You might have heard the term “legally blind” before. Legal blindness does not mean that a person is totally blind; in fact very few people have no sight at all.
A person who is “legally blind” has a visual acuity of 6/60 or less, or a visual field of less than 10 degrees, or both.
What is visual acuity?
the first measure is visual acuity. Visual acuity defines the ability to read and detect objects at a distance. It is measured using the vision chart, which everyone is familiar with.
The chart has lines of letters in different sizes. The person is then asked to read the letters from top to bottom. The top line has a very large letter, which a person who has normal sight would be able to see at 60 metres. However a person with a severe vision impairment may only be able to see this at 6 metres.
Similarly, for the subsequent lines, a person with normal sight would be able to see the letters at 36, 24, 18, 12, 9 and 6 metres respectively. Normal vision means that a person can read the “6 metre” line (usually the second bottom line) at the correct distance from the chart.
This means this person sees at 6 metres what other people with normal vision would see at 6 metres (6/6 vision). This is also known as 20/20 vision (6 metres or 20 feet). People with less than normal vision would have their vision described as 6/12 or 6/24 etc. depending on which line they could see on the chart.
People who are legally blind are people whose visual acuity or sharpness (with glasses or contact lenses, if needed) is 6/60 or worse in the better eye. This means that a person who is legally blind can see an object at a 6 metre distance, as compared with a person with normal sight who could see the same object at 60 metres. This means that they can only read the top line on a vision chart.
What is visual field?
Secondly, when people look straight ahead, they can normally detect objects to either side, or above or below the direction in which they are looking. This measurement is called the visual field. In people with normal sight, this is measured as 170 degrees. A person is “legally blind” if the combined visual field for both eyes is less than 10 degrees.
Some conditions, such as Retinitis Pigmentosa (RP) affect mainly the visual fields, while Stargardt disease and Macular Degeneration (MD) affect mainly visual acuity or central vision. Although some people with RP and MD develop a complete loss of all sight, most retain at least limited vision. Depending on the degree of this limitation, the person may have legal blindness but will still have the capacity to get around and do things.
Source: Retina Australia