The visual difficulties often known as visual stress are thought to be caused by a specific wavelength of light in white...
The visual difficulties often known as visual stress are thought to be caused by a specific wavelength of light in white backgrounds over-stimulating a particular area of the visual cortex in the subject, and causing visual disturbance and discomfort such as headaches and sometimes migraines. Research (Wilkins 1992) indicates that reading and comprehension can be severely impeded in about 5% of the population, but is affected to a degree in as many as 25%. Filtering out the "problem" wavelength with the right colour for the individual has made reading more accessible for thousands of children and adults. Please note: colored overlays and reading rulers do not "cure dyslexia." They are a form of ergonomic support for children and adults with a visual difficulty that is often (though not always) associated with it. Crossbow Education have sold over 2 million reading rulers since designing and patenting the product.