Color Blindness: Classification, Tests and Significance
Author(s): Rathi M., Sachdeva. S., Dhull C.S., Dhattarwal S.K., Gupta S.R., Mam A
Vol. 6, No. 3 (2006-07 - 2006-09)
(1)Rathi M., (2)Sachdeva. S., (3)Dhull C.S.,
(4)Dhattarwal S.K., (5)Gupta S.R., (6)Mam A.
(1)Reader, (2)Lecturer, (3)Professor & Chief, (5)Resident, (6)Resident, RIO,
(4)Associate Prof. Deptt. of Forensic Medicine, PGIMS Rohtak
Color vision tests are used for a wide variety
of purposes. Some of these include the rapid
screening of congenital red-green defects in
industry, transportation, military & gazetted
government posts. The classification of
discrimination ability within the population of
congenital red-green defects is used for job
assignment purposes.
It is essential for the benefit of both employer
& employee that the color vision requirements of
a job be adequately described. On the basis of
these professional requirements & observer
capabilities a decision can be made about whether
an individual’s color vision is suitable for
performing the particular duties encountered in
daily work situations. Such practical assessment
of the relevant color qualifications helps to
prevent the inappropriate allotment of
manpower. A majority in this regard is the lack
of precise checklists of color vision requirements
for different jobs; there are no guidelines to help
employers establish color requirements for a given
job. Broadly speaking, however many occupations
can be divided into three categories depending
on the quality of color vision required:
a) those excluding major color defective
observers
b) those requiring representative color vision
c) those requiring good color discrimination
Normal observers can match all hues by the
appropriate mixture of three colored lights.
Hence, normal observers are known as
trichromats. In order to define normal
trichromacy and diagnose color defects, we have
available some special color matches that make relatively quick and easy to perform, compared
with full spectrum color matching. Instruments
that allow us to evaluate these special matches
in the population are called anomaloscopes. There
have been three such special matches used to
test color defects: the Rayleigh match or equation,
the Pickford-Lawoski match or equation and the
Engelking-Trendelenburg match or equation. Of
these matches, the Rayleigh match and Pickford-
Lawoski 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
classification of these defects. It 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 670nm and 545nm.
We can, statistically define two subtypes of
normal trichromats whose color vision may make
them unsuitable for jobs in color-sensitive
industries:
a) The deviant color normal observer is the
normal trichromats whose Rayleigh equation lies
within normal range but with the midpoint
displaced more than ± 2 standard deviations
from the mean of average observers.
b) The weak color normal observer is one whose
Rayleigh equation midpoint is within normal
range for the population. Color-weak observers
comprise 20 percent of the normal population.
The Pickford-Lawoski Equation: a special
match that is available in some anomaloscopes
is the match of a white light to a mixture of
470nm & 585nm lights. The match was designed
to evaluate the effect of ageing and has also
proved important in evaluating color defects
acquired in eye diseases.
Engelking Trendelenburg Equation: this
equation involves the match of 490nm to a mixture of 470nm & 517nm. This match was
designed by Engelking & modified by
Trendelenburg in order to evaluate congenital
blue-yellow color defects.
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 there are four
subcategories of anomalous trichromats. Each
subcategory is defined by use of the
anomaloscope.
Simple Protanomalous Trichromats
(Genetic entity PA): these people need a higher
ratio of red to green primary than normal
trichromats in the Rayleigh equation.
Simple Deuteranomalous Trichromats
(Genetic entity DA): these people need a higher
ratio of green to red primary than normal
trichromats in the Rayleigh equation.
Extreme Protanomalous Trichromats
(Genetic Entity EPA): this group accepts a wide
range of red-green ratios, usually including the
ratio accepted by normal trichromats and perhaps
one of the primaries (usually red).
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).
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 500nm; generally a red
and a blue are used. On the basis of Rayleigh
equation dichromats can be differentiated into
protanopes and deuteranopes.
Protanopes (genetic entity P): this group
shows loss of sensitivity to long wavelengths.
