The properties and limitations of assistive technology for people with visual impairments
Preliminary
1. Definitions
1.1 Blindness
Blindness is defined primarily for two purposes:
- Epidemiological;
- Functional.
a) Epidemiology
In epidemiology blindness is defined in order to classify the level of visual impairment and so it is defined with reference to two criteria:
- Visual acuity; and
- Field of vision.
Classically, visual acuity is defined in terms of what a person can see at a given distance; one is classified as blind if the visual acuity allows a person to see at a short distance what the unaffected person sees at a much greater distance; this is expressed as a ratio.
Thus, a person is said to be blind if he sees at, say, three metres what an unaffected person sees at 60 metres, expressed as 3/60. Normative vision is expressed as a perfect number, i.e. 60/60, or, in colloquial terms ‘20/20 vision'.
This criterion describes vision in the better eye and takes account of correction - the use of spectacles to correct the inefficiency of the lens to accommodate.
The definition, alternatively, takes account of the field of vision.
Thus, the World Health Organisation [i] definition of blindness is that:
A person is blind if the visual acuity after correction in the better eye is 3/60 or less, or if the field of vision is 15° or less
b) Functionality
Functional definitions are used by administrators to define those who are entitled to a benefit or the clients who can benefit from organisations whose objective is to assist blind people. In the United Kingdom blindness is defined [ii] as a condition which means that:
A person so blind as to be unable to perform any work for which eyesight is essential
c) Notes
- Blindness does not mean a complete lack of light perception; most people defined as blind have some useful residual vision;
- Blindness is largely related to ageing;
- The visual acuity definition which defines vision at a distance of three metres was largely developed for an agricultural environment: it does not say anything about functionality inside three metres, where most computer access takes place; and it does not deal with the ability to see moving objects.
d) Short Acronym
Because many people who are defined as blind have residual vision and because many people outside the visual acuity definition of 3/60 have difficulty in seeing on-screen displays, I will use the term VIP to describe blind and visually impaired people.
1.2 Assistive Technology
a) Assistive Technology
Assistive technology is any service or device which mitigates the loss of visual acuity or visual functionality
This might be a magnifying device or a guide cane. In the ICT context, assistive technology is defined as:
ICT Assistive Technology is hardware and software compatible with a generic system which mitigates a visual acuity or visual functionality loss
b) Types of Assistive Technology
- Refreshable braille;
- Braille embossing;
- Synthetic speech;
- Magnification of on-screen elements.
c) Notes
- Manipulable text can be translated into braille code and rendered on a refreshable braille device showing up to 80 characters at a time but usually showing 40 characters on a desk top device and 32 or fewer on a portable device;
- Translated braille code can be embossed on a braille printer in standard braille publishing formats;
- "Screen magnification" is a shorthand term used to describe the ability to modify print size, font, leading, kerning, justification foreground/background contrast and colours.
1.3 Accessibility
There has been something of a turf war between those who are concerned with ICT usability and accessibility. I think this is largely redundant. I define usability as:
a) Usability
The extent to which an entity lends itself to effective task completion
b) Accessibility
I further define accessibility as:
That aspect of usability which specifically refers to the mitigation of an impairment
c) Alternative Formats
Alternative formats are renditions of text in:
- Braille;
- Modified print;
- Audio where there is not a commercial unabridged audio edition;
- Large print and synthetic speech.
[i] World Health Organisation: http://www.who.int/en/
[ii] UK definition of blindness http://www.opsi.gov.uk/RevisedStatutes/Acts/ukpga/1948/cukpga_19480029_en_7