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M. Buckland and N. Fogt
com) include stereotests. Other systems that
include stereopsis testing are digital visual acuity
systems such as M&S technologies SPV (contour, https://www.mstech- eyes.com), Thomson
Test Chart (random dot stimulus, https://www.
thomson-software-solutions.com), Reichert
ClearChart 4P (https://www.reichert.com), and
the Hoya EyeGenius (https://eyegeniushoya.
com/. Many of these systems have yet to be eval-
uated for reliability and sensitivity. StereoTAB is
a random dot stereo test app for iPad/iPhone to
measure stereopsis using anaglyphic glasses
(apps.apple.com).
The ASTEROID (Accurate STEReotest On a
mobile Device) system from Newcastle
University (https://www.ncl.ac.uk/business- and-
partnerships/expert- solutions/licensing/asteroid)
is in development as a game-based stereoacuity
test with a (global) disparity range of 12–1200″
[46]. Other systems in development for measuring stereoscopic depth include the eRDS system,
that uses a global target in a two-alternative force
choice method, and the Vivid Vision Virtual
Reality system, which has a range of 15–2400″ of
local stereoacuity [47].
8.9 Summary ofClinical
Application
andInterpretation
Stereopsis testing is an important part of vision
screening in addition to visual acuity and refraction for visual conditions, including uncorrected
refractive error, strabismus, and anisometropic
amblyopia [28]. Stereoacuity testing is often performed early in the ocular examination. Having a
positive stereo response, in particular with global
stereopsis testing, suggests that visual acuity is
relatively equal (monocular asymmetry in visual
acuities affects stereopsis more than bilateral
symmetrically reduced acuities), that the vision is
reasonably clear, and that bifoveal xation exists
(at least at the time and distance of testing).
Strabismic and amblyopic patients have worse
thresholds on global stereopsis tests than on local
stereopsis tests [11]. Cooper reported that in testing individuals with a random dot stimulus, all
patients with constant strabismus (with or without amblyopia) failed, as did all people with
microtropia [33]. Some but not all patients with
anisometropic amblyopia or intermittent strabismus passed. Characteristics of microtropia can
include a strabismic deviation of less than 5°,
anomalous retinal correspondence, motor fusion,
and foveal suppression [48]. An important advantage of a random dot stereogram over a line (contour) stereogram is that patients with microtropia
will not achieve stereopsis using a global (random dot) stereopsis stimulus, but these patients
may demonstrate local stereopsis [49] (perhaps
worse than 100s of arc) [48].
strabismus amblyopia, anisometropic amblyopia,
strabismus or signicant refractive error is
detected, prompt referral to an eye care provider
for management of these conditions is required
as earlier interventions are associated with better
binocular outcomes. Implementing newly developed therapies, such as perceptual learning
approaches to improve stereopsis should also be
considered. While it has been thought that there
is a relatively short critical period for the development of stereopsis that ends at a very young
age, recent studies have demonstrated that stereopsis, in some cases, may be developed or
improved even in adulthood [14]. Additionally,
orthoptic training for vergence disorders in children and adults can reduce xation disparity,
likely leading to improved stereoscopic depth
perception [19]. The potential stereopsis that may
be attained following strabismus surgery can be
estimated using a synoptophore in which the
location of the stimuli seen by each eye is placed
at the strabismic angle of deviation [16].
other ways. Birch et al. reported that patients
with infantile esotropia who had undergone strabismus surgery but had no stereopsis after surgery were far more likely to need another surgery
than those who demonstrated stereopsis postoperatively. In addition, patients who were corrected optically for accommodative esotropia but
had no stereopsis after this correction needed surgery to a far greater extent than children who
demonstrated stereopsis after correction [50].
