Color blindness is used in colloquial terms to
refer to the difficulty in telling colors apart but a more correct
term would be color vision defect. Color blindness is a misnomer
because only a small percentage of people are unable to see any
color. Color vision is important in many everyday tasks, such a
driving a car (does that traffic light mean "stop" or "go"?). Persons
with color vision defects may be at a disadvantage in school or
at work. In fact, lack of normal color vision may limit career opportunities.
For example, normal color vision is vital in such jobs as color
printing and color photography. Abnormal color vision can even be
dangerous in certain situations, such as in rail and water navigation.
For this reason, persons with color vision defects are excluded
by law from certain occupations.
What causes color vision defects?
Color vision depends on the absorption of light by visual pigments
contained within specialized cells in the eye called photoreceptors.
There are two types of photoreceptors: rods and cones. Rods, which
provide vision in dim light, have no ability to distinguish between
colors. Cones are responsible for color vision. There are three
different types of come pigment in the normal eye. Color vision
occurs within the visual part of the brain compares electrical signals
from the different types of cones. Defects in color vision are either
inherited or acquired. Inherited defects result mainly from missing
or incorrect visual pigments. There are different types of inherited
defects, with different levels of severity. Color vision defects
sometimes can be acquired, as a result of eye disease or normal
aging or as a side effect of certain medications. In acquired defects,
other parts of the eye besides cones and cone pigments may be affected.
What are the types of inherited color vision
There are three groups of inherited color vision defects: monochromacy,
dichromacy and anomalous trichromacy. The last two groups are subdivided
into red-green and blue-yellow types of defects. Inherited red-green
color vision defects are more common in males (1 to 8 percent depending
on race) than in females (about 0.4 percent). Inherited blue-yellow
defects are rare in either sex.
Monochromacy - Rod monochromats, or
complete achromats, are truly "color
blind" since they cannot distinguish any hues (e.g., blue, green,
yellow and red). They see only different degrees of lightness. For
them, the world appears to be shades of gray, black and white. They
also have poor visual acuity, aversion to bright light and nystagmus
(an involuntary, rapid movement of the eyes).
To have rod monochromacy, someone must inherit a gene for the disorder
from both parents. This condition occurs in approximately 1 in 30,000
of the population.
Dichromacy is a less severe form of color defect than monochromacy.
Dichromats can tell some hues apart. Dichromacy is divided
into three types: protanopia, deuteranopia and tritanopia.
Protanopia and deuteranopia
are red-green defects. Persons with red-green defects have difficulty
distinguishing between reds, greens and yellows but can discriminate
between blues and yellows. Protanopes often can name red and green
correctly because green looks lighter to them than red.
Males have red-green defects if they inherit a defective gene from
their mother. Affected males pass the defective gene to all of their
daughters but none of their sons. Females who inherit only one defective
gene are carriers of that gene. Females who inherit the gene for
red-green defect from both parents are affected.
Hereditary tritanopia is a blue-yellow
defect. Persons with blue-yellow defects cannot see the difference
between blues and yellows but can distinguish between reds and greens.
Tritanopia is somewhat rare (affecting between 1 and 15,000 and
1 in 50,000) and occurs equally in both sexes. Triatnopes usually
have fewer problems in performing everyday tasks than do those with
Anomalous Trichromacy - The ability of anomalous
trichromats to distinguish between hues is better than dichromats
but still not normal. Red-green anomalous trichromacy is subdivided
into protanomaly and deuteranomaly.
Both types are inherited in the same way as for red-green dichromacy.
The severity can range from mild to extreme. Some persons with the
mildest forms may not even realize their color vision is abnormal.
A third type of anomalous trichromacy is tritanomaly.
This condition is more often acquired than inherited.
How are color vision defects diagnosed?
Specialized color vision tests can easily detect color vision
defects. Pseudoisochromatic plate tests
are commonly used to screen for inherited color vision defects.
In this group of tests, a pattern of colored dots forms a number
or letter against a background of other colored dots. Persons with
normal color vision can discern these patterns but hose with color
Arrangement tests are a second type
of color vision test. They are used to measure the severity of inherited
color vision defects or to test for acquired color defects. The
person being tested must arrange color chips in order of similarity.
Can color vision defects be cured?
No cure exists for inherited color vision defects since they are
caused by missing or incorrect visual pigments. Acquired color vision
defects can be corrected sometimes if the underlying cause can be
Special aids have been developed to help persons with color vision
defects distinguish some of the colors that cause them trouble.
These devices include specially tinted contact lenses and eyeglasses.
However, these aids do not provide normal color vision and therefore
should be used with caution.