Aeromedical Factors

5. When the page is about 16–18 inches from you, the black X should disappear completely because it has been imaged onto the blind spot of your right eye. (Resist the temptation to move your right eye while the black X is gone or else it reappears. Keep staring at the airplane.) 6. As you continue to look at the airplane, keep moving the page closer to you a few more inches, and the black X will come back into view. 7. There is an interval where you are able to move the page a few inches backward and forward, and the black X will be gone. This demonstrates to you the extent of your blind spot. Figure 17-16. Night vision. 8. You can try the same thing again, except this time with your right eye covered stare at the black X with your expose the rods to the image. This can be done by looking 5° left eye. Move the page in closer and the airplane will to 10° off center of the object to be seen. This can be tried in disappear. a dim light in a darkened room. When looking directly at the light, it dims or disappears altogether. When looking slightly off center, it becomes clearer and brighter. Another way to check your blind spot is to do a similar test outside at night when there is a full moon. Cover your left When looking directly at an object, the image is focused eye, looking at the full moon with your right eye. Gradually mainly on the fovea, where detail is best seen. At night, the move your right eye to the left (and maybe slightly up or ability to see an object in the center of the visual field is down). Before long, all you will be able to see is the large reduced as the cones lose much of their sensitivity and the halo around the full moon; the entire moon itself will seem rods become more sensitive. Looking off center can help to have disappeared. compensate for this night blind spot. Along with the loss of Empty-Field Myopia Empty-field myopia is a condition that usually occurs when flying above the clouds or in a haze layer that provides nothing specific to focus on outside the aircraft. This causes the eyes to relax and seek a comfortable focal distance that may range from 10 to 30 feet. For the pilot, this means looking without seeing, which is dangerous. Searching out and focusing on distant light sources, no matter how dim, helps prevent the onset of empty-field myopia. Cones active Night Vision There are many good reasons to fly at night, but pilots must keep in mind that the risks of night flying are different than during the day and often times higher. [Figure 17-16] Pilots who are cautious and educated on night-flying techniques can mitigate those risks and become very comfortable and proficient in the task. Night blind spot Night Blind Spot It is estimated that once fully adapted to darkness, the rods are 10,000 times more sensitive to light than the cones, making Rods active them the primary receptors for night vision. Since the cones are concentrated near the fovea, the rods are also responsible Pilots must look 5°–10° off center of the for much of the peripheral vision. The concentration of cones object in order for the object to be seen. 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 Figure 17-17. Night blind spot. 17-22