Colour Blindness
Information
_________________________
Titles in
this page
What
is colour blindness?
Frustrations
and Dangers for the Colour Blind
Clinical information about colour blindness
How colour
blindness works
What is
colour blindness?
- Colour blindness (colour
vision deficiency) is a condition in which certain colours cannot be
distinguished, and is most commonly due to an inherited condition.
Red/Green colour blindness is by far the most common form, about 99%, and
causes problems in distinguishing reds and greens. Another colour
deficiency Blue/Yellow also exists, but is rare and there is no commonly
available test for it.
- Depending on just
which figures you believe, colour blindness seems to occur in about 8% -
12% of males of European origin and about one-half of 1% of females. I did
not find any figures for frequency in other races. Total colour blindness
(seeing in only shades of gray) is extremely rare.
- There is no
treatment for colour blindness, nor is it usually the cause of any
significant disability. However, it can be very frustrating for
individuals affected by it. Those who are not colour blind seem to have
the misconception that colour blindness means that a colour blind person
sees only in black and white or shades of gray. While this sort of
condition is possible, it is extremely rare. Being colour blind does keep
one from performing certain jobs and makes others difficult.
- Weather forecasts -
especially the Weather Channel - where certain colours just can not be
distinguished on their weather maps.
- Also, maps in
general because of the colour coding on the legends.
- Bi-colour and tri-colour
LEDs (Light Emitting Diodes): Is that glowing indicator light red, yellow,
or green?
- Traffic lights, and
worst of all, Caution lights: Colour blind people always know the position
of the colours on the traffic light - in most states, Red on top, Yellow
in the centre, Green (or is that blue?) on the bottom. It isn't good when
we go to a city or state where they put traffic lights horizontal - it
takes a couple of days to get used to that one! But caution lights present
an entirely different problem. In this situation there is only one light;
no top or bottom, no right or left, just one light that is either red or
yellow - but which is it?
- Getting in the sun
with your girlfriend: So, you're out in the boat or on the beach with your
girlfriend and soaking up the rays. But I can't tell until far too late if
I'm getting red - or if she is. If I can tell it's
red, by that time it's fire engine red and a painful sunburn is already
present.
- Colour observation
by others: "Look at those lovely pink flowers on that shrub". My
reply, looking at a greenish shrub "What flowers?"
- Purchasing
clothing: I've got some really neat colours of clothes. Not everyone
appreciates them like I do though; they seem to think the colours are
strange. I just don't know why!
- Kids and crayons: Colour
vision deficiencies bother affected children from the earliest years. At
school, colouring can become a difficulty when one has to take the blue
crayon -and not the pink one- to colour the ocean.
- Test strips for
hard water, pH, swimming pools, etc.: A colour blind person is generally
unable to :
- interpret some
chemical reactions
- see that litmus
paper turns red by acid
- identify a
material by the colour of its flame such as lead blue or potassium purple
- interpret the
chemical testing kits for swimming pool water, test strips for hard
water, soil or water pH tests - all of which rely on subtle colour
differences and a band of similar colours to compare against.
- Cooking and foods:
- When cooking, red
deficient individuals cannot tell whether their piece of meat is raw or
well done. Many can not tell the difference between green and ripe
tomatoes or between ketchup and chocolate syrup.
- Some food can even
look definitely disgusting to colour deficient individuals. For example,
people with a green deficiency cannot possibly eat spinach which to them
just look like cow pat. They can however
distinguish some citrus fruits. Oranges
seem to be of a brighter yellow than that of lemons.
- Are you wearing
lipstick? Many colour blind people cannot tell whether a woman is wearing
lipstick or not. More difficult to handle for some is the inability to
make the difference between a blue-eyed blonde and a green-eyed redhead.
Clinical information about colour blindness
- Cones (colour
sensitive receptors) containing single visual pigments selective for red,
green, and blue light, are present in the normal human eye. Disturbances
of colour vision will occur if the amount of pigment per cone is reduced
or if one or more of the three cone systems are absent.
- Although defective colour
vision may be acquired as a result of another eye disorder, the vast
majority of colour blind cases are hereditary - present at birth. The gene
for this is carried in the X chromosome. Since males have an X-Y pairing
and females have X-X, colour blindness can occur much more easily in males
and is typically passed to them by their mothers.
- Colour blindness is
rooted in the chromosomal differences between males and females. Females
may be carriers of colour blindness, but males are more commonly affected.
- Colour blindness is
a malfunction of the retina, which converts light energy into electrical
energy that is then transmitted to the brain. This conversion is
accomplished by two types of photoreceptor cells in the retina: rods and
cones.
