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The speed with which they read the text depended on the colour
and it did so in a similar way for all five observers. The larger the
difference between the chromaticities of the light under which they
were reading and the light they had selected as most comfortable, the
slower the reading speed. It did not appear to matter whether the
difference in chromaticity was due to a difference in hue (hue: CIE huv)
or strength of colour (saturation: CIE suv). The reading speed decreased
with increasing difference in chromaticity (hue, saturation or both)
from that selected as optimal. The graphs for the five individuals are
shown in Figure 7.4. The decrease shown in the large panel in Figure
7.4 has a mean and variance based on the average parameters of the
five individuals. Generally, the reading speed falls off in such a way that
when the difference in chromaticity exceeds 0.06 the reading speed is
little better than under “white” light (i.e., light with a correlated colour
temperature of about 4000K).
Vision, Reading Difficulties and Visual Stress
190
Figure 7.4. Data from five individuals who habitually wore coloured
glasses. The first two columns show the reading speed (without glasses) of
passages of randomly ordered words under light of different
Chapter 7 Do coloured filters work?
191
chromaticities. The position of each point shows the chromaticity of the
light, and the diameter of the point is directly proportional to the reading
speed. Contours representing similar reading speed have been fitted by
interpolation using a computer algorithm (triangulation). The contours
are similar in Session 2 despite fewer data, showing reliability. The data
have been replotted in the lower graph in terms of the difference in colour
between the subjectively optimal colour and the colour under which the
reading speed was measured. The data have been fitted by Gaussian
functions. The graphs show that a difference in chromaticity of about 0.06
is sufficient to eliminate most if not all advantage conveyed by colour. The
chromaticity of a spectrally uniform surface viewed through each
individual’s coloured lenses and illuminated by incandescent light (CIE
Type A), fluorescent light (CIE Type F3) or daylight (CIE Type D65), are
shown in the first column by the letters A, F and D, respectively. The
arrowheads on the y-axis show the reading speed under white (CIE Type
F3) light. The large panel shows the average function. Redrawn from
Wilkins et al.
11 12
Although the mean data in the large panel in Figure 7.4 are useful
to give a general perspective, it is also instructive to compare the data
from each individual in the small panels in the third column of Figure
7.4. These show considerable differences between individuals, both in
the degree of benefit from colour (y-axis) and in the precision of colour
choice (x-axis). Considering the precision, for participant OR, there is a
difference of approximately 0.04 UCS chromaticity between the
optimal colour and the most similar colour where the benefit levels off
to its minimum. In contrast, for participant EB the equivalent
chromaticity difference is approximately three times greater. This
illustrates an observation that is commonly made by clinicians, that
some patients show remarkable precision on testing with the Intuitive
Colorimeter, but others do not. Of course, there is no way for a clinician
Vision, Reading Difficulties and Visual Stress
192
to predict the precision that an individual will require until the
individual is tested. The system needs to provide the precision required
for the most sensitive patient.
These findings are of practical significance for at least two reasons.
Firstly, they help to explain why patients can choose a specific colour
for their lenses under one lighting condition, and the lenses are
nevertheless useful under different lighting. If the optimal chromaticity
is chosen under white (halophosphate) fluorescent lighting, the
reading speed under other types of lighting can be calculated using a
function similar to that shown in the large panel in Figure 7.4. The
calculations reveal that most tints selected under white fluorescent
light (CIE type F2 and F3, used in the Intuitive Colorimeter Mark 2 and
Mark 3 respectively), should continue to offer some increase in reading
speed under incandescent lighting (Illuminant A) and daylight
(Illuminant D65). The exceptions are those tints with purple hue, which
exaggerate the differences between light sources by selectively
transmitting light at each end of the visible spectrum.
Secondly, the functions in Figure 7.4 also permit an estimate of the
number of lenses that are necessary in any tinting system. Any
estimate is based on that system having a given percentage efficacy
compared to a theoretical system that offers an infinite number of tints
and could in principle obtain the optimal chromaticity under any given
light. The ramifications are discussed further in Chapter 9.
