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Low Vision
Low Vision
Contents
Types of Low Vision
The visual acuity of people with low vision varies widely, but, in general, low vision is defined as a condition in which a person's vision cannot be fully corrected by glasses, thus interfering with daily activities such as reading and driving. Low vision is more common among the elderly, but it can occur in individuals of any age as a result of such conditions as macular degeneration, glaucoma, diabetic retinopathy, or cataracts. Each of these conditions causes different types of effects in a person's vision, however, here are a few generalizations:
Important
- People with low vision often use screen magnification software.
- People with low vision often benefit from higher contrast.
- People with low vision sometimes override the document's default background and font colors.
It helps to see through the eyes of someone who has low vision. Below are a few graphics simulating the experience of people with different types of low vision.
Macular degeneration
The macula is near the center of the retina, which is the area in the back of the eye. The aging process and the thinning of the tissues of the macula cause the most common form of macular degeneration,"dry" macular degeneration. The result is a gradual loss of vision. "Wet" macular degeneration occurs when abnormal blood vessels at the back of the eye begin to leak fluid or blood and blur central vision, often resulting in rapid loss of vision. In either case, the person's central area of sight is affected the most, making it difficult to see objects that the person is looking at directly. The images below are a simulation of the effect of macular degeneration. Text can appear broken and unclear.
Experience it for yourself
To get a better idea of what macular degeneration is like, hold up your hand about 12 inches (about 30
cm) from your eyes, so that you can't see straight in front of you, but you can see around the edges of your hand. Now, without
looking to the sides, try to read something in your peripheral vision. Remember to keep looking straight at your hand! This
exercise shows how difficult it can be for individuals with advanced macular degeneration. The problem isn't so pronounced for
people with lesser degrees of macular degeneration, but the basic idea is the same.
Glaucoma
Glaucoma is caused by an increase in pressure inside the eye, which causes damage to the optic nerve. The end result is often the opposite of the effect of macular degeneration: the loss of peripheral vision and a blurry central area of vision. It can be particularly difficult to read text because text seems faded as well as blurry. Some people have compared the effects of glaucoma to looking at everything through a straw.
Diabetic Retinopathy
One of the effects of long-term diabetes can be the leaking of retinal blood vessels, causing dark patches in the field of vision where the leaks occur. Text can appear blurred or distorted in these regions.
Cataract
Individuals with cataract have areas of opacity in the lens of their eyes which results in a blurred or hazy effect, especially in bright light. Text can appear to fade into the background. High contrast is especially important for people with advanced cataract.
Tips for Trainers: WebAIM Low Vision Simulation
The WebAIM low vision simulation (available in the full version of the WebAIM Guide to Web Accessibility Techniques and Concepts) demonstrates the most common low vision types and provides basic details about how a screen enlarger works. It demonstrates graphical text, low contrast, and horizontal scrolling. The simulation also includes tasks that you can have your learners attempt to complete.
Screen Magnifiers
The most common technology that people with low vision use is the screen magnifier. This is a software program that zooms in on a small area of the screen, allowing people with low vision to see it more clearly. Common screen magnifiers include ZoomText and MAGic. Some kinds of content are difficult to interpret when enlarged. For example, graphics that contain text can become blocky and pixilated, making the text difficult to understand. Take a look at the two images below. The one on the left is a screenshot of text that has been enlarged. The image on the right is a screenshot of text within a graphic that has been enlarged.
Important
To make text more legible when enlarged, use true text as much as possible, rather than text in graphics.
Tips for Trainers: Screen Enlargers
To give your learners an understanding of how screen enlargers can help someone with low
vision, demonstrate or have your training participants use a screen enlarger.
Access a page that has a combination of normal text and graphics that contain text. Point out how graphical text becomes pixilated and hard to read when enlarged, but real text is still easy to read, even when greatly enlarged.
Tips for Trainers: Brief Overview Comparing Screen Enlargers
Explaining what a screen enlarger is and how it works is not a good substitute for demonstrating an actual screen reader. An effective Web accessibility trainer should invest in at least one version of a screen enlarger, not only for their own information and testing, but also so they can also demonstrate the screen enlarger in their trainings.
The following programs are among the most common screen enlarger software packages available.
ZoomText - http://www.aisquared.com/Products/Products.htm- external
link
. ZoomText is used to magnify items on a computer screen for users who have visual disabilities. There are several
versions and add-ons that give increased functionality, including screen reading functionality. A trial
version- external link is available.
