Visual acuity is typically written as a fraction, representing how well a person can see at a specific distance compared to what is considered "normal" vision.
Understanding Visual Acuity Notation
The standard way to write visual acuity measurements, particularly when using charts like the Snellen chart, is in a fractional format. This fraction provides insight into your clarity of vision at a specific testing distance.
Based on the reference provided:
- The smallest line read is expressed as a fraction, e.g., 6/18.
- The upper number refers to the distance the chart is from the patient (e.g., 6 metres).
- The lower number (usually written next to the line on the chart) is the distance in metres at which a 'normal' eye is able to read that line of the chart.
This notation shows the relationship between the testing distance and the smallest detail a person can resolve compared to a benchmark of normal vision.
Components of the Visual Acuity Fraction
Let's break down the fraction format:
- Numerator: The distance at which the test was conducted. This is usually standardized, commonly 6 meters (in the UK and many other places) or 20 feet (in the US).
- Example: If the test is done at 6 metres, the numerator is 6.
- Denominator: The distance at which a person with "normal" vision could read the same line. A higher number here indicates poorer vision.
- Example: If you can only read a line that a normal eye could read at 18 metres, the denominator is 18.
Examples of Visual Acuity Scores
- 6/6 Vision: This is considered "normal" vision. It means that at 6 metres, you can read the line that a person with normal vision can read at 6 metres.
- 6/12 Vision: This means that at 6 metres, you can only read the line that a person with normal vision could read from 12 metres away. Your vision is less sharp.
- 6/18 Vision: Following the example from the reference, this indicates that at 6 metres, you can read the line that a person with normal vision could read from 18 metres away. This represents significantly reduced vision.
- 6/60 Vision: This often represents very poor vision, meaning that at 6 metres, you can only read the largest line on the chart, which a normal eye could read from 60 metres away.
Visual Acuity in Different Units
While 6-metre notation is common in some regions, 20-foot notation is prevalent in the US.
- 6 metres is approximately 20 feet.
- Therefore, 6/6 vision is equivalent to 20/20 vision.
- 6/12 vision is equivalent to 20/40 vision.
- 6/60 vision is equivalent to 20/200 vision.
In summary, visual acuity is written as a fraction comparing the test distance to the distance at which a normal eye reads the same line.
