Visual Stress

If you suffer from reading difficulties and experience distortions when looking at certain materials, particularly text, it is important to investigate whether or not you are struggling with visual stress, (also known as Meares-Irlen Syndrome, Irlen Syndrome or Scotopic Sensitivity Syndrome). The distortions include:  

  • blurring
  • movement of letters
  • words doubling
  • shadowy lines
  • shapes or colours on the page
  • flickering     

Many children and adults with reading difficulties can benefit from using coloured overlays, which when placed over text, have been found to reduce these distortions. They can enable text to be read more fluently and with less discomfort and fewer headaches. Each individual seems to need a different colour and one person’s preferred colour can change over time.

We have had lots of success with both children and adults, many of whom have seen distinct improvements in their ability to read. To book a coloured overlay appointment phone 01730 264258

of the population suffer from Visual Stress

Professor David Thomson explains in this video the benefits of a coloured overlay assessment using the ReadEZ software.  

The ReadEZ Screening System

The ReadEZ Screening Software simulates the effects of coloured overlays and spectacles by changing the background colour of the screen. To determine the optimum colour, a sample of text is displayed on the screen while the colour of the background is changed systematically.


  • "My new coloured overlay has made a big difference to my reading. I used to struggle with reading out loud at school but now I am much more confident, I can read quicker and my reading age has risen by a couple of years."
    Rebecca, aged 15
  • Book a Coloured Overlay Assessment
  • Book a Coloured Overlay Assessment
  • Book a Coloured Overlay Assessment

Click on the tab to find helpful answers to questions about coloured overlays and visual stress, provided by the following UK experts.


  • Liz Lewis and David Pointon, Norfolk Sensory Support Service, England

  • Lorna Scott, Orthoptics Department, Ayr Hospital, Scotland

  • Peter Irons, Walton Comprehensive School, Peterborough, England

  • Dr Bruce Evans, Institute of Optometry, London

  • Wendy Tweedie, MRC Applied Psychology Unit, Cambridge

  • Prof. Arnold Wilkins, Visual Perception Unit, University of Essex, Colchester


1. What are coloured overlays?

Coloured overlays are sheets of translucent or transparent coloured plastic that can be placed over a page of a book so as to colour the text beneath without interfering with its clarity.

2. What do they do?

Coloured overlays reduce the perceptual distortions of text that children sometimes describe. They enable some children to read text more fluently and with less discomfort and fewer headaches. Each individual seems to need a different colour, so it is important to assess the effects of a wide range of colours.

3. What proportion of children can benefit?

In several studies, children in county primary schools were individually shown a passage of text covered in turn by a variety of coloured overlays, including grey or clear overlays for comparison. About 20% of the children found one or other of the colours improved the clarity of the text. They continued to use an overlay of that colour without prompting for more than three months. They read more quickly with their overlay, both before and after they had become accustomed to its use.

4. How should an overlay be used?

The reader should place the sheet over the page, when reading. The text should be positioned to avoid reflections from the surface of the overlay caused by lighting. The overlay should not be creased, and it is a good idea to keep it in an envelope when it is not in use. Pupils should nevertheless feel free to touch the overlay in order to point when reading. If teasing is a problem, it may help for staff to explain to the class that the use of overlays to correct sight is similar to the use of glasses.

5. What are visual perceptual distortions?

Some people can experience distortions when they look at certain materials, particularly text. The distortions of text include blurring, movement of letters, words doubling, shadowy lines, shapes or colours on the page, and flickering. These distortions are characteristic of a condition that some have called Visual Stress (previouslyMeares-Irlen Syndrome, Irlen Syndrome or Scotopic Sensitivity Syndrome).

6. How can visual perceptual distortion be spotted?

Visual perceptual distortion should be suspected in children who have trouble learning to read, particularly if they report headaches and eye-strain from prolonged exposure to the page. If the child reports any illusory movement of the letters or words, or glare from the white paper, then treatment with coloured overlays or filters should be considered.

One possible question to ask is: “After you have been reading for a while, do the words or letters do anything different?”. If open-ended questions such as the above fail to provoke reports of distortions, more direct questions can be given. The child can be shown a page of text, and asked the following questions: “Do the letters stay still or do they move?”; “Are the letters clear or are they blurred (fuzzy)?”; “Is the page too bright, not bright enough or just about right?” ; “Does it hurt your eyes to look at the page or is it OK?”. Reports of movement, blurring and glare are more likely in children who benefit from overlays.

