Glossary of Vision and Eye Terms

by David A. Wallace MD


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A - B-C - D-E-F - G-H - I-J - K-L - M-N-O - P-Q - R-S - T-U-V - W-X-Y-Z

If you don't find the term you're looking for here, you will also find extensive ophthalmic online dictionaries and optical glossaries at,,  and

In an optical context, aberration is any perceived or real imperfection in the formation of a visual image. Aberrations can be regular and measurable, such as spherical aberration, chromatic aberration, and coma; or can be irregular, as might be noticed when looking through a pane of hand-blown glass. Instruments that measure aberrations of the visual system are called 'wavefront analyzers' or aberrometers. See also Higher-Order Aberrations , below.


Removal of tissue. In the context of excimer laser treatment, ablation describes the delicate and precise sculpting of corneal collagen by the laser. 'Ablation depth' corresponds to the depth (measured in microns) of tissue removed by treatment.


The process by which the human eye adjusts from distance to close focus. This involves a coordinated combination of multiple factors, including (a) Change in muscle tension within the ciliary muscle or ciliary body, (b) a change in pull on the lens by tiny fibers called zonules, and (c) a resulting increase in radius of curvature of the lens itself, which changes its optical power.

Aqueous Humor

The fluid that fills the eye behind the cornea, in front of the crystalline lens.


In an optical context, altering the curvature of a lens to adjust or correct for spherical aberration.  See Spherical Aberration , below.

An optical imperfection due to the cornea or lens of the eye being 'out of round' or not uniformly evenly curved. Example: the side of a donut is a 'toric' or astigmatic surface as the horizontal curve is different from the vertical curve. The difference between the two curvature radii equals the amount of astigmatism. Astigmatism is corrected by lenses ground in a cylindrical shape (as compared to concave or convex lenses which correct near- and farsightedness, respectively). For that reason, the astigmatism component of an optical lens is sometimes referred to as the 'cylinder'.

Binocular Vision

Fusion of the visual information from each eye to afford stereoscopic vision and depth perception. Some people do not have the capacity for binocular vision, and see things either exclusively with one eye, or one eye at a time, but never both at once.


Measurement of the internal dimensions of the eye, performed as a part of IOL power calculation . The ocular axial length measurement is used in one of several mathematical formulas to calculate the proper power of lens implant to be used in cataract surgery. Biometry can be performed either by ultrasonic instruments (called "A-scans") or by optical coherence tomography. Dr. Wallace is one of the co-inventors of the Bio-Pen , the smallest and most accurate instrument of its' kind.

A clouding or opacification in the crystalline lens of the eye. When the clouding becomes significant enough to impair vision, cataract surgery is often the treatment of choice.

Cataract Surgery

Removal of a clouded lens from the eye, and replacement by an artificial intraocular lens (see “IOL” below).

Ciliary Body

A round, muscle-like tissue behind the iris, responsible for accommodation (see above), and for production of the fluid that fills the anterior chamber, nourishing the inner lining of the cornea.


A trademarked name arising from the combination of the terms "collagen" and "polymer" that describes the material used in manufacture of the Staar Visian ICL. The ICL (for "internal collamer lens" is made in part from colagen and in part from some hydrogel polymer; hydrogel refers to the class of materials that are used to manufacture soft contact lenses.


Coma A lens aberration, resulting from different magnifications in the various lens zones, that occurs in that part of the image field that is some distance from the principal axis of the system. Extra-axial object points appear as short comet-like images.

Conductive Keratoplasty or "CK"

This technique uses radio-frequency energy to shrink corneal tissue. CK has for the most part fallen out of favor as effectiveness fades over time.

A type of vision test that measures ability to distinguish small gradations in contrast, or subtle differences between shades of gray. Contrast sensitivity does diminish slowly with advancing age. It is also recognized that most refractive surgery can result in reduced contrast sensitivity.

The clear front 'window' of the eye, in front of the colored iris. The cornea is responsible for about 2/3 of the total focusing power of the eye (and the lens contributes the rest). The cornea is the tissue sculpted by laser in refractive surgery including LASIK and PRK.

A computer-generated map of the surface contour and curvature of the cornea. Useful for detecting corneal astigmatism, smoothness of curvature, keratoconus, and/or other conditions that may warrant advising against excimer laser care with current technology.  in the 1990s, all corneal topography systems measured only the front corneal surface.  Beginning in about 2000, systems evolved for measuring both the front and back corneal surfaces.  Later-generation imaging systems are also referred to as corneal tomography systems.  All these devices intend to display corneal curvature, elevation, and thickness maps.


