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| CHAPTER 12. Night Operations
Night Vision Generally, most pilots are poorly informed about night vision. Human eyes never function as effectively at night as the eyes of nocturnal animals, but if humans learn how to use their eyes correctly and know their limitations, night vision can be improved signifi cantly. The human eye is constructed so that day vision is different from night vision. Therefore, it is important to understand the eye’s construction and how the eye is affected by darkness. Innumerable light-sensitive nerves called cones and rods are located at the back of the eye or retina, a layer upon which all images are focused. These nerves connect to the cells of the optic nerve, which transmits messages directly to the brain. The cones are located in the center of the retina, and the rods are concentrated in a ring around the cones. [Figure 12-6]
The function of the cones is to detect color, details, and faraway objects. The rods function when something is seen out of the corner of the eye or peripheral vision. They detect objects, particularly those that are moving, but do not give detail or color—only shades of gray. Both the cones and the rods are used for vision during daylight. Although there is not a clear-cut division of function, the rods make night vision possible. The rods and cones function in daylight and in moonlight; in the absence of normal light, the process of night vision is almost entirely a function of the rods. The fact that the rods are distributed in a band around the cones and do not lie directly behind the pupils makes offcenter viewing (looking to one side of an object) important during night fl ight. During daylight, an object can be seen best by looking directly at it, but at night a scanning procedure to permit off-center viewing of the object is more effective. Therefore, the pilot should consciously practice this scanning procedure to improve night vision. The eye’s adaptation to darkness is another important aspect of night vision. When a dark room is entered, it is diffi cult to see anything until the eyes become adjusted to the darkness. In the adaptation process, the pupils of the eyes fi rst enlarge to receive as much of the available light as possible. After approximately 5 to 10 minutes, the cones become adjusted to the dim light and the eyes become 100 times more sensitive to light than they were before the dark room was entered. About 30 minutes is needed for the rods to become adjusted to darkness; when they do adjust, they are about 100,000 times more sensitive to light than in the lighted area. After the adaptation process is complete, much more can be seen, especially if the eyes are used correctly. After the eyes have adapted to the dark, the entire process is reversed when entering a lighted room. The eyes are fi rst dazzled by the brightness, but become completely adjusted in a few seconds, thereby losing their adaptation to the dark. Now, if the dark room is reentered, the eyes again go through the long process of adapting to the darkness. Before and during night fl ight, the pilot must consider the adaptation process of the eyes. First, the eyes should be allowed to adapt to the low level of light. Then, the pilot should avoid exposing them to any bright white light that would cause temporary blindness and possibly result in serious consequences. Temporary blindness, caused by an unusually bright light, may result in illusions or afterimages until the eyes recover from the brightness. The brain creates these illusions reported by the eyes. This results in misjudging or incorrectly identifying objects, such as mistaking slanted clouds for the horizon or a populated area for a landing fi eld. Vertigo is experienced as a feeling of dizziness and imbalance that can create or increase illusions. The illusions seem very real and pilots at every level of experience and skill can be affected. Recognizing that the brain and eyes can play tricks in this manner is the best protection for fl ying at night. Good eyesight depends upon physical condition. Fatigue, colds, vitamin defi ciency, alcohol, stimulants, smoking, or medication can seriously impair vision. Keeping these facts in mind and taking appropriate precautions should help safeguard night vision. In addition to the principles previously discussed, the following actions aid in increasing night vision effectiveness:
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