Each colour is carefully determined by years of Irlen research. Not everyone is helped by the same colour or combination of colours. If you filter out the problem wavelength with someone who has this sensitivity the text becomes clearer, and headaches and migraines can be reduced.

It is often the case that one person will benefit from overlays of one colour, and lenses of another colour. If lenses are to be worn, it is important that a separate professional assessment is carried out. There are two widespread approaches to testing for coloured lenses. One is carried out by a specialist optometrist using the "Intuitive colorimeter", a piece of equipment designed by Professor Arnold Wilkins in conjunction with the Medical Research Council and marketed by Cerium Visual Technologies, which will identify precisely the chromaticity of the illumination of a piece of text most effective in increasing reading speed, giving a measurement that is then used in prescribing tinted lenses. A list of recommended optometrists who test for coloured lenses with the Intuitive Colorimeter can be found here Visual dyslexia needs to be objectively observed. Most sufferers are not aware that their experience of reading differs from that of other people. Fortunately, behaviour associated with visual stress can be quite easy to spot. A different approach is used by the Irlen Institute, working through a selection of lenses combining different coloured layers to arrive at an optimum tint. The system was developed by Helen Irlen under a U.S. federal research grant from 1980-1985. Both systems have their supporters and detractors. There is no doubt that coloured lenses, either from the Irlen or Wilkins systems, are of great benefit to a significant number of people. However it is also true to say, as has already been mentioned, that many children do not benefit from their lenses as much as they might as they can resist wearing them, particularly as they get older. An Optometrist's assessment is only free under the NHS in Scotland.Visual Stress can exist on its own or alongside other dyslexia-related difficulties and other SpLDs. This set of difficulties is sometimes called "visual dyslexia," but it is not actually dyslexia.

It was the most important moment of my life. For the first time in my life I could see text clear as glass. I was astounded…. All I could think to say was 'How did they do that?'" Visual dyslexia is a reality for about 30% of the population. It can cause headaches, migraines, reading discomfort and distortion of text that can vary from mild to so severe that reading is seriously impaired or even prevented.