Protanopes confuse blues with purples, bluegreens
with red-purples and light greens with
brown.
Deuteranopes (genetic entity D): this group
possesses spectral sensitivity similar to that of
normal observers. A deuteranopes will confuse
blues with blue-purples, blue-greens with purples
and greens with reddish purples.
Types of color vision tests:
Anomaloscopes: are optical instruments in
which the observer must manipulate stimulus
control knobs to match two colored fields in color
and brightness. The anomaloscope is the
standard instrument for the diagnoses of color
vision defects. When supplemented by
information from other color vision tests, the
results provided by this instrument permit the
accurate classification of all color deficiencies.
Plate tests: in this the observer must identify a
colored symbol embedded in a background (most
pseudoisochromatic plates); identify which of
four colors is most similar to a standard color, or
identify which circle matches a gray rectangle.
There are many types of pseudoisochromatic
tests and all provide efficient screening of
congenital red-green defects. Pseudoisochromatic
tests should be used primarily as screening tests
to divide people into normal and color-defective
populations; their diagnostic value is limited. At
present it is always better to look on information
from pseudoisochromatic plate tests as
providing a probable but not certain diagnosis.
Arrangement tests: in this the observer is
required to arrange color samples by similarity
in a sequential color series. Usually the colors are
mounted in caps, which are numbered on the
back and can be moved about freely during
performance. These are easy to administer and
can be used by untrained personnel.
Lantern tests: these are designed as practical
means for measuring the ability of seamen,
railway personnel and airline pilots to identify
and discriminate navigational aids ad signals.
Accordingly, these tests emphasize correct color
recognition as the important testing variable.
Their value lies in their simulation of the working
condition.
While the anomaloscope remains the only
clinical method for precise diagnosis of the
presumed genetic entities, many tests have been
devised for quick, inexpensive and efficient
screening of the color-defective population.
Screening tests are used to identify individuals
who may eventually require more extensive color
testing. Their usefulness is in the identification of
such individuals rather than the diagnosis of the
color defect.
The most effective test for rapid screening is
one of the validated plate tests designed for this
purpose including the Ishihara, Dvorine, AO H- R-R, and other series of pseudoisochromatic
plates that detect about 96% of the cases
confirmed by anomaloscope.
The anomaloscope is the only clinical
instrument for diagnosis and classification of the
presumed genetic entities of dichromacy and to
both simple and extreme anomalous trichromacy
as defined by Franceschetti.
Plate and arrangement tests are not entirely
successful at diagnosis of congenital red-green
defects.
There currently are no uniformly accepted
criteria for procedures to establish the precise
color vision requirements for various types of
jobs. Working group 41 therefore recommends
that color vision requirements for various jobs be
established individually within industry and the
military as needed.
REFERENCES:
- Abney WW (1906) Modified apparatus for the
measurement of colors and its application to the
determination of the color sensations. Phil Trans Roy Soc
(Series A) 205: 333-355.
- Alpern M, Moleller J (1977) The red and green cone
visual pigments of deuteranomalous trichromacy. J Physiol
266: 647-675.
- Bowman KJ (1973) The Farnsworth dichotomous test
- the panel D-15. Aust J Optom 56: 13-24.
- Boynton, RM (1979) Human Color Vision. New York:
Holt, Rinehart and Winston.
- Edridge-Green FW (1920) The physiology of vision
with special references to color blindness. London ; G.Bell
and Sons, ltd.
- Farnsworth D (1957) The Farnsworth-Munsell 100-
hue test for the examination of color discrimination.
Baltimore, Md.: Munsell Color Company, Inc.
- Linksz A ( 1964) An essay on color vision and clinical
color vision tests. New York: Grune and Stratton.
- Ohta M (1961) Study on the generalized color
discrimination test. Report I. Acta Ophthalmol (Jpn) 65:
512-19.
- Pickford RW (1951) Individual differences in color
vision. London: Routledge and Kegan Paul.
