Dictionary of terms relating to
eyesight and vision

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A condition of the crystalline lens, in which the normally clear lens becomes clouded or yellowed, causing blurred or foggy vision. Cataracts may be caused by aging, eye injuries, disease, heredity, or birth defects. Surgery is a treatment option. The affected lens is removed and is replaced with a substitute (implant) lens or with a special type of contact lens. Generally the success rate of cataract surgery is over 90%, if the eye is otherwise healthy.


An instrument used in orthoptics/vision therapy to train binocular skills and accommodation skills. The Keystone Correct-Eye Scope is an example of a Cheiroscope.

Ciliary Body

A structure directly behind the iris of the eye and contains the ciliary muscle. (See diagram of the eye)

Ciliary Muscle

A band of muscle and fibers that are attached to the lens that controls the shape of the lens and allows the lens to accommodate (change focus).


A congenital malformation (birth defect) in which part of the eye does not form due to failure of fusion of an embryonic feature called the intraocular fissure. The resultant coloboma can be likened to a missing slice from a pie that may involve a number of different structures within the eye including the choroid, iris, lens, optic nerve, and retina.

A coloboma can occur as an isolated defect in an otherwise normal baby, or it can be part of a multiple congenital malformation syndrome such as the cat-eye syndrome (named after the coloboma which gives the eye something of a feline look). Colobomas are also seen in the aniridia-Wilms tumor association (the concurrence of undergrowth of the iris of the eye and Wilms tumor of the kidney); and the trisomy 13 syndrome (a disastrous disease due to an extra chromosome number 13).

Color Perception Test

A test that measures the ability to identify and distinguish colors.

Color Vision Deficiency

Also known as Colorblindness. It is the absence of or defect in the perception of colors. Color vision is based on perception of red, green, and blue. If there is a defect in the perception of one of these colors, a color will be perceived as if it were composed only of the other two colors. Based on the color or colors for which there is defective perception, a person may suffer from red, green, or blue blindness. Color blindness in which all colors are perceived as gray is termed monochromasia. For people with the common, inherited, types of color deficiency there is no cure.

Comitant Strabismus

A condition in which the magnitude of deviation remains essentially the same in all positions of gaze and with either eye fixating.

Computer Vision Syndrome (CVS)

The complex of eye and vision problems related to near work that are experienced during or related to computer use. Its symptoms include eyestrain, dry or burning eyes, blurred vision, headaches, double vision, distorted color vision, and neck and backaches. The condition is caused by various internal and external factors. Treatment options may include prescription glasses and/or vision therapy.


A receptor cell which is sensitive to light and is located in the retina of the eye. It is responsible for color vision.


An inflammation of the conjunctiva, the transparent layer covering the inner eyelid and the white portion (sclera) of the eyeball. Conjunctivitis can be caused by a virus, bacteria, or fungus (infectious conjunctivitis, or "pink eye", may be contagious); by allergies to pollen, fabrics, animals, or cosmetics (allergic conjunctivitis); or by air pollution or noxious fumes such as swimming pool chorine (chemical conjunctivitis). Symptoms include red or watery eyes, blurred vision, inflamed inner eyelids, scratchiness in the eyes, or (with infectious conjunctivitis) a puss like or watery discharge and matted eyelids. Conjunctivitis is usually treated with antibiotic eye drops and/or ointment.

Contrast Sensitivity Test

Contrast sensitivity tests measure the degree to which this ability has been lost. Unlike the Snellen visual acuity test, which measures the ability to see objects (or letters) of different sizes, a contrast sensitivity test measures two variables, size and contrast. The ability to detect objects of different sizes at lower contrasts is expressed as a contrast sensitivity function (CSF). The test determines the person's contrast detection threshold, the lowest contrast at which a pattern can be seen.

Typically, the best scores of CSF are recorded for medium-sized objects when their contrast is low. The smallest objects can be detected only when their contrast is very high. Imagine an image comprising vertical black and white stripes. If the stripes are very thin, individual stripes will not be visible. Only a gray image is visible. As the stripes then become wider, there is a threshold width from where it is possible to distinguish the stripes.

The fact that larger objects require higher contrast is explained not by how the eye gathers information but by how the brain processes that information. The brain is relatively insensitive to what neurologists call "low spatial frequencies." Contrast sensitivity readings are presented as a curve, which plots the lowest contrast level at which a person can detect an object of a given size. The higher the contrast sensitivity, the lower the contrast level at which an object can be seen.


The ability to use both eyes as a team and to be able to turn the eyes inward to maintain single vision up close.

Convergence Excess (CE)

A clinical condition in which the eyes have a tendency to turn excessively inward when viewing an object at a near distance. Symptoms may include visual fatigue while reading or using a computer, occasional blurred or double vision, and inability to comprehend or concentrate while reading. Clinical signs include: greater esophoria at near than distance, high AC/A ratio, and a high lag of accommodation. Can be improved with vision therapy and/or glasses. (See "Esophoria")

Convergence Insufficiency (CI) (clinical condition)

The inability of the eyes to turn inward and/or sustain an inward turn. Symptoms include eye strain with reading and using a computer, headaches, loss of comprehension, difficulty concentrating, blurred or double vision, and eye fatigue. Clinical signs include: near point of convergence of greater than 4 inches (10 cm), greater exophoria at near than at distance, and low AC/A ratio. Vision therapy is an effective treatment option. For more information, please click here. To review the American Optometric Association's guidelines for convergence insufficiency, please click here. (See "Exophoria")


The transparent, blood-free tissue covering the central front of the eye (over the pupil, iris, and aqueous humor) that initially refracts or bends light rays as light enters the eye. Contact lenses are fitted over the cornea. (See diagram of the eye)

Corneal topgrapher

The Corneal Topographer is an instrument that maps the unique curvatures of your cornea.  This instrument takes numerous measurements, including your astigmatism.  For new or current rigid gas permeable contact lens wearers and soft contact lens wearers, the corneal topographer can provide the doctor the most advanced information to better fit your eyes. 

Cover Test

A test of eyeball alignment in which each eye is covered with an occluder (eye cover) and then uncovered to observe eye movements. For more information and a diagram, please click here.

COVTT- Certified Optometric Vision Therapy Technician.

To be certified an individual must be employed by a Fellow (FCOVD), provide documentation of 2000 hours or 2 years of direct clinical experience in vision therapy; or 1000 hours of clinical experience if the individual holds an AA degree or higher with emphasis in the behavioral sciences. Submit written answers to a series of Open Book Questions dealing with various aspects of vision function, testing and therapy. Pass an extensive written and oral examination evaluating the candidate's knowledge and clinical abilities in behavioral vision, vision development and vision therapy. COVTTs must obtain at least 6 hours of continuing education annually in functional / developmental / behavioral vision care.

Crystalline Lens

Transparent disc located behind the iris which changes shape to focus on objects at different distances from the eye. (See diagram of the eye's lens)

Cycloplegic Refraction

One method available to eye doctors to determine the eye's refractive error and the best corrective lenses to be prescribed if needed. The eye is dilated with the muscles of accommodation (eye focusing muscles) being temporarily paralyzed with specialized eye drops or spray (Atropine, Homatropine, Cyclogyl, or Mydriacyl). This is a good method for non-responsive or non-communicative patients such as young children. The technique of retinoscopy is used with this method. (See "Retinoscopy")

Cylinder Lens

An ophthalmic lens that has at least one non-spherical surface. Used to correct astigmatism. The values are typically from -0.75 to -1.25. The cylinder measurement is given with a "-" sign. (Please note that the sign for myopia (nearsightedness) is also "-".)