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21. Tyrrell R, Holland K, Dennis D, Wilkins A. Coloured overlays, visual
discomfort, visual search and classroom reading. J Res Read.
1995;18(1):10 - 23.
22. Wilkins A J, Jeanes RJ, Pumfrey PD, Laskier M. Rate of Reading Test: its
reliability, and its validity in the assessment of the effects of coloured
overlays. Ophthalmic Physiol Opt. 1996;16:491–7.
23. Wilkins A, Allen P, Monger L, Gilchrist J. Visual stress and dyslexia for the
practicing optometrist. Optom Pract. 2016;17:103–12.
24. Gilchrist JM, Allen PM, Monger L, Srinivasan K, Wilkins A. Precision,
reliability and application of the Wilkins Rate of Reading Test.
Ophthalmic Physiol Opt. 2021;41:1198–208.
25. Wilkins A, Lewis E, Smith F, Rowland E, Tweedie W. Coloured overlays
and their benefit for reading. J Res Read. 2001;24:41–64.
26. Scott L, McWhinnie H, Taylor L, Stevenson N, Irons P, Lewis E, et al.
Coloured overlays in schools: Orthoptic and optometric findings.
Ophthalmic Physiol Opt. 2002;22:156–65.
27. Allen PM, Dedi S, Kumar D, Patel T, Aloo M, Wilkins AJ. Accommodation,
pattern glare, and coloured overlays. Perception. 2012;41:1458–67.
28. Bouldoukian J, Wilkins AJ, Evans BJW. Randomised controlled trial of the
effect of coloured overlays on the rate of reading of people with specific
learning difficulties. Ophthalmic Physiol Opt. 2002;22(1):55-60.
29. Firth AY, Machin J, Watkins CL. Tilt and reading speed. J AAPOS.
2007;11:52–4.
30. O’Leary CI, Evans BJW. Double-masked randomised placebo-controlled
trial of the effect of prismatic corrections on rate of reading and the
relationship with symptoms. Ophthalmic Physiol Opt. 2006;26:555–65.
31. O'Leary, C. I., et al. "The effect of low refractive corrections on rate of
reading." Optometry in Practice 2014;15(3):87-100.
32. Ousler, G. W., 3rd, et al. "Optimizing Reading Tests for Dry Eye Disease."
Cornea 2015;34(8):917-921.
33. Ridder, W. H., et al. "Evaluation of reading speed and contrast sensitivity
in dry eye disease." Optom Vis Sci 2013;90(1):37-44.
34. Yammouni, R. and B. J. Evans (2020). "An investigation of low power
convex lenses (adds) for eyestrain in the digital age (CLEDA)." J Optom
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35. Jang, Y. (2016). "Mobile colored overlay application to Korean WRRT with
detailed color control." International Journal of Applied Engineering
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36. Zeri F, Tavazzi S, Punzi M, Miglio F, Evans BJW, De Luca M. New Italian
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Chapter 6 Coloured filters for visual stress– early studies
179
Chapter 7
Do coloured filters work?
Chapter abstract
Research on the use of coloured filters for people with visual stress
is described in this chapter. There is a sizeable body of research, and
the chapter concentrates on systematic reviews and randomised
controlled trials. The use of coloured lenses in those with reading
difficulties remains controversial, and reasons for the controversy are
discussed. It is argued that it is only in a minority of individuals with
reading difficulties (those with visual stress) that the treatment is likely
to be successful.
Tinted lenses and coloured lighting
Evidence for the efficacy of tinted lenses prescribed with the
Intuitive Colorimeter (Chapter 6) has accumulated over the 25 years
since the launch of the system, although there remain unresolved
issues.
open trial involving patients who responded to reports in the media.3
1 2
The accumulation of evidence began in a small way with an
Open trial
In the open trial, volunteers who responded to an advertisement
selected a chromaticity of light that reduced perceptual distortion of
text viewed in the Intuitive Colorimeter. Coloured lenses that provided
the chosen chromaticity under conventional lighting were then offered
to the volunteers (free of charge), and a year later at the end of the trial,
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024
A. J. Wilkins and B. J. W. Evans, Vision, Reading Difficulties and Visual Stress,
https://doi.org/10.1007/978-3-031-65568-5_7
181
more than 80% were still wearing their lenses and reporting benefits.
The benefits included greater comfort/clarity when reading (58%) or
reduced headaches (47%).3 The trial was suggestive but could not be
conclusive because of the role that placebo effects can play in studies
of this kind. A double-masked trial that controlled for placebo effects
was needed, whereby both patients and clinicians were unaware of
whether the patients were wearing a genuine or sham tint. Such a trial
was difficult to undertake because patients already knew the colour of
the lens they had selected as beneficial.
