General terms related to sight and low vision:
See also: AFB Conditions
Sighted – a person is considered to be “sighted” if they have “normal” or average vision, even if it must be corrected using glasses or contacts. If the person is capable of achieving better than 20/200 vision in either eye, with or without correction, then they are not legally blind, and are considered to be sighted.
Legally blind – Most government agencies and health care institutions agree that legal blindness is defined as a visual acuity (central vision) of 20/200 or worse in the best seeing eye or a visual field (peripheral vision) that is limited to only 20 degrees. https://www.essilorusa.com/newsroom/what-does-it-mean-to-be-legally-blind
Visually Impaired – a person is visually impaired if their vision is worse than 20/200. The term can be used for any level of visual impairment, including total blindness.
Light perception or light projection – the ability to perceive the difference between light and dark, or daylight and nighttime. A person can be incapable of seeing anything more than just light and dark. Sometimes a person can see the direction from which light emanates. The ability to perceive light but little else will have educational and functional implications
Low vision – a condition in which visual acuity is 20/70 or poorer in the better-seeing eye and cannot be corrected or improved with regular eyeglasses. In functional terms, low vision is uncorrectable vision loss that interferes with daily activities. It is better defined in terms of function, rather than [numerical] test results.
Technical terms related to sight and low vision:
Acuity – how clearly a person sees when looking at eye charts. Eye charts measure visual acuity. Acuity is usually expressed in numbers (e.g, 20/20 or 20/200). The bottom number refers to how well a person can see the charts compared to a person with normal (20/20) vision. A person with 20/80 vision sees at 20 feet what a person with normal vision can see at 80 feet. A person can have a different visual acuity in each eye.
Visual Field – the total area that a person sees when looking at an object straight ahead, as measured in degrees of a circle. Normal visual field is about 180 degrees. People can have visual field losses anywhere in their visual field (upper, lower, nasal – toward the nose, and temporal – away from the nose).
Peripheral Vision – this refers to the vision that you have at the edges of your visual field. Peripheral vision is not detailed vision; it detects motion and objects in lower light levels. Some people may have limited peripheral vision (limited visual field).
Terms relevant to accommodations for visual impairment:
Night vision – the ability to see in the dark and in low light levels. In some eye conditions, a person may see well enough in good lighting to read normal print, but be unable to navigate safely at night without special instruction.
Glare – a shine or reflection that reduces the ability to see. Light from windows, flourescent lighting, light bouncing off shiny floors and tables, etc., can produce glare and make it harder to see. This will have educational and mobility implications.
Contrast – how well colors or black and white stand out against each other. Higher contrast makes things easier to see. For example, black against white, and navy against yellow are easier to see. Brown against navy blue would result in poor contrast. Print is of poor contrast when it is light or blurry. Some persons see better when certain colors are used.
Visual clutter – when a page is cluttered with too much print or graphics, and has little white space, then it may be too cluttered for a person to take in all of the information on the page or to find certain information on the page.
Viewing distance – how far the person is away from what they are looking at. The farther away, the smaller the object or text will appear, but more of it may be seen. Although a person may need to be more than a foot away to see another’s entire face, even at that distance, features may be blurry. The goal of the lesson will dictate the viewing distance. A person with very low vision would not understand what an elephant is if they got close enough to see it’s skin clearly; they would need to back away to see all of the elephant as a whole, although perhaps not clearly.
Accommodations to light changes – how well a person can adjust to seeing when going from a dimly lit room out into the bright sunshine, or vice versa. Eyes may adjust quickly or very slowly to changes in lighting. A person who has difficulty accommodating may need to move very slowly when going to a place with different lighting.
Visual fatigue – when eyes feel tired or strained, they may function more poorly than usual.
Font style and font size – the size and style of the print on the page. A person will low vision may require larger print size (measure in points), and a font style that is very simple and without flourishes that increase the cluttered look of the print.