- Pokorny J, Smith VC (1976) Effect of field size on
red-green color mixture equations. J Opt Soc Am 66:
1522-24.
Dr Manisha Rathi
Reader, Regional Institute Of Ophthalmology,
PGIMS, Rohtak, Phone: 01262-279679
Fax: 01262-279780
E-Mail: manisharathi@hotmail.com
(1)Rathi M., (2)Sachdeva. S., (3)Dhull C.S., (4)Dhattarwal S.K., (5)Gupta S.R., (6)Mam A.
(1)Reader, (2)Lecturer, (3)Professor & Chief, (5)Resident, (6)Resident, RIO, (4)Associate Prof. Deptt. of Forensic Medicine, PGIMS Rohtak
Color vision tests are used for a wide variety of purposes. Some of these include the rapid screening of congenital red-green defects in industry, transportation, military & gazetted government posts. The classification of discrimination ability within the population of congenital red-green defects is used for job assignment purposes.
It is essential for the benefit of both employer & employee that the color vision requirements of a job be adequately described. On the basis of these professional requirements & observer capabilities a decision can be made about whether an individual’s color vision is suitable for performing the particular duties encountered in daily work situations. Such practical assessment of the relevant color qualifications helps to prevent the inappropriate allotment of manpower. A majority in this regard is the lack of precise checklists of color vision requirements for different jobs; there are no guidelines to help employers establish color requirements for a given job. Broadly speaking, however many occupations can be divided into three categories depending on the quality of color vision required:
a) those excluding major color defective observers
b) those requiring representative color vision
c) those requiring good color discrimination
Normal observers can match all hues by the appropriate mixture of three colored lights. Hence, normal observers are known as trichromats. In order to define normal trichromacy and diagnose color defects, we have available some special color matches that make relatively quick and easy to perform, compared with full spectrum color matching. Instruments that allow us to evaluate these special matches in the population are called anomaloscopes. There have been three such special matches used to test color defects: the Rayleigh match or equation, the Pickford-Lawoski match or equation and the Engelking-Trendelenburg match or equation. Of these matches, the Rayleigh match and Pickford- Lawoski 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 classification of these defects. It 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 670nm and 545nm. We can, statistically define two subtypes of normal trichromats whose color vision may make them unsuitable for jobs in color-sensitive industries:
a) The deviant color normal observer is the normal trichromats whose Rayleigh equation lies within normal range but with the midpoint displaced more than ± 2 standard deviations from the mean of average observers.
b) The weak color normal observer is one whose Rayleigh equation midpoint is within normal range for the population. Color-weak observers comprise 20 percent of the normal population.
The Pickford-Lawoski Equation: a special match that is available in some anomaloscopes is the match of a white light to a mixture of 470nm & 585nm lights. The match was designed to evaluate the effect of ageing and has also proved important in evaluating color defects acquired in eye diseases.
Engelking Trendelenburg Equation: this equation involves the match of 490nm to a mixture of 470nm & 517nm. This match was designed by Engelking & modified by Trendelenburg in order to evaluate congenital blue-yellow color defects.
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 there are four subcategories of anomalous trichromats. Each subcategory is defined by use of the anomaloscope.
Simple Protanomalous Trichromats (Genetic entity PA): these people need a higher ratio of red to green primary than normal trichromats in the Rayleigh equation.
Simple Deuteranomalous Trichromats (Genetic entity DA): these people need a higher ratio of green to red primary than normal trichromats in the Rayleigh equation.
Extreme Protanomalous Trichromats (Genetic Entity EPA): this group accepts a wide range of red-green ratios, usually including the ratio accepted by normal trichromats and perhaps one of the primaries (usually red).
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).
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 500nm; generally a red and a blue are used. On the basis of Rayleigh equation dichromats can be differentiated into protanopes and deuteranopes.
Protanopes (genetic entity P): this group shows loss of sensitivity to long wavelengths. Protanopes confuse blues with purples, bluegreens with red-purples and light greens with brown.
