking through the very center of your spectacle lens. If you look through the periphery of your lens, you might end up seeing partly through the lens and partly through the edge of the lens - this is decentration. Decentration can cause various symptoms including edge glare or even monocular double vision. Other factors such as the normal size of the pupil, whether it is dark out (your pupil will enlarge), or the size of the ablation zone will affect the severity or presence of symptoms.

Descemet's Layer

Layer of cells in the cornea above the corneal endothelium.

Depth Perception

The ability of the vision system to perceive the relative positions of objects in the visual field.  This ability to determine the distance of an object is provided by the two eyes receiving slightly different images due to their wide positioning on the face.  The two images are compared by the brain to calculate distance.

Diabetes Mellitus

A chronic metabolic disorder characterized by a lack of insulin secretion and/or increased cellular resistance to insulin, resulting in elevated blood levels of simple sugars (glucose) and including complications involving damage to the eyes, kidneys, nervous system and vascular system.

Diabetes Type I

Insulin dependent, resulting from destruction of the insulin producing pancreatic islet cells.

Diabetes Type II

Non-insulin dependent, resulting from tissue resistance to insulin.

Diabetic Retinopathy

Diabetic retinopathy damages the tiny blood vessels that supply the retina. In the early stages of this disease-called non-proliferative or "background" retinopathy, the retinal vessels weaken and develop bulges that may leak blood or fluid into the surrounding tissue. 

Diffuse Lamellar Keratitis

Also called Sands of Sahara due to the appearance of dunes of sand in the cornea with advanced stages.  DLK is an inflammation under the LASIK flap of the cornea which is believed to be caused by a response to the presence of sterile infiltrates in the flap interface.


The complication occurs in the early post-operative period, vision is hazy and the eye is painful and teary. There is also an intolerance of bright light.


A process by which the pupil is enlarged. Allows more light to the interior of the eye.


A unit of measure of the refractive power if a lens. A one-diopter lens will focus parallel light rays one meter from the lens and a two-diopter lens will focus one-half of a meter from the lens. A plus 1.0 diopter lens is convex and will converge the light rays so they focus as a visible image 1 meter past the lens. A minus 1.0 diopter lens is concave and will diverge or spread light. The minus lens will not actually focus as a visible image on an optics table. Its image is known as a virtual image and if the diverging rays were followed to their point of origin, they would focus one meter in front of the minus lens.


A condition in which a single object is perceived as two; also called double vision. Usually with both eyes open as in binocular diplopia, but can be with only one eye as in monocular diplopia.


An abbreviation for Diffuse Lamellar Keratitis.


Abbreviation for doctorate of osteopathy.

Doctorate of Osteopathy

A certificate affirming that the holder has achieved the training required to be a medical doctor through a school of osteopathy.

Dominant Eye

The dominant eye is the eye that looks directly at an object. The non-dominant eye is the eye that looks at an object from the side. 

Double Image

Also called ghosting.  If you look at a clock and some of the numbers have a lighter image just off to the side, this is a typical double image problem.


Tiny yellow or white deposits of debris that accumulate within Bruch's membrane (of the retinal pigment epithelium). Common after age 60 and an early sign of age-related macular degeneration.

Dry Eyes

A condition where either not enough tears are produced or tears dissipate too fast.


Abbreviation for Epithelial Basement Membrane Dystrophy.  See Anterior Basement Membrane Dystrophy.


Abbreviation for Extracapsular Cataract Extraction.


An outward bulging of the cornea due to internal pressures and/or a weakened cornea.




Refractive condition in which no refractive error is present and distant images are focused sharply on the retina with no need for corrective lenses. Perfect vision or 20/20.


A prefix meaning within or inside.


An inflammation within the eye. Inflammations may be caused by organisms such as bacteria or may be sterile as in immune disorders. Endophthalmitis usually indicates an infectious disease, but occasionally occurs as a complication of surgery.


The inner layer of cells on the inside surface of the cornea.


Secondary refractive surgery treatments made to refine or improve the original visual result. Outcome predictability is reduced at higher corrections. Higher corrections and wider optical zones require deeper sculpting and consequently undercorrection and overcorrection are more common. Enhancement treatment by contrast is usually a small correction and usually has higher outcome predictability.


Surgical removal of the eye.


A watering of the eyes due to excessive secretion of tears or to obstruction of the lacrimal passages.


The outer surface layer of the cornea, like the epidermis or outer layer of the skin.

Epithelium Basement Membrane Dystrophy

See Anterior Basement Membrane Dystrophy

Epithelial Ingrowth

A complication of refractive surgery when epithelial cells under the flap begin to grow and multiply causing visual abnormalities and if sever, loss of visual acuity.  The most common treatment is lifting the flap, removing the cells, irrigation of the interface, and repositioning of the flap.


The position of the eyes in an over-converged position compensated by the external eye muscles so that the eyes do not appear turned inward.


