Color Vision Deficiencies
The next time you go strawberry
picking, imagine how much harder it would be if the fruit were the same color as
the leaves. If you are a man, there is a 10% chance that they are! So is this a
problem you need to worry about?
Strawberries. Strawberries as they would appear to someone who is red/green colorblind.
Roughly 1 in 10 men are fully or
partly color blind. This means that one of the three types of color detectors in
their eyes is either faulty or missing altogether. The condition is hereditary
and sex-linked: fathers will pass the gene to their daughters (but not their
sons) and mothers can pass it to all their children. However, because women can
be unaffected carriers, men are at least 20 times more likely than women to
develop color deficiencies.
Color vision defects can also be acquired, as a result of disease, side effects of certain medications, or through normal aging processes, and these deficiencies may affect parts of the eye other than the photoreceptors.
It is important to remember that people with color deficiencies generally can see most colors; they just have trouble distinguishing between some shades of red and green. Apart from making terrible strawberry pickers, people who are color blind are excluded from certain jobs for safety reasons. For example, they cannot be airline pilots, policemen or ship captains. Their everyday lives are also fraught with occasional minor hazards: how to match clothing, how to decide whether the power indicator on the stereo is red or green, and how to choose an appropriate color scheme for decorating the house.
Normal cones and pigment sensitivity enable an individual to distinguish all the different colors as well as subtle mixtures of hues. This type of normal color vision is known as trichromacy and relies upon the mutual interaction from the overlapping sensitivity ranges of all three types of photoreceptor cone.
A mild color vision deficiency occurs when the pigment in one of the three cone types has a defect, and its peak sensitivity is shifted to another wavelength, producing a visual deficiency termed anomalous trichromacy, one of three broad categories of color vision defect.
Dichromacy, a more severe form of color blindness, or color deficiency, occurs when one of the pigments is seriously deviant in its absorption characteristics, or the particular pigment has not been produced at all.
The complete absence of color sensation, or monochromacy, is extremely rare, but individuals with total color blindness (rod monochromats) see only varying degrees of brightness, and the world appears in black, white, and shades of gray. This condition occurs only in individuals who inherit a gene for the disorder from both parents.
Dichromats can distinguish some colors, and are therefore less affected in their daily lives than monochromats, but they are usually aware that they have a problem with their color vision. Dichromacy is subdivided into three types: protanopia, deuteranopia, and tritanopia. Approximately two percent of the male population inherits one of the first two types, with the third occurring much more rarely.
Protanopia is a red-green defect, resulting from loss of red sensitivity, which causes a lack of perceptible difference between red, orange, yellow, and green. In addition, the brightness of red, orange, and yellow colors is dramatically reduced in comparison to normal levels. The reduced intensity effect can result in red traffic lights appearing dark (unlit), and red hues (in general), appearing as black or dark gray. Protanopes often learn to correctly distinguish between red and green, and red from yellow, primarily based on their apparent brightness, rather than on any perceptible hue difference. Green generally appears lighter than red to these individuals. Because red light occurs at one end of the visible spectrum, there is little overlap in sensitivity with the other two cone types, and people with protanopia have a pronounced loss of sensitivity to light at the long-wavelength (red) end of the spectrum. Individuals with this color vision defect can discriminate between blues and yellows, but lavender, violet, and purple cannot be distinguished from various shades of blue, due to the attenuation of the red component in these hues.
Individuals with deuteranopia, which is a loss of green sensitivity, have many of the same problems with hue discrimination as do protanopes, but have a fairly normal level of sensitivity across the visible spectrum. Because of the location of green light in the center of the visible light spectrum, and the overlapping sensitivity curves of the cone receptors, there is some response of the red and blue photoreceptors to green wavelengths. Although deuteranopia is associated with at least a brightness response to green light (and little abnormal intensity reduction), the names red, orange, yellow, and green seem to the deuteranope to be too many terms for colors that appear the same. In a similar fashion, blues, violets, purples, and lavenders are not distinguishable to individuals with this color vision defect.
Tritanopia is the absence of blue sensitivity, and functionally produces a blue-yellow defect in color vision. Individuals with this deficiency cannot distinguish blues and yellows, but do register a difference between red and green. The condition is quite rare, and occurs about equally in both sexes. Tritanopes usually do not have as much difficulty in performing everyday tasks as do individuals with either of the red-green variants of dichromacy. Because blue wavelengths occur only at one end of the spectrum, and there is little overlap in sensitivity with the other two cone types, total loss of sensitivity across the spectrum can be quite severe with this condition.
A number of standard tests for color vision have been developed. The Ishihara pseudoisochromatic plates are one such test. They can be used as a quick screening tool to determine whether an observer has a red-green color vision deficiency. The Ishihara test does not screen for blue cone (tritan) defects. Below are a few of the 38 plates which comprise this test.
A person with normal color vision sees a number seven in the circle on the left.
The majority of color deficient
observers see no number at all
Those with normal vision see the number thirty-five in the circle above. A person with protanopia sees only he number five. A person with deuteranopia sees the number three. People who are partially color deficient will see both numbers but one more distinctly than the other.
majority of observers with red-green deficiencies see the number 73, 5, and 45. The
majority of observers with normal color vision see nothing at all!
This is because normals' sense of color is actually masking the subtle
brightness differences which color deficient observers use to see the number.
Card # 1 - Try and
find a circle, star, and/or square.
Answer: Color deficient
individuals should see the yellow square. Color normal individuals should see
the yellow square and a "faint" brown circle.
Card # 2 - Try and find
a circle, star, and/or square.
Answer: Color deficient
individuals should see the yellow circle. Color normal individuals should see
the yellow circle and a "faint" brown square.
Try and find a dog, boat,
balloon, or car (as shown in the below demonstration card) in Card # 3.
Card # 3
Answer: Color deficient
individuals should see nothing. Color normal individuals should see a
"faint" brown boat.
Life's minor frustrations (and occasional dangers) for the color blind:
In spite of the limitations, there are some visual acuity advantages to color blindness, such as the increased ability to discriminate camouflaged objects.
Outlines, rather than colors, are responsible for pattern recognition, and improvements in night vision may occur due to certain color vision deficiencies. In the military, colorblind snipers and spotters are highly valued for these reasons.
As a natural part of the aging process, the human eye begins to perceive colors differently in later years, but does not become "colorblind" in the true sense of the term.
Aging results in the yellowing and darkening of the crystalline lens and cornea, degenerative effects that are also accompanied by a shrinking of the pupil size. With yellowing, shorter wavelengths of visible light are absorbed, so blue hues appear darker. As a consequence, elderly individuals often experience difficulty discriminating between colors that differ primarily in their blue content, such as blue and gray or red and purple.
At age 60, when compared to the visual efficiency of a 20-year old, only 33 percent of the light incident on the cornea reaches the photoreceptors in the retina. This value drops to around 12.5 percent by the mid-70s.
For any detailed visually guided tasks on which performance varies with illumination, an older person requires extra lighting.
Aging causes a dramatic slowing in dark adaptation. This age related delay in dark adaptation may also contribute to night vision problems commonly experienced by the elderly.