A coloured overlay is a coloured transparent plastic sheet placed on top of reading or copy text. It’s purpose is to minimise the discomfort of glare from a white page and to mask the visual perceptual effects that can make reading unpleasant or even impossible for some individuals.
This can improve clarity, eye movement control, ease of concentration and comfort. It is not known for certain how the coloured overlays work but some interesting theories have evolved from neuro-physiological research.
It should be remembered that the coloured overlay is simply a temporary tool to aid the progress of reading. The symptoms masked by the overlay are generally the result of an underlying deficiency in functional control over the visual system. More permanent improvements are likely with lens and vision therapy and the overlay may then no longer be required.