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CHAPTER 2
CLASSIFICATION OF COLOR VISION DEFECTS
_
This section describes how an individual's color vision is characterized
on the basis of color-matching performance and chromatic discrimination
capacity. The characterization of normal color vision, which occurs
in about 90 percent of men and 99 percent of women, is described first.
NORMAL COLOR VISION
Colorimetric Definition
There are many ways of producing a given hue sensation.* For example,
a ~yellow. can be produced by a monochromatic radiation {S89 nm) or by
the additive mixture of a yellow-green (545 nm) and a yellow-red (670
nary). White is produced by a continuous source containing radiations of
all visible wavelengths, such as the sun, or it may be produced by a
mixture of as few as two wavelengths, for example, 475 nm (blue) and
575 nm (yellow}. When we look at a yellow or white object, or at any
color field at all, we have no way of knowing the spectral composition
of the physical stimulus.
The Color-Matching Experiment
In a typical example of a color-matching experiment, the observer sees
a circular field formed by two semicircular half-fields (Figure 2-1~.
The half-fields contain different isolated spectral bands. In the
particular example we have chosen, the upper half-field contains a
narrow spectral band centered at 545 nm (yellow-green light) and a
*A discussion of the facts of calorimetry is beyond the scope of this
report. Information that is necessary for an understanding of the
design of color vision tests is described in the appendix. Readers
interested in a more complete discussion of calorimetry may find it in
Bouma (1972), 80yn ton (1979), Graham {1965J, LeGrand (1965), Pokorny et
al. (1979), Wright (1946, 1972), and Wyazecki and Stiles (1967~.
4
OCR for page 5
s
545 nm`,.~
/
/ M IXTURE
/ COLOR
TEST COLOR J
670 nm
~589 nm
FIGURE 2-1 View of the field as seen by the observer, who sees a
circle with a black dividing line. The top half of the field appears
red, orange, yellow, yellow-green, or green depending on the relative
amount of 670 nm and 545 nm light. The bottom half appears yellow.
narrow spectral band centered at 670 nm (yellow-red light). The lower
half-field contains a narrow spectral band centered at S89 nm (reddish
yellow light). By appropr late adjustment of the quantities of 54 5 nm
and 6 70 nm 1 ights, an observer can make the whole field appear to be
the same color. The halves of the split field contain dissimilar
spectral radiations and yet are seen as the same by the observer.
Pairs of such stimuli are known as metamers.
Normal observers can match all hues by the appropriate mixture of
three colored lights. Hence, normal observers are known as trichromats
(tristhree, chroma=color). The match usually requires that one of the
three mixture primaries, as they are called, be subtracted from rather
than added to the mixture f ield. Because light cannot be physically
subtracted, the third primary is added to the test field; the task thus
requires matching the mixture of the test color and one mixture primary
to the mixture of the remaining two primaries. Different normal
trichromats will use slightly different amounts of the primaries to
match various hues, but it is the general similarities among normal
observers rather than the comparatively small differences that allow us
to classify an observer, whose color vision we are evaluating, as either
normal or abnormal.
Some Special Matches
In order to define normal trichromacy and diagnose color defect, we
have available some special color matches that make use of only two
primaries. These matches are relatively quick and easy to perform,
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6
compared with full spectrum color matching. Instruments that allow us
to evaluate these special matches in the population are called
anomaloscopes. Historically there have been three such special matches
used to test color defect: the Rayleigh match or equation, the
Pickford-Lakowski match or equation, and the Engelking-Trendelenburg
match or equation. Of these matches, the Rayleigh match and
Pickford-Lakowski match are the most frequently used today.
Rayleigh Equation. The Rayleigh equation differentiates normal
trichromats from observers with congenital red-green color defect and
allows classif ication of these defects. The Rayleigh equation is a
special type of color match that involves matching a spectral light
near 589 nm to a mixture of spectral or nearly spectral lights near 670
nm and 545 nm, as shown in Figure 2-1. {The exact wavelengths have
differed in various instruments. Wavelenoths seven here are for the
Nagel Model 1 anomaloscope in current production). When the 670 nm and
545 nm mixture primaries are added together, normal trichromats see a
full range of hues from yellow-green, yellow, orange, to yellow-red,
depending on the proportion of 670 nm to 545 nm primary in the mixture.
The observer's task is to adjust this Proportion so that the mixture
field exactly matches the 589 nm f ield
~ ~ ~ The observer may also adjust
the luminance of the 589 nm field to achieve an exact match. A normal
trichromat can make the match quickly and reliably; there is a unique
proportion of 670 nm and 545 nm, which is matched to 589 nm.
