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Visual Acuity Application

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Visual Acuity
Visual Acuity (VA) is a measure of the clarity of central vision - the spatial resolution of the visual processing system. It is dependent both on the sharpness of the retinal image and the sensitivity of the visual system.

When testing VA, chart characters are black symbols (usually letters) against a white background and the distance between the eye and the testing chart is sufficient to approximate to infinity

Snellen Acuity
Traditional Snellen acuity can be expressed in feet (imperial) or metres (metric)
In the expression, 6/12 vision, the 6 is the distance in metres between the subject and the chart while the 12 means that the subject can read the chart from 6 metres away as well as a normal person could read the same chart from 12 metres away.

The letters on the 6/6 line of the Snellen chart are composed of five lines that are separated by a visual angle of one minute of arc. From 6 metres this corresponds to lines that are spaced 1.75mm apart making a 6/6 letter equal to 5 minutes of arc or 8.75mm

The imperial equivalent of 6/6 vision is 20/20 vision. At 20 feet or 6 metres, a normal human eye is able to separate lines that are one arc minute apart. Vision of 6/6 or 20/20 is considered normal – although most individuals are able to resolve smaller letters – usually 6/5 or 20/17.

Decimal Acuity
The Snellen fraction can also be expressed as a decimal. 6/6 or 20/20 has a decimal fraction of 1. Better Visual Acuity such as 6/5 has a larger number (1.2) and lower acuity such as 6/12 results in a lower number (0.5)

LogMAR
The name LogMAR is derived from the Logarithm of the Minimum Angle of Resolution. In contrast to the Snellen chart, the sizes of the letters progress systematically in geometric progression. The letter size of each line is designated as the logarithm to the base 10 of decimal visual acuity, so the 6/6 (or 20/20) line is LogMAR 0.00 and the 6/60 (20/200) line is LogMAR 1.0.

On a LogMAR chart the space between lines and letters change in proportion and there are five letters on each line. This allows for a more consistent sampling of visual acuity between lines. Each letter has a score value of 0.02 log units and since there are 5 letters per line, the total score for a line on the LogMAR chart represents a change of 0.1 log units.

Because of this consistency LogMAR is the instrument of choice for research and is advocated by many eye care practitioners for general clinical use. The chart was designed by Ian Bailey and Jan Lovie in 1980 at the National Vision Research Institute of Australia.