Henderson and colleagues note that six of eight UK charities endorse the use of coloured filters by children with reading difficulties and counsel a “more balanced” approach . These authors fail to consider one obvious explanation for the endorsement which is that these charities are in close contact with their members, some of whom provide feedback on the benefits from coloured filters. Henderson and colleagues also make the assumption that coloured filters are claimed to be a treatment for reading difficulties. A more commonly held view is that coloured filters reduce symptoms in individuals with visual stress . Visual stress seems to affect about one third of people with dyslexia [3;4]. This means that studies that ask whether coloured filters treat reading difficulties will probably find that they do not. Reviews that ask the same questions, such as that by Albon & colleagues  will also be likely to produce a negative finding.
We know of over 20 studies of the effects of overlays on reading rate, and few of them were included in the review by Albon et al, which was concerned exclusively with reading disability, and which has not appeared in the peer-reviewed literature. The second review which the authors cite  covers just 2 studies (both are listed twice), neither study involving filters designed to sample colours systematically. The second study that Galuschka et al cite  is inappropriate, with a sister-paper reporting on reading performance . Both studies cited by the review used the Irlen proprietary diagnostic process which is different to the MRC patented system used by eyecare professionals . The Irlen method seems to over-diagnose visual stress (an inference from the findings of Richie et al ) and over-diagnosis would weaken statistical power.
It is not possible to undertake a study of overlays using a double-masked (double-blind) design, given that the participant typically chooses the colour optimal for them. However the 20+ studies that have been published in peer-reviewed journals include controls for placebo effects by using clear overlays, grey overlays, overlays of complementary colour, overlays of favourite colour, and a variety of motivational instructions, none of which provide for an improvement in reading rate as great as that observed with an overlay chosen as optimal for the clarity of text.
We accept the point that Henderson et al make that the diagnosis of visual stress is challenging and more work is certainly needed in this field, including the development of better diagnostic tests [11;12] and larger RCTs. Some suggestions for research design have been summarised elsewhere . In the meantime, it would seem a sensible precaution for children who struggle with reading to be asked if words appear to blur or move, and if reading causes a headache. For children who report these symptoms, a conservative approach is required. The College of Optometrists guidelines advocate ruling out conventional visual factors and screening with coloured overlays. Overlays cost very little. They are beneficial for a few, but those few deserve to be helped. If a child finds an overlay helpful for a sustained period and an improvement is noted by their teachers and parents then they can be tested with the MRC-patented Intuitive Colorimeter [13;14] to see if coloured lenses would help to a greater extent. Practitioners who can undertake the necessary assessment can be found via the Society for Coloured Lens Prescribers (www.s4clp.org) whose members have subscribed to a Code of Conduct. Given the occasional dramatic benefit, it is appropriate for the dyslexia charities to make the public aware of this intervention.
 Henderson LM, Taylor RH, Barrett B, Griffiths PG. Editorials: Treating reading difficulties with colour. Br Med J 2014 Aug 19;349:g5160.
 Allen PM, Evans BJW, Wilkins AJ. The uses of colour in optometric practice to ameliorate symptoms of visual stress. Optometry in Practice 2012;13(1):1-8.
 Kriss I, Evans BJW. The relationship between dyslexia and Meares-Irlen Syndrome. J Res Reading 2005;28(3):350-64.
 Singleton C, Henderson LM. Computerized screening for visual stress in children with dyslexia. Dyslexia 2007 May;13(2):130-51.
 Albon E, Adi Y, Hyde C. The effectiveness and cost-effectiveness of coloured filters for reading disability: a systematic review. West Midlands Health Technology Assessment Collaboration; 2008.
 Galuschka K, Ise E, Krick K, Schulte-Korne G. Effectiveness of treatment approaches for children and adolescents with reading disabilities: a meta-analysis of randomized controlled trials. PLoS ONE 2014;9(2):e89900.
 Robinson GL, Foreman PJ. Scotopic sensitivity/Irlen Syndrome and the use of coloured filters: a long-term placebo-controlled study of reading strategies using analysis of miscue. Perceptual & Motor Skills 1999;88:35-52.
 Robinson GL, Foreman PJ. Scotopic sensitivity/Irlen Syndrome and the use of coloured filters: a long-term placebo-controlled and masked study of reading achievement and perception of ability. Perceptual & Motor Skills 1999;88:83-113.
 Allen PM, Evans BJW, Wilkins AJ. Vision and Reading Difficulties. London: Ten Alps, 2010.
 Ritchie SJ, Della SS, McIntosh RD. Irlen colored overlays do not alleviate reading difficulties. Pediatrics 2011 Oct;128(4):e932-e938.
 Allen PM, Gilchrist JM, Hollis J. Use of visual search in the assessment of pattern-related visual stress (PRVS) and its alleviation by coloured filters. Invest Ophthalmol Vis Sci 2008 May 9.
 Hollis J, Allen PM. Screening for Meares-Irlen sensitivity in adults: can assessment methods predict changes in reading speed? Ophthalmic Physiol Opt 2006 Nov;26(6):566-71.
 Wilkins AJ, Nimmo-Smith I, Jansons JE. Colorimeter for the intuitive manipulation of hue and saturation and its role in the study of perceptual distortion. Ophthal Physiol Opt 1992;12:381-5.
 Wilkins AJ, Evans BJW, Brown J, et al. Double-masked placebo-controlled trial of precision spectral filters in children who use coloured overlays. Ophthal Physiol Opt 1994;14:365-70.
Competing interests: Bruce Evans works as a community optometrist and a small proportion of his patients are treated for visual stress and purchase coloured filters. He also lectures on this topic, with some lectures funded by Cerium Visual Technologies who manufacture the Intuitive Colorimeter and coloured filters.