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most sharp. While the normal
field of vision for each
eye is about 135 degrees
vertically and about 160 degrees
horizontally, only the
fovea has the ability to perceive
and send clear, sharply
focused visual images to
the brain. This foveal field
of vision represents a small
conical area of only about 1
degree. To fully appreciate
how small a one-degree field is, and to demonstrate foveal field, take a quarter from
your pocket and tape it to a flat piece of glass, such as a window. Now back off 4
½ feet from the mounted quarter and close one eye. The area of your field of view
covered by the quarter is a one-degree field, similar to your foveal vision.
Now we know that you can see a lot more than just that one-degree cone. But,
do you know how little detail you see outside of that foveal cone? For example,
outside of a ten-degree cone, concentric to the foveal one-degree cone, you see only
about one-tenth of what you can see within the foveal field. In terms of an oncoming
aircraft, if you are capable of seeing an aircraft within your foveal field at 5,000
feet away, with peripheral vision you would detect it at 500 feet. Another example:
using foveal vision we can clearly identify an aircraft flying at a distance of 7 miles;
however, using peripheral vision (outside the foveal field) we would require a closer
distance of .7 of a mile to recognize the same aircraft. That is why when you were
learning to fly, your instructor always told you to “put your head on a swivel,” to
keep your eyes scanning the wide expanse of space in front of your aircraft.
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Types of Vision
Photopic Vision. During daytime or high intensity artificial illumination conditions,
the eyes rely on central vision (foveal cones) to perceive and interpret sharp
images and color of objects.
Mesopic Vision. Occurs at dawn, dusk, or under full moonlight levels, and is
characterized by decreasing visual acuity and color vision. Under these conditions,
a combination of central (foveal cones) and peripheral (rods) vision is required to
maintain appropriate visual performance.
Scotopic Vision. During nighttime, partial moonlight, or low intensity artificial
illumination conditions, central vision (foveal cones) becomes ineffective to maintain
visual acuity and color perception. Under these conditions, if you look directly
at an object for more than a few seconds, the image of the object fades away completely
(night blind spot). Peripheral vision (off-center scanning) provides the only
means of seeing very dim objects in the dark.
Factors Affecting Vision
The greater the object size, ambient illumination, contrast, viewing time, and atmospheric
clarity, the better the visibility of such an object.
During the day, objects can be identified easier at a great distance with good detail
resolution. At night, the identification range of dim objects is limited and the detail
resolution is poor.
Surface references or the horizon may become obscured by smoke, fog, smog,
haze, dust, ice particles, or other phenomena, although visibility may be above Visual
Flight Rule (VFR) minimums. This is especially true at airports located adjacent
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to large bodies of water or sparsely populated areas where few, if any, surface references
are available. Lack of horizon or surface reference is common on over-water
flights, at night, and in low-visibility conditions.
Excessive ambient illumination, especially from light reflected off the canopy, surfaces
inside the aircraft, clouds, water, snow, and desert terrain can produce glare that
may cause uncomfortable squinting, eye tearing, and even temporary blindness.
Presence of uncorrected refractive eye disorders such as myopia (nearsightedness
— impaired focusing of distant objects), hyperopia (farsightedness — impaired focusing
of near objects), astigmatism (impaired focusing of objects in different meridians),
or presbyopia (age-related impaired focusing of near objects).
Self-imposed stresses such as self-medication, alcohol consumption (including
hangover effects), tobacco use (including withdrawal), hypoglycemia, and sleep deprivation/
fatigue can seriously impair your vision.
Inflight exposure to low barometric pressure without the use of supplemental oxygen
(above 10,000 ft during the day and above 5,000 ft at night) can result in hypoxia,
which impairs visual performance.
Other factors that may have an adverse effect on visual performance include:
windscreen haze, improper
illumination of
the cockpit and/or instruments,
　
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