Two statistics are taken on the primary mixture: the range and the
midpoint of the matches. The matching range includes all of the ratios
Of 670 nm and 545 nm that a given observer can match to the 589 nm.
Usually little change can be made in the mixture ratio without upsetting
the color match, and the matching range is termed narrow. The midpoint
of the matching range is the 670 to 545 nm ratio, which lies in the
center of the range.
In population studies of normal trichromats, the distribution of
match midpoints describes a bell-shaped or normal curve including only
a rather narrow group of settings. An observer who has a unique match
that falls within this range can be excluded f rom the most f requent
categor ies of color defective vision--the congenital red-green color
defects. We can, however, statistically define two subtypes of normal
trichromats whose color vision may make them unsuitable for jobs in
color-sensitive industries (Figure 2-2~. The deviant color normal
observer is the normal trichromat whose Rayleigh equation lies within
normal range but with the midpoint displaced more than + 2 standard
deviations from the mean of average observers. Deviant color normal
observers comprise 4 percent of the normal population. The weak color
normal observer is one whose Rayleigh equation midpoint is within
normal range but whose matching range is more than twice the most
frequent range (modal value) for the population. Color-weak observers
comprise 20 percent of the normal population.
The Pickford-Lakowski Equation. A special match that is available
in some anomaloscopes is
source) to a mixture of
1
match of a white light (from a tungsten
nm and 585 nm lights. The match was
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RED-GREEN COLOR EQUATI ON | CLASSI FICATI ON
. I' I I ' NORMAL TRICHROMATS
~ 1 1 1
I - ~[ ~ NORMAL
1 1 1 1
I = I I _ I RED-GREEN DEVIANT
1 1 ~ - 1
I ~ I COLOR-WEAK
. ,
1 1 1 1
I I ~ ~ ~ ~ SIMPLE ANOMALOUS TRICHROMATS
1 1 1 1
1 1 1 1
I I I I PA (PROTANOMALOUS)
1 1 1 1
I I I I _ DA (DEUTERANOMALOUS)
1 1 1 -
I lil ~ I ~
I I I I EXTREME ANOMALOUS TRICHROMATS
1 1 1 1
~ ,, I EPA (EXTREME PROTANOMALOUS)
I
1 1 1 1
EDA (EXTREME DEUTERANOMALOUS)
I III I ~
I I I I DICHROMATS
. . ! P (PROTANOPE)
.
~ ~ ~ ~ D (DEUTERANOPE)
. . ~ ~ ~ _
-3 SD , +3 SD , STATISTICAL PARAMETERS
. .
0.35 1.0 3.0 ANOMALOUS QUOTIENTS
.
FIGURE 2-2 Characteristic red-green matches made by normal trichromats
and observers with congenital red-green defects.
Adapted from Lakowski (19691.
designed to evaluate the effect of aging (Pickford, 1968) and has also
proved important in evaluating color defects acquired in eye diseases
(Lakowski, 1972~.
Engelking Trendelenburg Equation.
This equation involves the match
of 490 nm to a mixture of 470 nm and S17 nm. This match was designed
by Engelking and modified by Trendelenburg in order to evaluate con-
genital blue-yellow color defects (Engelking, 1925; Trendelenburg,
1941~. Other investigators have used different wavelengths. As for
the Rayleigh match, the normal trichromat makes a unique match. The
match may be strongly affected, however, by the inert pigments of the
eye. These inert pigments are the lens and a pigment that occurs in
the back of the eye called the macular pigment. The lens and macular
pigment absorb short-wavelength light (400-500 nary). Different indi-
1
-
r
OCR for page 8
8
viduals show great variability in the amount of light absorbed by these
pigments. One result of this variability is that the Engelking-
Trendelenburg equation shows a wide distribution of match midpoints in
the normal population, thereby decreasing the utility of the match as a
test for the abnormal color vision.
Chromatic Discriminative Ability
There is considerable variability among color-normal observers in their
ability to discriminate small differences in hue or saturation. This
fact may be demonstrated in a number of ways. In color match perform-
ances, different observers have different matching ranges. Observers
with good color discrimination tolerate little change in the mixture
ratio and have narrow matching ranges. Observers with poor color
discrimination have wide matching ranges. A clinical method of
estimating chromatic discrimination is the Farnsworth-Munsell 100-hue
test.
