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usually change the chromaticity sufficiently to eliminate the benefit in
reading speed, according to the model described in Chapter 7 and
shown in Figure 7.5.
Some chromaticities are available only with dark lenses that have a
transmission less than 5%. These dark lenses were disregarded as
impractical, leaving the range of colours (gamut of chromaticities)
shown by the shaded area in Figure 9.1.
Figure 9.1. Gamut of tints available using conventional CR39 dyeing
techniques, including only those having photopic transmission greater than
5%. The area of the gamut is 0.08.
The number of trial tints required in any system for ophthalmic
tinting depends on the accuracy with which it is desired to approximate
the chromaticity a patient chooses as improving the clarity and/or
comfort of their vision. For example, in Figure 9.2 the optimal
arrangement of chromaticities available in lenses is shown by the
points, a different point for each lens. The chromaticity chosen by the
patient as optimal is shown by the cross. If there are many lenses, as
Vision, Reading Difficulties and Visual Stress
240
in Figure 9.2a then the closest available lens chromaticity is closer on
average, than when there are just few lenses, as in Figure 9.2b. Any
chosen chromaticity can be approximated by a few imprecise tints or
many precise tints, so what degree of accuracy is required? The
necessary accuracy has been estimated in two ways. The first is based
on the repeatability with which a given chromaticity is chosen by
symptomatic patients. The second is based upon the effect on reading
speed when the chromaticity departs from optimal.
The variability with which a patient chooses a chromaticity as
comfortable for vision provides an estimate of the range (gamut) of
chromaticities over which a patient will tolerate different colours as
beneficial. For the symptomatic patients studied by Aldrich et al.1
(those 16 patients conforming to the Delphi criteria2), the average
difference between the chromaticity chosen on one assessment and
on a second assessment administered immediately afterwards was
0.036 with a standard deviation of 0.023.1 If one were to repeat the
assessments a large number of times, it is reasonable to suppose that
the selected chromaticities would be distributed in the CIE UCS
diagram according to a bivariate normal distribution. The average
difference between two points in a bivariate normal distribution is the
square root of pi times the standard deviation of this normal
distribution. The average difference was 0.036 so the standard
deviation of the u’ and v’ coordinates can therefore be estimated to be
0.036/√π = 0.0203, with lower and upper confidence limits (estimated
from the standard deviation of the mean given earlier) of 0.016 and
0.025 respectively.
In a maximally efficient system, the available trial lenses would have
chromaticities that would tesselate (tile) the chromaticity space evenly,
at the centres of hexagons, as shown in Figure 9.2c. The greater the
precision required, the greater the number of trial lenses and the
Chapter 9 Management of visual stress
241
smaller the size of the hexagons. If it is required for there to be a trial
lens having chromaticity within 1 standard deviation of any chosen
chromaticity, then the minimum number of trial lenses required to
tesselate the gamut shown in Figure 9.1 has been estimated to be 77.1
This estimate may be suitable for most patients, but what about those
few patients whose reading speed is greatly influenced by colour?
Figure 9.2. Optimal tessellation of a chromaticity surface with (a) a large
number of trial lenses, (b) a small number; (c) hexagons tessellating a
surface.
In research described in Chapter 7, five patients who routinely used
coloured filters for reading were asked to select a colour of light
optimal for the clarity of text.
3,4
They were then asked to read randomly
ordered high frequency words aloud as quickly as they could when the
text was lit with each of a succession of randomly chosen colours. On
average the reading speed decreased as the chromaticity of the light
departed from the chromaticity chosen as optimal, and it did so
according to the functions shown in Figure 7.4. As can be seen, the
beneficial effect of colour in increasing the speed of reading is reduced
by about half when colour departs from optimal by 0.03 (when the
CIELUV colour difference is 39; for discussion see Appendix to
Reference 3). As noted in Chapter 7, it seems that some individuals
require a more precise colour than do some others.
A general tinting system has to be able to accommodate the
Vision, Reading Difficulties and Visual Stress
242
individuals who require precision, even if many individuals do not. In
Chapter 9 Management of visual stress
243
practice, it would be difficult to manufacture a set of tinted trial lenses
with chromaticites so exact as to be positioned at centres of regularly
arranged hexagons. Instead, it is simpler to have a modest number of
trial lenses but to ensure that they may be efficiently combined to give
different tints. In the Intuitive Colorimeter system, the number of
combinations of trial lenses is more than 6000 and they differ in
chromaticity from one another by no more than 0.02. The functions
shown in Figure 7.4 suggest that the effect of a chromaticity difference
of 0.02 on reading speed is small. (A chromaticity difference of 0.02
corresponds to a CIELUV difference of 26; see Appendix to Reference
3).
The above estimates apply to lenses. When lenses are worn, the
entire scene is coloured and the eyes can adapt to the difference in
colour and the lower light levels. Different considerations apply in the
case of overlays because the eyes remain partially adapted to the white
lighting and the light level and colour of both the overlay and its
surround.
The nature, number and size of overlays
There are now a large number of overlay systems on the market,
including the 10 Intuitive Overlays, the 12 Cerium Overlays, the 10
Crossbow overlays, the 8 from Phoetya and the 15 reading rulers from
Studyteq. The Intuitive Overlays can be combined in pairs, as described
in Chapter 6 to offer a choice of 30 different shades of colour, see
Figure 6.13. The Cerium Overlays and the Crossbow Overlays can
similarly be combined and offer 32 and 30 shades respectively,
although some are then rather dark. The only system of overlays to
have a substantially greater number of overlays are those from the
National Reading Styles Institute (NRSI), which are not designed to be
used in combination.
Figure 9.3. Four commercially available systems of overlays. The position of
each point indicates the chromaticity of the overlay in the CIE UCS diagram.
