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safely in the aviation environment.
11.8.10 The section of Annex 1 dealing with colour perception states that the applicant shall be required to
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-49
demonstrate ability to perceive readily those colours the perception of which is necessary for the safe
performance of his duties. Precise physical and physiological criteria cannot be given because of the large
number of variables in different viewing situations.
11.8.11 Simple practical tests such as the ability to name correctly signal flare or signal light colours give
information only about the specific test situation and are of limited value.
Physiology of colour perception and colour deficiency
11.8.12 Colour is a subjective phenomenon. The three subjective attributes of colour vision are:
Hue this is an attribute associated with the dominant wavelengths of the spectrum and
refers to how we perceive an object’s colour, e.g. red, yellow, blue.
Saturation also called “chroma”, this refers to the vividness or dullness of a colour and indicates
the degree of absence of whiteness.
Lightness also called “value”, this refers to the luminous intensity of a coloured light or the
amount of light the colour reflects, and it distinguishes between the lightness and
darkness of a colour.
These three attributes are not mutually independent.
11.8.13 Like other visual functions, colour perception is only possible when certain stimulus thresholds are
reached. To be identified, a coloured object must be large enough and bright enough to exceed those
thresholds. Location in the visual field, duration of exposure and contrast with the surround are also important.
During normal bright illumination (photopic vision) the peripheral retina is less colour-sensitive than the central
retina. In dim illumination (scotopic vision) only the retinal rods are functioning, and colour perception is not
possible.
11.8.14 Under normal circumstances the human eye responds to the part of the electromagnetic spectrum
between 380 nm (violet) and 750 nm (red) although at very high intensities this range may be increased. This
is the visible spectrum. The ability of the eye to distinguish between different wavelengths is the basis of that
part of colour vision called hue discrimination, i.e. what colour is the object. This ability to distinguish between
different hues varies in different parts of the visible spectrum. Near the limits of the spectrum, particularly at the
red end, large differences in wavelength are necessary to produce a perceptible change in hue. Near the centre
of the spectrum the sensitivity of the eye is maximal and in the regions around 495 nm (blue-green) and 595 nm
(orange-yellow) wavelength differences as small as 1 nm can be detected.
11.8.15 In 1895, Johannes von Kries (1853-1928), professor of physiology at Freiburg (Germany),
elaborated on the work of his predecessors to lay down the principles of the duplicity theory of vision which
suggests two distinct types of visual activity in the retina — a rod-mediated mechanism which operates at low
light-levels and is achromatic, and a cone-mediated mechanism which operates at high light-levels and is
responsible for colour perception. Low light-level vision is called scotopic, and high light-level vision is called
photopic. Most of our normal viewing activity takes place between these extremes, involves both rods and
cones and is called mesopic vision.
11.8.16 Colour perception, like other visual functions, is a complex process involving both retinal and
III-11-50 Manual of Civil Aviation Medicine
occipital visual cortical activity. The traditional trichromatic theory (the Young-Helmholtz theory10), while it does
not explain certain things like colour constancy, coloured shadows and some coloured after-images, does
explain most of the observed facts about colour vision and is useful in understanding colour vision defects. The
theory was proved in a 1983 experiment when microspectrophotopic readings of single eye cone cells were
obtained.http://en.wikipedia.org/wiki/Young%E2%80%93Helmholtz_theory - cite_note-0
11.8.17 There are three populations of retinal cones. One contains a visual pigment with maximum
sensitivity in the red portion of the spectrum, the second contains a pigment with maximum sensitivity in the
green portion of the spectrum and the third group has pigment with maximum sensitivity in the blue portion of
the spectrum.
11.8.18 There is some overlap in the spectral sensitivity curves but basically one can think of red sensitive
cones, green sensitive cones and blue sensitive cones. By appropriate stimulation of these three types of
cones, all spectral colours can be perceived.
　
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