Double-masked randomised controlled trial (RCT)
In the photograph in Figure 7.1, taken at dusk, the blue of the sky is
clearly visible as such and differs from the yellow of the rooms on the
left lit with incandescent light, and the white of the room on the right
lit with bright fluorescent light. We are not usually aware of these
differences in the colour of the illuminating light because we do not see
the colours side by side, as in the photo. Instead, everything we see is
coloured by the illuminating light and we remain unaware of the colour
of the light itself because we adapt to it. This adaptation is a component
of the mechanisms of colour constancy – a neural process that enables
us to see surface colours as constant despite changes in the spectrum
of the light falling on them and consequent changes in the light they
reflect.
The process of adaptation permitted the design of a double-masked
study4 in which patients were offered coloured lenses without being
aware of whether the colour was or was not the colour they had earlier
selected as best reducing the distortions of text they perceived.
Patients were allowed to adapt to the colour of light that illuminated
text in the Intuitive Colorimeter (described in Chapter 6). The processes
of adaptation meant that unless the colour was strongly saturated,
Vision, Reading Difficulties and Visual Stress
182
patients had little idea of the colour of the illuminating light. When the
light was strongly saturated, patients became aware of the colour but
did not usually appreciate that to reproduce this colour under normal
room lighting a strongly saturated lens was needed.
Figure 7.1. Window at dusk showing the differences in the colour of
lighting in the sky and in the rooms (incandescent left, fluorescent right).
The colour appearance of the most beneficial lenses (tinted with the
optimal colour as selected in the colorimeter) could not therefore be
easily recognised. This separation of the colour appearance of tinted
lenses from the colour appearance of the coloured light that gave the
therapeutic effects permitted a double-masked study to be conducted.
Each participant was provided with two pairs of spectacles, only one of
which provided the chosen chromaticity (under typical lighting);
patients and clinicians were not aware of which pair was which.
Additional precautions were taken to ensure that participants could
not work out which pair of coloured glasses matched the colour they
had chosen in the colorimeter. For example, they never saw the two
pairs side by side and there was an interval of at least two weeks after
the first pair were sent for retinting before the participant saw the
Chapter 7 Do coloured filters work?
183
second pair of coloured lenses.
Figure 7.2 Double masked study.2 Chromaticities of individual
participants’ active lenses (square points) are connected by a line to the
chromaticites of the control (sham) lenses. The length of the lines
represents the difference in chromaticity between the two. Redrawn from
Wilkins et al.4
The children who participated in the study selected their optimal
colour in the Colorimeter using a standard protocol. The protocol
involved increasing the saturation to a modest level at a given hue, over
the course of about 5 seconds, then waiting about 5 seconds before
returning the colour to white. Twelve hues were presented in this way
in turn and the good hues shortlisted. At each of the shortlisted hues,
the full range of saturation was explored, and the best combination of
hue and saturation selected by successive comparison. Hue and
saturation were then repeatedly altered in turn by small amounts in
order to find the optimal combination of hue and saturation. Having
obtained the optimal combination, the hue was gradually changed
Vision, Reading Difficulties and Visual Stress
184
until the child reported the distortions starting to reappear, and this
setting provided a suboptimal placebo control. Spectacle lenses were
made to match each setting, the chosen setting and the suboptimal
placebo setting, and one pair, selected at random by an independent
collaborator, was glazed into frames and sent to the child. Figure 7.2
shows a summary of the chromaticities. Of course, the researchers
could not be sure the control lens was actually inert, but it was likely
that on average the active lenses were more effective than the control.
The children and their parents were asked to keep diaries in which they
noted any symptoms of eye-strain or headache.
At the end of the investigation, the diaries revealed statistically
significantly fewer symptoms when children had been wearing the
active tint, see Figure 7.3. The active and placebo tints were closely
similar in colour: they differed on average by only 6 times the smallest
colour difference that can just be noticed when two coloured surfaces
are placed side by side. Therefore, the trial demonstrated not only that
the active tint was more beneficial than the control, but also that the
tint needed to be selected with precision for optimal effect: similarly
coloured lenses did not reduce symptoms as much as the lenses that
were precisely tailored to the individual’s needs.
The study included many individuals who did not start the diaries
and many who did not complete the study, which was a weakness.
However, for those who completed the study there were significantly
fewer symptomatic days when the optimal colour was worn than
symptomatic days when the sub-optimal colour was worn. This can be
seen in Figure 7.3; there are more squares above than below the
diagonal. The figure also highlights a second approach to analysing the
data. In this type of research, called a crossover study, each participant
acts as their own control. The large body of data provided by the daily
diaries allowed the researchers to calculate, for each individual
Chapter 7 Do coloured filters work?
185
participant, whether they had statistically significantly fewer
symptomatic days with the optimal colour than with the sub-optimal
control colour. The solid square symbols indicate the participants for
whom there were fewer symptomatic days with the optimal colour
than with the sub-optimal, to an extent that is not likely to be
attributable to chance (p<0.05). For every case, the benefit was with the
optimal and not the sub-optimal colour.
