Coloured filters are tools, such as coloured lenses or coloured overlays, which block specific light frequencies. Coloured lenses are tinted, non-optical lenses which can be put into spectacle frames. Coloured overlays are clear plastic sheets that can be placed on top of reading materials, such as books or newspapers.
Coloured filters are designed to block the specific wavelengths of light to which an individual is sensitive. So, for example, an individual might be especially sensitive to blue or yellow light and therefore use an appropriate filter to reduce its effects.
Coloured filters are used to help individuals with visual processing difficulties and/or visual stress. People with visual processing difficulties are especially sensitive to lights, glare, patterns, colours, and contrast. People with visual stress may experience perceptual distortions and discomfort, especially when reading printed text.
According to Lewis et al (Date unknown)
“The reader should place the [overlay] sheet over the page, when reading. The text should be positioned to avoid reflections from the surface of the overlay caused by lighting. The overlay should not be creased, and it is a good idea to keep it in an envelope when it is not in use.
”Children who persist in using their overlay usually find coloured glasses more convenient to use. Glasses can help with writing, whereas overlays cannot. The degree of precision in the choice of colour is critical for obtaining the best results, and the precision available with lenses is far greater than with overlays. Perhaps for this reason glasses often give better results.
“It is essential to realise that the appropriate colour for use in glasses is not the same as that in overlays. For example, a child may choose a yellow overlay and benefit from blue lenses. The colour of the lenses can only be assessed by optometrists or orthoptists who use the Intuitive Colorimeter, or by the use of a very large number of coloured trial lenses. Other methods of selecting coloured lenses may be less likely to select the optimal colour.”