Handwriting – a person with a visual impairment may have more difficulty reading handwriting rather than typed print. A person with low vision may have difficulty reading their own handwriting as well. Some tools that can help include bold pens and pencils (20/20 pens are similar to Sharpies), using print rather than cursive, using a computer or other note-taker (or even audio recordings) for taking notes, etc. When giving a person something to read that has been written by hand, remember to use large, clear letters (not cursive) and leave plenty of white space on the page.
Paper – special paper with wide bold lines can help a person with low vision write in straight lines.
Clipboards – can be used to hold papers close to your face without having to bend over a table to see the writing on the page, as well as keep papers organized.
Low Vision Devices
Low vision devices are tools that make it easier for a person with low vision to see text and objects.
Magnifier – a device used for viewing things up close, to look through to make things bigger. The stronger the magnification, the bigger things will look, but also the less of it that can be seen at one time. There are many different styles of magnifiers and choosing the best one for the tasks is critical. There are hand-held and stand, flat and dome, pocket, large and small magnifiers and they come in many different powers. Magnifiers can also be mounted on glasses lenses for the purpose of reading, but those should not be worn while walking around for safety reasons. The use of magnifiers can affect posture, visual fatigue and other factors, for good or bad – each person is unique.
Telescope – used for viewing objects that are farther away, to look through to make things bigger. These also come in different forms, from hand-held to mounted on glasses. They can be used to read small amounts of information far away, as on a chalkboard in a classroom, or to read street signs. A person cannot see a lot of information using a telescope, so it would not be appropriate to use one for copying many notes from a chalkboard, but rather used as a spotting tool.
CCTV, Closed Circuit TV – CCTV’s are devices that contain a camera to view objects or papers on a flat tray, and then display that image onto a TV (or computer) screen or display. A CCTV can be a very large, bulky all-in-one device that is not easily moved unless on a cart. CCTV’s are used to enlarge the text on paper that is printed in “normal” size and not otherwise accessible (readable) to the person with low vision. CCTV’s can also be used to get a better, clearly look at small objects that are not flat, as long as they fit into the area available between the camera and the tray, and that the focus mechanism works for that. Be aware that writing using the CCTV can be difficult if the writing hand is blocking what the camera can see on the page.
Medical terms relating to eyes and brain structures relevant to vision:
See also: Medicine Net Terms
Eye – The organ of sight. The eye has a number of components. These components include but are not limited to the cornea, iris, pupil, lens, retina, macula, optic nerve, choroid and vitreous.
Cornea – clear front window of the eye that transmits and focuses light into the eye.
Iris – colored part of the eye that helps regulate the amount of light that enters the eye. The iris can get wider to let more light in, or smaller to let in less light.
Pupil – dark aperture in the iris that determines how much light is let into the eye. The pupil is actually a hole in the iris through which the light can reach the retina.
Lens – transparent structure inside the eye that focuses light rays onto the retina. The lens can change its shape in response to the light in order to focus the light on the retina.
Retina – nerve layer that lines the back of the eye, senses light and creates impulses that travel through the optic nerve to the brain.
Macula – small area in the retina that contains special light-sensitive cells and allows us to see fine details clearly.
Choroid – thin vascular layer between the sclera and the retina that supplies blood to the retina and conducts arteries and nerves to other structures in the eye.
Sclera – white outer layer of the eyeball. At the front of the eye it is continuous with the cornea.
Vitreous humor – clear, jelly-like substance that fills the middle of the eye.
Optic Nerve – the nerve that connects the eye to the brain and carries the impulses formed by the retina to the visual cortex of the brain.
Rods – photoreceptor cells in the retina of the eye that can function in less intense light than cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in peripheral vision.
Cones – photoreceptor cells in the retinas of the eye. They respond differently to light of different wavelengths, and are thus responsible for color vision and function best in relatively bright light, as opposed to rod cells, which work better in dim light.
Visual cortex – the part of the cerebral cortex that receives and processes sensory nerve impulses from the eyes.