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At low light levels the cones are inactive. The rods are active and
there is little appreciation of colour. At higher light levels colour is seen
because the L- M- and S-cones absorb different quantities of light. Each
of the cone types has a broad sensitivity curve, indicating a sensitivity
to radiation over a wide range of different wavelengths. Light sources
(e.g., lamps or bulbs) emit radiation differently at various wavelengths
and this is represented on a graph as the spectral power distribution
(Figure 6.3). The broad spectral sensitivity of cones means that
different light sources that have different spectral power distributions
can appear the same colour. For example, the two light sources with
the very different spectral power distributions in Figure 6.3 will appear
the same colour because they simulate the L, M and S cones similarly.
The light sources are said to be metameric.
Chapter 6 Coloured filters for visual stress– early studies
159
Figure 6.3 Spectral power from two sources of white light, one daylight
(CIE D50), top, and the other a multi-band fluorescent lamp (CIE F10),
bottom. The light sources have similar illuminance and colour
appearance but very different spectral power distributions.
Figure 6.3 makes it clear that there is a many-to-one mapping from the
spectral energy of a light source to its colour (more technically its
chromaticity). There are other more subtle influences on perceived
colour that have to do with the appearance of directional light, but by
far the greatest contribution to perceived colour is the relative energy
captured by the three classes of cone, which is directly related to
chromaticity. (This is a simplification because there are other
photoreceptors in the retina that may have a part to play in
photophobia, see Chapters 8 and 11.)
Colour can be represented in a diagram standardised by the CIE
(Commission Internationale de l'Eclairage). In this uniform chromaticity
scale (UCS) diagram shown in Figure 6.4, all the colours have the same
luminance. It is not possible to represent the UCS diagram perfectly on
a page of a book because the saturation of colour is stronger than can
be shown with printing pigments, and because the more saturated the
printing pigment the darker it is. Any point in this diagram represents
a colour that derives from a given ratio of energy captured by the three
cones. The brightness is not represented (it forms a third dimension
coming out of the page).
The CIE UCS diagram in Figure 6.4 is usually plotted in Cartesian
coordinates u’ and v’. However, it can also be thought of as a polar plot
in which the hue can be represented by the angle at which the point
occurs relative to the white and the strength or saturation of a colour
is then represented by its distance from white. White is near the centre
Vision, Reading Difficulties and Visual Stress
160
(a white in which all visible wavelengths have equal energy has
coordinates u’=0.211, v’=0.474). At the edge of the plot are the spectral
colours from monochromatic light, the strongest colours that can be
generated. (The colours of the rainbow are spectral colours but they
do not appear saturated because they are mixed with white light from
the sky.) When an object (technically, a black body) is heated to high
temperatures, the colours that are produced are described in terms of
the Planckian locus, illustrated by the curved black line in Figure 6.4.
Most conventional sources of light, both natural and artificial lie close
to this line.
Figure 6.4. CIE 1976 Unform Chromaticity Scale (UCS) diagram, showing the
spectrum locus (400-700nm) around the edge, and the colours obtained
when a black body is heated. These lie on the Planckian locus shown by the
curved solid line. Most conventional sources of light, both natural and
artificial, lie close to the Planckian locus. Adapted from Wikipaedia.11
Chapter 6 Coloured filters for visual stress– early studies
161
The Intuitive Colorimeter system
In the 1970s, Wilkins investigated the striped patterns that can over-
stimulate the visual system in susceptible individuals triggering
epileptic seizures.
12,13
By the early 1980s, this work had linked such
pattern sensitivity with the visual discomfort that many people
experienced when reading.
14,15
You are now viewing striped patterns:
those formed by the lines of text, and finer (higher spatial frequency)
patterns formed by the vertical strokes of letters within the words
(prominent in the word minimum).
