There are many children struggling at school because their poor sight has not been detected

Anita Lightstone, RNIB

Studies carried out by Dr David Thomson of City University, London, between 1997 and 2001 found that among school-age children, between 17% and 22% had some form of undetected vision problems.

Anita Lightstone, RNIB's Head of Low Vision and Prevention, said: "This research supports what RNIB has suspected from anecdotal evidence.

"There are many children struggling at school because their poor sight has not been detected.

"We want everyone to be aware on Eye Test Action Day that eye tests are vital so that children can achieve their full potential at school."

Lesson problem

Sight problems can have an immediate impact on school work - children may not be able to see the blackboard or read text books properly.

In one case reported to the RNIB, an optometrist was referred a seven-year-old boy by an educational psychologist, because he was having difficulties concentrating or interacting with other people.

The optometrist said: "When I tested his sight it was immediately clear that he was extremely long-sighted with some astigmatism.

"After wearing glasses for a month his reading improved dramatically and he was socialising with other children at school."

'False diagnosis'

Professor Gary Rubin, Professor of Visual Rehabilitation at University College London, told BBC News Online that while such severe consequences of uncorrected vision defects were rare, parents should make the effort to have their children's eyes tested.

There can be some rather dramatic ways in which poor sight can lead to permanent and incurable loss of vision

Professor Gary Rubin, University College London

He said: "There can be some rather dramatic ways in which poor sight can lead to permanent and incurable loss of vision.

"These problems are normally correctible with glasses, and failing to do so could mean it is difficult for the child to see the blackboard.

"However, a more dramatic instance might be a child who is failing to develop reading skills and language skills and is falsely diagnosed as mentally disabled.

But he added: "The prevalence of this kind of problem is very, very low".

Even pre-school children can be tested for signs of visual problems.

"Preferential looking" tests involve checking the child's reaction to patterned or plain cards, as a child will always be drawn to a card with a pattern it can see in preference to a plain one.

Another test involves checking the "red-eye" of the child - a reflection of the blood vessels in the back the eye which often crops up in flash photography.

If the eye is not achieving proper focus, the "red eye" will appear as a crescent rather than a disc.

NHS sight tests are free to people under 16 and over 60. People who are on benefits and low incomes, or with certain medical conditions also qualify for free tests.