Any imperfection in the representation, reproduction, or perception of a signal. Visual distortion can be caused by aberration in an optical lens. Certain visual distortions can result from refractive surgery, including increased glare, flare, and/or halo around point sources of light such as street lights or headlights. Shadowing, ghosting, and areas of blur are also types of distortion.


Thinning of tissue in an uncontrolled, progressive, or unanticipated fashion. Corneal ectasia can have adverse optical consequences, as abnormally thin tissue can be biomechanically and optically unstable. Keratoconus (see below) is a type of corneal ectasia, characterized by thinning and steepening of the central or paracentral cornea. A much rarer condition, called keratoglobus, is characterized by ectasia of the peripheral cornea. Ectasia is thought to be aggravated by eye-rubbing in some, but not all cases. Ectasia is rarely associated with laser sculpting surgery, but when it occurs can cause untoward visual consequences and may be difficult to treat.


General medical term referring to the lining cells on the inner surface of any living tissue. In relation to the cornea of the eye, this means the delicate cells on the inside surface. These cells serve a ‘pump function’ in that they pump fluid out of the cornea and into the aqueous humor. If the endothelial pump function is impaired, the cornea would swell and lose its’ optical clarity, turning instead a milky white.


General medical term referring to the lining cells on the outer surface of any living tissue. Specifically in relation to the cornea of the eye, this means the protective cells lining the outer surface, covering the collagen layer.

Excimer Laser

The type of laser currently used for the vast majority of vision-correction surgery including PRK and LASIK. Developed by IBM in the late 1970's to aid in microchip fabrication, as the UV-wavelength laser has greater theoretical resolution than photolithography techniques using visible light. The excimer laser was adapted for vision correcting eye surgery shortly thereafter. The laser emits a 'cool' beam of UV light to gently sculpt the cornea with exquisite precision, and without heating, injuring, shrinking, or otherwise distorting surrounding tissue.

The opposite of nearsightedness, also called hyperopia. Farsighted persons may see better at distance than close up, but still require optical correction with convex lenses.

Higher-Order Aberrations

Subtle, regular and measureable imperfections in a lens or optical system that is not corrected with simple sphero-cylindrical lenses. An astronomer and mathematician in the early 20th Century named Fritz Zernike developed a series of polynomial equations to describe low- and higher-order aberrations in lathe-cut lenses. These Zernike polynomials form the basis for analysis and description of aberration patterns in many current systems designed for wavefront analysis of the human eye, including the Alcon Custom Cornea ; Bausch & Lomb Zyoptix and Visx WaveScan systems.


Medical term for farsightedness.


The colored part of the eye that controls how much light enters, by adjusting pupil size (the pupil being the opening in the center of the iris). Analogous to the diaphragm in the lens of a camera.

Internal Collamer Lens; a device implanted inside the eye. ICL placement is advised primarily for very high amounts of myopia correction (beyond that attainable reliably by LASIK), and in cases where corneal thickness is not adequate to support laser sculpting for the degree of correction desired. 

Intra-Ocular Lens: A small optical lens implanted inside the eye, typically as a part of modern cataract surgery. Without IOL implantation, very thick "coke-bottle" lenses would be necessary to restore vision. With IOL implantation, it is possible not only to clear up the blurring and distortion caused by the cataract, but also to correct any existing refractive error (nearsightedness, farsightedness, etc.).


Distortion of the corneal curvature that causes optical distortion. Usually due to focal thinning, biomechanical weakness, excess eye rubbing, genetic factors, or combinations of these causes. Absent treatment, keratoconus can be progressive, such that it can cause progressive visual compromise. Mild to moderate keratoconus can be treated with specially-fit contact lenses. In the past, significant to advanced keratoconus was only treatable by corneal transplant surgery. In the current era, a strengthening treatment called UV Corneal Cross-Linking or CXL can stabilize affected tissue, preventing further progression; though this treatment cannot restore the cornea to a non-distorted state.  A combination of CXL and specially-fit contact lenses (custom-fit RGP, hybrid lenses, or scleral RGPs) can often restore excellent vision.


An instrument used to measure curvature radius of the cornea.