The research began with an exploration of the physiological
mechanisms whereby visual stimuli can provoke seizures in patients
with photosensitive epilepsy.16 Using an electroencephalograph (EEG)
Wilkins showed that photosensitive patients were often sensitive not
only to flickering light but also to geometric visual patterns with specific
characteristics. In most healthy observers the patterns evoked curious
visual phenomena: illusions of colour, shape, and motion, see Figure
6.5. He showed that individuals with migraine were unusually
susceptible to these phenomena. There were several links between the
illusions people reported and the headaches they had. For example,
people who reported frequent headaches tended to report more
illusions; the illusions were more pronounced in the 24 hours before
the start of a headache, and if the pain was on one side of the head the
illusions tended to occur on one side of the pattern when the observer
looked at its centre.15
The patterns that provoked most illusions were striped patterns
that could trigger seizures in patients with photosensitive epilepsy.
Other patterns, that did not have the specific characteristics, did not
induce so many illusions and did not show the links with headaches.15
Further experimentation revealed that if the characteristics of the
patterns were changed, so too was the likelihood of illusions. Indeed,
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162
the number of illusions varied with the characteristics of the pattern in
exactly the same way as the likelihood of seizures. Particular patterns
of stripes were the worst, the sort that op-artists such as Bridget Riley
employ to produce strong visual effects. Figure 6.5 shows an example
of a grating pattern from an escalator stair tread that can give rise to
perceptual instability of the kind seen in some op art.
Figure 6.5. Escalator stair tread.
Horizontal lines of text resemble a striped pattern, more evident if
the page is blurred. When asked to report the distortions seen in a page
of text when looking at a letter in the centre people reported
shimmering of the lines and colours, just as when observing a pattern
of stripes.17 Figure 6.6 shows the record sheet from an observer who
reported seeing the rhomboid lattice, commonly seen in patterns of
stripes.
Chapter 6 Coloured filters for visual stress– early studies
163
Figure 6.6. An observer’s report of a coloured rhomboid lattice seen when
a page of text was fixated. (a) closely spaced text (b) widely spaced text.
Rhomboid lattices are often reported in high contrast gratings and their
size depends on the grating spatial frequency. Here the rhomboid was
reported as yellow in (a) and green in (b). Redrawn from Wilkins and
Nimmo-Smith.17
When the ‘‘stripes’’ produced by lines of text above and below those
being read were covered by a mask, see Figure 6.7, the clarity of text
improved, particularly for people who were susceptible to illusions in
patterns of stripes.14 The mask, known as a typoscope, also reduced
the abnormal EEG activity when patients with photosensitive epilepsy
were reading.18
There were early reports of coloured glasses, usually blue, being
effective in reducing photosensitive seizures, so when Irlen’s findings
were publicised in the British press, Wilkins thought there might be a
connection. He constructed an instrument that illuminated text with
coloured light to study the patients who reported benefit from
coloured filters. At first, he mixed the light from three lamps (red,
green, and blue) but patients found difficulty adjusting the mixture to
produce the colour that they found to be most helpful. This way of
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164
mixing colours is not very intuitive: for example, red and green light
add together to give yellow. He invented an alternative instrument that
permitted the separate manipulation of the intuitive dimensions of
colour: hue, saturation (strength of colour) and brightness. The
instrument subsequently became known as the Intuitive Colorimeter.19
The horizontal lines on which the words lie when
printed in a paragraph of text resemble a pattern of
stripes. Patterns of stripes such as the pattern in
Figure 6.5 can induce illusions of motion, and
sometimes of colour and shape. Similar illusions are
sometimes reported in text. If you compare the clarity
of this line with that of the identical line in the passage
below you can assess the effect of reducing the
pattern of stripes by masking the lines you are not
currently reading. In this example the lines are simply
reduced in brightness and contrast. Masks of this kind
are known as typoscopes, and they reduce eye-strain
and seizures from reading.
