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Testing the sensory status in strabismus
11.7.44 The presence of fusion, diplopia or suppression can be determined with the Worth four-dot test
(W4D) which uses a box illuminated from the inside and presenting four dots — a red one at the top, a green
dot at either side and a white dot at the bottom (Figure 11-15). The test can be done at 6 m (20 ft) or at near,
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-47
and small flashlight Worth four-dot tests are available.
11.7.45 The applicant wears spectacles having a red lens on one side and a green lens on the other. These
lenses can be reversed. With the red lens in front of the applicant’s right eye and the green in front of his left
eye the following results may be described:
a) Five dots – two red and three green = diplopia
b) Four dots with the bottom one described as being a combination of red and green or changing
from red to green and back = fusion
c) Two red dots only = suppression of the left eye
d) Three green dots only = suppression of the right eye
Figure 11-15. Worth four-dot presentation
Convergence
11.7.46 Convergence is an act by which the eyes are turned towards each other in order to maintain
binocular vision when near objects are regarded. There is an approximate relationship between convergence
and accommodation. The unit of convergence is the metre angle which is the amount of convergence required
to view an object 1 m away. In ordinary clinical work it is usually sufficient to measure convergence by having
the applicant focus on a small target which is brought progressively closer to the eyes until diplopia is reported
or the examiner sees that fusion cannot be maintained and one eye deviates outwards. As an approximate
value, this “near point of convergence” is measured in cm. Normal values are usually between 6 and 8 cm. If the
near point is 10 cm or more, the convergence is insufficient.
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III-11-48 Manual of Civil Aviation Medicine
Evaluation of significant defects of binocular vision
11.7.47 The proper evaluation of an applicant with significant ocular muscle imbalance who does or who
might experience diplopia, asthenopia or both, requires referral to an appropriate vision care specialist for an
orthoptic evaluation to determine the applicant’s fusional amplitudes. This is done by measuring the applicant’s
ability to maintain fusion when the retinal image in one eye is moved either with prisms or with a major
amblyoscope (synoptophore).
11.7.48 More detailed advice on the evaluation of significant defects of binocular vision is given in the
Attachment to this chapter.
11.8 COLOUR VISION
Introduction
11.8.1 The increasing use of colour-coded information in flight information display systems means that
adequate colour perception continues to be important for flight crew and air traffic controllers.
11.8.2 The traditional conventions “red for danger or stop” and “green for safety or go” are in common use
worldwide and unlikely to change in the foreseeable future.
11.8.3 In addition, aviation personnel need to be able to distinguish colours on charts and in the terrain.
11.8.4 The colours most widely used on the flight deck, in the aircraft cabin, on external airborne lighting,
in air traffic control instruments and on aerodrome runways are red, green, yellow, orange, blue, cyan, magenta
and white.
11.8.5 Deficient colour vision is often referred to as colour blindness, but this is an inaccurate use of a
term which refers to monochromatic vision. Colour-blind individuals are very rare and, in addition to their
monochromatic vision, they generally have poor visual acuity, nystagmus and photophobia.
11.8.6 Individuals with less severe colour vision defects are common — some eight per cent of all males
and about 0.8 per cent of all females will fail the more stringent colour perception tests. More than 99 per cent
of these will have red-green deficiencies.
11.8.7 While it is unfortunate that the ability to distinguish red and green is the most common variety of
colour vision defect, it does not mean that every applicant with a red-green colour deficiency must be denied
a licence.
11.8.8 Because colour perception is a purely subjective phenomenon, it is impossible to know exactly
what sensation an individual has when viewing light of a particular wavelength. What can be demonstrated is
that individuals with colour vision defects are unable to distinguish variations in colour that are readily apparent
to a person with normal colour vision.
11.8.9 There are all grades of colour vision defect from subtle to severe, and the question which arises is
how much of a colour vision defect can be allowed before an individual must be considered unable to operate
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