In children, once a binocular disorder such as
Measures of stereopsis can be important in
8 Clinical Measurement ofStereoacuity
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Stereoacuity can play a role in the management of an amblyopic patient. For example, during amblyopia treatment in which the patient’s
visual acuity has stabilized but the stereoacuity is
absent or highly decient, this could result in an
extended management period. Because of the
variability in stereotesting, it can be difcult to
determine the amplitude of changes in stereoacuity that can be considered signicant or clinically
meaningful. Adams et al. determined that a
change of approximately 2 octaves (i.e., a change
in the stereopsis threshold by a factor of 4) was
necessary to exceed the test–retest variability
(Preschool Randot, near Frisby stereotests,
Frisby-Davis distance (FD2), distance Randot
stereotests) [32].
8.10 Conclusion
The presence and quality of stereoscopic depth
perception can signicantly impact an individual’s quality of life; therefore, assessing stereopsis
and applying interventions to improve stereopsis
are important in clinical practice. Commonly
used clinical stereopsis tests vary in the method
by which the disparity is created: global/cyclopean (random dot stereogram) or local/contour,
the method by which dichoptic viewing is
achieved (e.g., anaglyphic stimuli, polarized
stimuli, lenticular technology), the number of
choices at each disparity level (e.g., twoalternative forced choice or four alternative
forced choice) and the maximum and minimum
disparities available. Thus, normative values
attained on one stereotest are unlikely to match
those of other stereotests. Clinicians should
therefore use the same stereotest for initial and
post-intervention stereopsis assessments.
In terms of screening patients for visual abnormalities, this is best performed with random dot
stimuli (global stereopsis) rather than contour
stimuli (local stereopsis). There is no single test
of stereopsis to screen for vision disorders such
as strabismus and amblyopia adequately, and
therefore screenings should include a test of
(global) stereopsis along with tests of visual acuity and refraction.
It may be possible to overcome some of the
limitations of printed stereoacuity tests with
computer-based measurements. These methods
may allow for the presentation of disparities at
more levels, more opportunities to test depth
detection at a particular disparity multiple times
(thereby improving test–retest variability), and
the ability to apply psychophysical approaches
(e.g., the staircase method) that allow for a more
accurate estimate of stereoscopic thresholds [11,
16]. Lastly, computer-based techniques may
allow for assessing thresholds for motion in
depth, a visual characteristic that is potentially
important in everyday life.
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Color Vision
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AmithavikramR.Hathibelagal
9.1 Introduction
Color vision testing is important and should be
done routinely, especially in children, to screen
for color vision deciency (CVD). Most individuals are unaware of this condition, and it is usually detected when they fail occupational-related
color vision tests, leading to adverse psychological consequences [1]. Some experience difculties in day-to-day activities such as driving,
identifying a suitable matching pair of clothes,
etc. This highlights the importance of assessing
color vision regularly and providing suitablecareer counseling to people with color vision
issues. CVD is an X-linked condition [2], and
therefore, its prevalence is higher in males
(Caucasians [3]: 8% and Indians [4]: 3%). This
chapter will focus briey on the history of the
development of color vision tests, the principles
of color vision, and their application in the development of color vision tests.
9.2 History
One of the oldest reports on congenital color
vision deciency comes from the famous English
chemist John Dalton (1766–1844) [5], who had
A. R. Hathibelagal (*)
Brien Holden Institute of Optometry and Vision
Sciences, Prof. Brien Holden Eye Research Centre,
L V Prasad Eye Institute, Hyderabad, India
abnormal color vision (deutan defect). The
development ofmoderncolor vision tests can be
traced back to a train accident (the Lagerlunda
collision in Switzerland in 1875); however, it
was not conclusively proved that it was caused
by the train operator’s abnormal red/green CVD
[6]. Nevertheless, this incident triggered the
need to assess color vision (in different professions as a part of the recruitment process) and
led to the subsequent development of color
vision tests.
9.3 Principles ofColor Vision
andIts Testing
In humans, three classes of cone photoreceptors
mediate color vision: short-wavelength sensitive
cones (S cones; peak sensitivity: 440 nm),
medium-wavelength sensitive cones (M cones;
peak sensitivity: 530nm), and long-wavelength
sensitive cones (L cones; peak sensitivity:
560nm) [7, 8].