The position of the eyes in an over-converged position so that the non-fixating eye is turned inward.

Excimer Laser

An argon-fluoride laser that emits ultraviolet light that is emitted in pulses at a wavelength of 193 nm. The term Excimer comes from the concept of an energized molecule with two identical components or excited dimer (contracted to one word exci-mer). Each pulse of this his "cool" laser removes 1/4000 millimeter of tissue from the targeted surface by breaking intra molecular bonds in collagen molecules. It would take about 200 pulses from an Excimer laser to cut a human hair in half. This laser was originally developed for use in the microprocessor industry and later found its application in vision correction.


The position of the eyes in an over diverged position compensated by the external eye muscles so that the eyes do not appear turned outward.


The position of the eyes in an over diverged position so that non-fixating eye is turned outward.

External Ocular Muscles

The six muscles that turn the eyes to position them for viewing.

Extracapsular Cataract Extraction

A surgical procedure that removes the cataractous lens of the eye but leaves the posterior lens capsule intact.


A clear fluid.


Abbreviation for Fellow of the American College of Surgeons.


Common term for hyperopia.


Abbreviation for Food and Drug Administration.

Feeder vessel treatment

Stopping retinal bleeding by closing the blood vessels that supply the leaking blood vessels.


The process the body uses to create scar tissue.

Flap & Zap

Slang term for LASIK.

Fluorescein Angiography

A test to examine blood vessels in the retina, choroid, and iris. A special dye is injected into a vein in the arm and pictures are taken as the dye passes through blood vessels in the eye.

Fluorescein Staining

A tiny amount of a fluorescein dye is placed in the tear film. The color will make microscopic dots appear where the eye has become dry.  Excessive staining is an indication of dry eye.

Flying Spot

This is a method of applying excimer laser energy.  Rather than applying all excimer energy in a broadbeam across the entire ablation area, or the energy in variable spots sizes across the ablation area, a very small spot of excimer energy is applied in rapid succession at different locations across the ablation area.

Focusing Power Of The Eye

As light enters our eye, it must be brought to a focus on the retina in order to perceive a clear image. About two thirds of the focusing power of the eye comes from the cornea, the rest comes from the lens inside the eye. As the light enters the eye, it is focused a fixed amount by the cornea. As the light passes through the pupil, the lens then adjusts the focus a variable amount with the exact amount of focusing power applied dependent on the distance of the object being viewed. Objects at near like a book or knitting require more power than distance objects like movies or traffic signs.

Food and Drug Administration

The federal agency of the United States government responsible for the evaluation and approval of medical devices.  The FDA does not evaluate surgical procedures that to not require a new medical device.     .


The central part of the macula that provides the sharpest vision.


Molecules that have been implicated as one causative factor in the stimulation of abnormal cellular reproduction (cancer) and cellular destruction (aging).

Functional Visual Disability

The degree to which a visual error interferes with a person's ability to perform normal daily activities, such as reading, driving at night, or performing hobbies.


The interior lining of the eyeball, including the retina, optic disc, and macula. This portion of the inner eye can be seen during an eye examination by looking through the pupil. G


Abbreviation for refractive surgery complications glare, arc, starburst, and halo.


This is a profile of how excimer energy is applied.  Used primarily on flying spot lasers, more energy is applied to the center of the spot than at the outer edges.

Gene therapy

A method using genes (sequences of DNA) to treat disease.


This is a common term for double images.  If you look at a clock and some of the numbers have a lighter ghost image just off to the side, this is ghosting. 


A disease characterized by increased pressure within the eyeball. more If not diagnosed and treated, glaucoma may lead to optic nerve damage, loss of visual field, gradual vision impairment, and sometimes blindness.


A diagnostic procedure using a mirror/lens device placed directly upon the cornea that is used to view the drainage area through which aqueous fluid exits the eyeball.

Gray Box Laser

Common term used for a laser imported from outside the United States. This type of acquisition is not approved by the FDA. 


A known complication of refractive surgery that causes images from light sources to blur with circles radiating out from the center.  Halos also occur naturally without refractive surgery.


An opacification or cloudiness of the normally clear cornea.   Any build up of inflammatory infiltrates (white blood cells), extra moisture, scar tissue, or foreign substances (like drugs) can cause a clouding of the cornea.


Day blindness.  Often caused by clouding or opacity of one or more of the normally clear ocular tissues.

Humanitarian Device Exemption

A Humanitarian Device Exemption from the Food and Drug Administration authorizes the use and marketing of a device that is intended to benefit patients in the treatment of conditions that affect fewer than 4,000 individuals.


Also known as farsightedness or longsightedness.  Hyperopia occurs when the eyeball is too short from front to back, or the eye's focusing mechanism is too weak, causing light rays to be focused behind, rather than on, the retina. People with hyperopia have difficulty seeing objects close up. This refractive abnormality requires a plus (positive or convex) lens for correction.


