One of the kids I tutored for reading really seemed to benefit from colored overlays. I was timing him (words read aloud per minute), and had been timing him everyday all along. When I put on the overlay, his reading speed went up pretty dramatically. I don't know if he had "Irlen Syndrome" but he said that without the overlay,the words looked like they were superimposed on each other and the overlay made them clearer. I think he had some strabismus/double vision. Probably what he really needed was an eye exam and maybe there would have been something else that could have helped, like surgery or prism glasses. He refused to use the overlays in class, I don't think he wanted to stand out and look "different" (he also had some major behavioral issues). I tried the overlays on many of the other kids I worked with (most had tracking issues or reading problems suggestive of mild dyslexia),and most said the overlays made the print easier to see, but their reading speed didn't actually improve. Hope this helps.
