Night vision is of particularly great significance in aviation. Aviators' perception of objects in the night sky is vital to flight during night hours.
Do our eyes function at night just as they do during the day?
No.
The whole dynamics of optical perception change during the night and other scenarios of dimly lit environments. Sensory cells called rods and cones, which are responsible for the images produced in our brain, work in cohesion in daylight conditions.
However, at night, under lighting conditions lower than the moonlight, the cones are useless and visual perception is then the sole responsibility of rods. It is then up to the rods to acquire, sense and transmit the image to the optic nerve.
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Dark Adaptation
The process in which the human eye programs itself to adapt to a dark environment is referred to as dark adaptation.
The eye doesn't instantaneously start functioning efficiently in a dark environment. It may take 30 to 45 mins or even more, for the eye to adjust itself in the dark environment. During this time period, the sensitivity of the rods is increased 100,000 times! These cells, are highly sensitive to light sources, but have poor resolution and cannot distinguish colors.
When this eye, adapted to the dark environment, is momentarily exposed to a bright light source, the cells adapt to the bright environment. Adaptation to bright environments from a darker one, is a process completed in a few seconds.
The Role of Night Vision in Aviation
The following is of great significance in terms of aviation:
- The arrangement of rods in the periphery of the eye, leaves only the peripheral vision of a pilot, usable and dependable during the night. Since the cones are unable to perform in such low-light conditions and they are highly concentrated in the central region of the retina termed as the foveola, there occurs a blind spot in the pilot's visual field at night, which can prove fatal if taken lightly or if not corrected for via off-center viewing techniques.
- The immediate adaptation of the eye to bright lights, in only a matter of seconds, can result in flash blindness. Even a momentary exposure to any bright source of light can cause the eye to readjust and when the bright light disappears, the eye is left blind, trying to readjust to the dark environment again, which takes about 30 to 45 mins. In order to prevent flash blindness, care is taken that the pilots are not exposed to any bright light during night flying.
- Scenarios such as; when everything the pilot sees is dark or very poorly lighted, outside the cockpit, may lead to certain misunderstandings and night illusions. Misunderstanding ground lights for stars, wrong perception of the horizon, miscalculating the altitude due to brightness of runway lights, distance illusion phenomenon, black hole illusion and auto-kinetic effect are just a few common night illusions.
- well educated about all the night illusions.
- they are all required to dark-adapt their eyes before night flights (whenever practical).
- aviators use off-center viewing techniques to scan the sky.
- they avoid bright lights at night.
Resources
Federal Aviation Administration. Airplane Flying Handbook.
Oxford Aviation Services. Joint Aviation Authorities Airline Transport Pilot’s License Theoretical Knowledge Manual.