MAGic - http://www.freedomscientific.com/fs_products/software_magic.asp- external
link. MAGic for Windows is a flexible screen enlarging and screen reading (optional) software program from the developers of
JAWS. MAGic has many features that make Web navigation easier. A trial version of MAGic- external link is available. Pricing starts at around $395 without screen reading support.
Opera - http://www.opera.com/- external link. Opera is a Web browser with screen
enlarging functionality. A free, ad-supported version is available. Opera is a great tool because it is both very easy to learn and
use, and it is free.
Tips for Trainers: Use Opera as Screen Enlarger
Have your training
participants use Opera to view an enlarged Web page. Opera can be downloaded for free at http://www.opera.com/- external
link. Two versions are available, one with Java and one without. The non-Java version will work for most training
situations.
To enlarge the Web page, use the drop-down box to the right of the address bar. A common screen enlarger setting is 400%-600%. Ask the participants to share their experiences using the screen enlarging software. Emphasize the difference between real text and graphical text. Be sure to point out that the screen enlarger user must often scroll horizontally to see the entire page's content.
High Contrast
Sites with low contrast can be difficult to read for people with low vision. Some poorly designed sites on the Web have bad color combinations such as blue links on black backgrounds, red text on green backgrounds, or other combinations that are not easy on the eyes for anyone, but especially not for people with low vision. There's no hard rule as to how much contrast is enough, but use your best judgment. However, it's usually not too difficult to tell when color combinations do not contrast adequately.
Experience it for yourself
Here are some ideas for roughly simulating low contrast vision:
-
turn down the contrast on your monitor
-
look at your monitor through a tissue or a piece of tracing paper (this technique is more successful on laptop and flat LCD monitors than on full-size monitors)
Important
To the extent possible, maximize the contrast of your Web pages, including graphics, fonts, and backgrounds.
Tips for Trainers: Contrast Example - Opera and User Style Sheets
The Opera browser - http://www.opera.com/- external link allows you to create user-specified page
styles. These styles can be applied to any page on the Web with a click of a button. Develop several user style sheets that change
the text and background colors and demonstrate what several pages look like when the colors are user-specified. Encourage learners
to design pages that do not rely on color alone to convey content and are useable when displayed with various background and
foreground color schemes.
Overriding Font and Background Colors
Some people with low vision will change the settings in their operating system and/or browser to not only enlarge the text, but to increase the contrast of the text in relation to the background. Some people like to have a black background with white or yellow text. Others prefer to have a white or yellow background with black text. These are the most common settings, but there other people prefer other high contrast settings. See the images below for examples of how these settings can appear:
Here's something to remember:
Important
To allow people to customize their contrast settings, it is better to put as much text as possible in true text format, rather than in graphics.
Horizontal Scrolling
This last point is not so much a matter of accessibility as it is usability. You have probably come across Web sites that require you to use your horizontal scrollbar to see the content on the right of the screen, even though you had your browser window maximized. This can be a bit annoying to people with perfect vision, but it is even more so for people who use screen magnifiers and are forced to scroll even further to the left and right inside of the small enlarged space they are viewing.
Important
To the extent possible, use percentages, rather than absolute units (e.g. pixels), in your document layout.
The general rule when designing for low vision is to make everything configurable. If the text is real text, users can enlarge it, change its color, and change the background color. If the layout is in percentages, the screen can be widened or narrowed to meet the user's needs. Configurability is the key.
Key Concepts
Important
| Challenges | Solutions |
|---|---|
| Text in graphics does not enlarge without special software, and looks pixilated when enlarged | Limit or eliminate text within graphics |
| Users may set their own font and background colors | Allow them to do so by using as much real text as possible, rather than text within graphics. |
| Screen magnifiers reduce the usable window size | To reduce that amount of horizontal scrolling, use relative rather than absolute units (e.g. use percentages for table widths instead of pixels) |
Tips for Trainers: User Experiences
Share any experiences you may have had with low vision users. When possible, invite someone who has low vision to your training to share their perspective and insight. Learning directly how inaccessible Web content affects a specific person helps developers to understand the importance of accessible design.
Tips for Trainers: Fast Facts on Low Vision
- It is estimated that approximately 12 million Americans have some form of vision disability that cannot be corrected by glasses.
Lighthouse - external link
- 100 million Americans are visually disabled without corrective lenses. University of Washington Dept. of
Ophthalmology Statistics on Blindness and Blinding Diseases in the United States- external
link
- Vision problems affect one in 20 (nearly 5 million) preschool-age children, ages 3-5, and 25 percent (12.1 million) of school-age
children, ages 6-17. Braille Institute- external link
