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11.3.16 In an ordinary office, the luminance of lightly coloured walls are approximately:
Moderately lit room 15 – 30 cd/m2
Ordinarily lit room 30 – 60 cd/m2
Brightly lit room 60 – 100 cd/m2
11.3.17 The luminance of the white parts of a transparent visual acuity chart mounted in an examination
box and illuminated from behind is 200 – 500 cd/m2. The additional luminance given by the ambient room
lighting is insignificant.
11.3.18 Visual acuity testing should be done in a well-lit room. The ordinary room lights in the examining
room should be left on. Extremes of room illumination, either very dark or very bright, may affect visual acuity
measurements and should be avoided.
Exposure time
11.3.19 Exposure time is not important during ordinary clinical measurement of visual acuity because the
times used are well above the threshold values of about 0.5 second.
11.3.20 However, exposure time does become important in the dynamic visual environment of many flight
operations. Angular motion exceeding 10 degrees/second produces significant blurring of vision. During the
final seconds before touchdown the pilot relies on ground references when manoeuvring the aircraft, even with
automated landing systems. With a landing speed of 145 kt (approximately 270 km/h) a surface area of 500 m2
is all that can be inspected without exceeding the critical angular velocity (angular velocity at which visual acuity
begins to deteriorate).
Physical contrast
11.3.21 The physical contrast between an object and its background is a limiting factor in the resolving
power of the eye. The contrast threshold is the just noticeable difference between an object and its background.
Reflection from clean white paper is about 75 per cent while that from a black symbol is about 5 per cent. The
contrast is (75–5)/75 or 93 per cent providing excellent visibility. Old faded charts or improperly maintained
vision chart projectors may present contrast of only around 30 per cent which is a critical value in maintaining
optimum visual acuity. The importance of proper test equipment is obvious.
11.3.22 There are many situations in aviation where contrast is different from that normally used during
clinical testing. Low light levels causing reduced vision are the most frequent but very high light levels can
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-9
produce glare sufficient to reduce visual acuity.
Pupil diameter
11.3.23 Low light conditions cause dilatation of the pupil with resulting spherical aberration and blurring of
the retinal image. This blurring may stimulate accommodation with induced myopia further reducing distance
visual acuity. High light levels, up to a certain value, induce miosis. This reduction of the optical aperture leads
to increased depth of focus and, by masking of the refractive errors of the eye, to improved acuity.
Test objects and symbols
11.3.24 A wide variety of visual test objects are in use throughout the world. An attempt to have the Landolt
rings adopted internationally as the test symbol was unsuccessful, and most of the designated medical
examiners in ICAO’s Contracting States continue to use the Snellen letters or variations based on the same
visual angle. Since the object of the examination is to assess the applicant’s general visual efficiency and refer
those who do not meet the required standards for further evaluation, the use of Snellen type symbols is
acceptable. The letters of the alphabet vary widely in their legibility, e.g. “L”, “I” and “T” are much easier to
identify than “G”, “R” and “B”. For this reason, uniform symbols such as the Landolt rings or the letter E oriented
in different directions are better test objects from a scientific point of view.
11.3.25 Vision test charts of different designs show a wide variation in the number, appearance and choice
of symbols in each row and the progression in size of the symbols. These variations introduce discrepancies in
visual acuity measurements. To determine whether or not the visual requirements of Annex 1 are met, an
internationally uniform standard such as the Landolt rings or the E-test is desirable. The chart should have, as
a minimum, symbols representing the following levels of visual acuity: 6/60 (20/200, 0.1), 6/12 (20/40, 0.5), 6/9
(20/30, 0.7) and 6/6 (20/20, 1.0). A minimum of ten symbols should be available for the 6/12, 6/9 and 6/6 levels.
Figure 11-1 shows examples of Landolt rings together with other symbols commonly used.
11.3.26 A chart of Landolt rings should contain ten symbols in each row with random gaps at 3, 6, 9 and
12 o’clock. Two different charts should be available to prevent possible memorization.
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Manual of Civil Aviation Medicine 2(38)