CONGENITAL SEX-LINKED COLOR VISION DEFECTS
Observers with congenital red-green color defects comprise about 10
percent of the U.S. male population, thus 4 percent to S percent of the
U.S. general population (Paulson, 1973~. These observers are of
concern to the armed forces and transportation industries, because
their defects may inhibit their ability to function appropriately.
Red-green defects have X-chromosome-linked recessive inheritance.
This term refers to traits carried on the sex chromosomes. The female
has two X chromosomes, one inherited from the mother and one from the
father; the male has one X chromosome from the mother and one Y
chromosome from the father. X-chromosome-linked recessive inheritance
is specific to genes occurring at a locus on the X chromosome for which
no corresponding gene occurs on the Y chromosome. Thus, in
X-chromosome-linked recessive inheritance, the male who inherits a
defective gene from his mother will always show the defect because he
has no normal gene to oppose expression of the defect. The female,
however, must inherit a defective gene from both parents to show the
defect. Females who have one defective gene are called carriers. The
affected male who gives an X chromosome to daughters and a Y chromosome
to sons will pass the defective gene to all of his daughters and to
none of his sons. Assuming that the mother has two normal X
chromosomes, the daughters will be carriers. The offspring of
marriages between color-defective males and color-defective females
will have higher incidences of color-defective males, color defective
females, and carrier females. Comprehensive reviews of the genetic
aspects of red-green defects are given by Bell (1926), Francois and
Verriest (1961), Waardenburg (1963), Kalmus (1965), Jaeger (1972), and
Franceschetti et al. (1974~.
Color matching using the Rayleigh match, allows us to differentiate
the various red-green color defects. There are two major subdivisions
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9
of color defect observers according to the severity of the defect;
anomalous trichromats and dichromats. Within each of these classes we
find two qualitatively different types of red-green defect, the protan
defects and the deutan defects. The classif ication given below is that
of Franceschetti (1928~.
Anomalous Trichromats
These observers comprise about 7 percent of the U.S. male population.
Color-Matching Classification
Anomalous trichromats, like normal trichromats, need three primaries
for color mixture to match the spectrum, but their matches differ from
those of normal trichromats. According to Franceschetti (1928) there
are four subcategories of anomalous trichromats. Each subcategory is
defined by use of the anomaloscope.
Simple Protanomalous Tr ichromats . The simple protanomalous
tr ichromat is given the designation PA. This term refers to a presumed
genetic entity. The observers who comprise 1 to 2 percent of the U.S.
male population need a higher ratio of red to green primary than normal
tr ichromats in the Rayleigh equation (see Figure 2-2~. The mixture
half-field that the protanomalous trichromat accepts as a color match
to the yellow test field would appear orange to the normal trichromat.
In addition, long-wavelength (red) spectral lights appear dim; we say
that protanomalous trichromats have a Long-wavelength luminosity loss.
Simple Deuteranomalous Trichromats (Genetic Entity DA). These
observers comprise about 4 to 5 percent or the U.s. male population;
they need a higher ratio of green to red pr imary than normal tr ichro-
mats in the Rayleigh equation (Figure 2-2 ~ . The mixture half-f ield
that the deuteranomalous trichromat accepts as a color match to the
yellow test field would appear greenish yellow to a normal trichromat.
In addition to the deviation of the match midpoint, the ranges of
the matches made by simple protanomalous and simple deuteranomalous
trichromats are often wider than for normal trichromats (see Figure
2-27. Although a given anomalous trichromat may have as narrow a
matching range as that of a normal trichromat (Hurvich, 1972; Alpern
and Moeller, 1977), it is more usual to find that the matching ranges
are two to three times wider than those of normal trichromats (Willis
and Farnsworth, 1952; Helve, 19723.
F:~rtr~m— Prc~tan~malour: lair trichromats (Genetic Entitv EPA) . This group
accepts a wide range or rea-green ratios, usually Inc~ua~ng cuo ratio
accepted by normal trichromats and perhaps one of the primaries
(usually redJ. The extreme protanomalous trichromat also shows reduced
sensitivity to long-wavelength spectral light.
t
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10
Extreme Deuteranomalous Trichromats (Genetic Entity EDA). These
observers also accept a wide range of red-green ratios, including the
normal match and perhaps one of the primaries (usually green).
Extreme protanomalous trichromats and extreme deuteranomalous
trichromats differ from the corresponding simple anomalous trichromats
in the width of the matching range (see Figure 2-2 ~ . Extreme anomalous
tr ichromats always have a wide matching range that usually includes the
normal match.
