|
||||||||||||||||
|
||||||||||||||||
|
||||||||||||||||
|
| ||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 97
APPENDIX
UNDERSTANDING TEST DESIGN
In order to understand how a color test is designed and why it works,
it is helpful to understand how normal color vision is represented by
color mixture. Normal color mixture data allow specification of color
surfaces according to an important visual property: the condition under
which two color samples will appear identical to a normal observer.
COLOR MATCH TNG
As described in Chapter 2, in a color-matching experiment an arbitrary
color is matched in visual appearance to a mixture of primary colors.
When identical in visual appearance, the two color fields that have
dissimilar spectral distribution are called metamers.
A fundamental property of normal human color vision is that it is
possible to find a metamer for any spectral hue by variation of only
three primary colors. The terms trichromat (a three-color mixer) and
trichromacy (the property of being a three-color mixer) come from this
property of normal vision.
For spectral lights, the primaries and the spectral light are
,
arranged In pairs so that the spectral light and one primary match the
remaining two primaries. Thus, except at the primaries themselves, the
appearance of the mixture fields will not be like the spectral hue or
any of the primaries. The importance of the experiment relates not to
the appearance of the hue but to the equivalence of hue.
One way of presenting the results of color mixture experiments is
in a chromaticity diagram. A diagram called the x,y chromaticity
diagram was devised by the Commission Internationale of Eclairage (CIE)
based on the average color matches of many color-normal observers.
Figure A-1 shows the x,y chromaticity chart for the ~standard. 1931 CIE
observer. An isosceles triangle completely encloses the experimentally
determined chromaticity diagram. The spectral wavelengths are
represented around the perimeter of the chromaticity diagram, which is
called the spectrum locus, and equal energy Awaited occurs in the
center. Since the chromaticity diagram represents only color matches
and not hues, the hue names added to the chart in Figure A-1 are
provided for the convenience of those who are not familiar with the
appearance of different wavelength regions of the spectrum. Saturation
97
t
OCR for page 98
y
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
02
0.1
o
CYA N ~
500
1 520`
GREEN ~ ~~S30
510 '/ ~0 YELLOW-GREEN
4 basso
~60
Y E LLOW
ED
RIO
49d ~
W/
' ~
,80 ~
z 1 470~.5
_ _ _
VIOLET 400
1 ~ ~ ~ 1
~ · 1 ~ ·
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
~~ '
X
FIGURE A-1 The CIE (x,y) chromaticity diagram.
Source: Pokorny et al. (1979, by permission.
is not specified by the chromaticity chart, but in general it can be
said that saturation along any line from the center to the spectrum
locus increases with the distance from the center. The line connecting
the coordinates for 380 nm and 700 nm is identified as the line of
nonspectral purples. All real colors may be represented within the
boundaries formed by the spectrum focus and the purples. Mixtures of
any two chromaticities can be represented by ~ straight line joining
the pair of mixture lights on the diagram. Each mixture is a point on
the line specified by the relative amounts of the two components of the
mixture.
OCR for page 99
99
REPRESENTATION OF DEFECTIVE COLOR VISION
IN THE CHROMATICITY DIAGRAM
The dichromat is an observer who requires only two primaries for
spectral color matching. For example, the dichromat can match any
spectral color to a mixture of a blue primary (e.g., 450 nm) and a red
primary (650 nary).
The color-matching data of protanopes and deuteranopes can be
plotted on the normal chromaticity diagram. The procedure, however, is
theoretically correct only if dichromacy is a reduced form of normal
trichromacy; that is, if the dichromat is simply lacking one of the
normal discriminative mechanisms. For dichromats, some mixture lines
on the x,y chromaticity diagram represent a series of colors that cannot
be discriminated from one another (so-called isochromatic or confusion
lines). Sets of such isochromatic lines are shown in Figure A-2: the
upper panel gives isochromatic lines of protanopes; the lower panel
gives those of deuteranopes. By associating the coordinates in the x,y
chromaticity diagram with their usual series of color appearances to
normal trichromats, we can describe approximately which colors are
confused by the two types of dichromats. For both protanopes and
deuteranopes, one isochromatic line lies on the spectrum locus from 540
nm to 700 nm; protanopes and deuteranopes are said to confuse spectral
yellow-greens, yellows, oranges, and reds. From other data we know
that, as a general rule, protanopes confuse reds with dark browns; pale
blues with purples and magentas; blue-greens, whites, and reds; light
greens with light browns (fawn). Deuteranopes confuse red, orange, and
light browns; blues, violets, and blue-purples; blue-greens, whites,
and purples; light greens, magentas, and purple-reds.