A deficiency of oxygen supply to a tissue.


A disease of unknown origin or without apparent cause.

Idiopathic macular degeneration

An extremely rare form of macular degeneration that affects people in their twenties and thirties.

Intraocular lens (IOL)

Plastic lens that may be surgically implanted to replace the eye's natural lens.

Intraocular Pressure (IOP)

Pressure within the eye.  High intraocular pressure is called glaucoma.

Intrastromal Corneal Ring Segments

Trade name Intacs; the implantation of small slivers of plastic at the outer edges of the cornea to flatten the center of the cornea and reduce myopia.  Intacs have also been found to have therapeutic effect for keratoconus patients.

Intravitreal bubble

A gas bubble sometimes introduced into the eye to displace or stop retinal bleeding. This technique is also commonly used in the repair of retinal detachment.


See Intraocular Lens


See Intraocular Pressure


Pigmented tissue lying behind the cornea that gives color to the eye (e.g., blue eyes) and controls amount of light entering the eye by varying the size of the pupillary opening.


Abbreviation for Joint Commission on Accreditation of Healthcare Organizations.

Joint Commission on Accreditation of Healthcare Organizations

The Joint Commission evaluates and accredits more than 18,000 health care organizations and programs in the United States.     . K


An abnormal scaring of damaged tissue. 


Surgical excision (removal) of any portion of the cornea.


Inflammation of the cornea.


A prefix indicating relationship to the cornea.


An inflammation (infectious or auto-immune) of the cornea and conjunctiva.


A disorder that involves a thinning of the central cornea.   The normally round shape of the cornea becomes distorted. A cone-like bulge develops, resulting in significant visual impairment.


The measurement of the curvature of the cornea.


A refractive surgical technique where a partial thickness circular flap of cornea is removed, frozen, reshaped on a lathe and replaced upon the cornea. The lathe can shape either a convex or a concave lens.


A surgical incision of the cornea as in Radial Keratotomy.

Lacrimal Gland

The small almond-shaped structure that produces tears; located just above the outer corner of the eye.


See Laser Assisted sub-Epithelium Keratomileusis.


An acronym for Light Amplification by the Stimulated Emission of Radiation. Laser light is different from ordinary light in that it is composed of one color (wavelength) traveling in one direction and each light wave is traveling in step with the adjacent wave making the laser light more powerful by a factor of millions. This energy is carried by the wave in "packets" called photons.

Laser Assisted In-Situ Keratomileusis

An advanced laser procedure combining ALK and PRK to reshape the central cornea, thereby decreasing or eliminating myopia, hyperopia, and astigmatism. The refractive surgeon uses an automated microkeratome to shave off a thin, hinged layer of the cornea. This flap is then lifted like a hinged door and the exposed surface is reshaped using the excimer laser. After altering the corneal curvature, the flap is replaced and adheres without stitches.

Laser Assisted sub-Epithelium Keratomileusis

Laser Assisted Epithelium Keratomileusis is the detachment of the epithelium with the use of an alcohol solution that softens the epithelium and allows it to be rolled back into a flap.  The flap of epithelium is then be repositioned over the cornea following excimer ablations.

Laser Thermal Keratoplasty

A non-excimer laser refractive surgery.  The office-based instrument  applies two rings of laser energy to the midperiphery of the cornea.  Each ring gently heats collagen in the cornea to change corneal shape.   The application of energy is accomplished without physically contacting the cornea with instrumentation or other apparatus.


Abbreviation for Laser Assisted In-situ Keratomileusis.

Legal Blindness

In the U.S., (1) Visual acuity of 20/200 or worse in the better eye with corrective lenses. (2) Visual field restricted to 20 degrees diameter or less (tunnel vision) in the better eye. Note these criteria are used to determine eligibility for government disability benefits and do not necessarily indicate a person's ability to function.


A transparent double convex (outward curve on both sides) structure between the iris and the vitreous humor. Two structures of the eye focus light onto the retina. The first is the cornea or front surface of the eye that provides about 65% of the focusing power of the eye. The human lens is located behind the iris and in front of the vitreous humor and provides the remaining focusing power for the eye. In younger patients (usually below age 45) the lens is able to adjust it's power allowing the eye to change it's focal length from distance to near.

Lenticular Astigmatism

When the crystalline lens is not symmetrical. 


The visible borderline between the clear cornea and the white sclera of the eye. The conjunctival layer that covers the globe also joins at the limbus.


Abbreviation for Laser Thermal Keratoplasty.


A vitamin that might reduce the risk of macular degeneration, but which has not yet been proven to do so.

Low Vision

Visual loss that cannot be corrected with eyeglasses or contact lenses and interferes with daily living activities.

Macro Striae

Wrinkles in the LASIK flap.