Color has three attributes: hue, saturation, and
value. The interplay of these three factors is used
in the design of the color vision testing. The color
names we use daily are called hues (e.g., blue,
green, or red). The different shades of a given hue
are called saturation; for example, dull red (less
saturated) vs. bright red (more saturated), etc.
Individuals with CVD typically require higher
saturation levels to identify the hues and distin-
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024
T. Das, P. Satgunam (eds.), Ophthalmic Diagnostics, https://doi.org/10.1007/978-981-97-0138-4_9
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guish them correctly. The underlying principles
of color vision testing are based on color confusion axes—the colors lying on a given color confusion axis are indistinguishable for those with
CVD. The orientation of the confusion axes in
the color space varies with the type of
CVD.Noise(static or dynamic) is introducedin
colour vision tests to ensure that the discrimination ability tested isbased on chromatic contrast,
not luminance contrast.
9.4 Types ofColor Vision
Deciencies
Color vision deciency refers to a condition in
which one or more cone class(es) are missing or
their functionis altered or completely absent.The
peak spectral sensitivity of the affected cone
class is altered relative to normal trichromats
(individuals with normal color vision).
Depending on the severity and cone class
affected, CVD can be broadly categorized as
anomalous trichromacy, dichromacy, or monochromacy (Fig.9.1).
Anomalous trichromacy is a milder form of
CVD in which the peak spectral sensitivity of the
L or M cone is abnormal (corresponding toprotanomaly or deuteranomaly, respectively).
Relative to normal trichromats, there is a reduced
wavelength gapbetween the peak spectral sensitivities of L and M cones, which can adversely
affect color discrimination, mostlyin thewavelength range between 500 to 700nm. Tritanomaly
is rare, because there is a large gap between the
spectral sensitivities of either L or M cones and
the S cone spectral sensitivity.
Dichromacy is a relatively severe form than
anomalous trichromacy. In dichromats, one type
of cone is missing or lacks complete function
(protanopia—L cones missing; deuteranopia—M
cones; and tritanopia—S cones).
Monochromatism refers to a condition in
which only one photoreceptor class is functioning or present (either the rod or cone). Conditions
with L- and M-cone monochromacy are much
rarer; S -cone monochromatism(also calledBlue
cone monochromatism; BCM) is a type of cone
monochromacy where the L and M cones are
missing and functional S-cones are present.Patients with BCM tend to have normal blueyellow colour vision and absence of red-green
colour vision. Rod monochromatism is also
called achromatopsia, where only rod function is
present. Its symptoms include extremely poor
color perception (almost in shades of gray), poor
visual acuity, and photophobia.
9.5 Tests forColor Vision
Deciency
Based on the testing design and principles, the
tests for CVD can be broadly categorized as follows: (a) pseudoisochromatic tests, (b) color
arrangement tests, (c) color-matching tasks, and
(d) threshold-based digital color vision tests. The
following section will discuss each of the test categories and a fewimportant examples for each of
them.
Fig. 9.1 A owchart showing the different types of color vision deciencies based on the affected photoreceptor
classes
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9.5.1 Pseudoisochromatic Test
Design Pattern
The plates are called “pseudo” isochromatic
because they appear falsely isochromatic (The
word ‘Iso’ is derived from the Greek root isos,
which means equal/same) to an individual with
CVD.Such tests can be rapidly administered and
are mainly used for screening red-green (R-G)
defects. The pseudo isochromatic plates typically
incorporate four different test designs, namely
transformation, vanishing, hidden-digit design,
and classication plates.
9.5.1.1 Ishihara Plates
Ishihara (named after Professor Shinobu Ishihara
from Tokyo, Japan) plates are the most used color
vision screening test globally. Their popularity as
a screener is due to their simplicity and high sensitivity in detecting color vision deciency. The
Ishihara test can perform well even in challenging light environments. Arabic numerals/tracings
are used in the foreground, and the background
consists of dots of varying shape, chromaticity,
and lightness to reduce the role of luminance
(brightness changes) in detecting the target. In
each version, the Ishihara test uses all four test
designs to screen and identify the type of R-G
defects (Table 9.1). Colored dots are chosen
around the red region where the protan and deutan confusion axes are similar. Thus, isolating
protan from deutan using the Ishihara test is challenging compared to other pseudoisochromatic
tests.
