The consultant has rules to follow when completing the Certificate of Vision Impairment (CVI), the form used in England. Other certification forms use similar rules.
These rules take into account:
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visual acuity: your central vision, the vision you use to see detail
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visual field: how much you can see around the edge of your vision, while looking straight ahead.
Your visual acuity is measured by reading down an eye chart while wearing any glasses or contact lenses that you may need. This is known as a Snellen test. Your field of vision is measured by a 'field of vision test'.
Measuring your visual acuity and visual field helps the ophthalmologist to decide whether you are eligible to be registered as severely sight impaired (blind) or sight impaired (partially sighted).
Snellen scale
Visual acuity is measured using the Snellen scale. A Snellen test usually consists of a number of rows of letters which get smaller as you read down the chart.
On the Snellen scale, normal visual acuity is called 6 / 6, which corresponds to the bottom or second bottom line of the chart. If you can only read the top line of the chart then this would be written as 6 / 60. This means you can see at 6 metres what someone with standard vision could see from 60 metres away.
The figures 6 / 60 or 3 / 60 are how the result of a Snellen test are written. The first number given is the distance in metres from the chart you sit when you read it. Usually this is a 6 (for 6 metres) but would be 3 if you were to sit closer to the chart, ie 3 metres away.
The second number corresponds to the number of lines that you are able to read on the chart. The biggest letters, on the top line, correspond to 60. As you read down the chart, the numbers that correspond to the lines get smaller, ie 36, 18, 12, 9 and 6. The bottom line of the chart corresponds to the number 6. Someone with standard vision who can read to the bottom of the chart would have vision of 6 / 6.
For example, a person with standard vision would be able to read the second line on the chart when 36 metres away. However, if you had a Snellen score of 6 / 36, you would only be able to read the same line at 6 metres away. In other words you need to be much closer to the chart to be able to read it. Generally the larger the second number is, the worse your sight is.
Results of your vision test
The ophthalmologist uses a combination of both your visual acuity and your field of vision to judge whether you are eligible to be registered, and at which level. If you have a good visual acuity, you will usually have had to have lost a large part of your visual field to be registered as severely sight impaired (blind) or sight impaired (partially sighted).
Alternatively, if you have all your visual field, you will usually have to have a very poor visual acuity to be registered as severely sight impaired (blind) or sight impaired (partially sighted).
Generally, to be registered as severely sight impaired (blind), your sight has to fall into one of the following categories, while wearing any glasses or contact lenses that you may need:
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visual acuity of less than 3 / 60 with a full visual field
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visual acuity between 3 / 60 and 6 / 60 with a severe reduction of field of vision, such as tunnel vision
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visual acuity of 6 / 60 or above but with a very reduced field of vision, especially if a lot of sight is missing in the lower part of the field.
To be registered as sight impaired (partially sighted) your sight has to fall into one of the following categories, while wearing any glasses or contact lenses that you may need:
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visual acuity of 3 / 60 to 6 / 60 with a full field of vision
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visual acuity of up to 6 / 24 with a moderate reduction of field of vision or with a central part of vision that is cloudy or blurry
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visual acuity of up to 6 / 18 if a large part of your field of vision, for example a whole half of your vision, is missing or a lot of your peripheral vision is missing.
More Information
For more about the vision criteria for registration, contact our Eye Health Information Service.
Tel: 0303 123 9999
Email: eyehealth@rnib.org.uk
This information gives general guidance only and is not an authoritative statement of the law.