One confusion line passes through equal energy white. It indicates
precisely which chromaticities and in particular which monochromatic
light (neutral point) can be completely matched with the equal energy
white. Chromaticities represented above this line are said to appear
"yellow" with increasing saturation; those below the line are ~blue, n
also with increasing saturation. Accordingly, the visible spectrum is
said to appear as shades of yellows and blues to observers with
protanopia or deuteranopia.
A word of caution is in order. The CIE standard observer represents
a person with average visual photopigments, lens, and macular pigment
absorptions. The isochromatic lines are thus similarly indicative of
those expected for an average group of dichromats whose ocular media
have characteristics similar to those of normal trichromats. For a
single dichromatic observer, the "isochromatic" contours may look quite
different from those of Figure A-2, just as a single normal observer
will make color matches that differ from the group average. Neverthe-
less the isochromatic lines are useful in the design and evaluation of
screening tests for color defects and in indicating what kinds of colors
will be confused by protanopes, deuteranopes, and tritanopes. Protano-
malous and deuteranomalous trichromats are said to make color confusions
that are qualitatively similar to, although of less severity than, the
corresponding dichromat (Farnsworth, 1943~. This similarity forms the
basis for many tests for color defect. Farnsworth also introduced the
-
t
OCR for page 100
oB
0.6
0.2
100
09 1 1 1 1 1 [--- 1 ~ I
520
~,`30
5'0 ~ \~540
0.7 _
05 500 it\
Y 0.4 _
0.3 _
_ _
1 _
0.1
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
\
O _
\ \\~570
4------~20
480\~
470 an/
460 ~ _
L , 40O, , , ~ , , ,
O 01 02 0.3 0.4 0.5 06 0.7 08
X
:
O _
-0.1
-0.2
-0.3
-0.4
boo ~ i
. .
O 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 O.9 1.0 1.1 1.2 1.3 1.4
X
FIGURE A-2 Confusion lines for protanopes (upper panel) and
deuteranopes (lower panel).
Source: Pokorny et al. {1979), by permission.
1
e
OCR for page 101
101
term protan to characterize protanopes and protanomals and the term
deutan to characterize deuteranopes and deuteranomals. The data of
tritanopes may also be plotted on the CIE (1931) x,y chromaticity
diagram; their isochromatic lines converge on the "blue" corner of the
diagram.
THEORY OF TEST CONSTRUCTI ON
Pseudoisochromatic Plate Tests
Most pseudoisochromatic plate tests are constructed empirically. The
colors are selected on a trial-and-error basis, and only those plates
shown to have high diagnostic efficiency are retained. The trial-and-
error procedure is necessary because the surface mode of presentation
complicates plate design. Factors such as form, size, glossiness,
texture, and glitter will affect the readability of the plates.
Stilling (1873) designed the first plates, using great ingenuity and
finesse. Applying information obtained from two color-defective
assistants (one red-green blind, the other blue-yellow blind) and
following Hering's color theory (see Hering, 1964) of opposing pairs,
he succeeded in constructing a series of plates in which figures
composed of one set of variegated color dots appeared on a background
of corresponding confusion colors. For instance, he used red-orange
dots on a dull yellow background and yellow-green dots on an orange-
brown background to detect red-green deficiencies, and pale blue dots
on a pale yellow-green background to detect yellow-blue deficiencies.
Since Stilling's time, various types of plate tests have been
constructed. Although some tests are better than others, all make use
of four basic plate designs. The first, a vanishing test plate, is the
simplest and most frequently used. The colors of the figure and
background are confused by certain types of dichromats (i.e., the
colors will fall on a given confusion line). A defective observer
fails to read the figures that are clearly discerned by normal
observers. The figure is said to have ~vanished. for the defective
observer. In modern tests (such as the Tokyo Medical College test),
the color distance between the colors of f igure and background is
varied; these tests are designed both to screen and to quantify the
defect. An observer cannot recognize any of the plates, no matter how
great the color distance between the figure and background, is assumed
to have a severe color defect. If the plates with the greatest
distance between figure and background can be read but those with
intermediate and small color difference cannot be read, the defect
may be designated as medium; and if only the plates with the smallest
color difference are confused, the defect is mild. This principle is
illustrated in Figure A-3 with reference to the screening and quanti-
tative red-green plates of the Tokyo Medical College. However, the
ability of a given color-defective observer to read a set of plates
depends not only on the observer's chromatic discriminative ability but
also on how appropriate the selected confusion colors are for that
observer.