Stein and colleagues13 have challenged the idea that so many tints
are necessary. Stein initially took the view that just the colour yellow
was required and carried out research comparing a yellow filter with a
blue filter, with the hypothesis that the blue filter was a suitable
(unhelpful) control. Stein reported that “to our surprise, we found that
some children actually benefited more from wearing the blue filters
than from the yellow filters.” Stein interpreted this result as indicating
Chapter 7 Do coloured filters work?
193
that two colours, one yellow and the other blue are needed. He did not
investigate an alternative explanation, that colour needs to be
individually prescribed.
Stein’s view is at variance with the work of Palomo-Alvarez & Puell14
who found no effect of yellow lenses on binocular vision,
accommodation, ocular motor scanning and reading speed in children
with reading difficulties. Further evidence against Stein’s view comes
from Robinson and Foreman who found that individually prescribed
coloured filters were more helpful than giving a standard blue to all
participants (see above).
Repeatability of colorimetry
No subjective clinical test is perfectly repeatable. When people
undertake the same test on two occasions, there will always be people
who perform differently on the second occasion than the first. The
above discussion of the need for precision assumes that the
colorimetry assessment is reasonably repeatable. This was called into
question by Suttle et al.15 who classified colorimetry as having “poor”
repeatability because the number of Just Noticeable Differences (JNDs)
between two measurements was “large”. JND refers here to the
difference that is just noticeable when two coloured surfaces are
observed side by side, as is the case with coloured overlays. When two
surfaces are simultaneously visible, very small differences in
chromaticity are discernible. It is not clear from the paper by Suttle et
al. how the JNDs were derived. During colorimetry, coloured lights are
presented successively rather than simultaneously. Any comparison
involves not only adaptation to the colour but memory for colours
previously shown. This inevitably increases variability. JND is a
subjective measure, whereas chromaticity difference is a physical
Vision, Reading Difficulties and Visual Stress
194
three classes of cones. For this reason, we use it here in preference.
In a study by Aldrich et al.,16 colorimetry was performed twice in
immediate succession with different examiners and different
colorimeters. Participants had signs and/or symptoms of visual stress,
80% of whom met diagnostic criteria for visual stress that were
introduced after the study was started (see below).17 Participants were
asked, during colorimeter testing, to select the chromaticity that best
reduced discomfort and perceptual distortion of text. Tinted lenses
that provided the chosen colour increased the rate of reading on the
Wilkins Rate of Reading Test (WRRT; Chapters 6 and 9). The rate was
measured relative to tinted lenses that were 0.07 different in CIE UCS
chromaticity and acted as a placebo control. Because the colorimetry
assessment was undertaken twice it was possible to assess its
reliability. The two settings were of similar colour (average UCS
chromaticity difference 0.043), see Figure 7.5, from which Aldrich et
al.16 estimated that the standard deviation of the u’ and v’
chromaticities was 0.02. The previous estimates for the chromaticity
limits of efficacy of a tint of 0.07 are therefore more than three times
the standard deviation of repeated measurements.
measure that is directly related to the relative energy captured by the
Chapter 7 Do coloured filters work?
195
Figure 7.5 The chromaticities obtained in the first colorimeter assessment
are marked by a point. They are connected by a line to the chromaticities
from the second assessment. Broken lines represent the participants
whose consistency in colour choice was rated as poor by both examiners
separately. In the first assessment saturation was constrained to 30, so
the second assessment usually gave chromaticities that were more
saturated (i.e., further from equal energy white, shown by the cross).
