Colorblindness by Rebecca Grant
Five to 8% of the men and 0.5% of the women of the world
are born colorblind. That is as high as one out of twelve
men and one out of two hundred women. The human eye sees by
light stimulating the retina (a neuro-membrane lining the
inside back of the eye). The retina consists of Rods and Cones.
The rods, located in the peripheral retina, grant us our night
vision, but can not distinguish color. The cones, located
in the center of the retina (called the macula), operate
poorly at night but allow us to perceive color during daylight
conditions.
The cones each contain a light sensitive pigment, which is
sensitive over a range of wavelengths. Each visible color
is a different wavelength from approximately 400 to 700 nm.
Genes contain the coding instructions for these pigments,
and if the coding instructions are incorrect, the wrong pigments
are produced, and the cones are sensitive to varied wavelengths
of light, resulting in a color deficiency. The colors people
see are completely dependent on the sensitivity ranges of
these pigments.
There are many of misconceptions about colorblindness. The
idea is anyone labeled as "colorblind" only sees black and
white – comparable to watching a black and white movie or
television.
It is extremely rare to have monochromasy - complete absence
of any color sensation or, totally color blind.
There are many different types and degrees of colorblindness
or more correctly called - color deficiencies.
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People with
Trichromasy vision have normal cones and light
sensitive pigment. These people are able to see the
numerous different colors and subtle mixtures of them
by using the cones sensitive to one of three wavelengths
of light - red, green, and blue. A mild color deficiency
is present when one or more of the three cone's light
sensitive pigments are not right and their peak sensitivity
is shifted. These deficiencies include protanomaly and
deuteranomaly. A more severe color deficiency is present
when one or more of the cone’s light sensitive pigments
is severely shifted. These include protanopia and deuteranopia.
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Protanomaly occurs
in one out of 100 males. Protanomaly is referred to as
"red-weakness." Any red colors or shades seen by
a normal observer appears more weakly by the protanomalous
viewer, both in terms of its saturation, or depth of color,
and its brightness. Red, orange, yellow, yellow-green,
and green appear shifted in hue towards green, and all
appear paler than they do to the normal observer. The
red component that a normal observer sees in a violet
or lavender color is so weakened for the protanomalous
observer that he may fail to detect it, and therefore
sees only the blue component. Hence, to him the color
that normal people call "violet" may look like just another
shade of blue. Under poor viewing conditions, such as
driving in bright sunlight or in rainy or foggy weather,
it is easily possible for protanomalous individuals to
mistake a blinking red traffic light for a blinking yellow
or amber one. In addition, these individuals may also
fail to distinguish a green traffic light from the various
"white" lights in storefronts, signs, and streetlights.
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Deuteranomaly affects five out of 100 males.
The deuteranomalous person is considered "green weak".
Similar to the protanomalous person, he is poor at
discriminating the small differences of hues in the
red, orange, yellow, and green region of the spectrum.
He makes errors in the naming of hues in this region
because they appear shifted towards red for him. Deuteranomalous
individuals do not have the loss of brightness problem
faced by many protanomalous persons. From a practical
stand point though, many protanomalous and deuteranomalous
people breeze through life with very little difficulty
doing tasks that require normal color vision. Some
may not even be aware that their color perception
is in any way different from normal.
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| Dichromasy affects
two out of 100 males and can be divided into protanopia
and deuteranopia. These individuals normally know they
have a color vision problem and it can effect their
lives on a daily basis. They see no perceptual difference
between red, orange, yellow, and green. All the colors
that seem so different to the normal viewer appear to
be the same color for this two percent of the population.
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Protanopia occurs
in one out of 100 males. For the protanope man, the
brightness of red, orange, and yellow is more reduced
compared to normal. This difference can be so pronounced
that reds may be confused with black or dark gray, and
red traffic lights may appear to be extinguished. These
men may learn to distinguish reds from yellows and greens
primarily based on the apparent brightness or lightness,
not on any perceptible hue difference. Violet, lavender,
and purple are indistinguishable from various shades
of blue because their reddish components appear so dimmed
they are invisible, e.g. Pink flowers, reflecting
both red light and blue light, may appear just blue
to the protanope. |
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Deuteranopia
affects one out of 100 males. The deuteranope suffers
the same hue discrimination problems as the protanope,
but without the abnormal dimming. The names red, orange,
yellow, and green really mean very little to him aside
from being different names that every one else around
him seems to be able to agree on. Similarly, violet,
lavender, purple, and blue, seem to be too many names
to use logically for hues that all look alike to him.
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It should be obvious there are several different kinds and
degrees of color vision deficiencies. Protanomalous or deuteranomalous
individuals can usually pass as a normal observer in everyday
activities. They may make occasional errors in color names,
or may encounter difficulties in discriminating small differences
in colors,
but usually they do not perform very differently from the
normal except on color vision tests.
The protanope and deuteranope, on the other hand, can be
severely color deficient. The real problem, as a protanope
or deuteranope may see it, is there are far too many hue names
used by most people without any obvious basis for using one
instead of another. Why call something "orange" when it does
not look different in any way from something else called green,
tan, beige, or any of several other color names?
It is important to get children’s vision tested at an
early age. Here are some examples of colorblind tests given
at a doctor’s office:
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