The World Health Organization (WHO) classifies impaired vision into five categories:
- Low vision 1 is a best corrected visual acuity of 20/70.
- Low vision 2 starts at 20/200.
- Blindness 3 is below 20/400.
- Blindness 4 is worse than 5/300
- Blindness 5 is no light perception at all.
A visual field between 5° and 10° (compared with a normal visual field of about 120°) goes into category 3; less than 5° into category 4, even if the tiny spot of central vision is perfect.
- Total Blindness: the student who cannot see at all and who should use his other senses in learning.
- Legal Blindness: the student whose vision acuity is 20/200 in the better eye and the vision field is 20º or less.
- Partially sighted: the student whose vision acuity is from 70/20 to 200/20 after rectification.
- Low vision: the student who has one problem or more in vision and whose vision could not be rectified by the glasses and might affect his daily life activities.