The small, sensitive area of the central retina; provides vision for fine work and reading.

Macular Edema

The collection of fluid in and under the macular portion of the retina causing swelling.

Macular hole

A microscopic hole that can appear in the macula.

Macular pucker

A thin membrane growing on the retina that contracts and distorts the retina, resulting in blurred and distorted vision.

Macular translocation

A surgical procedure that relocates the macula away from leaking blood vessels.

Manifest Refraction

An examination of the eye to determine refractive error while the natural crystalline lens is able to accommodate.


Most older technology excimer laser machines employ different kinds of masks to customize, refine, and smooth the corneal surface. Many machines have an internal constricting diaphragm (like a camera f-stop mechanism) as an internal mask. Variable rotation of the masks is used to deal with astigmatism.


Abbreviation for medical doctorate.


Nasal or towards the nose.

Medical Doctorate

A certificate affirming that the holder has achieved the training required to be a medical doctor through a school of medicine.


A shortened name for Aesculap-Meditec.

Mesopic Pupil Size

The size of a pupil under medium light conditions such as daylight or a well lit room.

Micro Striae

Also called Bowman's Crinkles.  Very small wrinkles in the flap caused by Bowman's layer constricting.


The term micrometer has replaced the term micron that was used before 1967.


A unit of length equal to one-millionth of a meter.


A surgical device that is affixed to the eye by use of a vacuum ring.  When secured, a very sharp and thin metal blade slides across the front of the cornea and shaves a small amount of the cornea at a predetermined depth.


Abnormally small size of eye.


A nanometer, or 10 to the -9 power meter. The term millimicron was used before 1967.


Pupillary constriction.


A technique to limit the effects of presbyopia by correcting one eye for near vision and the other for far vision.

Munnerlyn Formula

Named for engineer Charles R. Munnerlyn, Ph.D, who pioneered the technology for vision correction based on the excimer laser.  This is the basic formula to calculate the amount of tissue that needs to be removed to affect refractive change. 


Pupillary dilation.


Also known as nearsightedness or shortsightedness. Myopia occurs when the eyeball is too long from front to back, or the eye's focusing mechanism is too strong, causing light rays to be focused in front, rather than on, the retina. People with myopia have difficulty seeing objects far away. This refractive abnormality requires a minus (negative or concave) lens for correction. N

Myopic degeneration

An abnormality similar to macular degeneration that can occur in severely myopic people.


Abbreviation is nm. 10 to the -9 power meters. The term nanometer has replaced the term millimicron that was used before 1967.


Toward the nose.

National Committee for Quality Assurance

An independent, non-profit organization that certifies credentials verification organizations and accredits managed care organizations.     .

National Eye Institute

A division of the US federal government's National Institutes of Health.  Approximately three-fourths of US eye research funding originates at the National Eye Institute.     .


Abbreviation for National Committee for Quality Assurance.

Near Point Of Accommodation

The closest point in front of the eyes that an object may be clearly focused.

Near Point Of Convergence

The maximum extent the two eyes can be turned inward.


Common term for myopia.


Abbreviation for National Eye Institute.


The formation of new blood vessels, often fragile and inappropriate for the location.  Long-term use of contact lenses can starve the cornea of oxygen, causing neovascularization as the body attempts to provide oxygen through blood vessels.


Ophthalmic equipment company.     .

Night Blindness

Common term for nyctalopia.


Night blindness.  Often caused by a deficiency of the retina due to a lack of vitamin A.


An eye condition characterized by rapid, jerky eye movements. O

Occult choroidal neovascularization

A type of set macular degeneration characterized by slower leakage of fluids under the retina.

Ocular histoplasmosis

Scars in the choroids resulting from infection by a fungus.

Ocular Hypertension

Elevated intraocular pressure.

Ocular Rosacea

The cause of ocular rosacea is not understood but its symptoms can leave the eyes feeling irritated and "gritty". Patients may experience redness, tearing or crusting on the lashes. 

Off Label Use

A term used for procedures that are legal, but are not specifically approved for a particular device or drug.  An example is LASIK.  The excimer laser is FDA approved for PRK, but not for LASIK, which is a combination of ALK and PRK.  The LASIK procedure comes under the definition of the practice of medicine (also called scope of practice), so the FDA does not necessarily need to approve the use of the excimer laser specifically for LASIK.  An easier example is the scalpel.  The scalpel is not specifically approved for all procedures that may require a scalpel, but if a medical doctor determines the use of this FDA approved tool is appropriate, then it is okay with the FDA.  Some excimer lasers have received FDA approval specifically for LASIK, but this is more for marketing purposes than to accommodate any legal requirement.


Anything to do with the eye.

Ophthalmic Imaging Systems

Ophthalmic equipment company.     .


