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Figure 6.11. Chromaticities available with the Intuitive Colorimeter. The
hue angle is shown by the radiating “spokes” and the saturation by the
concentric “rings”. The blue line shows the Planckian locus, see Figure 6.4
for explanation.
All the colorimeters since the Mark 1 use a total of eight filters, seven
coloured and one grey. Although only three filters are needed to
provide a large range of colours (televisions use red, green and blue,
for example), the advantage of seven filters is that the power of the
light at each wavelength (i.e. the spectral power distribution) is then
close to that obtained when the tinted lenses are worn under white
light.20 This is despite the fact that the colorimeter adds light from
various filters to obtain a particular chromaticity, whereas a subtractive
mixture is used when trial lenses are combined to reproduce that
chromaticity. The similarity of spectral power distribution with the
Colorimeter and with the trial lenses means that practitioners and
patients who have a colour vision deficiency (Chapter 2) can use the
instrument.
Chapter 6 Coloured filters for visual stress– early studies
169
arrangement of early models is replaced by electronics: eight rows of
white LEDs are covered by seven coloured filters and one neutral filter
of similar transmission. The number of LEDs lit behind each filter is
controlled by a tablet computer so that the resultant light is a mixture
of one or two coloured filters and a grey. The proportion of light from
the grey filter controls the saturation. The resultant combination of
coloured and grey light is mixed by multiple reflection in a chamber
with reflective surfaces.
Experience with early versions of the Intuitive Colorimeter made it
obvious just how specific a colour was required to best reduce the
distortions of text. It was only with the specific colour selected using
the colorimeter that patients reported maximum benefit. If the efficacy
of coloured filters were to be examined experimentally, a tinting
system was required that could potentially provide any colour with
sufficient precision. Such a tinting system was designed with the help
of Tim Noakes of Cerium Visual Technologies
8,9
.
With the tinted trial lenses only two dyes are necessary to obtain
any shade (Figure 6.12). For example, a yellow-green is produced with
a combination of yellow and green trial lenses, and a red is produced
with a combination of rose and orange trial lenses. The lenses are
arranged in pairs, one for each eye. For each colour, five pairs provide
a series of increasing saturation. The deposition of dye doubles from
one pair to the next in the series. This means that the saturation of
colour can be increased in very small increments by an efficient
combination of the lenses.
In the latest version of the Intuitive Colorimeter, the mechanical
Vision, Reading Difficulties and Visual Stress
170
Figure 6.12. Periphery: Graphs of the spectral transmission of the trial
lenses that accompany the Intuitive Colorimeter. Centre: the chromaticities
they provide on their own (coloured dots) and when combined (black dots).
The five pairs of lenses provide 25 = 32 possible combinations and
32 levels of saturation. The 32 levels of saturation of one dye can be
combined with the 32 levels of lenses from dyes of neighbouring
colours, giving a total of 7167 [7 x 32 x 32 - 1] possible combinations of
trial lenses, (more if the most saturated versions with rose and purple
dyes are included). All the combinations give slightly different shades
of colour, providing close approximations to any possible Colorimeter
setting, together with many colours that are more saturated than those
in the Colorimeter.
The dyes and lenses were chosen to meet the following design
parameters: (1) it was necessary to approximate any chromaticity to
Chapter 6 Coloured filters for visual stress– early studies
171
within the smallest difference that can be discriminated; (2) because
there are many different spectral transmissions that will provide a
given chromaticity, dyes were selected from those available so that the
transmission was as high as possible and (3) the transmission varied
with wavelength as smoothly as possible.
The second feature, to provide maximum transmission, means that
the system was designed to produce the lightest tint that alleviates
symptoms. This is why only adjacent colours are combined. With
tinting systems that use a different approach a patient might be given
a tint that is helpful, but is darker than necessary. Although this is
unlikely to do harm, it will be cosmetically more disturbing for the
patient.