Corneal sculpting surgery wherein a microkeratome is used to expose the collagen layer ("stroma") for optical sculpting purposes. Before the development of excimer lasers, surgery of this type was performed by (a) removing completely a superficial disc of cornea (called the "cap"); (b) placing the cap in a microscopic lathe; (c) freezing the tissue; (d)lathe-carving the desired optical correction into the inside surface; (e) thawing the tissue; and (f) suturing the cap back in place. Modern LASIK is much more elegant, more precise, and less invasive.


Surgical replacement of corneal tissue. Lamellar (layered) keratoplasty can be performed to augment existing thin tissue, but has distinct optical limitations due to the interface between donor and host tissue. Penetrating keratoplasty (also referred to as corneal transplant) can be done to remove abnormal, damaged, diseased or optically distorted central cornea, replacing it with healthy tissue from a donor. Advanced keratoconus is now among the most common reasons for corneal transplant surgery. This treatment is a very effective method of restoring normal anatomy, thickness and optical function to the eye. Penetrating keratoplasty is also (very rarely) warranted to treat complications of LASIK and/or PRK. cornea.

Laser-Assisted Sub-Epithelial Keratomileusis. Basically, a fancy name for PRK. Corneal sculpting using the excimer laser is performed after removing the surface lining cells of the cornea (called the epithelium).

Laser-ASsisted In-situ Keratomileusis: Currently the most popular procedure for vision correction. A microkeratome is used to make a thin flap of the front corneal tissue which is then folded open like the cover of a book. Treatment with excimer laser is then applied to the underlying collagen to correct the eye's prescription. The flap is then returned to its' original position.


Used to focus light. An object characterized by a uniformly curved surface or surfaces that refracts, or changes the path of light passing through, in a predictable fashion. There is a lens inside the human eye, made of protein, which helps us adjust our focus from far to near, and vice versa. The human lens supplies about 1/3 of the total focusing power of the eye.


Surgical removal of a the crystalline lens of the eye. This can be performed as a treatment for cataract (usually in combination with IOL implantation), or as one of several methods available to correct significant refractive errors of the eye including high hyperopia and high myopia.


Laser Thermo-Keratoplasty. This is a technique using laser energy to heat (and thereby shrink) corneal tissue, thereby altering corneal curvature and the refractive state of the eye. A circular pattern of 8 or 16 spots is placed in the mid-periphery of the cornea. LTK is only able to correct small amounts of farsightedness. Though recently (2002) approved by the FDA for very limited amounts of farsighted correction, there is significant disagreement about whether the results are stable over time.  LTK is now virtually obsolete.


The precision microsurgical instrument that is used to create the flap in LASIK and other types of 'lamellar' (i.e. layered) corneal surgery.

Mono from the Greek word for "one" describes the optical situation where one eye is corrected for distance, one for near. This has proven to be an acceptable option for many (but not all!) individuals who desire to avoid reading and distance glasses for as long as possible. At LA Sight, we demonstrate this to all individuals over 45 or so who come in to discuss laser care. Satisfaction with laser care is extremely high if people understand presbyopia, and the advantages and limits of monovision. We find that our success rate is even higher if we allow prospective patients to experience monovision correction in soft contact lenses as a part of the process of choosing whether or not to have monovision-style treatment.

Medical term for nearsightedness.

Myopia; the condition wherein a person can see better close-up than at a distance. The commonest optical condition worldwide -- approximately 40% of people are nearsighted. Corrected by concave lenses.


Eye care professional with a medical degree and specialty residency training in eye health, in treatment of eye disease, and surgery. Performs eye surgery including laser treatment.


In the U.S., this identifies an eye care professional trained to grind prescription optical lenses, and insert such lenses into eyeglass frames. An optician is not trained to measure the prescriptive needs of the eye.


Eye care professional with special training in eye health, prescription and fitting of eyeglasses and contact lenses, and in evaluating candidacy for vision correcting surgery such as LASIK and lens implant care.


A corneal topography instrument formerly manufactured Bausch & Lomb Surgical. The Orbscan provides detailed topographic analysis of the front corneal surface (like other topography devices). In addition, it measures topography of the posterior surface , and provides accurate mapping of corneal thickness across the entire cornea, unlike ultrasonic pachymeters (which only measure at one point).


An instrument that measures the thickness of the cornea, helpful in evaluation of corneal health, identification of certain corneal disease states, and in evaluation of candidacy for excimer laser surgery. Dr. Wallace is one of the inventors of the  Pach-Pen pachymeter.