1
r
OCR for page 102
102
be\
4
y
-
3
~0
FIGURE A-3 Construction of "vanishing types plate as applied to the
Tokyo Medical College Test.
Based on data from Lakowski (1969
~ .
The second type of plate design is the qualitatively diagnostic
type. It is an extension of the vanishing type, except that two
clusters of colored dots are used to print two digits or symbols against
a common background. For a red-green plate, one of the digits or
symbols should be visible to the deuteranope and the other visible to
the protanope. The inclusion of these plates theoretically allows the
examiner to distinguish the different types of dichromats, but in
practice this idea is not well realized. Figure A-4 illustrates this
design with respect to plate number 13 of the diagnostic series in the
AD ERR test.
The third type of plate design is the transformation plate. This
is perhaps the most interesting and cleverly designed of the pseudoiso-
chromatic plates. Both normal and defective observers can see a figure
in the plate, but each identifies a different one. For example, on
plate 5 of the Ishihara test (5th edition), the normal observer sees a
.5" composed of yellow-green and light green dots on a background of
light and dark orange and pink dots, whereas the red-green dichromat
reports seeing a rather neutral .2. on a ~warm. background of colored
OCR for page 103
103
.4
.3
\
Figt:
Wit
\ .C
\
.2
.3
FIGURE A-4 Design of quantitatively diagnostic plate from AD H-R-R
test (plate 13~.
Source: Lakowski internal communication, Visual Lab., U.B.C., (1976~.
dots. The design is accomplished by the strategic placement of colored
dots that cluster in four locations on the chromaticity diagram, as
shown in Figure A-S. For a normal observer, whose reference point for
color is the position of Illuminant C, two of the clusters constitute
the green figure and two constitute the orange background. The con-
fusion lines for red-green dichromats, however, indicate that their
reference point for color appears to be in the red-purple areas of the
chromaticity diagram. The positions of the four clusters relative to
the confusion lines show that half of the normal figure and half of the
normal back- ground become the alternative ~figure,. and the other half
of the figure and background'become the ~background. for the dichromat.
The fourth type of plate design is the hidden-digit plate. Hidden-
digit plates are designed so that dichromats, but not normal observers,
can see the intended figure. In the previous three types of plates,
colors for figure and background are separated by large color differ-
ences. This is not so in the hidden-digit plate, in which the use of
three different colors and small variations in saturation prevent
normal observers from seeing a figure but allow observers with
-
OCR for page 104
-
y
Greer,/
Normal/ \<
rigure
.4 .
.3
\
Normal Or)
_>kground
~' ~ \ .~
/ ~~` Background
//~~~ Prowar
figure
FIGURE A-S Analysis of Transformation type. of plate from Ishihara,
5th edition (plate 5 ~ .
Based on data from Lakowski (1969~.
red-green color defects to do so. The latter perceive two color
groupings that are distinct enough from each other to follow two
separate i~ochromatic lines: the more saturated orange, khaki, and
yellow-green dots form the background, and the less saturated pinks,
grays, and greens form the figure. Figure A-6 shows the loci for
colors used in plates ,0 and 11 of the Ishihara plates (Sth edition).
The ability to read the hidden-digit plates (itself an error} depends
on the degree of red-green defect; those whose defects are more severe
read these plates more frequently. The ability of normal observers to
read them seems to be a function of age. About half of the normal
subjects between 20 and 30 years of age read hidden-digit plater
easily, but these plates are hardly ever read by subjects over SO or by
young children.
f
t
OCR for page 105
es ~
A
y
3
Yellow Green
\ /
Khaki
r art ' \
/ \
/~
In figure \
FIGURE A-6 Colorimetric data for ~hidden-digit. plates from Ishihara,
5th edition (plates 10-11~.
Based on data from Lakowski (1969~.
Arrangement Tests
Most modern arrangement tests use Munsell colors, the chromaticities of
which can be displayed in the CIE chromaticity diagram. From this
display we can predict the expected behavior of observer's with
congenital color defects.
