To the layperson, blindness indicates a complete loss of sight. The legal definition of blindness, however, has two components.
Firstly, normal vision is tested in two main ways. The 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 normal person would be able to see at 60 metres. However a person with a severe vision disability may only be able to see this at 6 metres. Similarly, for the subsequent lines, a normal person 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 used to be called and is the same 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.
Thus, legally blind individuals are those whose visual acuity or sharpness (with glasses, if needed) is 6/60 or worse in the better eye. This means that the legally blind person can see an object at a 6 metre distance, as compared with the normal sighted person who could see the same object at 60 metres. This means that she/he can only read the top line on a vision chart.
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 normal sighted people, this is measured as 170 degrees. A person is "legally blind" if the combined visual field for both eyes is less than 10 degrees.
The following examples explain effects of limited visuaI field. If a typical two-lane road is 12 metres wide, a person with 10 degrees of vision can see from one side of the road to the other only by looking 20 metres down the road (without scanning from side to side). By comparison a person with 140 degrees of vision can see from one side of the road to the other at only 10.5 metres.
When the person with 10 degrees vision travels at 60kph, a period of 4 seconds lapses between seeing something at a point on the side of the road and actually passing that point. This means that, after a side street has been seen to be clear 70 metres down the road, there will be 4 seconds during which something can come out of the side street without being seen.
When looking at the floor from a height of 1.5 metres, a person with 10 degrees of vision can see within a circle 26 cm across, whereas a person with 140 degrees of vision can see within a circle about 2 metres across. This makes a big difference when looking for an object on the floor!
Thus, a person who is "legally blind" may have a visual acuity of 6/60 or less, or a visual field of less than 10 degrees, or both.
RP affects mainly the visual fields, while Stargardt disease and Macular Degeneration (MD) affect mainly visual acuity. 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.
A "legally blind" person is entitled to Disability Support Pension (Blind) from the Commonwealth Government.