The black points represent the single overlays and the white points
combinations of two overlays, one upon another.
In Figure 9.3 the chromaticities of overlays in four systems are
shown. The points are surrounded by circles, the colour of which
indicates the colour of the overlay. The radius of the circles is
equivalent to a difference in chromaticity of 0.02 (CIELUV colour
difference of 26). When the chromaticity difference was greater than
Vision, Reading Difficulties and Visual Stress
244
0.02 the reading speed was less than 80% of optimum, according to the
model offered in Chapter 7, Figure 7.5. However, this model was
developed for coloured light not coloured surfaces, which may limit its
applicability (see next section). In Figure 9.3, uncoloured areas lying
outside the circles represent chromaticities for which there is a
distance in chromaticity to the nearest overlay greater than 0.02. The
greater the uncoloured area, the greater the chances, in principle, that
an individual who requires a particular colour is unable to be helped
by any overlay in the overlay system because none of the overlays is
near enough to the optimal chromaticity. The light and dark grey
shaded areas in Figure 9.3 indicate chromaticities that are available
only with filters that provide a reflectance less than 20% and 10%
respectively, and are therefore too dark to be generally acceptable.
Note that the chromaticities shown in Figure 9.3 are those under
light with uniform energy throughout the visible spectrum. As can be
seen from Figure 9.4, the chromaticities cluster similarly but over a
different region of the diagram under daylight (CIE D65), fluorescent
lighting (CIE Type F3) and incandescent lighting (CIE Type A).
Figure 9.4. The chromaticities of light reflected through the Intuitive
Overlays under three types of lighting: left panel, daylight (CIE Type D65),
centre panel ‘white’ (halophosphate) fluorescent lighting (CIE Type F3) and
right panel, incandescent light from a filament lamp (CIE Type A).
Chapter 9 Management of visual stress
245
is lit with lighting similar to that normally experienced by the patient.
In classrooms and offices this is usually “white” fluorescent light
(Correlated Colour Temperature ~4000K) with a light level of at least
300lux. Incandescent sources available in consulting rooms are not a
suitable light source for use in an overlay assessment. Some schools
and offices are now lit with LED lighting, with a colour temperature
typically about 4000K.
As mentioned in the previous section, electric lighting from gas
discharge lamps can have a very uneven spectral power distribution,
and it is therefore preferable if the spectral reflectance of overlays
varies smoothly with wavelength. The various overlays on the market
differ little in this respect, see Figure 9.5.
It is therefore important to select an overlay colour when the room
Vision, Reading Difficulties and Visual Stress
246
Figure 9.5 Spectral reflectance of overlays in three systems.
Because the change in the chromaticity of an overlay with
illumination can be greater than the change in chromaticity from one
Chapter 9 Management of visual stress
247
overlay to another, there are likely to be diminishing returns from
increasing the number of overlays or combinations of overlays. The 30
or so chromaticities available with the overlays shown in Figure 9.5
appear to provide an adequate compromise between cost and
availability, although this issue has not been the subject of formal
study. (Note that the above limitations apply to a lesser extent with
lenses, where it is possible to adjust the colour precisely. Departures
from a personal optimum due to changes in lighting generally have
only a small effect. Purple tints are an exception because they transmit
light at both ends of the visible spectrum.)
Waldie and Wilkins5 compared the increase in reading speed using
three sizes of overlay. All sizes were of the same chosen colour, but one
was just sufficient to cover the text being read, but not the white
margins that surrounded it, one covered the entire page including the
text margins, and one covered the text whilst the surround was
covered by an overlay of complementary colour. All three conditions
were associated with an equivalent increase in reading speed,
suggesting that the overlay needs only to be of a size sufficient to cover
the text being read. Overlays smaller than this are available as “reading
rulers” from Crossbow Education, but these require the reader to move
them down the page as they read, which can sometimes disrupt the
acquisition of information from the page by those readers who are
relatively fluent.
The original five Crossbow reading rulers were shown to have no
effect in increasing reading speed by Smith and Wilkins.6 Intuitive
Overlays cut to similar size and used as reading rulers did increase
reading speed, however. The difference between Crossbow reading
rulers and Intuitive Overlays was attributed to the limited range of
colours available with the original five Crossbow reading rulers. This
inference was supported by unpublished work in which children were
Vision, Reading Difficulties and Visual Stress
248
offered a subset of the Intuitive Overlays and their reading speed
increment was then less than with the full set. Overlay systems with
few shades of colours may be inadequate, but the number of shades
necessary depends on a compromise between cost, testing time,
availability, and the variability due to lighting, as mentioned above.
Crossbow have now increased their range of colours, and the figures
in this section pertain to their most recent range.
A coloured overlay or coloured paper provides one surface colour
among many. The eyes will tend to remain adapted to white(ish) light
and to the mix of visible surface colours. It is from this mix that the
colour of any illumination is derived (by neural computation that is
poorly understood) so that the colour appearance of surfaces remains
unchanged under different lighting. With coloured paper the issues are
the same as with a coloured overlay, except that an adequate choice of
colour is usually less readily available, and the precision with which
such choice can be made seems to be critical for the increase in reading
speed in some individuals.
Are the chosen overlays and lenses the same colour?
When spectacle lenses are worn, most of the visual field is coloured
(depending on the lens size). The eyes adapt to the colour in a manner
similar (but not identical) to the adaptation that occurs when the light
source is itself coloured. The conditions of adaptation are different in
the case of an overlay, which provides one surface colour among many
visible to the user, all illuminated with white light. The difference
between overlays and lenses as regards the conditions of adaptation
may be one possible reason why there is only a weak association
between the colour chosen as optimal for use as an overlay and the
colour chosen as optimal in a lens, see Figure 9.6.
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249