A medical doctor who specializes in the diagnosis and medical or surgical treatment of eye diseases. To become an ophthalmologist one must first obtain a medical degree (M.D. or D.O. in North America) and then complete further specialty training. A medical degree typically takes 7 or 8 years university training and a specialist certification an extra 3 years in the USA or 4 years in Canada. Specialist MD's are usually divided into "medical" and "surgical" disciplines. Ophthalmology is technically a "surgical" discipline. Some ophthalmologists chose not to perform surgery and can be called "medical ophthalmologists". An ophthalmologist may also prescribe eyeglasses and contact lenses.


Examination of the internal structures of the eye using an illumination and magnification system.

Optic Cup

The white, cup-like area in the center of the optic disc.

Optic Disc

The circular area (disc) where the optic nerve connects to the retina. Also know as the optic nerve head.

Optic Nerve

The optic nerve is a bundle of nerve fibers, about the diameter of pencil, which passes through the back of the eyeball and connects to the nerve fiber layer of the retina. It can be observed directly with an instrument called an ophthalmoscope. The optic nerve carries the visual messages from the photoreceptors of the retina to the brain.

Optical Zone

The area of the eye through which light passes to the retina.   To reach the retina light must pass through the cornea, aqueous humor, crystalline lens, and vitreous while passing through the pupil.  The optical ablation zone is the area where a laser has created full refractive error correction.  See also Transition Zone.


An expert in the art and science of making and fitting glasses. The optician may also dispense and/or fit contact lenses, depending on local licensing practices.

Optometric Refractive Surgery Society

Optometrist membership organization.     .


A doctor of optometry is a non-medical primary eye health care provider who specializes in the examination, diagnosis, treatment, management, and prevention of diseases and disorders of the visual system. Optometrists today most often complete a bachelor of science degree (with very specific requirements) followed by an optometry degree program that requires four to eight years to complete. Patient contact begins in the second professional year with the final year being a full 12-month internship. Many optometrists include the provision of contact lens and spectacle devices as part of their daily practice. The optometrist may also prescribe medications, depending on local licensing practices.


Abbreviation for Optometric Refractive Surgery Society.


A non-surgical procedure using contact lenses to alter the shape of the cornea to effect a change in the refractive error.


A known complication of refractive surgery where the expected amount of correction is more than desired. Overcorrection often occurs where healing regresses less vigorously than predicted.


A process in which certain by-products of oxygen react with nearby molecules. It is thought to cause damage to tissues.


An ultrasonic procedure used to measure the thickness of the cornea.


Abbreviation for Photoastigmatic Refractive Keratectomy


Non-inflammatory swelling/elevation of the optic nerve often due to increased intracranial pressure or the presence of a tumor.

Penetrating Keratectomy

A surgical procedure when a button-like full thickness segment of the cornea is removed and replaced with a donor cornea from another person.

Peripheral Vision

The ability to see objects and movement outside of the direct line of vision.


See Photoablative Inlay

PERK Study

A multi-center study of radial keratotomy (RK) outcomes funded by the National Eye Institute. Ten-year results of RK patients who had the surgery in 1983 were published in Archives of Ophthalmology in October 1994.


The study found that seven out of ten patients who had the operation on both eyes no longer wore or required corrective lenses ten years after surgery, and 85% of patients saw 20/40 or better without glasses. It concluded that RK is a "reasonably safe operation" that can "effectively reduce but not eliminate myopia".


The study also found that 43% of patients experienced a hyperopic shift following surgery over the ten-year period. These patients typically experienced an improvement in their vision as their refractive error moved closer to 20/20. It is for this reason that ophthalmic surgeons will often intentionally undercorrect their patients, then monitor the rate of healing before performing a follow-up enhancement procedure.


Patients whose vision had achieved full correction or near full correction through surgery and who experience a hyperopic shift could become farsighted and need to wear glasses.


A cataract surgical procedure which uses an ultrasonic vibration to shatter and break up a cataractous lens, making it easier to remove. The vibration is delivered by an irrigation-aspiration instrument. In a survey of ASCRS members in 1994, 86% preferred the phacoemulsification cataract removal technique over the extracapsular cataract extraction technique. The technique was invented by Charles D. Kelman, MD, and was first published in 1967.


Meaning that the natural crystalline lens of the eye is present.

Phi-motion Angiography

Phi-motion angiography employs a scanning laser to capture rapid-sequence images of blood vessels underlying the retina.


An optical instrument containing many lenses which is used to determine the required power of glasses or refractive error.

Photo Therapeutic Keratectomy

The use of an excimer laser to remove surface tissue of the cornea for medical or optical treatment reasons.


The "cold" process of tissue removal which occurs with excimer laser radiation in the 200nm wavelength range. This far-ultraviolet wavelength possess light photons so powerful that the molecular bonds of the target tissue both break down and have sufficient extra kinetic energy to fly off the surface; hence ablation. Microscopic pictures show incredibly precise cuts with no evidence of tissue burning in adjacent tissue.