7. How are visual perceptual distortions caused?

The cause of the distortions is not known with any certainty. Some authors have hypothesised that the distortions are due to a dysfunction, perhaps a hyper-excitability, of nerve cells in the visual cortex, an area of the brain at the back of the head. Individuals with migraine, and those with migraine in the family, are particularly susceptible to the distortions.

8. What is Visual Stress (Meares-Irlen Syndrome)?

The term Visual Stress is sometimes used to refer to the collection of symptoms and signs of visual fatigue when reading that are reduced when colour is used as therapy. Other terms are Meares-Irlen syndrome, Irlen syndrome or Scotopic Sensitivity Syndrome (SSS). (The syndrome is not yet widely recognised by the medical and scientific communities, and there is no universal agreement on its name.) The symptoms of visual perceptual distortion in children with reading difficulty were first described by Olive Meares, but have been listed by Helen Irlen, as follows.

9. What are the symptoms of Visual Stress?

Some of the main symptoms are:
glare from the page
headaches when reading
sore eyes when reading
movement/blurring of print
(onset of symptoms varies and may depend on lighting conditions, style of text and quality of paper).

10. What are the signs of Visual Stress?

Some of the signs may be:
rubbing eyes
excessive blinking
poor concentration
inefficient reading
difficulty in keeping place

11. Which texts show a benefit from colour?

Overlays can increase the speed of reading, although with conventional text the improvement may only be apparent after ten minutes continuous reading when the child would begin to tire were an overlay not used. If the text is closely spaced, the benefit is more immediate. Books with large text are less of a problem, and may not need an overlay.

12. Which children benefit?

The children who benefit may be good readers who tire quickly, but more often they have difficulty reading. They usually suffer visual discomfort when reading and, when questioned, will often report perceptual distortions of the text. These distortions usually include apparent movement or blurring of the letters and words. Often there is a family history of migraine.

13. Does visual perceptual distortion relate directly to learning difficulties, or dyslexia?

Children with reading difficulty are more likely than others to report visual perceptual distortion, and to benefit from coloured overlays. A smaller proportion of good readers also show similar benefits. Individuals with dyslexia may have difficulties with visual perception, but usually also have difficulties of a linguistic nature which need to be addressed separately.

14. Why can children have “perfect eye sight” and still experience distortion?

An optometrist will report “perfect eye sight” when someone can see a letter chart without needing refractive correction (glasses), and when there are no (orthoptic) problems of co-ordination between the eyes. The perceptual distortions may occur quite independently of any refractive error, although they are often, but not always, associated with a mild binocular vision difficulty (i.e. a difficulty in moving the eyes together, keeping the direction of gaze appropriately co-ordinated). In most cases the binocular difficulties do not appear to be the basis for the distortions.

15. Does visual perceptual distortion occur in families, and if so, why?

Many traits run in families and visual perceptual distortions are no exception.

16. Does visual perceptual distortion cause writing to deteriorate?

The visual perceptual distortions that people experience can affect all aspects of visual function, but they are more likely when the visual material has many similar contours (letters). Text is unlike natural scenes in that it is composed of many identical elements. These are at their most confusing in small closely spaced printed text, but they also occur in handwritten work.

17. Do children need coloured overlays or coloured glasses permanently?

It seems that children benefit most from colour if it is offered as soon as any reading difficulty is suspected, before the cycle of failure has begun. Many 7-year-olds appear to use coloured overlays for a year or two and then discard them as unnecessary. This may be because the acquired familiarity with text then makes the distortions less distracting.

18. Can overlays or glasses harm the eyes?

Just as some colours are reported as being beneficial, others are often reported to be uncomfortable. Individuals sometimes show a marked aversion to these uncomfortable colours. Provided the appropriate colour is chosen, it seems unlikely that overlays can have any detrimental effect. The possible long-term effects of wearing coloured glasses are unknown at present.

19. Should coloured glasses be worn all the time?

In our view, children should be free to wear the glasses if they find it helpful to do so. They should not becoerced into wearing them if they would not otherwise do so, or bullied and teased so that they refrain from wearing them. The response to colour is subjective and individual, and the wearer is the best person to judge whether there is any benefit.

