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11.8.19 Colour vision defects occur when there is deficiency in one or more of the three cone pigments,
and there are all grades of severity of the defects.
11.8.20 Normal individuals have all three types of cones with normal amounts of their respective pigments
and use all three mechanisms in colour perception and colour matching. Such individuals are normal
trichromats. A normal trichromat is able to match any given hue by using an appropriate mixture of red, green
and blue light.
11.8.21 The commonest type of colour vision defect is one in which the individual has all three types of
cones but one type is deficient to some degree (Table 11-5). Such individuals are anomalous trichromats. They
fall into three groups:
a) red deficient = protanomalous trichromat
b) green deficient = deuteranomalous trichromat
c) blue deficient = tritanomalous trichromat.
11.8.22 In dichromatism the affected individuals have only two colour-sensing mechanisms and can match
any coloured or white light by a mixture of two other coloured lights taken from near the two ends of the
spectrum. They accept colour-matches made by normal observers but they also make matches unacceptable
to the trichromat. There are three types of dichromats:
a) Protanopes — who lack the retinal long-wavelength sensitive pigment; have a reduced
sensitivity to red light (that is black for them). They see no colour in red and blue-green.
b) Deuteranopes — who lack the mid-wavelength sensitive pigment. They have normal
sensitivity to light and for them green and red-purple are seen achromatic.
c) Tritanopes — a rare type where probably the short-wavelength pigment is missing. Blue-violet
is achromatic to them.
Table 11-5.
10 After Thomas Young, English physician and physicist (1773-1829) and Hermann Ludwig Ferdinand von
Helmholtz, German physiologist (1821-1894).
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-51
Classification of colour vision
Normal trichromatism (normal colour vision)
Congenital colour vision defects
Dyschromatopsia
Anomalous trichromatism
Protanomaly
Deuteranomaly
Tritanomaly
Dichromatism
Protanopia
Deuteranopia
Tritanopia
Achromatopsia
Rod monochromatism
Cone monochromatism
Acquired colour vision defects
11.8.23 The third group consists of monochromats who may be rod monochromats or cone monochromats.
Both deficiencies are extremely rare, are associated with severe visual problems and need not be considered
further in an aeromedical context.
11.8.24 Congenital deficiencies in the blue sensitive mechanisms — tritanopia and tritanomaly are also
rare and are seldom of practical importance.
Prevalence and distribution of colour vision defects
11.8.25 There are interesting variations in the prevalence of colour vision defects. Caucasians have the
highest prevalence. African Americans, Japanese and Chinese have about half this prevalence, and the lowest
rate is found in native Africans.
11.8.26 The breakdown of the various defects in Caucasian men is approximately as follows:
a) deuteranomalous trichromatism — 4.6 per cent
b) protanomalous trichromatism — 1.0 per cent
III-11-52 Manual of Civil Aviation Medicine
c) deuteranopia — 1.4 per cent
d) protanopia — 1.2 per cent.
11.8.27 Persons with colour vision defects have difficulty distinguishing colours which are easily
distinguished by normal persons. The degree of difficulty varies with the severity of the defect.
11.8.28 These difficulties will be worse when light levels are low and when the colours are unsaturated.
The main concern in the aviation environment is the risk of confusion between red, white (yellow) and green
signals.
11.8.29 The problem with colour vision standards for pilots and air traffic controllers is that there is very
little information which shows the real, practical implications of colour vision defects on aviation safety. Ideally
one would select only applicants with normal colour vision as measured by the most discriminating tests. This
policy would deny licences to a significant number of individuals who might be able to function safely in the
aviation environment. The question is where to draw the line. Many Contracting States simply define as
acceptable those applicants who obtain a certain score with an authorized set of pseudo-isochromatic test
plates, others accept as “colour safe” colour-deficient applicants who pass certain additional tests.
Tests for colour vision
11.8.30 Tests for colour vision fall into three categories:
a) chromatic confusion plates or discs
b) colour lantern tests
c) anomaloscopes.
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Manual of Civil Aviation Medicine 2(63)
