Copyright 1995, 1996 Larry Bickford. All Rights Reserved.


This is an alphabetized list of commonly used terms relating to the anatomy and function of the human vision system. You may find this a useful reference when reading the articles in the EyeCare Connection or other texts and journ als.

There are currently around eighty entries and more are added periodically as necessary. You may wish to view and print this list in a smaller size as your browser might allow.


the ability of the eye to focus objects closer than optical infinity, approx. 20 ft (6m)

Accommodative insufficiency:

loss of or less than age expected normal amounts of accommodation


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


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


often called "lazy eye" it is a unilateral (occasionally bilateral) condition in which the best corrected vision is poorer than 20/20 (6/6) in the absence of any obvious structural anomalies or ocular disease

Amsler grid:

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

Anterior ocular segment:

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


micronutrients which destroy or neutralize free radicals, molecules which have been implicated as one causative factor in the stimulation of abnormal cellular reproduction (cancer) and cellular destruction (aging).


The watery fluid produced by tissues inside the eye and which fills the forward chamber of the eye.

ARMD: age related macular degeneration:

destruction and loss of the photoreceptors in the macula region of t he retina resulting in decreased central vision and, in advanced cases, blindness


a refractive anomaly caused by unequal refraction of light in different meridians, resulting in a distorted and blurred image.

Automated perimeter:

A computer-driven device used to plot defects in the visual field. Usuallly this is a large hemisphere shell into which the patient's head is placed. 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.


a member of the carotinoid family of vitamins, a precursor to vitamin A

Binocular vision:

the simultaneous use of the two eyes. Normal binocular vision yields a stereoscopic image and parallax-induced depth perception

Blind spot, physiological blind spot:

The ONH does not itself contain photoreceptors, and therefore its location at the back of the eye is a blind spot. This small area can be measured and in glaucoma, as the nerve fibers die, the blind spot tends to enlarge and enlongate. This is one of the diagnostic hallmarks of glaucoma.


a change in the structure of the crystalline lens which results in loss of light and therefore image transmission to the retina.

Color vision:

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


inflammation of the conjunctiva. May be caused by bacteria, virus, allergens or chemical or UV exposure


the thin transparent membrane lining the exposed eyeball and inside surface of the eyelid


the turning of the eyes inwards/outwards so that they are both "aimed" towards the object being viewed


the front-most surface of the eye. The transparent "window" and primary focusing lens of the eye.

Corneal curvature:

the shape of the front of the eye

Crystalline lens:

a transparent disk located behind the iris which focuses light onto the retina. The lens has the ability to vary its power to accomodatively focus light from objects closer than optical infinity.

Depth perception:

the ability of the vision system to preceive the relative positions of objects in the visual field.

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 dama ge to the eyes, kidneys, nervous system and vascular system

Diabetes type I: IDDM

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

Diabetes type II: NIDDM

non-insulin dependent, resulting from tissue resistance to insulin


a condition in which a single object is perceived as two; also called double vision.


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


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


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


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

Extra-ocular/external ocular muscles:

the six muscles that turn the eyes to position them appropriately for viewing objects in various locations in the observed environment


abnormally increased intraocular pressure resulting in optic nerve damage and loss of visual field


A viewing procedure utilizing a mirror/lens device placed directly upon the cornea that is used to view the drainage area called "the angle" through which aqueous fluid exits the eyeball.


farsightedness, long-sighted (UK): a refractive condition in which light entering the eye is focused virtually behind the retina, resulting in a blurred image in the non-accommodated eye.


a deficiency of oxygen supply to a tissue

Intraocular pressure (IOP):

the fluid pressure within the eye created by the continual production and drainage of aqueous fluid in the anterior chamber.


the disk of pigmented muscle tissue located in the anterior chamber behind the cornea and anterior to the lens, the center of which is the pupil


The restriction or blockage of blood flow through a blood vessel. Ischemia is a causative agent of certain heart attacks and strokes and is involved in various types of visual field losses.


inflammation of the cornea


a member of the carotinoid family of vitamins, similar to beta-carotene

Macular edema:

the collection of fluid in and under the macular portion of the retina


pupillary constriction


pupillary dilation


Nearsightedness, short-sighted (UK): a refractive condition in which light entering the eye is focused short of the retina, resulting in a blurred image

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 inwards


the formation of new blood vessels, often fragile and inappropriate for the location

Nerve fibers/axons, retinal:

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

Ocular hypertension:

Elevated fluid pressure. The normal pressure is about 10 to 20mmHg, with the majority of people falling between 13 and 19. Over 20 is considered "suspicious" over 24 "cautiously concerned" and warranting immediate investigation, and over 30 is considered to be "urgent" and a potential emergency situation.

Optic nerve, optic nerve head (ONH):

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.


examination of the internal structures of the eye using an illumination and magnification system.
Orthokeratology: o-k: a non-surgical procedure using contact lenses to alter the shape of the cornea to effect a change in the refractive error


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


non-inflammatory swelling/elevation of the optic nerve often due to increased intracranial pressure or space-occupying tumor


sensitivity to light

Photorefractive keratectomy:

PRK: a surgical procedure using an excimer laser to remove corneal tissue to effect an alteration of the refractive error


a (usually) irritation caused degeneration of the conjunctiva resulting in a thickening and yellowing of the normally thin transparent tissue

Posterior optical segment:

the part of the eye posterior (behind) to the crystalline lens, including the vitreous, choroid retina and optic nerve

Posterior Vitreous Detachement (PVD):

the separation of the vitreous body from it1s attachment from the retinal surface due to shrinkage from degenetative or inflamatory conditions or trauma. Also an age-related condition


a condition in which the accommodative system fails to allow for the focusing of near objects, resulting from age-related changes in the crystalline lens or cilliary muscle


the center hole in the iris through which light must pass to reach the back of the eye.

Pupillary response:

the constriction and dilation of the pupil due to stimulation by light or accommodation

Radial keratotomy:

RK surgical procedure using diamond scalpel to create incisions into the epithelium of the cornea which result in the flattening of the tissue and thereby altering the refractive error


determination if optical refractive errors of the eye

Refractive error:

the degree to which images received by the eyes are not focused on the retina, e.g.: myopia, hyperopia, astigmatism


the neurological tissue at the posterior of the eye, containing the rods and cones which receives light and converts it to electrical signals for transmission via the optic nerve to the brain

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 in the early stages.


an area of partial or complete loss of vision surrounded by an area of normal vision, as what can occur in advanced ARMD


the ability to perceive three dimensional depth


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


a procedure for the measurement of intraocular pressure. One of the tests for glaucoma

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, a t 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 th e processes which result in ARMD and cataracts and causes exposure keratitis

Vision therapy:

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

Visual acuity:

the clearness of vision which depends upon the sharpness of the retinal image

Visual field:

the area or extent of space visible to an eye in a given position of gaze. There is a central visual field--that is directly in front of us, the target at which we are looking, and a peripheral visual field---that which we perceive in our "side vision". The fields of each eye partly overlap. We do not perceive the blind spots from each eye because the area that is missing in one eye is present in the other.


the gel-like transparent fluid substance filling the posterior four fifths of the globe between the cystalline lens and retina