The third design feature above, smooth transmission curves, is
apparent in the peripheral graphs in Figure 6.12. This minimises the
different colours that result under different types of artificial lighting
having different spectral peaks. For example, Figure 6.3 shows the
spectral power from two light sources with the same colour: daylight
and fluorescent light. If a coloured filter happens to absorb light in part
of the spectrum where there is a peak from the fluorescent lighting,
the effective colour will be different from that obtained with daylight.
Apart from the Irlen system and the Intuitive Colorimeter system,
there are now several systems of tinted lenses on the market. For
example, the ReadEZ system has 12 differently coloured clip-on lenses
and two saturations (lightness) giving a choice of 24. In addition, a few
optometric practices choose to dye their own lenses. The research
described in Chapter 7 suggests that any system of tinted trial lenses
needs to be able to provide a minimum of 77 different shades of colour
in order to be sure of obtaining a tint that maximizes the benefit. This
is possible with the Intuitive Colorimeter system.
Vision, Reading Difficulties and Visual Stress
172
Cerium (Intuitive Colorimeter), and ChromaGen systems. Tinted
spectacle lenses are far more commonly prescribed, however, due to
cost, convenience, and the fact that tinted lenses are used mainly for
reading. Tinted lenses of whatever kind are usually prescribed for
children only after a successful period of overlay use (see Chapter 10).
Another approach, which is facilitated by modern digital devices (e.g.,
computers and tablets), is to change the background screen colour.
This is described further in Chapter 9.
Intuitive Overlays
The Irlen Institutes initiated screening in schools using coloured
overlays - sheets of coloured plastic placed over a page of text to colour
the text beneath without interfering with its clarity. The chromaticities
of the Irlen overlays were measured by Wilkins and it was found that
they do not sample chromaticity systematically.
Wilkins reasoned that if individuals need different colours, it would
be advisable to prepare a set of overlays that sampled chromaticity
systematically, so that any required chromaticity could be closely
approximated. He designed the Intuitive Overlays with this in mind,20
nine coloured and one grey. The nine coloured overlays have
chromaticities distributed evenly on the circumference of a circle in the
CIE UCS diagram (inner points in Figure 6.7), indicating that they have
similar saturation and evenly distributed hue. They can be combined,
two at a time, by placing one upon another to provide stronger (more
saturated) colours, with chromaticities lying on a circle with greater
radius, (outer points in Figure 6.7). The overlays differ in their
reflectance. Some are inevitably lighter than others: yellow attenuates
light only at the short wavelength end of the spectrum whereas blue is
darker because it attenuates light over most of the spectrum, see
Figures 6.12 and 6.13. As might be expected, double overlays are
Precision tinted contact lenses can be prescribed using the Irlen,
Chapter 6 Coloured filters for visual stress– early studies
173
darker than single overlays. The CIE UCS diagram does not represent
these differences in darkness.
Figure 6.13. (Centre plate) Centre: Uniform Chromaticity Scale diagram (CIE
1976) showing the chromaticities of the nine colour Intuitive Overlays, inner
ring of white points, and grey overlay (central point). The chromaticities are
those of the overlay when it is in contact with a spectrally uniform (white)
surface. The chromaticities of double overlays formed by placing one
overlay on top of another are shown by the outer ring of points. The grey
points are the chromaticities of two overlays of identical colour. The crosses
mark the chromaticities of two overlays of neighbouring colours. The lines
Vision, Reading Difficulties and Visual Stress
174
connect the chromaticities of the double overlays with those of constituent
single overlays. Periphery: Graphs showing the reflectance as a function of
wavelength for each of the overlays, using the axes of the graph of the
spectrum, lower left. Note that the reflectance functions and chromaticities
shown are for the overlays themselves, and take no account of the
illumination.