Chromatic Discriminative Ability
As indicated by the matching width, many anomalous trichromats show a
loss of color discrimination compared with normal trichromats. There
is, however, considerable variability among both protanomalous and
deuteranomalous tr ichromats in the manifestation of such discrimination
loss, with some of these observers showing no loss of color discrimina-
tion. Some anomalous trichromats may make more errors on the
Farnsworth-Munsell (FM) 100-hue test than normal trichromats. Their
errors do not occur randomly but occur for hues where their color
discrimination is poorest. The Farnsworth-Munsell 100-hue test is
discussed in detail in Chapter 3, Existing Tests.
Dichromats
Dichromats comprise 2 to 3 percent of the U.S. male population.
Color-Matching Classification
Dichromats require only two primaries to match spectral colors. They
can match all spectral colors by a suitable mixture of two primaries
located on either side of 500 nm; generally a red and a blue are used.
Furthermore, when spectral colors and small fields are used, Dichromats
can match all wavelengths above 540 nm to a single wavelength. In the
Rayleigh match they can match either the 545 nm primary, the 670 nm
primary, or any mixture of these primaries to spectral yellow (see
Figure 2-2~. On the basis of the radiance at which they set the
brightness of the yellow half-field in the Rayleigh match, Dichromats
can be differentiated into Protanopes and deuteranopes.
Protanopes {Genetic Entity P). Protanopes comprise 1 percent of
the U.S. male population and show loss of sensitivity to long
wavelengths. On the Nagel Model 1 anomaloscope, Protanopes match
spectral red to very dim levels of spectral yellow; they match spectral
green to brighter levels of spectral yellow. Protanopes confuse blues
with purples, blue-greens with red-purples, and light greens with brown.
e
Deuteranopes (Genetic Entity D). Deuteranopes, who comprise 1
percent of the U.S. male population, possess a spectral sensitivity
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11
similar to that of normal observers. On the Nagel Model 1 anomaloscope.
they match spectral red and spectral green to approximately the same
radiance of yellow. A deuteranope will confuse blues with blue-purples,
blue-greens with purples and greens with reddish purples.
Chromatic Discriminative Ability
Dichromats have virtually no wavelength discrimination above 540 nm.
They are thought to see the spectrum as shades of blues and yellows
separated by a neutral region of grays in the spectral region around
495 to 500 nm. Dichromats usually make more errors on a color
discrimination test such as the FM 100-hue test than a normal or
anomalous trichromat. Their errors occur for those hues where their
discrimination is poorest and show profiles similar to those of the
corresponding anomalous trichromats (see Chapter 3~.
AUTOSOMAL DOMINANT TRITAN DEFECT
In addition to the X-chromosomal-linked color defects, there are some
very rare hereditary color defects. The tritan defect is one of these
rare defects (minimum frequency estimated to be between 1/13,000 and
1/6S, 000 [Kalmus, 1965;~. The defect shows autosomal dominant
inheritance, a term which refers to traits carried on any but the sex
chromosomes. In dominant inheritance the defect occurs if only one
defective gene is inherited. Autosomal dominant inheritance is
characterized by a high frequency of the defect (50%) in the male and
female children of an affected parent. The severity of the defect can
be quite variable from one family member to another.
The tritan defect is characterized by a lack of function of the
mechanism that allows normal observers to discriminate colors that
differ by the amount of violet or yellow they contain. The dichromatic
state of the tritan defect is termed tritanopia. A tritanope can match
all spectral colors to a mixture of two primaries, usually located on
either side of 565 nm, and will have a wide matching range on the
Engelking-Trendelenburg and Pickford-Lakowsk~ equations. The tritanope,
however, will have a reliable match on the Rayleigh equation, and the
match will fall within the distribution of matches made by normal
trichromats unless, of course, there is a concomitant red-green defect
(Pokorny, Smith and Went, 1981~.
It has proved difficult to demonstrate a defect comparable to
anomalous trichromatism in observers with tritan defect, primarily
because of the normal interobserver variability in the Engelking-
Trendelenburg equation. There are, however, many cases of ~incomplete
tritanopia, n consistent with the typical variability observed in
autosomal dominant inheritance. Specialized tests of color matching
are required to differentiate the tritanope, incomplete tritan, and
tritanomalous observer.
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12
ACQUI RED COLOR VI S ION DEFECT S
Normal Color Vision Changes with Age
Test performance that depends on detection of small differences in
color is at its best in young adults in their early twenties. Fewer
observers above the age of 25 show excellent color discrimination.
