Visual acuity
The Assessment of Vision
Vision is assessed, by formally testing the acuity of both distant and near vision, visual fields, (that is, how far you can see all around at the side, while looking straight ahead) checking the health of the eye and looking at the retina. Binocular vision is vision using both eyes and monocular vision is vision using one eye. Distant and near vision are tested by using the Snellen’s Test Types. Each eye is tested individually. The Ophthalmologist or Optometrist will always do a test of binocular vision during the test for visual acuity. However, if this result is not available, the better of the two monocular visual acuities should be used.
Most reports will provide monocular acuities i.e. vision in each eye independently and so the VA of the one eye should be used.
It may be helpful to give some examples:
| Left eye | Right eye | Binocular vision |
|---|---|---|
| 6/6 | 6/6 | 6/6 |
| 6/12 | 6/6 | 6/6 |
| 6/12 | No vision | 6/12 |
The rule is therefore if this information is available when monocular vision is recorded, take the better of the two eyes to give what is in effect binocular vision. If information is not available then we need to obtain VAO and an optometrist report is likely to be the best source.
For distant vision:
The person sits or stands at 6 metres, and reads down the vision chart from the largest letter at the top, to the smallest letter at the bottom. The chart is a large card or a lighted box, which displays the letters.
If a person can only see the top letter, their vision is described as 6/60; that means that, while at a distance of 6 metres they an only see what a person could normally see at a distance of 60 metres.
However, if they can see the letters on the second bottom line, their vision will be described as 6/6 (they can see with equal clarity at 6 metres what another person with unimpaired vision standing at 6 metres will see), or the smallest letters on the bottom line, their vision will be described as 6/5, which means that they can clearly see at 6 metres, what a person with unimpaired vision, standing at 5 metres can see. 6/5 vision would be better than average.
The vision test card, and light box, should be well illuminated.
The visual acuity is tested firstly without, and then with the use of corrective spectacles, or contact lenses.
If the person cannot see the numbers on the card, the person is moved to 3 or 4 metres from the card, and tested. If this is not possible, counting fingers, and hand movements (at 30cm), or light perception are recorded. In these cases the reports and the Certificate of Visual Impairment may describe the visual acuity in the following manner:
- NPL – no perception of light
- PLO – perception of light only
- HM – hand movements
- CF – count fingers
Snellen Visual Acuity Chart
Snellen’s chart with equivalent LogMar measurements
LogMar is a scale that expresses visual acuity as a decimal. It is usually used for statistical purposes and is rarely used in clinical practice.
| Snellen 6 metres |
Snellen 3 metres |
LogMar |
|---|---|---|
| 6/60 | 3/30 | 1.0 |
| 6/48 | 3/24 | 0.9 |
| 6/38 | 3/19 | 0.8 |
| 6/30 | 3/15 | 0.7 |
| 6/24 | 3/12 | 0.6 |
| 6/19 | 3/9.5 | 0.5 |
| 6/15 | 3/7.5 | 0.4 |
| 6/12 | 3/6 | 0.3 |
| 6/9.5 | 3/4.8 | 0.2 |
| 6/7.5 | 3/3.8 | 0.1 |
| 6/6 | 3/3 | 0.0 |
For Near vision:
Near vision is tested by using a test card and each eye is tested individually. The card has number of printed paragraphs with print of varying sizes. Each paragraph is described in terms of “points” measuring the body of the print – where a “point” is 1/72 of an inch. In a common test, N48 is the largest type, and N5 is the smallest, which an unimpaired eye can see, held at a comfortable reading distance, (usually 14 inches), from the eyes.
This type is N12.
Snellen Test Card
Amended April 2011
