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and Mizar in the constellation Ursa Major. He measured the distance between the two stars as approximately
one minute of arc at the eye. This unit — 1 minute of arc — is the unity of visual acuity; it corresponds to a
retinal distance of 4 microns (micrometres, μm). A visual acuity of unity indicates a power of resolving detail
1 Swinging flashlight test: Swinging the flashlight back and forth between the two eyes identifies if one pupil
has less light perception than the other. Shine the flashlight at one eye noting the size of both pupils. Then
swing the flashlight to the other eye. If both pupils now dilate then that eye has perceived less light stimulus
than the opposite eye, indicating a defect in the sensory or afferent pathway.
2 After Hjalmar Schiøtz, Norwegian physician (1850-1927).
III-11-4 Manual of Civil Aviation Medicine
subtending one minute of arc at the eye. It is usually expressed as 6/6; an acuity of a half as 6/12 and so on.
This definition of visual acuity is the basis of the optotypes most widely used today. The first test chart of this
type was published by the Dutch ophthalmologist Herman Snellen in 1862.
11.3.2 Test distances usually of 5 or 6 m (16 or 20 ft) are used, as this distance constitutes infinity for the
normal eye and practically no accommodation is required to see clearly. The optotype is constructed so that the
gaps between the letter components subtend an angle of one minute of arc at the prescribed distance.
“Normal” visual acuity is defined as 6/6 (or 20/20 or 1.0). The numerator refers to the test distance and the
denominator to the distance at which a “normal” eye could just see that particular size letter. Conventionally,
a variety of different letters of the alphabet are used in test charts or projectors. This presents problems
because some letters are more difficult to read than others (e.g. B is more difficult to identify than L), and the
recognition of letters is a perceptual process which may vary from one individual to another. The use of symbols
such as Landolt rings or a series of the letter “E” oriented in different directions has the advantage of presenting
symbols of uniform difficulty and of not requiring knowledge of the names of the letters. See Figure 11-1.
11.3.3 Under optimum conditions many normal persons have visual acuity better than 6/6; measurements
between 6/3 and 6/5 are usual. Nevertheless, 6/6 is widely accepted as a satisfactory level for performing most
visual tasks. In addition to the resolving power of the eye there are other factors which affect visual acuity.
These include physical contrast between the test symbols and their background, light adaptation status of the
retina, exposure time of the test type, pupil size, clarity of the ocular media, and the state of the sensorium.
Figure 11-1. Visual symbols in common use
11.3.4 At very low levels of illumination the visual acuity will be poor. As illumination increases visual
LANDOLT RINGS ALTERNATE SYMBOLS
a = Ring size
b = Stroke size
c = Gap size
c
a
b
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-5
acuity improves up to a certain level beyond which there is no further gain. Figure 11-2 illustrates the
relationship between visual acuity and background luminance. Background luminance refers to the white
background of the test chart against which the test symbols are viewed, it does not refer to the luminance of the
walls of the examining room. Visual acuity improves significantly when the background luminance increases
because this enhances the contrast between the black symbols and the white background. Above a
background luminance level of 80 cd/m2, the visual improvement is minimal; this level is the recommended
minimum for visual acuity test charts.
Photometric units
11.3.5 To understand the visibility of objects some knowledge of photometric units is helpful. The term
radiometry applies to measurements of the entire spectrum of radiant energy. Photometry applies to
measurements of the visible portion of the electromagnetic spectrum (380 to 750 nm).
11.3.6 Before describing the more common photometric units, it should be mentioned that the term
brightness refers to the subjective impression of a range of sensations varying from very dim to brilliant. It is a
perception originating in the rods and cones of the retina. It is a complex sensation which is non-linear and
dependent on the state of dark or light adaptation of the retina. Brightness cannot be measured in physical units
and is not the same as luminance.
Figure 11-2. Visual acuity and background luminance
11.3.7 The following are the more important physical units dealing with light:
1/100 1/10 1 10 100 1 000 cd/m2
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