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Instrument Flying Handbook Menu>Human
Factors>Vision Under Dim and Bright
Illumination
Under
conditions of dim illumination, aeronautical charts and aircraft
instruments can become unreadable unless adequate cockpit lighting
is available. In darkness, vision becomes more sensitive to
light; this process is called dark adaptation. Although exposure
to total darkness for at least 30 minutes is required for complete
dark adaptation, a pilot can achieve a moderate degree of dark
adaptation within 20 minutes under dim red cockpit lighting.
Red light distorts colors, especially on aeronautical charts,
and makes it very difficult for the eyes to focus on objects
inside the aircraft. Pilots should use it only where optimum
outside night vision capability is necessary. White cockpit
lighting should be available when needed for map and instrument
reading, especially under IMC conditions.
Dark adaptation is impaired by exposure to
cabin pressure altitudes above 5,000 feet, carbon monoxide inhaled
through smoking and from exhaust fumes, deficiency of Vitamin
A in the diet, and by prolonged exposure to bright sunlight.
Since any degree of dark adaptation is lost within a few seconds
of viewing a bright light, pilots should close one eye when
using a light to preserve some degree of night vision. During
night flights in the vicinity of lightning, cockpit lights should
be turned up to help prevent loss of night vision due to the
bright flashes.
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