The process of tissue destruction accomplished by visible light radiation. Tissue is broken down by the light and "clots" as if it were cooked.

Photodynamic treatment

The use of laser beams to activate special dyes in order to stop retinal bleeding.


A painful but temporary condition of the eye caused by intense light sources, such as an arc welder.


Sensitivity to light.

Photorefractive Keratectomy

A surgical procedure using an excimer laser to reshape the central cornea to a flattened shape for people who are myopic and a more curved surface for people who are hyperopic. Photorefractive Keratectomy techniques may also be used to correct astigmatism.


The process of tissue destruction as occurs with infra red light radiation.

Phthisis Bulbi

Shrinking of eyeball following injury, infection, or disease.

Pigment epithelial detachment

A split occurring in Bruch's membrane fills with fluid and causes a dome-shaped detachment of the pigment epithelium underlying the retina, leading to visual distortion or other symptoms. Pigment epithelial detachments are often associated with macular degeneration.

Pigment epithelium rip

A severe condition caused by a tear in the pigment epithelium, leading to sudden loss of vision. Pigment epithelial rips are often associated with macular degeneration but can also result from direct trauma to the eye.


A yellowish spot seen on the white of the eye at the junction of the clear cornea and white sclera of the eye. These lesions are usually caused by ultraviolet radiation. The white surface of the eye cannot "tan" and therefore cannot protect itself from sunburn.


Abbreviation for Penetrating Keratectomy.


No refractive error.  Normal vision.  A diopter of 0.

Polypoidal choroidal neovascularization

Small swellings within the walls of blood vessels under the retina burst and cause damage to the retina.


Back surface or behind.

Posterior Capsular Opacification

Opacification of the posterior lens capsule. Sometimes called a "secondary cataract". Often a consequence of modern cataract surgery. It occurs when a thin membrane of tissue grows over the remaining capsule following cataract surgery, and can develop in as many as half of all cases between several months and several years after surgery. Most often treated using the YAG laser to ablate a hole in the capsule.

Posterior Chamber

The space between the back of the iris and the front face of the vitreous; filled with aqueous fluid.

Posterior Optical Segment

The part of the eye behind to the crystalline lens, including the vitreous, choroid, retina, and optic nerve.

Posterior Vitreous Detachment

The separation of the vitreous body from the retinal surface due to shrinkage from degenerative or inflammatory conditions or trauma. Often an age-related condition.


Part of the normal process of aging. As a person becomes older, we begin to lose the flexibility of the natural crystalline lens of the eye and weakness of the ciliary muscle. Presbyopia actually starts at about age ten. Most people do not begin to experience the effects of presbyopia until their forties. This loss of flexibility limits the ability of the eye to change its point of focus from distance to near. Because of this normal process, people begin to wear bifocals or other reading correction. The one advantage of mild myopia is the ability to remove your glasses after the onset of presbyopia and continue to read. Mild myopia effectively counteracts presbyopia. Refractive surgery does not change affects of presbyopia.


An abbreviation for Photorefractive Keratectomy.


When the natural crystalline lens of the eye has been replaced with an artificial lens during cataract surgery or Clear Lens Replacement.


A scarring condition of the cornea, caused by exposure to intense sunlight.


Abbreviation for photo Photo Therapeutic Keratectomy.


A growth of scar tissue on the cornea. These lesions like pinguecula are caused by an ultraviolet burn to the surface layer or epithelium.

Punctal Plugs

Small silicone or plastic plugs that are inserted into any or all of the punctum.


Tear drains around the eye.  There are two in the upper lid and two in the lower lid.


Appears as a small black dot in the center of the iris. The pupil changes its diameter in response to changes in ambient lighting. The pupil varies the amount of light reaching the retina and the depth of focus of the eye.

Pupillary Response

The constriction and dilation of the pupil due to stimulation by light or accommodation. Q R


Pertaining to the radius or line from a circle center to the circumference of the circle.

Radial Keratotomy

A surgical procedure that permanently alters the shape of the cornea by placing microscopically thin relaxing incisions in the peripheral cornea. The incisions cause the central portion of the cornea to flatten, thereby reducing the power of the cornea.


Sites in the brain that allow the attachment of certain drugs, making them active and able to produce the desired results.


Manufacturer of ophthalmic equipment for Conductive Keratoplasty.     .


(1)  A test to determine the best eyeglasses or contact lenses to correct a refractive error (myopia, hyperopia, or astigmatism).

(2)  The bending of light by the use of lens or other material.

Refractive Error

The degree to which images received by the eyes are not focused on the retina (myopia, hyperopia, or astigmatism) measured in diopters.

Refractive Surgery

Any surgical procedure which permanently alters the focusing power of the eye in order to change refractive errors.  Refractive surgery may include corneal surgery such as LASIK, LASEK, PRK, Intacs, CK, LTK or lens surgery such as CLR or Phakic IOL


A return by the cornea toward the original refractive state. 