Color discrimination loss shows a characteristic pattern: discrimina-
tion on the blue-yellow axis is more affected than discrimination on
the red-green axis. Thus, normal trichromats will show considerable
widening of their matching ranges for the Engelking-Trendelenburg and
Pickford-Lakowski matches as their age increases (Lakowski, 1958; Ohta
and Kato, 1976) whereas the matching range for the Rayleigh equation
shows little or no change with age. On the FM 100-hue test, more errors
are made by older observers, who may show blue-yellow discrimination
loss (Ohta, 1961; Lakowski, 1962; Verriest, 1963; Frill and
Schneiderman, 19 64 ~ .
The loss of discriminative ability with age is primarily but not
solely attributable to the aging process in the lens of the eye
(Lakowski, 1962~. The lens gradually becomes denser and may accumulate
screening pigments, which usually absorb short-wavelength light
strongly (the lens may appear yellowish). As a consequence less light,
especially short-wavelength light, reaches the retina.
These age effects are not trivial. The discriminative loss with
age may be important in certain job situations.
Effect of Disease, Injury, and Drugs
Most individuals with defects in color perception have hereditary color
defects that do not progress. However, color defects may occur
secondary to disease or injuries. Such defects are termed acquired
color defects. Acquired defects may be caused by disease or injury
affecting the eye, the optic nerve, or the visual cortex. Some acquired
defects result from primary hereditary retinal disorders and probably
should be termed developmental color vision defects. Cataracts, too,
may cause color defects. Drugs may cause toxic effects on the eye,
with some loss of vision and color vision.
The acquired defects usually involve discrimination loss and may
occur prior or secondary to loss in visual acuity. The common clinical
methods of testing color vision are based on tests designed to evaluate
the hereditary defects, using observers with normal visual acuity. The
assessment of acquired color defects may be complicated by low visual
acuity, presence of an undiagnosed congenital color defect, or other
concomitant problems. However, the evaluation of color vision in eye
disease can be diagnostically important and is common clinical practice
in Europe. (Discussion of the measurement and etiology of acquired
color vision defects is given in Grdtzner, 1972, and Pokorny et al.,
1979~.
It is helpful to summarize differences between acquired and
congenital color defects (see Table 4-2~. Congenital color defects
usually involve both eyes. There is usually no visual complaint or
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13
evidence of other abnormal visual function. The congenital defects are
likely to involve red-green discrimination and to show X-chromosomal-
linked inheritance. These observers may name many object colors
correctly, since they were constrained in childhood to use the termin-
ology of the color normal observer, and they tend to use whatever cues
possible to do so (Jameson and Hurvich, 1978~. In comparison, acquired
or developmental color defects may differ in severity in the two eyes
and are usually accompanied by decreased vision and other evidence of
eye disease. These defects are more likely to involve discrimination
loss on the blue-yellow axis.
PHYSICAL FACTORS AFFECTING COLOR VISION
Color vision is not static. Color appearance, color matches, and color
discrimination are affected by changes in illumination and field of
view.
Illumination
All color tests are designed to be administered at specific
illuminations. Color discrimination is best at medium or moderate
levels of illumination. At very high levels (glare), the apparent
saturation of colors is decreased. Most observers find glare sources
uncomfortable. At very low levels of illumination, discrimination
deteriorates in a characteristic manner. If the FM 100-hue test is
given at 1/lOOth of the recommended test illumination, observers may
show increased errors, primarily on a blue-yellow axis.
Field Size
The size of the field of view is also important in color vision. The
larger the field of view, the better the color discrimination. Normal
trichromats show small but systematic changes in the Rayleigh match as
field size is changed (Pokorny and Smith, 1976~. Reduction in field
size to one-quarter degree of visual angle (the size of a tip of a
matchstick at arm's length) from the 1° to 2° (the size of a nickel at
arm's length) of a typical anomaloscope will lead to decreased
discrimination. Anomalous trichromats and dichromats show considerable
improvement in discrimination as field size is enlarged. The wide
matching ranges that occur with the usual anomaloscope field become
narrower when a sufficiently large test field (8°) is used. Many
dichromats will not accept a full matching range on an anomaloscope
when the field is 8° (Smith and Pokorny, 19777.
Population statistics are all based on the use of anomaloscope
fields of 1° to 2°. Furthermore, most color screening tests depend for
their success on discrimination loss in anomalous trichromats and
dichromats. Such tests may be less effective if the observer is
allowed to increase the field of view by bringing the samples closer.
The ability to present a large field in an anomaloscope, however, may
be useful in studying acquired color defects.
Representative terms from entire chapter:
normal trichromats