Redrawn from Aldrich et al.16
One of the difficulties that has beset research in this area is that of
defining individuals with visual stress. For example, the dotted lines in
Figure 7.5 are large because these few individuals were not
symptomatic and did not conform to the criteria subsequently
specified by Evans et al.17 and outlined in Chapter 10. Obviously if
individuals are examined who do not experience the symptoms of
visual stress then the subjective colorimeter assessment cannot be
expected to be repeatable. The criteria used by Suttle et al.15 to identify
visual stress were less stringent than the criteria of Evans et al.17
Vision, Reading Difficulties and Visual Stress
196
It was noted above that even for individuals who have visual stress,
there is variation between people in the precision required for the
selection of the optimal colour. Optometrists are familiar with a similar
effect when they test for refractive error: some people will tolerate only
the exact optical prescription, requiring a surprising degree of
precision, while other individuals are relatively vague and tolerant. For
both refractive error and visual stress, the reason why people differ so
much is not fully understood. With visual stress, it is not known
whether this simply reflects differences in personality, a difference in
the physiological mechanism underlying visual stress (Chapter 8), or
both. Whatever the reason for the variation between people, a
consequence of this is that the data from some individuals are likely to
be less repeatable than from others. Understandably, researchers tend
to make generalisations about the group they have studied, typically
measuring the average result. For clinicians, and others caring for
people with visual stress, each person should be treated as an
individual and their own needs for precision explored. The procedure
for testing with the Intuitive Colorimeter allows for this: in the later
stages of the procedure the step size for making changes is reduced
until the patient cannot discriminate between the effect of different
chromaticies.
Assessments of repeatability rely on two assumptions. These are
that there is one optimal colour and that this will not change in the
interval between the two measurements. These assumptions were
addressed by Evans in two papers in 2018 describing case studies.
18 19
It was noted that, for most people with visual stress, the symptoms are
maximally alleviated by colours from one region in colour space,
symptoms are worse with the opposite (complimentary) colour, and
colours between these two extremes tend to have lesser effects on
symptoms. Evans presented a different type of case, described as
Chapter 7 Do coloured filters work?
197
uncommon, who reported two loci of benefit in different regions of
colour space.18 He noted “Some patients report that one colour
alleviates certain symptoms, while another improves different
symptoms. The patient initially may choose the colour that helps the
symptom that they find most bothersome and then, over time, another
symptom predominates resulting in a change in colour.” These cases
are sometimes described by clinicians as having “twin-peaks”. It might
be helpful for future research on repeatability of colorimetry to avoid
such cases, or at least to study them separately.
What determines the colour optimal for comfort
and for reading?
When an object is heated it gives off radiation. It glows red hot at
temperatures of about 1000K and then as the temperature is increased
to 2000K the colour changes to orange. At 4000K the object is white hot
and at 8000K it is blueish. The colour of the object depends on the
change in the spectrum of radiation and this changes with temperature
according to Planck’s law. The chromaticities follow the Planckian locus,
shown by the curved black line in Figure 7.6.
The lighting we are normally exposed to in everyday life varies in
colour in exactly the same way, ranging from the rose of the setting
sun to the blue of the sky at times of day other than dawn and dusk.
When asked to choose a colour comfortable for reading most healthy
individuals who do not have visual stress choose chromaticities that lie
close to the Planckian locus. This is as might be expected: if people are
asked to choose a colour of lighting they will select a colour within the
range that they will experience with everyday light sources. The
chromaticities chosen vary considerably: some people choose an
orange hue and others a blue, but mostly the choices are close to the
Vision, Reading Difficulties and Visual Stress
198
Planckian locus, see the left panels of Figure 7.6.
Figure 7.6. Chromaticites of light chosen as comfortable for viewing text.
The data are from separate studies of different patients by Aldrich et al20
and Vieira et al.21 For the control population the chosen chromaticites
cluster around the Planckian locus. For the individuals who experience
migraine with aura, strongly saturated coloured light is chosen, distant
from the Planckian locus. Reproduced under STM Permissions Guidelines
from Wilkins et al.23
The colour choice is different in patients who experience migraine
with aura. When asked to choose lighting of a colour comfortable for
reading, nearly all patients with migraine aura choose a colour they
would never normally experience. They choose instead strongly
saturated chromaticities well away from the Planckian locus.
20-22
In
these experiments the participants in both groups were instructed in
exactly the same way. Clearly, the patients who have migraine with
aura, like those with visual stress, are basing their choice of colour on
something they are experiencing that is different to the experience of
the control group.
Chapter 7 Do coloured filters work?
199