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Figure 9-5. The reduction of atmospheric pressure that accompanies flying can produce physical problems for scuba divers.
Figure 9-6. The human eye.
The
rods and cones (film) of the retina are the receptors which record the image and transmit it through the optic nerve to the brain for interpretation.Rods and ConesFovea(All Cones)The pupil (aperture) is the opening at the center of the iris. The size of the pupil is adjusted to control the amount of light entering the eye.PUPILLight passes through the cornea (the transparent window on the front of the eye) and then through the lens to focus on the retina.CORNEALensIrisRod ConcentrationRetinaOptic Nervefor registering images. Light enters through the cornea at the front of the eyeball, travels through the lens, and falls on the retina. The retina contains light sensitive cells that convert light energy into electrical impulses that travel through nerves to the brain. The brain interprets the electrical signals to form images. There are two kinds of light-sensitive cells in the eyes: rods and cones. [Figure 9-6]
The cones are responsible for all color vision, from appreciating a glorious sunset to discerning the subtle shades in a fine painting. Cones are present throughout the retina, but are concentrated toward the center of the field of vision at the back of the retina. There is a small pit called the fovea where almost all the light sensing cells are cones. This is the area where most “looking” occurs (the center of the visual field where detail, color sensitivity, and resolution are highest).
While the cones and their associated nerves are well suited to detecting fine detail and color in high light levels, the rods are better able to detect movement and provide vision in dim light. The rods are unable to discern color but are very sensitive at low light levels. However, a large amount of light overwhelms the rods, and they take a long time to “reset” and adapt to the dark again. There are so many cones in the fovea that the very center of the visual field has virtually no rods at all. Therefore, the middle of the visual field is not very sensitive in low light. Farther from the fovea, the rods are more numerous and provide the major portion of night vision.Vision in Flight
Of all the senses, vision is the most important for safe flight. Most of the things perceived while flying are visual or heavily supplemented by vision. As remarkable and vital as it is, vision is subject to some limitations, such as illusions and blind spots. The more a pilot understands about the eyes and how they function, the easier it is to use vision effectively and compensate for potential problems.
The eye functions much like a camera. Its structure includes an aperture, a lens, a mechanism for focusing, and a surface
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Figure 9-8. Night blind spot.
Cones active
Rods activeNight blind spot
Figure 9-7. The eye’s blind spot.center can help compensate for this night blind spot. Along with the loss of acuity (sharpness) and color at night, depth perception and judgment of size may be lost.
Balloon pilots, while not normally conducting flight operations at night, can experience similar issues when flying in low light conditions, particularly if there is haze or reduced visibility. In those instances where the balloon is operated at night, such as during a night “glow” or tether, night vision can be immediately destroyed by the light from the burner. It may take several minutes for the pilot to recover his or her vision, time in which complete awareness of his or her
The area where the optic nerve enters the eyeball has no rods or cones, leaving a blind spot in the field of vision. Normally, each eye compensates for the other’s blind spot. Figure 9-7 provides a dramatic example of the eye’s blind spot. Cover the right eye and hold this page at arm’s length. Focus the left eye on the X in the right side of the windshield, and notice what happens to the balloon while slowly bringing the page closer to the eye.Empty-Field Myopia
Another problem associated with flying at night or in reduced visibility is empty-field myopia, or induced nearsightedness. With nothing on which to focus, the eyes automatically focus on a point just slightly ahead of the aircraft. Searching out and focusing on distant light sources, no matter how dim, helps prevent the onset of empty-field myopia. Night Vision
It is estimated that once fully adapted to darkness, the rods are 10,000 times more sensitive to light than the cones, making them the primary receptors for night vision. Since the cones are concentrated near the fovea, the rods are also responsible for much of the peripheral vision. The concentration of cones in the fovea can make a night blind spot in the center of the field of vision. To see an object clearly at night, the pilot must expose the rods to the image. This can be done by looking 5° to 10° off center of the object to be seen. This can be tried in a dim light in a darkened room. When looking directly at the light, it dims or disappears altogether. [Figure 9-8]
　
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