The chromaticities of the Farnsworth Panel D-15 are shown in Figure
A-7 together with confusion lines of the deuteranope and protanope.
Since the confusion lines connect pairs of cape that are identical or
closely similar for a given dichromat, the expectation in that the
protanope or deuteranope will make a characteristic arrangement of
caps, connecting caps that oppose each other in the color circle but
that lie on the appropriate confusion line. For protanopes a possible
arrangement is:
P 15, 1, 14, 2, 13, 3, 12, 4, 11, 5, 10, 6, 9, 7, 8.
OCR for page 106
106
0.8
0.6
0.4
0.2
o
me
0.6
0~4
0
a
I 1 1 ~ --1 ' -1
lo_
' . \
_ \
4 - 4
_
\ 1...~
_ \ P 15 ~
. ~ 1 1. 1. "1
0 0.2 0.4
-
-
-
1. 1 1 1
0.6 0.8
-\ ~
_
1 1 . 1 ~ 1 1 1 ~ 1
0 0.2 0.4 0.6 0.8
X
FIGURE A-7 Chromaticity coordinates of caps in the Farnsworth Panel
r)-15 Test. Confusion lines are indicated by dashed lines for protanopes
{upper panel) and deuteranopes (lower panel). An expected cap
arrangement is indicated by solid 1inese
Figure prepared by Pokorny and Smith for this report.
OCR for page 107
107
For deuteranopes a possible arrangement is:
P. 1, 15, 2, 14, 3, 13, 4, 5, 13, 11, 6, 10, 7, 9, 8.
These major axes are indicated on the score sheet together with the
axis for a tritanope. A fourth axis, that for the achromat, has also
been defined (Sloan, 1954). A typical arrangement for an achromat is:
P' 1, 2, 3, 4, 5, 15, 16, 14J 13' 7, 12' 11' 8, 10, 9.
A similar analysis may be applied to the two arrangement tests designed
by Lanthony.
Figure A-8 shows the position of the FM 100-hue test caps in the
CIE diagram, together with confusion lines of protanopes and
deuteranopes in the FM 100-hue test. For administration of the test,
the boxes are presented one at a time, so that color confusions across
the color circle are not allowed. The errors occur for locations where
a confusion line is tangent to the color circle of the FM 100-hue
test. The bipolar error axis that occurs on the FM 100-hue test is
therefore orthogonal to the confusion axis. For example, the major
confusion axis for the deuteranope is green and red-purple. On the FM
100-hue test, the errors occur at the orthogonal axis, namely, for
yellow, yellow-red, blue, and purple-blue caps. Similarly, for the
protanope, the major confusion axis is red and blue-green. Errors on
the FM 100-hue test occur for yellow, green-yellow, purple-blue, and
purple caps. This rotation of the error axis is clearly noted when
Panel D-15 confusions and FM 100-hue test errors are plotted on the
same diagram (Pinckers, 1971~.
Anomaloscope
The experiment that became known as the Rayleigh equation was first
described by Rayleigh (1881} and consisted of mixing monochromatic
yellow-red with yellow-green to match a monochromatic yellow. He
described three methods to obtain this equation--two involving spectral
colors and one involving colored discs combined by a rotating prism.
Nagel was the first one to use a direct vision spectroscope with
spectral colors.
An apparatus such as Rayleigh's or Nagel' ~ (or indeed any other
kind of device with similar functions) is usually called an
anomaloscope, that is, an instrument for specifying anomalous color
vision. Today the anomaloscope is used as an instrument capable of
measur ing var iations in color vision for normal, anomalous, and
dichromatic observers, not only in the classic Rayleigh equation but
also in other combinations of two lights to match a third. To fulfill
such objectives the anomaloscope has to be validly designed, reliably
administered, and the data obtained from it must be correctly
quantified. The design of an anomaloscope depends on the choice of
primaries used for the color mixtures, the areas that such a mixture
will subtend at the retina, the level of luminance of mixture obtained,
OCR for page 108
108
_ I I I I I 1 1~
0.8
0.6
0.4
0.2
o
/`
'A '
\
I ... - in, \
., .
_,_ ______
I
I
,:
\
At,
_ _ ``
. I I I I I I I I l
0 0.2 0.4 0.6 0.8
X
FIGURE A-8 Chromaticitie~; for the 85 caps of the FM 100-hue test.