20. Should children with binocular difficulties who are undergoing orthoptic eye exercises or other medical treatment continue to use overlays?

In our opinion, overlays can be used regardless of any simultaneous eye exercises or medical treatment. However, since visual perceptual distortions can sometimes be caused solely by binocular vision problems, it is often sensible to have these corrected first. If the distortions remain then coloured filters need to be tried.

21. How long should overlays be used before coloured glasses are considered?

There are many factors involved. First, are the overlays obviously beneficial? If so, only a short trial period, say six weeks, is necessary, particularly if headaches have been reduced but not eliminated, and if untidy writing continues to be a problem. Under these circumstances glasses may further reduce the headaches and may well improve the handwriting.
If, on the other hand, the response to overlays is less marked, it seems sensible to see whether the child continues to use overlays without prompting for, say, a school term or longer, before considering coloured glasses. Coloured glasses are more expensive than overlays, and it may be wise to wait before incurring the cost.
Another factor to consider is the age of the child. It is often difficult to assess a child for coloured glasses below the age of 8.

22. Are coloured glasses necessary?

Children who persist in using their overlay usually find coloured glasses more convenient to use. Glasses can help with writing, whereas overlays cannot. The degree of precision in the choice of colour is critical for obtaining the best results, and the precision available with lenses is far greater than with overlays. Perhaps for this reason glasses often give better results.

23. Are glasses the same colour as overlays?

It is essential to realise that the appropriate colour for use in glasses is not the same as that in overlays. For example, a child may choose a yellow overlay and benefit from blue lenses. The colour of the lenses can only be assessed by optometrists or orthoptists who use the Intuitive Colorimeter, or by the use of a very large number of coloured trial lenses. Other methods of selecting coloured lenses may be less likely to select the optimal colour.

24. Why are glasses a different colour from overlays?

When you wear glasses everthing you see is coloured, but you are often unaware of the colouration because you adapt to it and make allowances for it (for example, the colour of light from a normal household light bulb is very yellow in comparison to daylight, but you are never aware of this). When you use an overlay only part of what you see is coloured and the eyes are adapted to white light. The way that the brain processes what you see in the two circumstances is very different.

25. What is the best method for combining overlays?

The best method is that which most efficiently covers the largest number of possible colours.
The Intuitive Overlays used in recent research were scientifically developed so that similar colours can be combined two at a time in a simple yet thorough way. If these overlays are used and all suggested combinations are tried, a wide range of colours will have been efficiently and systematically sampled.

26. How reliable is the choice of overlay colour?

When tested a second time, individuals may sometimes choose a different colour, but it is usually a similar colour. The choice of colour may appear random, but it is not!

27. What does one do if a child reports a large range of colours beneficial, but cannot make a consistent choice?

Beneficial colours should be compared side by side. If the choice remains unreliable, then one of the chosen colours should be given a trial for a period of a week, followed by one of the other colours.

28. Do children change their preferred colour?

Children sometimes seem to change their preferred colour. The reason for this is not known.

29. Does it matter if the child still sees areas of white page around the overlay?

Areas of white page may well affect the choice of colour. The conditions of the test should resemble those under which the overlay will be used. If neighbouring white pages are unlikely to be encountered when the overlay is used, they should be avoided during the test procedure.

30. Would it help children to work under lighting that is not fluorescent?

Schools are usually overlit by fluorescent and natural light. Care must be taken to avoid glare by shielding work surfaces from direct sunlight. Complaints of glare from fluorescent lighting should be taken seriously; they usually result from real rather than imagined problems. In 80% of classrooms fluorescent lighting emits high-frequency invisible flicker that can affect some people, particularly those with migraine. If headaches are attributed to fluorescent lighting, the individual should be seated where the fluorescent light is “diluted” by daylight or the relatively steady light from filament lamps.

31. Can adults be affected?

Yes. Although some people seem to “grow out” of the condition, many do not. The distortions may be less pronounced when reading becomes fluent and text ceases to be a meaningless collection of confusing shapes. Sadly, visual perceptual distortion is often not recognised in children and many sufferers enter adulthood without ever having been treated.

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37 Lavant Street, Petersfield, Hampshire, GU32 3EL, 01730 264258