Visual acuity is the measure of the sharpness or clarity of vision. Usually the vision is tested looking straight ahead but the athlete may need to move their head to achieve best acuity. Classification is based on the best acuity.
The two main tests used to measure visual acuity are Snellen and LogMAR.
Snellen charts are named after the Dutch ophthalmologist Herman Snellen, who developed the chart in 1862 and is traditionally the most common chart used for measuring visual acuity. The Snellen test involves the eye health practitioner asking the client to identify letters on a chart which get progressively smaller on each subsequent row. Perfect vision in a Snellen test would equal 6/6, which means the person can see at 6 metres what they ‘should be’ able to see at 6 metres.
6/24 for example, means the person can see at 6 metres what a normally sighted person could see much further away, that is at 24 metres.
The American still use feet and “20 /20 vision” means the person can see at 20 feet what they should be able to see at 20 feet.
The LogMAR chart is widely used around the world and is recognised to be the more accurate measurement of visual acuity, especially with lower visual acuities. For this reason, many sports prefer sight classifications to be measured with the LogMAR test, however, this chart is not commonly used by UK optometrists.
The visual field, which is measured in degrees, is the width of the area that the eye can see when focussed on a central point.
In addition to visual acuity, in order to classify sight within the 5 sight categories, a visual field test will need to be conducted.
A visual fields should be measured with each eye using appropriate correction. During 2019 visual fields criteria were further clarified and guidelines agreed by an expert classification group, for the need for athletes to provide a wider field assessment. The following guidelines were issued:
- The approved machines for classification are Humphrey Field Analyzer, Goldmann Perimeter or Octopus Interzeag
- Range of field tested must be at last 80 degrees ( Humphrey 120 field is therefore fine , Goldman and Octopus can also record at least 80 degrees field width)
- Central fields e.g. 30 degrees may occasionally be needed to provide additional information ( all 3 machines can record central field )
- When testing the field the head position must be that which gives the widest field
Using the more definite measurement guidelines and looking at the wider field may possibly result in some classification changes for a few athletes.
Categories by Snellen acuity measurement
This category includes: having no light perception in either eye, light perception and ability to perceive some movement in front of the eye but inability to recognise shapes.
Partially sighted B2 athletes will have limited vision in both eyes either in how far or how wide they can see.
This category includes being able to count fingers at 15 centimetres to a visual acuity of up to and including 2/60.
2/60 means somebody within this sight category would see the top letter of the vision chart at a distance of up to and including 2 metres. A normally sighted person would see that letter at a distance of 60 metres.
B2 also includes someone with a visual field of under 10 degrees even if their acuity is better than 2/60
B3 is the highest category used for most international & Paralympic sport and includes those with a level of vision better than 2/60 and up to and including 6/60.
It also includes those with a visual field of less than 40 degrees who may have acuity better than 6/60
Anyone with better vision than 6/60, and up to and including 6/24 would be within this sight category.
This category is the highest sight level used within VI sport for those having a visual acuity of better than 6/24, but not better than 6/18.
Better vision than 6/18.
Categories by LogMAR acuity measurement
Perfect vision (20/20) in a LogMAR test would be 0.0. The bigger the number the worse the vision so low vision is defined as best-corrected visual acuity worse than 0.5
Visual acuity poorer than 2.60. Visual field not applicable.
Better than 2.60 up to and including 1.50 or a visual field of 10 degrees or less.
Better than 1.50 up to and including 1.0 or a visual field of more than 10 degrees and less than 40 degrees.
Better than 1.0 up to and including 0.6.
Better than 0.6 up to and including 0.48.
Better vision than 0.48.
Sport specific classification
The current classification system was developed many years ago when visually impaired competitive sport was in its infancy. It is used across the board even though it is recognised that different levels of vision are needed for different sports. A lot of research will however be required to define scientifically how reduced levels of vision affect performance in different sports. Until the time that sport specific classification is available, we have to use the system described above.
To find out more about British Blind Sport sight classifications or to speak to a member of the BBS Classifications Team please call 01926 424247 or email firstname.lastname@example.org.