Dashed lines indicate axes of confusion lines for color-defective
observers.
Based on data from Lakowski (1969 ~ .
and the ability to vary with ease the purity and luminance of the test
field to which the mixture is being compared.
The choice of primaries for the Rayleigh equation has been well
established, the main principle being that they ought to be chosen with
the greatest separation in dominant wavelength between them. Thus in
the Nagel Model II, the mixture field originally consisted of a red
stimulus wavelength 671 nm [lithium line), and a green stimulus of
t
OCR for page 109
109
wavelength 536 nm {thallium line). However, for the Model I, a longer
wavelength was chosen (546 no, the mercury line). Chromaticities of
primaries and the test color on the Nagel Model I are shown in Figure
A-9. The primaries lie on the linear portion of the spectrum locus,
and, thus, on the confusion lines of red-green dichromats.
The choice of primaries for the Engelking-Trendelenburg equation is
not so well established. In the Nagel Model lI anomaloscope, they are
at 517 nm (for green) and at 470 nm (for indigo) , while in the
Pickford-Nicolson anomaloscope they are at 552 . 5 nm and 473 . 3 nm,
respectively. Because neither of these pairs lies on isochromatic
lines of tritanopes, desaturation of the blue-green test color is
necessary in testing people with tritan defect. Normal observers also
require differing amounts of desaturation. Because desaturation was
not available on the Nagel Model II, the Engelking-Trendelenburg
equation is rarely used.
The luminance of mixtures of primaries, especially at the most
frequently chosen ratio, must be well above the threshold level for
cone vision but preferably at the top of the mesopic range of vision,
although the critical luminance will depend primarily on the size of
the viewing aperture. The larger the subtense over 1.5° the higher
will be the luminance necessary. In the Nagel and Pickford-Nicolson
anomaloscopes, it is about 5 cd/m2.
The choice of test wavelength varies among instruments but should
correspond as nearly as possible to a region in which color
discrimination is good. Thus for the Rayleigh equation the wavelength
should be near 590 nm, while for the Engelking-Trendelenburg equation
it should be near 490 nm. With the Nagel and the Pickford-Nicolson
anomaloscopes, the dominant wavelengths for the yellow tests are at
589.3 nm (originally at the sodium line) and 584.3 nm, respectively;
for blue-green tests they are at 490 nm and 493.5 nm, respectively.
Furthermore the luminance of the test yellow must be variable. For
the Rayleigh equation, luminance variation is necessary to distinguish
between protanopes and deuteranopes. The red primary appears dim to
the protanope, and hence only a small amount of the yellow luminance is
required for a match; a deuteranope, however, for whom the red field is
as bright as it is for observers with normal vision, will require a
correspondingly higher luminance in the standard yellow to match this
pr imary. For the Engelking-Trendelenburg equation, it is important to
vary the luminance of the standard blue-green to accommodate age
changes in sensitivity and normal variation in ocular media.
The size of the stimulus field and its consequent subtense at the
retina is also important. Anomalo~copes vary in the field size that is
used, and the visual angle may vary considerably. In the Nagel Model
T. the visual angle is f ixed near 2°; in the Nagel Model II it can be
varied between about 1.5° and 3°. Additionally, the manufacturer has
supplied some Model I instruments that allow variation of field size.
In the Pickford-Nicolson anomaloscope, at a viewing distance of one
meter, the field varies from 0.5° to 3°, but a 1.5° field is most
frequently used. Red-green dichromats do not accept the classic
dichromatic matches when the field size extends to 8° (Smith and
Pokorny, 1977; Nagy and Boyn ton, 1979~. Thus the classification
e
OCR for page 110
0.8
0 7
0 6
0.4
03
02
01
o
O 1
-02
110
= , , ,
520
~530
510 ~ \~550
500 ~ 70 N
- 490~
470\
_
_
-0.3
-0.4
~~o~
1 . · , ~ I I I I · I · · ~ ~
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 13 14
X
FIGURE A-9 Chromaticities for primaries and test colors on the
Nagel Model I and the Pickford-Nicolson anomaloscopes.
Pokorny et al. (1979), by permission.
Of people with red-green defects may differ from instrument to
instrument when the size of target varies appreciably.
Representative terms from entire chapter:
confusion lines
