Glossary of Vision Loss Terms
The terms and definitions found in this glossary relate to vision and vision loss. Please note that this glossary is not comprehensive.
20/20 vision - A measurement of visual acuity (or clearness of sight) that most eye care professionals consider as the mean. The level of acuity is routinely measured by requiring an individual to identify various sizes of black symbols on a white background at a distance - an eye chart. The first numeral 20 defines the distance in feet from the individual. The second numeral 20 defines the distance from which a person can distinguish characters on an eye chart that is 20 feet away. Most people can identify characters on an eye chart at 20 feet. A person with 20/200 eyesight can see at 20 feet what a person with normal sight (20/20) can see when standing 200 feet away.
AMD - AMD is an acronym for Age-related Macular Degeneration, characterized by a worsening loss of central vision as a result of deterioration of the macula, which contains retinal cones that are required for sight. Learn more about AMD / Macular Degeneration
Amsler Grid - A black-lined grid on a white background, used for testing central sight field distortion and impairments that signal macular degeneration and other vision loss issues. See a graphic representation of an Amsler Grid.
Aqueous Humor - the clear fluid contained between the iris and cornea that provides necessary nutrients to the lens and cornea. When this fluid cannot drain, it creates unrelieved pressure that can result in Glaucoma and vision loss. Learn more about Glaucoma.
Astigmatism - An eye disorder caused by uneven curvature of the eyeball that motivates light rays to bend, targeting more than one point of the retina, instead of a single spot. This causes blurry vision, vague shadows on characters when reading, and slight double vision.
Blepharospasm - A condition that causes involuntary blinking to an excessive measure that the affected individual cannot see. Blepharospasm is considered as a form of functional blindness.
Braille - A tactile reading and writing system invented by Louis Braille in the 1820s and widely used by the blind and deaf-blind. Braille is made up of a system of raised dots that are felt with the fingertips. Braille makes reading, writing, note taking, and communicating possible for blind and deaf-blind users.
Braille display - A tactile device consisting of a row of rectangular cells, each of which contains a series of moveable pins. The pins are controlled electronically to move up or down to form Braille characters. The characters depict the information that appears on the text source, typically via a computer or notetaker device.
Cataracts - Commonly caused by aging in conjunction with diabetes, excessive exposure to UV rays, and sometimes caused by smoking and excessive steroid use, cataracts result in clouding or yellowing of the eye lens and, thus, blurred vision; halos around objects ( in particular, lights); glare; and diminished perception of color saturation. Learn more about Cataracts.
CCTV / Closed-circuit television - an older term for a video camera system that broadcasts a signal to a specific monitor (as opposed to a public broadcast source). The more common term for use for people with low vision is video magnifier.
CMV Retinitis - A visual impairment characterized by spots before the eyes (floaters), blurred and/or distorted vision, commonly experienced by persons with immune deficiencies.
Contrast sensitivity - A property that regulates the level at which the eye can detect differences in contrast between an object or character and its background - for instance, the contrast between a black letter on a white page.
Corneal Dystrophy - Classified under several categories, corneal dystrophy is a medical syndrome that results in loss of transparency of the cornea. Individuals who have this condition experience differing levels of blurred vision, light sensitivity, and vision loss.
Diabetic Retinopathy - The leading cause of vision loss in persons who are diagnosed with diabetes. Diabetic retinopathy is brought on by leakage of the retinal blood vessels and can develop into progressive blurred vision, double vision, floaters, visual distortion and, ultimately, some degree of blindness. Learn more about Diabetic Retinopathy.
Glaucoma - A sight impairment caused by unrelieved pressure inside the eye as a result of fluid build-up. Permanent impairment can range from loss of peripheral vision to severe vision loss. Individuals with glaucoma may experience increased frequency of headaches, blurred vision, halos around lights, difficulty seeing in the dark, and sometimes, a non-reactive pupil, pain, or even a swollen eye. Learn more about Glaucoma.
Grave’s Disease - An autoimmune disorder present in cases of thyroid abnormality. Grave’s Disease can affect vision, including eye discomfort, double vision, and degrees of vision loss.
Hyperopia / Hyperopic / Farsighted - A person is considered “farsighted,” when items at a distance are in focus, but close object are blurry. The medical term for farsightedness is hyperopia.
Keratoconus - An eye disorder that leads to degeneration of vision and various degrees of vision loss, ranging from blurred, doubled or distorted vision to severe vision loss that cannot be improved by glasses or other common optical aids. Keratoconus results from a severe form of astigmatism in which the cornea thins and reforms into a cone-shaped bulge.
Legally blind - Having 20/200 vision in the best eye with correction or a visual field of 20 degrees or less.
Low vision - A significant reduction of visual acuity that ordinary eyeglasses, contact lenses, and/or medical treatment cannot completely correct.
Macula - The small area in the retina that contains special cells that are especially sensitive to light. The macula enables people to see fine details clearly.
Macular Degeneration - See AMD.
Metamorphopsia - Distorted vision caused by impairments of the eye’s macula and/or retina. Objects may look nearer or farther than they really are, and lines may appear wavy or bent.
Multiple-view magnification camera - A portable, rotating closed-circuit camera that provides magnified views of objects from varying distances â€” from close-up self view magnification, to document views for reading, to the magnification of objects from across large rooms or auditoriums.
Myopia / Myopic / Nearsighted - A person is considered “nearsighted” or myopic when close up items are in focus, but more distant objects are blurry, by degree.
Notetaker - A portable electronic Braille device that enables blind users to take notes, create documents, and access applications. These devices usually provide either speech or Braille output or both.
OCR / Optical Character Recognition - Electronic conversion of images or printed text into machine-editable and readable text after capture by scanning, where it can be readable using speech, large print, or Braille.
Ocular Albinism - A condition in which lack of pigmentation in the eye results in light sensitivity, blurry vision, and varying degrees of vision loss.
Presbyopia - An eye disorder of middle age, usually first evidenced by some minor loss of vision - blurred letters while reading at a normal close-up distance. Clear vision degenerates over time into more significant loss of near sight, as the individual continues to age. It is usually caused by the natural loss of the elasticity of the eye.
Retinal detachment - A critical eye condition in which the retina - the part of the eye that receives light and translates it into signals that allow the brain to form images - separates from the underlying tissue. Some people may experience floaters, shadows over vision, very blurred vision, and/or unexplained flashes of light. Without immediate treatment, permanent damage may occur, including significant vision loss to blindness. Retinal detachment is most commonly found in persons who are aged, have severe myopia, have had cataract surgery, or have been diagnosed with diabetic retinopathy.
Retinitis Pigmentosa - a progressive visual impairment with various end-results, ranging from some significant vision loss to total blindness. Symptoms include night blindness and loss of peripheral vision. Retinitis pigmentosa is one of the most common forms of inherited retinal degeneration. Learn more about Retinitis Pigmentosa.
Scanning and reading - A means to convert text to speech or large print output text OCR software.
Screen magnification software - Software that interfaces with a computer’s graphical output to present enlarged images of monitor content.
Screen reader / Screen reading software - Software, such as JAWS, that interprets computer screen content and reads it aloud via synchronized speech.
Speech synthesizer - Software that works with a computer’s sound card to produce speech.
Video magnifier - A means of enlarging the appearance of text and images via a closed-circuit video camera system, by projecting magnified views from the camera’s field of focus to a computer monitor. See CCTV.