According to the FAA, a pilot's most important sensory asset to obtain reference information during flight is their eyes. All pilots know before they start flying airplanes whether or not we have normal uncorrected vision, or whether we may be near or far sighted. And those of us who wear glasses or contact lenses know we should always carry a spare pair of glasses with us when we fly, just to be safe.But vision in flight is far more important than a lesson in optics. Seeing involves the transmission of images from the exterior surface of the cornea to the interior surface of the retina and then the transfer of those signals to the brain.
The area where the optic nerve connects to the retina in the back of each eye is known as the optic disk. There is a total absence of cones and rods in this area, and, consequently, each eye is completely blind in this spot. Under normal binocular vision conditions this is not a problem, because an object cannot be in the blind spot of both eyes at the same time. On the other hand, where the field of vision of one eye is obstructed by an object (windshield post), a visual target (another aircraft) could fall in the blind spot of the other eye and remain undetected. This is known as the anatomical blind spot. The “Night Blind Spot” appears under conditions of low ambient illumination due to the absence of rods in the fovea, and involves an area 5 to 10 degrees wide in the center of the visual field. Therefore, if an object is viewed directly at night, it may go undetected or it may fade away after initial detection due to the night blind spot. The fovea is the small depression located in the exact center of the macula that contains a high concentration of cones but no rods, and this is where our vision is most sharp. While the normal field of vision for each eye is about 135 degrees vertically and about 160 degrees horizontally, only the fovea has the ability to perceive and send clear, sharply focused visual images to the brain. This foveal field of vision represents a small conical area of only about 1 degree. To fully appreciate how small a one-degree field is, and to demonstrate foveal field, take a quarter from your pocket and tape it to a flat piece of glass, such as a window. Now back off 4 ½ feet from the mounted quarter and close one eye. The area of your field of view covered by the quarter is a one-degree field, similar to your foveal vision. Now we know that you can see a lot more than just that one-degree cone. But, do you know how little detail you see outside of that foveal cone? For example, outside of a ten-degree cone, concentric to the foveal one-degree cone, you see only about one-tenth of what you can see within the foveal field. In terms of an oncoming aircraft, if you are capable of seeing an aircraft within your foveal field at 5,000 feet away, with peripheral vision you would detect it at 500 feet. Another example: using foveal vision we can clearly identify an aircraft flying at a distance of 7 miles; however, using peripheral vision (outside the foveal field) we would require a closer distance of .7 of a mile to recognize the same aircraft. That is why when you were learning to fly, your instructor always told you to “put your head on a swivel,” to keep your eyes scanning the wide expanse of space in front of your aircraft. Do you remember when you were learning to fly, and your instructor kept telling you to keep your head moving and your eyes moving as you scanned the instruments? Well, now you know why. Eyes are your most effective sensory asset, and need to be protected at all times. With summer nearly upon us it is time to remember our sunglasses, and original aviator sunglasses are the best for pilots. While there are many choices of sunglasses, the U.S. government issues Randolph aviator sunglasses to its military pilots and soldiers. More on vision tomorrow. Until next time keep your wings straight and level Hersch! JetAviator7 Sometimes, flying feels too godlike to be attained by man. Sometimes, the world from above seems too beautiful, too wonderful, too distant for human eyes to see . . . — Charles A. Lindbergh, 'The Spirit of St. Louis,' 1953.' ps: Don't forget to sign up for our newsletter "All Things Aviation" here!