Table 9.1 Different types of plates in each of the versions of the Ishihara color vision test
38-plate
Type of plate
Demonstration 1 1 1
Transformation
(Screening)
Vanishing
(Screening)
Hidden digit
(Screening)
Classication 22–25 16–17 10
Tracing 26–38 18–24 11–16
edition
2–9 2–7 2–9
10–17 8–13
18–21 14–15
24-plate
edition
16-plate
edition
The demonstration plate (Fig.9.2a) has varying lightness and hence can be seen by anyone
whose visual acuity is better than 20/200 (logMAR: 0.0), including a rod monochromat.
Transformation plates refer to the plates in which
the individual with CVD will misread the numbers as opposed to normal trichromats (Fig.9.2b).
Vanishing test design plates are those in which
individuals with CVD cannot read the numbers.
Hidden-digit plates can be read by individuals
with CVD but not by normal trichromats. The
classication plates are built so that the individual numbers are constructed from different color
confusion axes, allowing differentiation between
protan and deutans (Fig.9.2c). For example, in
the classication plate with the numeral “42” on
it, only “4” is seen by the deutans, and only “2” is
seen by the protans. This helps in categorizing
types of defects. Tracing plates are used for people who cannot read (Fig.9.2d).
The application of criterion for determining
‘pass/fail’ is important in the usage of Ishihara
plates.It is important for clinicians to note that
the number of plates read does not indicatethe
severityof colour vision loss [9]. There are three
editions of plates: the 38-, 24-, and 16-plate.
Even normal trichromats make a few errors in the
Ishihara plates. Therefore, if one keeps a strict
criterion of allowing no errors in the rst 25
plates in the 38-plate editions, then ~18% of normal trichromats also fail. Typically, 2 or fewer
errors in a 24-plate edition are considered normal. A summary of the pass/fail criteria in different versions of the charts [10] is shown in
Table9.2.
There are a few limitations to the Ishihara test.
One of the challenges is that it is easy to get a
copy, learn and memorize, and pass the test.
Using a wrong criterion of pass/fail can lead to
misdiagnosis. The ability of the Ishihara plates to
differentiate protans from deutans is limited.
Also, the tracing plates are not suitable for
children.
9.5.1.2 HRR Plates
The HRR plates are a type of pseudoisochromatic plates originally developed by Hardy,
Rand, and Rittler [11, 12] and it was named
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Fig. 9.2 Example of different designs ofplates from an Ishihara pseudoisochromatic test. (a) Demonstration plate. (b)
Transformation plate. (c) Classicationplate. (d) Tracing plate
Table 9.2 Criteria for pass and fail for different versions of the Ishihara plate
Criteria for scoring (number of errors) 38-Plate edition 24-Plate edition 16-Plate edition
Pass 4 or less 2 or fewer 2 or fewer
Fail 8 or more 6 or more 4 or more
after them. The rst edition of the HRR plates
was published in 1955 by the American Optical
Company (USA), and the recent editions (third,
1991, and fourth edition, 2002) were developed
with Neitz and Bailey, and marketed by
Richmond Products (USA) [13]. Unlike the
Ishihara plates, the background noise in HRR
plates is chosen on either side of the white
region (where the confusion axes of protans and
deutans are divergent); therefore, these plates
can distinguish protans from deutans with more
sensitivity. In addition, HRR plates can also
ab
cd
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differentiate between R-G and blue-yellow
(B-Y) defects. The rst four are demonstration
plates (Fig.9.3a, b are examples of two demo
plates). The minimum acuity required to view a
demonstration plate is 20/200. The remaining
test can be averted if the patient cannot identify
symbols in the demo plates. Three symbols (triangle, circle, and cross) are used as a target in
any given plate. The screening plate uses a vanishing design of the pseudoisochromatic plates.
