Humans regard vision as their most valuable sense, yet they fail to appreciate what a remarkable and complicated organ the human eye is. The human eye has reached the absolute threshold of sensitivity to light and, under certain ideal conditions, can see a candle flame at a distance of over 15 miles. All eyes--including a cat's--are alike in that none of them can see in total darkness. Most people are completely uninformed about night vision and feel that there is nothing to be learned about the subject.
However, it is important for the pilot to understand the construction of the human eye, since it is so constructed that to see effectively at night it must be used differently from the daytime. The eye might be compared to a camera, since it consists of a lens that focuses the image upon the retina which corresponds to film in the camera. The retina is made up of various layers of cells, among them the significant cells for vision--the rods and cones. The cones, of which there are more than seven million in each human eye, are packed closely together in the very center of the retina. The rods are concentrated in a ring around the cones.
The function of the cones is to detect color, details, and far away objects. The rods function when something is seen out of the corner of the eye. They detect objects, particularly moving ones, but do not give detail or color, only shades of gray. Both the cones and the rods are used for vision during daylight and bright moonlight. In the absence of these, the process of vision depends almost entirely upon the rods. When the rods become adjusted to darkness--a process which requires about 30 minutes--they become about 100,000 times more sensitive to light than they were during daylight. However, the fact that rods are distributed in a band around the cones makes "off center" viewing important during night flight.
During daylight an object is best seen by looking directly at it, but at night a scanning procedure to permit "off center" viewing of the object is more effective. The pilot should consciously practice this scanning procedure to improve night vision. Smoking, the presence of carbon monoxide, hypoxia, Vitamin A deficiency, and the use of certain drugs adversely affect the eyes' night vision capabilities. It is important for the pilot to maintain good physical condition.
Cockpit lighting is also of extreme importance. While dim red lighting has the least adverse effect on night vision, it severely distorts colors. Older pilots may experience extreme difficulty in focusing the eyes on objects inside the cockpit.
The tendency in recent years has been toward the use of diffused white
or blue-white instrument lighting. In addition to night vision, the pilot
should also be aware of how to cope with illusions encountered during night
flight. Refer to Advisory Circulars 61-23B, "Pilot's Handbook of Aeronautical
Knowledge," and 61 21A, "Flight Training Handbook," and the Aeronautical
Information Manual for further information on the above subjects.
by Chuck Urquhart
Mr. Urquhart is a designated pilot examiner in the Pittsburgh, PA area.
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