The light sensitive layer of cells (rods and cones) on the inner back surface of the eye that converts light images into nervous impulses sent along the optic nerve for transmission to the brain.

Retinal Axons

The nerve fiber extensions of the retinal photoreceptors that form the nerve bundle called the optic nerve.

Retinal Detachment

A separation of the neural tissue of the retina from the pigmented epithelium layer and therefore the blood supply. Results in loss of vision in the detached area. Repairable with fair to good prognosis for vision if diagnosed in the early stages.

Retinal fibrosis

Scarring in the retina.

Retinal Pigment Epithelium (RPE)

The pigment cell layer that nourishes the retinal cells; located just outside the retina and attached to the choroid.

Retinitis Pigmentosa

The name given to a group of disorders of the Retina, all of which result in a progressive reduction in vision. 


Abbreviation for Radial Keratotomy.

Rods, Rod Cells

One type of specialized light sensitive cells (photoreceptors) in the retina that provide side vision and the ability to see objects in dim light (night vision). Also, see cones.

Saline Solution

A sterile salt solution used in cleaning, rinsing, and sometimes storing of contact lenses.

Sands of Sahara

See Diffuse Lamellar Keratitis.

Schlemm's Canal

The passageway for the aqueous fluid to leave the eye.


The tough, white, outer layer (coat) of the eyeball. With the cornea, it protects the entire eyeball.

Scleral Shell

Flush fitting thin artificial eye usually fit over phthisis bulbi.

Scotopic Pupil Size

The size of a pupil under low light conditions similar to light in a theater or during night driving.


An area of partial or complete loss of vision surrounded by an area of normal vision.

Secondary Implant

If no lens was placed in the eye at the time of cataract removal surgery, then a secondary procedure to implant an intraocular lens may be completed later.


Abbreviation for Society for Excellence in Eyecare.

Shirmer Test

Tiny strips of filter paper are placed in the patient's eyes just under the lids.  After five minutes the distance the tears have stained the paper is measured.  The greater the distance, the higher the natural tear production.

Side Vision

See peripheral vision.

Slit Lamp

A microscope using various magnifications combined with a strong light that can be focused into a slit for examining the eye.

Snellen Visual Acuity Test

The white chart with the big black E at the top and lines of letters that become increasingly smaller.  The Snellen Test is one of many tests used to determine visual acuity.  The term 20/20 means that the patient can see an item 20 feet away with the same clarity that a normally sighted person can see an item 20 feet away.  20/40 means that the patient can see an item 20 feet away with the clarity that a normally sighted person can see an item 40 feet away.  This is worse than normal vision.  20/10 means that the patient can see an item 20 feet away with the same clarity that a normally sighted person can see an item 10 feet away.  20/10 is better than normal vision.

Society for Excellence in Eyecare

Ophthalmologist membership organization.     .


See Sands of Sahara.


More commonly known as crossed-eyes, is a vision condition in which a person cannot align both eyes simultaneously under normal conditions. One or both of the eyes may turn in, out, up or down. An eye turn may be constant; when the eye turns all of the time. Strabismus may be intermittent; turning only some of the time, such as, under stressful situations or when ill.


A known complication of refractive surgery that causes images from light sources to blur with spikes radiating out from the center.  Starbursts also occur naturally without refractive surgery.

Stem cell

A type of primitive cell that can transform into and generate other cells.


The ability to perceive three-dimensional depth due to the distance between a person's two eyes.


A large class of pharmaceutical agents that chemically resemble cholesterol. Two better known types are anabolic steroids often used in athletics, and glucocorticoid steroids that are used to reduce inflammation.

Stiles-Crawford effect

The Stiles-Crawford effect (discovered in 1933) describes angular dependence of retinal sensitivity. Rays which enter the pupil near its center, which are parallel to retinal receptors, are more effective than oblique rays which enter the pupil near its margins. So, the light passing through the periphery of the pupil is less efficient at stimulating vision than the light passing near the center of the pupil. It is believed that photoreceptors act as light pipes, and more light gets down if it enters straight down (through the center of the cornea), rather than at a large angle (through the periphery of cornea).


Wrinkles in the corneal flap created for LASIK.  There are two kinds, macro striae and micro striae. 


Thickest layer of cells in the cornea.

Subretinal hemorrage

A condition caused by pooling of blood under the retina.


Upper eyelid depression.


Ophthalmic equipment company.     .

Sunrise Technologies

Ophthalmic equipment company.     .


The inability to perceive all of part of objects in the field of vision of one eye.

Tear Breakup Test

A test that determines the quality of the tears on the eye.  The doctor observes the tear film under the microscope while the patient avoids blinking until tiny dry spots develop. The longer the amount of time that passes before the tear film breaks up, the more stable the tear film. A good value is more than 10 seconds.

