Definitions and Glossary

Vision Impairment is the consequence of a functional loss of vision as a result of a disorder, disease or injury to the eye, optic nerve or brain.

A child or young person is thought to have a vision impairment if they have an eye condition that cannot be fully corrected using glasses.

The term vision impairment is used to cover a range of conditions and a range of levels of vision.  The term includes all blind and partially sighted people (


1. Definition of Visual Acuity from the National Sensory Impairment Partnership (NatSIP) - 2015

Distance vision is measured and expressed as a fraction denoting the size of print on a Snellen chart seen at a distance, for example:

  • 6/6: the child sees at 6 metres that which a fully sighted person would see at that distance (normal vision)

  • 6/12: the child see at 6 metres that which a fully sighted person would see at 12 metres (vision required for driving)
    6/18: the child sees at 6 metres that which a fully sighted person would see at 18 metres (impaired vision)

  • The terms such as mild, moderate, severe and profound are used by NatSIP to describe distance vision loss, these are defined as:


Mild Vision Loss - Within the range 6/12 to 6/18 Snellen/Kay (LogMAR 0.3-0.48)

Moderate Vision Loss - Less than 6/18 to 6/36 Snellen/Kay (LogMAR 0/5-0.78)

Severe Vision Loss - Less than 6/36 to 6/120 Snellen/Kay (LogMAR 0.8-1.3)

Profound Vision Loss - Less than 6/120 Snellen/Kay (LogMAR 1.32+)


2. Functional Vision

  • Can see to walk around

  • Can recognise faces

  • Can see print

  • Believed to have residual vision where formal testing is not possible

  • Defined as having sufficient vision for one of the following:

-independent mobility

-making social contacts

-near vision


3. Visual Field

  • Everything that can be seen by an eye in any one position

  • Includes central vision

  • Includes peripheral vision


Useful Links:

A glossary of eye conditions can be found at:

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