- The cones are
responsible for encoding colour. Each cone contains structures or visual
pigments sensitive to one of three wavelengths of light: red, green, and
blue. Normal persons are able to match all colours of the spectrum by
mixtures of only three fundamental colour sensitivities. Hence, the huge
variety of colours we perceive stems from the cone cells' response to
different compositions of wavelengths of light.
- Defects in colour
vision occur when one of the three cone cell colour coding structures
fails to function properly. One of the visual pigments may be present and
functioning abnormally, or it may be absent altogether.
- For practical purposes,
all colour-deficient individuals have varieties of red or green
deficiency. Blue deficiencies are very rare. Colour deficient patients are
not completely red or green blind. Compared to persons with normal colour
vision, they have some trouble differentiating between certain colours,
but the severity of the colour deficiency is variable.
- Colour blindness is
normally diagnosed through clinical testing. (See the Colour Blindness
Test - the most common test used) Although there is no treatment for colour
blindness, most colour deficient persons compensate well for their defect
and may even discover instances in which they can discern details and
images that would escape normal-sighted persons. At one time the U.S. Army
found that colour blind persons can spot "camouflage" colours
where those with normal colour vision are fooled by it.
How colour blindness
works
- The human eye sees
by light stimulating the retina (a neuro-membrane lining the inside back
of the eye). The retina is made up of what are called Rods and Cones. The
rods, located in the peripheral retina, give us our night vision, but can
not distinguish colour. Cones, located in the centre of the retina (called
the macula), are not much good at night but do let us perceive colour
during daylight conditions.
- Many people think
anyone labelled as "colour-blind" only sees black and white -
like watching a black and white movie or television. This is a big
misconception and not true. It is extremely rare to be totally colour
blind. There are many different types and degrees of colour-blindness,
really they are "colour deficiencies" since virtually no one is
truly blind to all colours.
- People with normal
cones and colour vision are able to see all the different colours and
subtle mixtures of them by using cones sensitive to one of three
wavelength of light - red, green, and blue.
- A mild colour
deficiency is present when one or more of the three cones functions
"poorly". A more severe colour deficiency is present when one of
the cones does not function at "all" or is missing.
- Protanomaly (one
out of 100 males):
Protanomaly is referred to as "red-weakness", an apt description
of this form of colour deficiency. Any redness seen in a colour by a
normal observer is seen more weakly by the protanomalous viewer, both in
terms of its "colouring power" (saturation, or depth of colour)
and its brightness. Red, orange, yellow, yellow-green, and green, appear
somewhat shifted in hue ("hue" is just another word for "colour")
towards green, and all appear paler than they do to the normal observer.
The redness component that a normal observer sees in a violet or lavender colour
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 colour
that normals call "violet" may look only like another shade of
blue.
Under poor viewing conditions, such as when driving in dazzling sunlight
or in rainy or foggy weather, it is easily possible for protanomalous
individuals to mistake a blinking red traffic light from a blinking yellow
or amber one, or to fail to distinguish a green traffic light from the
various "white" lights in store fronts, signs, and street lights
that line our streets. Do not let them adjust the colour on the
television, because it will look far to reddish or violet for the rest of
the family members.
- Deuteranomaly (five
out of 100 of males):
Let the deuteranomalous person adjust your television and he would add
more green and subtract red. He is considered "green weak".
Similar to the protanomalous person, he is poor at discriminating small
differences in hues in the red, orange, yellow, green region of the
spectrum. He makes errors in the naming of hues in this region because they
appear somewhat shifted towards red for him - difficulty in distinguishing
violet from blue.
From a practical stand point though, many protanomalous and
deuteranomalous people breeze through life with very little difficulty
doing tasks that require normal colour vision. Some may not even be aware
that their colour perception is in any way different from normal. The only
problem they have is passing a colour vision test.
- Dicromasy - can be
divided into protanopia and deuteranopia (two out of 100 males):
These individuals normally know they have a colour vision problem and it
can affect their lives on a daily basis. They see no perceptible
difference between red, orange, yellow, and green. All these colours that
seem so different to the normal viewer appear to be the same colour for
this two percent of the population.
- Protanopia (one out
of 100 males):
For the protanope, the brightness of red, orange, and yellow is much
reduced compared to normal. This dimming can be so pronounced that reds
may be confused with black or dark gray, and red traffic lights may appear
to be extinguished. They may learn to distinguish reds from yellows and
from greens primarily on the basis of their 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 are so dimmed as to be invisible e.g. Pink flowers,
reflecting both red light and blue light, may
appear just blue to the protanope.
- Deuteranopia (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.
Some of the information on this page was
taken from:
- An article by:
Diana H. Heath, M.D., a member of the Morton Plant medical staff,
specializing in ophthalmology
·
The webpage
written by Terrace L. Waggoner, O.D., Staff Naval Hospital
Pensacola