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COLOUR-BLINDNESS
There is a common misconception that anyone suffering from ‘Colour-blindness’ only sees
black and white. It is extremely rare for anyone to be totally colour-blind. (Monochromasy –
complete absence of colour sensation). There are different degrees of colour-blindness,
accurately called ‘colour deficiencies’.
People with normal cones and light-sensitive pigment are able to see different colours and
mixtures of them by using cones sensitive to one wavelength of light – red, green and blue. A
mild colour deficiency is present when one or more of the three cones’ light sensitive
pigments are not correct and the sensitivity is shifted. In most cases of colour-blindness, the
person has a different combination of cones than normal. To physically see colour, the cone
cells of the retina are the crucial components. People with normal colour perception have
three different cones. Each cone is tuned to see mostly either long wavelengths, which are
reddish; middle wavelengths, which are greenish; or short wavelengths, which are
bluish. These are referred to as the M-, L- and S-cones. Our minds tell us what colour we are
seeing by determining the ratio between signals from different types of cones. In colour-blind
people, either a cone is missing or has a different peak absorption from normal.
Congenital colour vision deficiencies most often affect the L-cones or M-cones. Full colourblindness and insensitivity to blue light are comparatively rare. L-cone and M-cone
deficiencies are known as ‘red-green colour-blind’, because of the inability to distinguish
between these two colours. Yellow-blue colour-blindness involves much rarer deficiencies
of the S-cones.
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 October 2001.
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Colour-blindness is primarily a congenital defect, but can be acquired as a result of disease or
accident. Parkinson’s Disease commonly induces symptoms of colour-blindness, as can any
disruption to the neural pathways between the vision centres of the brain.
Most congenital colour-blindness is caused by defects in the X chromosome. As females have
two X chromosomes and males have an X chromosome and a Y chromosome, colourblindness is much more common in males. Females must have a defect in both X
chromosomes before they exhibit colour-blindness. A female with the colour-blind defect in
one X chromosome is a carrier of colour-blindness. Male children of the female carrier are as
likely to be colour-blind as the male children of a male with colour-blindness. The male
children of a male with colour-blindness and a female carrier are extremely likely to be
colour-blind.
Approximately 5% - 8% of men and 0.5% of women in the world are born colour-blind. That
equates to 1 out of 12 men and 1 out of 200 women. People who are ‘red weak’ (protans) and
‘green weak’ (deutans) comprise 99% of this group.
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Anomalous Trichromacy causes problems in the L-cone and M-cone and is by far the most
common form of colour-blindness, but it is the least severe. It reduces the ability to
discriminate colours, but does not eliminate colour perception altogether.
Protanomaly, which affects 1 out of 100 males, is referred to as ‘red weakness’. Red, orange,
yellow, yellow-green and green appear somewhat shifted in hue towards green and all appear
paler than they would to a normal observer. This dimming can be so pronounced that reds
may be confused with grey and red traffic lights appear not to be operating. They may learn
reds from yellows and from greens on the basis of their apparent lightness, not on any
perceptual hue distinction.
The redness in a violet or lavender colour is so weakened for the protanomalous observer that
he may fail to detect it and therefore only sees the blue component. Pink flowers, reflecting
both red and blue light, appear blue to the protanope.
Deuteranomaly affects 5 out of 100 males and is referred to as green-weakness. These people
are poor at discriminating small differences in hue in the red, orange and green regions of the
spectrum. Hues in this region appear somewhat shifted towards red. However, an important
factor between deuteranomalous individuals and protanomalous individuals is that
deuteranomalous individuals do not have the loss of ‘brightness’ problem. To the
deuteranope, the names red, orange, yellow and green mean little. Similarly, violet, lavender,
purple and blue seem to be names to use logically for the hues that all look alike to him.
From a practical point of view, both protanomalous and deuteranomalous individuals can
breeze through life with very little difficulty. Some may not be aware that their colour
perception is different to normal. They only have a problem if they have to pass colour vision
tests.
Achromatopsia is a form of colour-blindness characterised by a total loss of colour vision. It
is fairly rare, but the effects are severe. They see best at low light levels. Achromatopes must
wear dark glasses in daylight or bright indoor areas.
Blue Cone Monochromacy is a condition affecting the L-cones and the M-cones, leaving just
the S-cones and the rods. Because S-cones do not contribute to the perception of brightness,
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blue cone monochromats have the same problems with lights as do achromatopes, although
they are able to distinguish a small range.
There are tests for colour vision deficiency using so-called pseudo-isochromatic plates, which
involve using dots of different chromaticity to produce patterns that can be identified as
numbers, letters or shapes. The first examples of these plates were those by Stilling (1st
edition, 1875) but the more popular set of plates for colour vision testing are those by
Ishihara (1st edition, 1917).
There are several types and degrees of colour vision deficiencies. Protanopes and
deuteranopes can usually pass as normal observers, but may occasionally make mistakes
when naming colours. They may have difficulty discriminating small differences in colours,
but usually they do not perform any differently from normal except in colour vision tests.
On the other hand, protanopes and deuteranopes may feel that other people use far too many
names for colours. Why call something ‘orange’ when it looks the same as something else
called green, tan, beige, etc.?
The Elite Human Nature System
 October 2001.
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