Deuteranopes (genetic entity D): this group possesses spectral sensitivity similar to that of normal observers. A deuteranopes will confuse blues with blue-purples, blue-greens with purples and greens with reddish purples.
Types of color vision tests:
Anomaloscopes: are optical instruments in which the observer must manipulate stimulus control knobs to match two colored fields in color and brightness. The anomaloscope is the standard instrument for the diagnoses of color vision defects. When supplemented by information from other color vision tests, the results provided by this instrument permit the accurate classification of all color deficiencies. Plate tests: in this the observer must identify a colored symbol embedded in a background (most pseudoisochromatic plates); identify which of four colors is most similar to a standard color, or identify which circle matches a gray rectangle.
There are many types of pseudoisochromatic tests and all provide efficient screening of congenital red-green defects. Pseudoisochromatic tests should be used primarily as screening tests to divide people into normal and color-defective populations; their diagnostic value is limited. At present it is always better to look on information from pseudoisochromatic plate tests as providing a probable but not certain diagnosis. Arrangement tests: in this the observer is required to arrange color samples by similarity in a sequential color series. Usually the colors are mounted in caps, which are numbered on the back and can be moved about freely during performance. These are easy to administer and can be used by untrained personnel.
Lantern tests: these are designed as practical means for measuring the ability of seamen, railway personnel and airline pilots to identify and discriminate navigational aids ad signals. Accordingly, these tests emphasize correct color recognition as the important testing variable. Their value lies in their simulation of the working condition.
While the anomaloscope remains the only clinical method for precise diagnosis of the presumed genetic entities, many tests have been devised for quick, inexpensive and efficient screening of the color-defective population. Screening tests are used to identify individuals who may eventually require more extensive color testing. Their usefulness is in the identification of such individuals rather than the diagnosis of the color defect.
The most effective test for rapid screening is one of the validated plate tests designed for this purpose including the Ishihara, Dvorine, AO H- R-R, and other series of pseudoisochromatic plates that detect about 96% of the cases confirmed by anomaloscope.
The anomaloscope is the only clinical instrument for diagnosis and classification of the presumed genetic entities of dichromacy and to both simple and extreme anomalous trichromacy as defined by Franceschetti.
Plate and arrangement tests are not entirely successful at diagnosis of congenital red-green defects.
There currently are no uniformly accepted criteria for procedures to establish the precise color vision requirements for various types of jobs. Working group 41 therefore recommends that color vision requirements for various jobs be established individually within industry and the military as needed.
REFERENCES:
- Abney WW (1906) Modified apparatus for the measurement of colors and its application to the determination of the color sensations. Phil Trans Roy Soc (Series A) 205: 333-355.
- Alpern M, Moleller J (1977) The red and green cone visual pigments of deuteranomalous trichromacy. J Physiol 266: 647-675.
- Bowman KJ (1973) The Farnsworth dichotomous test - the panel D-15. Aust J Optom 56: 13-24.
- Boynton, RM (1979) Human Color Vision. New York: Holt, Rinehart and Winston.
- Edridge-Green FW (1920) The physiology of vision with special references to color blindness. London ; G.Bell and Sons, ltd.
- Farnsworth D (1957) The Farnsworth-Munsell 100- hue test for the examination of color discrimination. Baltimore, Md.: Munsell Color Company, Inc.
- Linksz A ( 1964) An essay on color vision and clinical color vision tests. New York: Grune and Stratton.
- Ohta M (1961) Study on the generalized color discrimination test. Report I. Acta Ophthalmol (Jpn) 65: 512-19.
- Pickford RW (1951) Individual differences in color vision. London: Routledge and Kegan Paul.
- Pokorny J, Smith VC (1976) Effect of field size on red-green color mixture equations. J Opt Soc Am 66: 1522-24.
Dr Manisha Rathi
Reader, Regional Institute Of Ophthalmology,
PGIMS, Rohtak, Phone: 01262-279679
Fax: 01262-279780
E-Mail: manisharathi@hotmail.com