The development of the Intuitive Overlays enabled patients to
obtain a chromaticity close to that optimal for text clarity. It became
clear that an appropriately chosen colour could not only improve
perceptual clarity and reduce perceptual distortions but could reduce
visual fatigue. For some individuals and conditions, colour also can
improve reading and visual search, as described below and in later
chapters.
Although the overlays seemed primarily to prevent fatigue and
reduce visual symptoms, in one study a benefit in the speed of reading
conventional prose was obtained. The benefit occurred only after 10
minutes of continuous reading, when the children had started to tire.21
A limitation of relying on symptoms is their subjective nature, and a
less subjective method of assessing any benefit from the overlay was
required.
Rate of Reading Test
In 1996 Wilkins developed the Rate of Reading Test.22 This simple
test differed from conventional reading tests in several important
respects. Conventional reading tests aim to measure reading skill and
are designed not to be overly influenced by visual problems. Therefore,
the tests tend to use large well-spaced font, and some even use a
coloured background.
Chapter 6 Coloured filters for visual stress– early studies
175
achieve a different goal: to detect visual problems that affect reading
and yet to be relatively unaffected by reading skill. The patient is asked
to read a paragraph consisting of randomly ordered common words.
Each line has the same 15 high frequency words in a different order,
see Figure 6.8. Children who are poor at reading can succeed at the
task because the words are simple. Children often make errors of
transposition of words or omission of a line, but they are usually
unaware of their errors because the text is meaningless. A sense of
failure is therefore avoided.
come see the play look up is cat not my and dog for you to
the cat up dog and is play come you see for not to look my
you for the and not see my play come is look dog cat to up
dog to you and play cat up is my not come for the look see
play come see cat not look dog is my up the for to and you
to not cat for look is my and up come play you see the dog
my play see to for you is the look up cat not dog come and
look to for my come play the dog see you not cat up and is
up come look for the not dog cat you to see is and my play
is you dog for not cat my look come and up to play see the
Figure 6.14. A passage from the Wilkins Rate of Reading Test. The passage
is set in 9pt Times with 4pt spacing between words.
The words cannot be guessed from context (they have to be seen to
be read), and so visual errors are easy to measure. The text is printed
in a small font and the resultant fatigue means that it is possible to
show an increase in reading speed with overlays in about a minute. It
is simply necessary to ask an individual to read aloud for one minute
with the overlay and then for a further minute without the overlay, and
compare the speed. To assist in assessing consistency, however, the
test is usually given once with the overlay, once without, again without
and finally once again with the overlay. Because the test is
administered twice with and twice without an overlay it is possible to
The Wilkins Rate of Reading Test (Figure 6.14) was designed to
Vision, Reading Difficulties and Visual Stress
176
individual’s reading rate is not related to their mean reading rate. A
change of 15 words per minute is sufficient to indicate a change in
reading speed that cannot not reasonably be attributed to chance.
23,24
It was soon apparent that there are large differences in reading
speed between pupils, often those with the same scholastic attainment
in conventional tests of reading.
24 25
In individuals with similar reading
attainment, reading speed could differ by a factor of more than three.
Some of the differences in speed may be attributable to a difficulty
sometimes described as a rapid naming deficit25 (Chapter 1), and
others to visual problems (Chapter 2-5) including visual stress
(Chapters 7-8).
The Wilkins Rate of Reading Test was originally developed to
evaluate the effect of coloured filters on reading speed, and has been
widely used in research on this topic.
27 28
It has also been found to be
useful in assessing the effects of a wide range of visual interventions,
29
34
and has been transcribed into other languages.
35 36
Summary
The initial observations reported by Jansky, Meares and Irlen
2-5
have
been corroborated and extended in many subsequent studies. We now
know how to identify individuals who are adversely affected by visual
stress and how best to alleviate their difficulties using spectral filters.
These topics will be discussed in more detail in the next four chapters,
in which we will also address the controversy that continues to
surround this subject.
estimate the test-retest repeatability. The standard deviation of a given
Chapter 6 Coloured filters for visual stress– early studies
177
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