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Eye Health Glossary

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d-15

The Farnsworth D15 is a color vision test that is often used to identify severe color vision defects. It is a standard test used for assessing the visual abilities of military, police, and firefighter applicants.

dacryoadenitis

Inflammation of the lacrimal gland (tear-producing gland), which is located above and just to the outside of the eyeball. There are two types of dacryoadenitis: acute and chronic. Acute dacryoadenitis is usually caused by a bacterial or viral infection while chronic dacryoadenitis is most commonly due to an inflammatory disorder.

dacryostenosis

A congenital blockage or narrowing of the tear duct, which occurs in approximately 5% of babies. Tears are unable to flow into the lacrimal sac and are therefore backed up and build up on the surface of the eye. The combination of excess tears with dust and debris from the environment results in a mucoid discharge forming in the eyes. In most cases, an infant will outgrow this problem by the age of 9 months, but in rare cases where the duct does not open, minor surgery may be required.

depth perception

The ability to determine relative distances of objects in space. Depth perception requires the use of binocular vision.

descemet's membrane

A strong, thin, non-cellular layer of the cornea, located below the stroma and above the corneal endothelium.

deutan

Missing or altered M (green) cone, which is the cone photoreceptor that responds maximally to the middle wavelengths of the visible spectrum (shades of green light).

deuteranopia

A color vision defect in which the M cone (green cone) is either absent or has a wavelength sensitivity spectrum that is shifted to the point where it matches the wavelength sensitivities of the L cone (red cone). As a result, only shades of blue and yellow can be seen. For deuteranopes, the familiar color names of red, orange, yellow, and green all appear to look the same.

diameter

Length of a straight line through the center and between two points along the circumference of a circle. “Diameter” is one of three necessary numbers used for a contact lens prescription. The diameter determines the size of the contact lens that is needed to properly fit the eye of its wearer. For most people, the value is somewhere between 14.0 and 14.2 mm.

dichromat

A person who has only two types of functional cone photoreceptors and therefore cannot see all the shades and hues of color that a person with normal color vision can see. Learn more about Color Blindness.

diopter

The standard unit of measurement for lens power. It is defined as the inverse of the distance to its focal point in meters. Nearsighted (myopic) individuals will have a prescription with a minus sign in front of the dioptric power and farsighted (hyperopic) individuals will have a plus sign in front of the dioptric power.

diplopia

Double vision. There are two types of diplopia: pathological (due to disease or binocular vision problem) and physiological. Physiological diplopia occurs every day in all situations for people who have normal binocular vision. To test this, simply hold up your finger in front of your nose and look past it at the computer screen – do you see two fingers instead of one? This is physiological diplopia. Pathological diplopia can occur as a symptom of numerous vision disorders, including cataracts, strabismus, and myasthenia gravis.

divergence

Movement of the eyes in opposite directions. This only occurs when a person is switching views from a near target to a distance target (eg. from reading a book to looking across the room).

dominant eye

The eye that looks directly at an object; meanwhile, the other eye looks at the object from the side. A simple test to determine which eye is dominant: hold your finger about a foot in front of your face and line the finger up with a line some distance away (corner of the room, perhaps). Close one eye, then alternately close the other. Which eye were you using when you saw your finger jump away from the line? This eye is your non-dominant eye.

drusen

Tiny yellow deposits in the retina. There are two types of drusen: hard drusen and soft drusen. Hard drusen are small discrete deposits that are of the least concern because they represent the smallest impact on vision; however, hard drusen can progress to soft drusen, which are larger, with indistinct borders and can cause significant vision loss. Appearance of drusen is an early sign of age-related macular degeneration.

dry eye syndrome

Dry eyes is a common complaint of a large percentage of the population. Due to pollution, air conditioning, and prolonged computer use, few people these days are enjoying vision without dry eyes. Dry Eye Syndrome, however, is a pathological condition in which the tear film is so unstable that it cannot provide the eyes with adequate lubrication. Learn more about Dry Eye Syndrome and dry eye treatment.

dry macular degeneration (dry ARMD)

Also known as “atrophic” or “non-exudative” macular degeneration, dry ARMD is by far the most common of the two types of macular degeneration and is slowly progressive with little impact on vision. Learn more about dry macular degeneration.

dyslexia

The inability to read, write, or spell words, despite an ability to see the letters.

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