How big does a coloured overlay have to be?

Coloured overlays and coloured lenses can both increase reading speed, but when they do their colour is not necessarily the same, suggesting that the beneficial effects of a coloured filter might depend upon the area of the visual field that it colours. We investigated the effects of overlays on reading speed and varied the size of the overlay and the colour of the surround. Children who had been assessed with coloured overlays were required to read a passage of randomly ordered common words. The words were printed in black ink as a block of text positioned centrally on an A4 page of white paper in landscape orientation. The speed of reading was compared under four conditions: (1) without an overlay; (2) with an overlay of the chosen colour covering the entire page; (3) with the overlay cut so that it just covered the text but left the margin white; (4) with the overlay of the chosen colour covering the text but with the margin coloured a complementary colour, using a second overlay. The children who were using an overlay read more quickly with the overlay; those who were no longer using the overlay did not. Although the block of text covered less than half the page, the colour and nature of the margin did not affect reading speed significantly. These findings suggest that in order to be effective at improving reading speed an overlay needs to cover the text, but not necessarily the remainder of the page, which means that smaller overlays may sometimes be sufficient.