The horizontal lines on which the words lie when
printed in a paragraph of text resemble a pattern of
stripes. Patterns of stripes such as the pattern in
Figure 6.5 can induce illusions of motion, and
sometimes of colour and shape. Similar illusions are
sometimes reported in text. If you compare the clarity
of this line with that of the identical line in the passage
above you can assess the effect of reducing the pattern
of stripes by masking the lines you are not currently
reading. In this example the lines are simply reduced
in brightness and contrast. Masks of this kind are
known as typoscopes, and they reduce eye-strain and
seizures from reading.
Figure 6.7. A typoscope that masks the lines above and below those being
read. Some people, particularly those who report illusions in patterns of
stripes, find the text in the gap between the grey masks clearer to see than
the identical passage above.
Chapter 6 Coloured filters for visual stress– early studies
165
The Intuitive Colorimeter has several advantages for assessing the
subjective effects of colour: (1) hue, saturation, and brightness (more
strictly it is luminance, not brightness) can be varied separately and in
an obvious way; (2) the variation is continuous rather than discrete; (3)
the perceptual effects of colour can be studied while the patient’s eyes
are colour-adapted; (4) the assessment is quick and efficient, and (5) no
coloured surfaces are visible, which is important because the contrast
of coloured surfaces is affected by the colour of the illuminating light.
Colour adaptation, or chromatic adaptation, is one of the ways in
which the visual system adjusts to changes in illumination to help
preserve the appearance of object colours, so-called colour constancy.
Colour constancy refers to the stable appearance of object colours
despite the wide variation of light which might be incident upon and
reflected from an object and observed by our eyes. This effect is
experienced when entering a tent with coloured canvas or a room with
coloured lights: initially, colours seem unusual but within seconds, they
appear relatively normal. When looking through coloured glasses, such
colour adaptation will occur, but when looking at text under a coloured
plastic overlay only limited colour adaptation will take place because
colours adjacent to the overlay will be unaffected by the colour of the
sheet.
The various models of the Intuitive Colorimeter have evolved to
reflect the lighting predominant at the time. In the 1990s the lighting
typical in schools and workplaces was fluorescent. Therefore, in the
Mark 2 and Mark 3 Intuitive Colorimeters, a beam of white light from a
fluorescent lamp passed through a cylindrical filter assembly shown in
three-quarter view in Figure 6.8, and as a diagram in cross section in
Figure 6.9). Whereas the Mark 2 Intuitive Colorimeter used a
halophosphate fluorescent lamp (correlated colour temperature
3500K), the Mark 3 Intuitive Colorimeter used a more up-to-date multi-
Vision, Reading Difficulties and Visual Stress
166
phosphor fluorescent lamp (4000K). Once warmed up, the spectral
output of fluorescent lamps was relatively constant. “White”
fluorescent lamps had a “compromise” chromaticity mid-way between
the yellow of incandescent lamps and the blue of daylight.
Figure 6.8. Principle of the Mark 2 and Mark 3 versions of the Intuitive
Colorimeter. Light from a fluorescent lamp passes through a cylinder on
which are coloured and grey filters. The light passes into a viewing
chamber via a square aperture where it is mixed by multiple reflection
from the surfaces of the chamber. Rotation of the cylinder changes the
hue and translation of the cylinder along its axle changes the saturation.
Figure 6.9. Cross section of the Intuitive Colorimeter Mark 2 and Mark 3.
Chapter 6 Coloured filters for visual stress– early studies
167
Fluorescent lamps have been rapidly superseded by light emitting
diodes (LEDs) and the latest version of the Intuitive Colorimeter uses
LEDs, with seven coloured filters as in previous models, Figure 6.10.
The colour is varied by changing the brightness of the LEDs and the
number lit.
Because the source of light in the Intuitive Colorimeter has
resembled that typical in the workplace, the power at each wavelength
(the spectral power distribution) has been similar to that typically
experienced when coloured lenses are worn.
Figure 6.10. The Intuitive Colorimeter. Courtesy of Cerium Visual
Technologies.
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