The screening plates for B-Y are plates 5–6,
and R-G are fromplates 7–10. The classication (protan vs. deutan) and grading
platesforcolour vision severity(Fig. 9.3c, d)
are from 11 to 20. Individuals with color vision
deciency would require higher saturation levels to detect the targets reliably. Plates 21–25
are used for grading tritan defects. The correct
classication of protan/deutan occurs in 86% of
the cases. The HRR test works better in children because they can quickly identify shapes
than numerals.
Fig. 9.3 Example of plates from the HRR pseudoisochromatic test. (a, b) The demonstration plates. Panel (a)
contains no target. This plate gives a sense of perception
to the observer of how the plate appears when there is no
target as opposed to one in which a target is present. (b)
Two targets, the circle and the cross. Panels (c, d) show
the grading plates (the targets in panel (c) are more saturated than (d)) to categorize the severity of color vision
deciency
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9.5.1.3 General instructions for the
usage ofPseudoisochromatic
plates
The recommended test distance is 75 cm, and
each plate is shown perpendicular to the line of
sight for not more than 3seconds. The lighting
used for color vision testing should be D65,
whose color temperature is about 6500 K. The
light source of Illuminant C or similar use should
provide illumination in the order of 100–600 lux.
Yellow/tungsten light should not be used for testing as it can alter the appearance of the colors.
Care should be taken so that the plates are not
directly touched with the ngers and exposed to
sunlight. The book should be closed when not in
use.
9.5.1.4 Patient Instructions
andProcedure
Ishihara Plates
The patient is rst shown the demonstration
plate. If he/she answers correctly, then the test
proceeds further. The patient is instructed to read
the numbers on the chart, and each plate will be
displayed only for 3seconds.
HRR Plates
The patient is shown some colored symbols
and asked to answer the following questions:
(a) how many colored symbols are seen? (b) If
seen, then what symbols are present, and (c)
where are they located? Only when the participant gets the correct answer to all three questions the plate is judged as a “seen,” and the
patient is given a “pass” response; otherwise, it
is recorded as “not seen,” and the patient is
given a “fail.” The severity of HRR does not
agree signicantly with the anomaloscope classication; however, as a screening tool, it is
effective and has a high sensitivity of about
~96% in detecting CVD [13].
The other tests that use the principle of pseudoisochromatic plates are as follows:
• Dvorine test: a combination of pseudoisochro-
matic plates and naming tests
• Dalton pseudoisochromatic plates [14]: Cost-
effective tool for screening for CVD
• City University plates [15]
• CVTME (Color Vision Testing Made Easy):
for pediatric subjects [16]
9.5.2 Color Arrangement Tests
The color arrangement task involves arranging
colored caps in a sequential manner that represents gradual changes in colors from one to
another hue. In this test, individuals with CVD
will arrange colors in the palette in a sequence
that is perceptually very different from normal
trichromats. The errors in a sequence of numbers
behind the color caps indicate the magnitude of
the defect, and the pattern allows the identication of the defect type. The tests can detect both
congenital and acquired CVDs. The most common color arrangement tests are (a) the FM 100
hue test and (b) the D15 test.
9.5.2.1 FM 100 Hue Test
This test consists of 85 caps subdivided into 4
sets of boxes(22in rst box and 21in next three
boxes). In each box, the rst color is xed. The
last color of each box and the rst subsequent
color in the next box have the same chromatic
properties to allow continuation. Caps are chosen
to have the same brightness and saturation but
different hues. Caps are confused by individuals
with CVD for those colors that lie on the confusion axis.
Procedure: The participant is rst shown how
the colors are expected to be arranged. The starting cap is retained; the rest of the caps in the rst
set are removed from the box and placed loosely
in a random manner by the examiner, and the participant is instructed to arrange them in the natural sequence of changing colors, as shown
previously. The test is conducted under Macbeth
White light conditions (color temperature:
6500K). Once the given series of caps in one set
is arranged, a similar procedure is followed for
the subsequent boxes.
Scoring and analysis: The boxes are ipped
over to note the sequence of numbers on the back
of the color caps. There could be some minor
errors even for those with normal color vision.
There will be large errors in sequence in people
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