Irlen Syndrome is a visual-perceptual dysfunction which causes problems with reading, study and writing/spelling. It has been related to a retinal-sensory after-imaging effect, possibly associated with the magnocellular pathway, which may cause an overlapping of letters and words between consecutive eye fixations when reading or writing. Individuals with this problem may suffer from eye strain and fatigue while reading and inability to maintain focus. They also frequently report that print distorts while reading and writing, with blurring, shadowing and movement being the most common symptoms. People with Irlen Syndrome are usually inefficient readers and have to devote so much attention to the identification of individual words (due to print distortions) that they may not pay sufficient attention to the meaning of what is being read. Reading becomes an effort, with limits on the length of time they can read and maintain effective comprehension.

The syndrome is named after Helen Irlen, who was given the task in 1981 of helping undergraduate students with reading and learning disabilities at California State University, Long Beach. She observed that some of the students found a significant improvement in their reading ability when coloured plastic overlays were placed over printed material. A variety of colours which could be used in lenses were then identified and developed over the next five years.

The rationale for this work is that high levels of headache and visual symptoms are associated with dyslexia. Evidence is coming through which shows that strong visual stimulation can make life very difficult for dyslexic people e.g. black ink on white paper. Coloured lenses act by blocking the over-stimulation. This is the basis for the Irlen-Meares lenses. Arnold Wilkins uses a colorimeter that allows a child to select the colour that they find most useful in reducing the effects of glare. This colour can be made into a coloured overlay or built into a pair of glasses. For children who experience these problems, coloured overlays may well increase the rate of reading and reduce visual stress.