Tear film

A layer of fluid that bathes and lubricates the cornea.


Toward the ear.

TLC, The Laser Center

Chain of refractive surgery centers.     .


A procedure for the measurement of the fluid pressure inside the eye (intraocular pressure). One of the tests for glaucoma.


Ophthalmic equipment company.     .

Topographic Supported Customized Ablation

A system to use a topographical mapping system to guide the laser during refractive surgery.


To measure the high and low areas of a plane. See corneal topography.


Is the acronym for Topographic Supported Customized Ablation.  This means using a topographical mapping system to guide the excimer laser ablation.

Trabecular Meshwork

The spongy, mesh-like tissue near the front of the eye that allows the aqueous fluid to flow to Schlemm's canal then out of the eye through ocular veins.

Transition Zone

The area of laser ablation that changes for the full correction of the central ablation zone optical ablation zone to the original surface depth of the cornea.

Transpupillary thermotherapy (TTT)

The use of infrared light to stop retinal bleeding.


Corrective lenses that have three powers of correction.  Typically the majority of the lens is corrected for distance vision while a small area is corrected for near vision and another small area is corrected for middle vision.  Trifocals and bifocals are normally prescribed for individuals with presbyopia. U


Abbreviation for Uncorrected Visual Acuity.


Procedures using sound waves to measure certain portions or detect abnormalities within the eye.  

Ultraviolet Radiation

Radiant energy with a wavelength just below that of the visible light. UV-c is the shortest wavelength at 200-280nm and is absorbed by the atmosphere before reaching the surface. Extremely damaging to living tissue. UV-b, at 280-315nm is "burning rays" of the sun and is damaging to most living tissue. UV-a, at 315-400nm is "tanning rays" of the sun and is somewhat damaging to certain tissues. UV radiation has been described as a contributing factor to the processes that results in ARMD, cataracts, and causes exposure keratitis.

Ultraviolet (UV) light

The non-visible portion of the light spectrum with a wave length shorter than violet light.

Uncorrected Visual Acuity

The best vision measurement taken without the use of glasses or contact lenses.


A complication of refractive surgery where the expected amount of correction is less than desired. Undercorrection often occurs where healing regresses more vigorously than predicted.

Uvea, Uveal Tract

The middle coat of the eyeball, consisting of the choroid in the back of the eye and the ciliary body and iris in the front of the eye. V

Variable Spot

This is a method of applying excimer laser energy.  Rather than applying all excimer energy in a broadbeam across the entire ablation area, or with a very small flying spot of excimer energy applied in rapid succession at different locations across the ablation area, the energy is applied in variable spot sizes across the ablation area.

Vector Vision

Ophthalmic equipment company.     .

Vision Therapy

A treatment process for the improvement of visual perception and/or coordination of the two eyes for efficient and comfortable binocular vision (orthopedics, vision training, and eye exercises).

Visual Acuity

Clearness of vision. The ability to distinguish details and shapes of objects; also called central vision.

Visual Axis

The central area of the cornea, pupil, and lens that light passes through to reach the retina and be "seen".

Visual Field

The area or extent of space visible to an eye in a given position of gaze. The central visual field is directly in front and the target at which we are looking. The peripheral visual field is that which we perceive in our "side vision". The fields of each eye partly overlap.


Ophthalmic equipment company.     .

Vitreous hemorrhage

Hemorrhage within the vitreous of the eye.

Vitreous Humor

The transparent, colorless mass of gel that lies behind lens and in front of retina. W

Wave Length

The distance between the top of one wave and the top of the next wave.  The length of one complete wave of the argon fluoride excimer laser is 193 nm.  This wavelength is in the far ultraviolet end of the electromagnetic spectrum.

Wet macular degeneration

The abnormal increase in and leaking from blood vessels under the retina, leading to disturbances of the central field of vision.

YAG Laser

YAG is an abbreviation for neodymium yttrium-aluminum-garnet, the material used generate a short pulsed, high-energy light beam in the infra red wavelength of 1064 nm. The YAG laser is a surgical instrument that can be precisely focused by computer to cut, photovaporize, or fragment tissue. The YAG laser is used to treat posterior capsular opacification; a clouding of the remaining capsular tissue that develops postoperatively in as many as 75% of cataract removal operations. The tissue is vaporized with carefully controlled pulses of the YAG laser, and the surgery is performed on an outpatient basis. The common misconception that "lasers" are used to remove cataracts occurs because post-cataract patients eventually require YAG laser capsulotomy. Z


An antioxidant that neutralizes freeradicals and is important to the proper functioning of the body. Its role in macular degeneration is unknown.


The brand name for wavefront guided custom ablation on the Bausch & Lomb Technolas excimer laser.


The fibers that hold the lens suspended in position and enable it to change shape during accommodation.



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