One of the modern techniques of cataract removal, wherein clouded lens tissue is removed by ultrasonic fragmentation, through a very small incision.  This can be augmented, but not replaced, by laser-assisted cataract surgery

In our youth, the lens is very flexible and accommodates easily for close focus. As we mature, the lens grows in a fashion similar to that of a tree trunk. Layers of protein are deposited on the outside surface (like the rings on a tree trunk), causing the lens to become thicker, denser, and less flexible. When that occurs, close focusing becomes difficult, and people start using magnifying glasses to focus up close without eyestrain. This usually begins in the mid-forties and gets progressively worse for about 15 years.

Photo-Refractive Keratectomy, the vision-correcting procedure wherein excimer laser treatment is applied to the front corneal surface, just beneath the surface lining cells. PRK was the first excimer laser procedure developed, and is still performed in certain situations, but now enjoys less popularity than LASIK.


Radial Keratotomy, a type of refractive surgery now virtually obsolete. RK involved making radial incisions in the cornea, to correct myopia.

Measurement of the prescriptive needs of the eyes. Results in the optical prescription, which specifies the parameters to optimally correct your vision. Includes sphere (near- or far-sighted correction strength), cylinder (astigmatism correction strength), axis (angle of astigmatism correction, in degrees), and other factors. Measures the refractive error of the eyes.

Refractive Surgery

Also referred to as 'Vision Correcting' surgery: Any surgery intending to correct refractive errors of the eye.

The layer of nerve tissue inside the eye that senses light: analogous to the 'film' in a camera.

The long-standardized method of measuring vision sharpness using a chart with letters of different sizes. This test measures high-contrast visual acuity. Size of letters at a given distance correlates with angular resolution of the visual system. A measurement of 20/20 means that, at a viewing distance of 20 feet, the person being tested can see the same size letters as the optically "normal" eye.  A measurement of 20/40 means that the person being tested can see letters at 20 feet that the optically normal person can see at 40 feet.  For reference, 20/40 visual acuity is considered the minimum standard to obtain a passenger vehicle driver's license in most states including CA.  Legal blindness is defined as having best-corrected visual acuity of 20/400 or worse.

Spherical Aberration

Spherical Aberration A type of optical aberration resulting from failure of a lens (or optical system) to form a perfect image of a monochromatic, on-axis point source object. Spherical aberration is a form of 'higher-order' aberration. A lens can be perfectly spherical in curvature, but that does not mean that parallel light rays entering near the center of the lens will intersect the lens axis at the same point behind the lens as rays entering from more peripheral points. The differences in where these rays intersect determines the amount of spherical aberration. When rays from a point on the axis passing through the outer lens zones are focused closer to the lens than rays passing the central zones, the lens is said to have negative spherical aberration; if the outer zones have a longer focal length than the inner zones, the lens is said to have positive spherical aberration. Spherical aberration can be corrected by lenses with aspheric design.


In the context of ocular anatomy, stroma is the collagen layer of the cornea, accounting for about 90% of its' thickness and providing all of the structural and mechanical integrity of the cornea.


An Instrument that measures eye pressure. Important in recognition of risk for, and treatment of glaucoma. Dr. Wallace is one of the inventors of the  Tono-Pentonometer.


A portion of the electromagnetic spectrum just beyond visible range, near the dark blue and violet color range of visible light. A "UV" filter in eyeglasses can absorb (hence filter) ultra-violet light, affording UV protection from sunlight. An excimer laser emits energy in the ultraviolet range, but this energy is absorbed by surface tissue in the cornea and does not pass into the eye.

The gelatin-like material filling the back portion of the eye, behind the crystalline lens and in front of the retina.


A new term for the eye care profession, gleaned from optical engineering. 'Wavefront analysis' refers to a highly sophisticated and very complex method of measuring all of the optical properties of an optical imaging system (be it a camera, telescope, spy satellite or human eye), including all aspects that contribute to quality of image formation, such as lower- and higher-order aberration.

Zernike Polynomials

A series of mathematical formulae developed by astronomer Fritz Zernike to describe low- and high-order aberrations of an optical system. The complexity of these equations is beyond the ability of most refractive surgeons (and patients) to grasp. All current commercial wavefront analyzers provide a graphical display of aberrations in terms of their Zernike coefficients. No wavefront system currently available is capable of treating above the sixth radial order Zernike term.


The array of tiny fibers connecting the ciliary body to the crystalline lens of the human eye.