Visual Stress- Overlay Test

This service is offered at all Cargill’s practices.

Coloured overlays aim to reduce Visual Stress, the perceptual distortions experienced whilst reading text. This can increase reading fluency and speed, reduce headaches and increase the duration of comfortable reading.  The symptoms can occur despite normal vision.
Approximately 5% of the population are severely affected by Visual Stress and 20% to a lesser degree.

It is estimated that 1 in 5 people with dyslexia experience visual stress, but although they may co-exist, visual stress and dyslexia are different conditions.

Visual Stress is found to exist in many conditions including:

  • Dyslexia
  • Dyspraxia
  • Photosensitive Migraine
  • Photosensitive Epilepsy
  • Autism
  • ADHD

Visual perceptual distortions can be the experience of any combination of blurring, movement, doubling, shadows, flickering, seeing shapes, lines or colours when attempting to read text.

The term Meares-Irlen syndrome refers to a form of Visual Stress; it is the collection of symptoms or signs of visual fatigue when reading that are reduced when a colour is used. (Other terms include Irlen syndrome, Scotopic Sensitivity Syndrome SSS or Pattern Glare).

A representation of text as it may appear to a patient with visual stress.

“What I see” courtesy of Ffai Gritten MA Visual Artist.


  • movement of print
  • blurring of print
  • letters changing shape or size
  • letters fading or becoming darker
  • patterns appearing, sometimes describes as “worms” or “rivers” running through print
  • illusions of colour – blobs of colour on the page or colours surrounding letters or words
  • rapid tiring
  • headache or eyestrain


  • moving closer to or away from page
  • becoming restless
  • using finger as a marker
  • skipping words and lines
  • rubbing eyes and blinking excessively
  • low self esteem


After a full sight test, eye muscle and focussing assessment has been conducted, you or your child will be asked to view a page of random text whilst being shown a comparison of that text through different coloured overlays.  Colours which make the reading easier and more comfortable will be short-listed then the preferred colour determined.  An acetate sheet of the preferred colour will be issued, for a small charge, and your child will be asked to use this when reading over the next 4 weeks.  If the overlay is of genuine benefit then your child will continue to use it when reading, ruling out the placebo effect.
At this stage we can determine what coloured spectacle tint would also provide the same reading benefit, using our Colorimeter machine, based at our Canterbury & Ashford practices,  if spectacles are preferred to using an overlay.

Overlay sample:

Click a colour to sample the overlay effect:

'Mine is a long and a sad tale!' said the Mouse, turning to Alice, and sighing.

'It IS a long tail, certainly,' said Alice, looking down with wonder at the Mouse's tail; 'but why do you call it sad?' And she kept on puzzling about it while the Mouse was speaking, so that her idea of the tale was something like this....'

When booking your sight test please mention if you are interested in the I.O.O. coloured overlay test.

Please note that we are not Meares Irlen Screeners and we do not diagnose or treat dyslexia, but aim to help with Visual Stress symptoms that maybe associated with these conditions.

This service is offered at all Cargills practices.