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What does "legally blind" mean?

Silhouette of blind man with his guide dog

“Legally blind” means you’re essentially sightless in the eyes of the law if eyeglasses or contact lenses cannot correct your vision beyond specific standards.

How do we define who’s legally blind?

In the UK, your eye specialist (a consultant ophthalmologist) will decide if you can be certified as severely sight impaired (blind) or sight impaired (partially sighted). They will use criteria based on two areas:

1) Your visual acuity, this is your central vision and how well you can see detail. Often measured by looking at a chart in the examination room

2) Your visual field, this is how well you can see in the periphery whilst still looking straight ahead

Generally, to be certified as severely sight impaired (blind), your sight has to fall into one of the following categories, while wearing your glasses or contact lenses:

  • Visual acuity of less than 3 / 60 with a full visual field.

  • Visual acuity between 3 / 60 and 6 / 60 with a severe reduction of field of vision, such as tunnel vision.

  • 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 certified as sight impaired (partially sighted) your sight has to fall into one of the following categories, while wearing your glasses or contact lenses:

  • Visual acuity of 3 / 60 to 6 / 60 with a full field of vision.

  • 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.

  • Visual acuity of 6 / 18 or even better 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.

It’s good to know the definition of legal blindness because it can affect your ability to get a driver’s license or receive government disability benefits.

Measuring your visual acuity

When your optician asks you to read the smallest row of letters from across the room, you’re looking at the Snellen chart, the general standard for measuring clarity of eyesight.

The eye chart assumes a base viewing distance of 6 meters and compares your vision to the historical norm for most humans.

If you have 20/20 vision, (more commonly known as 6/6 in the UK) then the smallest letters you can read from 6 metres away match the normal 6 metre distance. By contrast, 6/60 vision means the letters you can read from 6 metres can be read from 60 metres by people with normal vision. Thus, you see only one-tenth of the norm.

Measuring your visual field

Some people can see small letters on an eye chart but can't see the person standing next to them. That’s because they have legal blindness due to poor peripheral vision. A wide visual field is crucial for activities like driving a car or crossing a busy street.

Visual field tests determine if you have a normal field of view that has no blind spots or unusual narrowing of your peripheral vision.

Peripheral vision has two parameters: lateral (side to side) and vertical (up and down). A maximum lateral field of view is nearly 180 degrees; distant objects remain visible from the right or left of the observer.

The normal vertical field is smaller at about 135 degrees.

How many people are legally blind?

According the NHS data, there are almost 2 million people living with sight loss in the UK. Of these, around 360,000 are registered as blind or partially sighted. Of these, it is believed that more than 25,000 are children under the age of 16 (as per RNIB statistics, 2018)

Causes of legal blindness

Though some people become blinded in accidents, a host of conditions can cause a visual disability or legal blindness.

Four leading causes of legal blindness in the UK are age-related macular degeneration (AMD or ARMD), cataracts, diabetic retinopathy and glaucoma.

Optic neuritis and neuropathy also can cause legal blindness, as can a number of congenital conditions such as congenital cataracts, infantile glaucoma and retinopathy of prematurity.

Keratoconus, a gradual thinning of the cornea, also can cause severe vision loss to the point of legal blindness.

Retinitis Pigmentosa is also a leading cause of sight loss with it estimated to be responsible for 10% of registered sight loss cases in the UK.

Resources for those who are legally blind

Legal blindness can make people eligible for special services and assistance.

If you are legally blind or you are a guardian or care provider for someone who is, seek the services of an optician who specialises in low vision.

Low-vision specialists typically are familiar with the latest vision aids such as magnifiers, telescopes and digital devices that can help you make the best use of any vision you still have.

These devices often help a person with legal blindness live more independently and enjoy activities that everyone else takes for granted.

Professionally trained guide dogs also are a great help for many people who are legally blind.

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