Figure 7.3 Percentage of days when the glasses were worn on which
symptoms of eye-strain or headache occurred. Each point represents a
participant, and the solid points represent participants for whom the
difference in symptoms was individually significant (p<0.05). Redrawn
from Wilkins et al.4
All research studies have their limitations and, as noted above, this
study suffered from attrition. This refers to the fact that of the 68 who
started the study, 15 did not finish and 16 failed to complete the
symptom diary (which was a quite an onerous commitment). It is
Vision, Reading Difficulties and Visual Stress
186
possible that some participants dropped out because they found
neither colour beneficial. It seems likely that some dropped out
because they found the control colour unhelpful, and this was known
to be the case for at least one participant. The problem of attrition
means that the researchers may have over-estimated the benefit from
coloured filters.
Another limitation is that, during recruitment in the early 1990s,
methods of detecting people with visual stress were in their infancy.
The Intuitive Overlays and Pattern Glare Test (Chapter 9) had not been
invented. In the trial, children were screened with an early set of
coloured overlays, which had only a limited range of colours.
Participants were included in the study if they chose to use an overlay
for at least three weeks. It is now known that this “loose” selection
criterion is likely to have included many people who would not meet
modern diagnostic criteria. This limitation, in contrast to that caused
by attrition, will cause the study to underestimate the benefit from
precision tinted lenses. Nowadays, stricter methods are used to detect
visual stress (Chapter 10). These methods are more likely to exclude
people who will not benefit from precision tinted lenses.
A limitation of all RCTs is that, to control for possible placebo effects,
the study will be necessarily undertaken using procedures that depart
from normal clinical practice.
5 7
This limits the extent to which the
results can be used to predict the effect of an intervention in clinical
practice. In the RCT described above, to ensure that participants did
not know which tint matched their optimal setting in the colorimeter,
no precision tinted trial lenses were used. In clinical practice, as
explained in Chapter 9, the optimal chromaticity found in the
colorimeter is checked using precision tinted trial lenses under the
type(s) of lighting that the patient is most likely to experience. The
omission of this stage in the RCT is likely to lead to an underestimation
Chapter 7 Do coloured filters work?
187
of the benefit from precision tinted lenses.
Australian study
Another randomised placebo-controlled trial was conducted by
Robinson and Foreman using Irlen lenses.8 Although, the lenses were
dispensed some time after the session in which the lenses were
selected, patients were able to see their lenses at the time of selection.
This means that, depending on participants’ memory skills, the trial
may not have been double-masked. Participants were allocated to one
of four groups, with at least 34 participants in each group. The first
group received the Irlen coloured lens for the entire duration (20
months) of the study. The second group wore a placebo lens for the
first 3-4 months and the Irlen lens subsequently. The placebo lens was
selected to be a similar colour to the Irlen coloured lens for that
individual. The third group wore a “one size fits all” blue lens for the
first 3-4 months and the Irlen lens subsequently, and a fourth group
wore no lens throughout the 20 months of the study. The Neale
Analysis of Reading test was used to measure reading rate, accuracy,
and comprehension. The reading assessments took place at the outset
of the study, when all four groups were broadly similar, and after 4
months, 8 months and 20 months.
The group that received the Irlen lens at the outset showed a steady
increase in reading comprehension which amounted to 4 years in the
20 months of the study. The second group that received the placebo
lens and the third group that received the blue lens for the first 3-4
months showed no increase in reading age during this period. Once
they received the Irlen lens they showed a rate of increase similar to
that of the first group. The groups were not matched for general ability.
The differential improvement in reading was only apparent for
comprehension, not rate or accuracy.
Vision, Reading Difficulties and Visual Stress
188
ChromaGen study
Harris used tinted contact lenses for people with colour vision
defects, arguing that when worn monocularly these improve colour
perception. Research indicates that these filters only have a small
subjective effect for people with colour vision defects, and should not
be worn in occupations that require good colour vision or night vision.9
Harris then extended this approach to individuals with dyslexia,
initially monocularly and then binocularly. He used an unconventional
method of tint selection, which variously resulted in (1) a coloured lens
in one eye only, or (2) different colours in the two eyes, or (3) similar
colours in the two eyes. Using clear contact lenses (with only a handling
tint) as a control, he showed that in 41 individuals with dyslexia tinted
contact lenses improved reading speed on the Wilkins Rate of Reading
test to a greater extent than the control.10 Although the study claimed
to be a double-masked placebo-controlled trial, it is not certain
whether the trial was masked because it seems likely that patients
would have been able to appreciate the difference between a contact
lens that was tinted and one that was not tinted.
Does the tint need to be precise?
The Wilkins Rate of Reading test (Chapters 6 and 9) was used to
examine the precision of colour necessary to improve reading speed.
Five patients who routinely wore coloured lenses were asked to read
in the Intuitive Colorimeter (without their lenses). The individuals were
asked to use the colorimeter to select the colour of light which best
improved the clarity and comfort of vision. They were then asked to
read passages of text (randomly ordered common words) aloud as
quickly as they could under light of different colours, randomly
selected on many trials over two sessions.
11 12
Chapter 7 Do coloured filters work?
189