Visual impairment describes vision that cannot be fully corrected by ordinary prescription lenses, medical treatment, or surgery. The term visual impairment includes conditions ranging from the presence of good usable vision to low vision or the absence of any sight at all – total blindness. The most common conditions are associated with aging: glaucoma, cataracts, macular degeneration, and diabetic retinopathy. Many terms are used when people refer to visual impairment. These terms are explained below.
- Total blindness: no light perception
- Severe low vision: visual acuity of 6/120 or worse (seeing an object within 6M , as compared with the normal range at a distance of 120M or above); or, visual field: 20 degrees or less
- Moderate low vision: vision acuity of 6/60 to better than 6/120
- Mild low vision: visual acuity of 6/18 to better than 6/60
Colour blindness is also one type of visual impairment (however, it is not fallen into the definition of a special educational need in Hong Kong). About two-thirds of people with visual impairment have multiple problems including cerebral palsy, epilepsy, hearing impairment and/or mental retardation.
Some of the assistive technologies for individuals with visual impairment include:
- Audio description for visual information, including video, photos and graphics on websites and mobile apps
- Electronic book players
- Electronic braille note-takers
- Magnifiers and programs for computers
- Optical character recognition (OCR) systems for scanning documents and converting the contents into accessible documents
- Screen readers
- Text-to-speech applications for digital texts