It’s half term and the waiting room is full. Reluctant children are sighing and scuffing the balding carpet as harassed parents barter good behaviour for afternoon treats. Outside the elegant, if shabby, 19th-century town house, double-deckers thunder towards south London. This is the clinic for specific learning difficulties at the Institute of Optometry. Each year the clinic, situated at the Elephant and Castle, sees about 1,000 children who are having difficulties with reading or writing at school and often complaining of headaches or sore eyes.

Sometimes the symptoms are hard to define. A child may get tired or lose concentration after a few minutes’ reading. They may lose their place in a book and re-read or skip lines, as well as read slowly. Some children describe letters “jumbling up” or “fizzing off the page”; they may already have been diagnosed as dyslexic.

Nevertheless, according to Dr Bruce Evans, senior lecturer and head of the clinic, many of his young patients will have passed their school’s eye test because they can read a chart at a certain distance and maintain clear vision close up for a short time. The danger is that parents and teachers do not make a connection between reading difficulties and eyesight.

Dr Evans’s research shows that children with specific learning difficulties are especially likely to suffer from weaknesses in binocular vision - the ability to make the eyes work together as a co-ordinated team.

He explains: “Treatment of these problems with eye exercises or spectacles, while not a cure for dyslexia, may make reading easier and help the child’s performance at school.”

As well as testing for binocular anomalies, eye health, refractive errors (short or long sight) and focusing ability, the clinic can also test for any benefit from using a coloured overlay - a transparent coloured plastic sheet which can be placed over text.

The controversial discovery that colour can help some people read print more easily was made by an American psychologist, Helen Irlen. It was treated with scepticism initially because Irlen’s methods were considered unscientific and her approach over-commercial.

However, in 1994, Dr Arnold Wilkins of the Medical Research Council carried out - with Dr Evans and others - a double-masked, placebo-controlled trial (the orthodox scientific measure of any treatment) using coloured spectacles. These were tinted to the particular shade which each child described as helpful in reading, and used along with “sham” glasses that were tinted a slightly different shade. To his surprise, Dr Wilkins found that some children benefited from the “real” coloured glasses. Particularly intriguing was the fact that different children required different colours, and the colour needed to be chosen with great precision.

From his latest research Wilkins estimates between one and three children in a typical primary school class of 30 will benefit from coloured lenses or an overlay. Among those who do so, the rate of reading (reading aloud a text of short, familiar words) rises on average by 8 per cent.

Neither Wilkins, nor anyone else, can yet explain why colour helps, but “the fact we don’t fully understand the mechanism doesn’t mean we shouldn’t use the treatment”, says Evans, stressing that the techniques used in his clinic are underpinned by scientific research. Indeed, the optometrists use a special instrument, the intuitive colourimeter, invented by Dr Wilkins, which generates an infinite number of colours to find the best solution for each child.

On this particular morning, Louis, 11, presents an ideal subject. He was originally sent along by the Dyslexia Institute two months earlier after he complained of headaches. On his last visit, his binocular vision was found to be perfect, so the optometrist in charge, Anita Lightstone, a lecturer in specific learning difficulties, tested him with a selection of coloured overlays. Louis’s reaction to using a green overlay was that “it stopped the letters going fuzzy”. Two months on he’s still using it for reading, from which Lightstone infers that he’s genuinely benefiting from the use of colour and merits further treatment.

Only a third of children given an overlay persist with it, so, before prescribing coloured glasses, it’s a useful way of screening out those who may be trying colour for novelty value. Today, Louis, impassive in his orange and black T-shirt, is taken into a dark room for an examination with the colourimeter.

Lightstone asks Louis whether the text is clearer under a varietyof different settings (and double-checks the consistency of his answers) to find the precise colour that helps - in this case, turquoise.

She explains that the colours used for the overlay and the glasses will differ because of “colour adaption”. The spectacles alter the colour of light coming into the eyes, whereas the overlay changes the colour of the text and nothing else.

Such is the demand for the clinic that the number of sessions has risen from one to five a week over the past two years. A growing number of children are being referred by psychologists as well as by teachers and parents (78 per cent are offered treatment, whether eye exercises, spectacles or colour overlays); no official referral letter is required. The institute is a charity, and charges Pounds 40 for a full eye examination for children and Pounds 40 for an assessment on the intuitive colorimeter.

For some children the results can be dramatic. The mother of one dyslexic 11-year old girl, Emily, who used to describe letters as “dancing on the page”, says her daughter’s blue-tinted glasses have changed their lives. Emily’s reading has improved significantly and, although she’s still two years behind, now “she picks up a book because she wants to and not because she has to”.

But, having sorted out one problem, Emily has run into another at school. Not with the children - they’re very protective - but with her teachers. In her last year at her north London junior school, the special needs teacher commented, “Oh I see we’ve got another affectation”; the headmistress asked Emily to “take those stupid glasses off”; and another said, “Will you kindly remove those sunglasses, we’re sitting indoors”.

This term Emily started secondary school, and her mother insisted a note was made in her records explaining about the glasses. Nevertheless, in her first week a teacher insisted that Emily took her “sunglasses” off. Her mother is exasperated. “Please don’t treat these children as freaks,” she says. “They have little enough confidence as it is. The institute says it can help them, they’re offered hope . . . and the last thing they need is to be humiliated at school.”

The Institute of Optometry (0171 407 4183) is holding a one-day course for teachers on vision and dyslexia on November 20.