Atlas Aviation - PILOT VISION

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PILOT VISION

Vision is a pilot's most important sense to obtain reference information during flight. Most pilots are familiar with the optical aspects of the eye. Before we start flying, we know whether we have normal uncorrected vision, whether we are farsighted or nearsighted, or have other visual problems. Most of us who have prescription lenses-contacts or eyeglasses-have learned to carry an extra set of glasses with us when we fly, just as a backup. But, vision in flight is far more than a lesson in optics. Seeing involves the transmission of light energy (images) from the exterior surface of the cornea to the interior surface of the retina (inside the eye) and the transference of these signals to the brain.

The Anatomy of the Eye

Light from an object enters the eye through the cornea and then continues through the pupil.
The opening (dilation) and closing (constriction) of the pupil is controlled by the iris, which is the colored part of the eye. The function of the pupil is similar to that of the diaphragm of a photographic camera: to control the amount of light.
The lens is located behind the pupil and its function is to focus light on the surface of the retina.
The retina is the inner layer of the eyeball that contains photosensitive cells called rods and cones. The function of the retina is similar to that of the film in a photographic camera: to record an image.
The cones are located in higher concentrations than rods in the central area of the retina known as the macula, that measures about 4.5 mm in diameter. The exact center of the macula has a very small depression called the fovea that contains cones only. The cones are used for day or high-intensity light vision. They are involved with central vision to detect detail, perceive color, and identify far-away objects.
The rods are located mainly in the periphery of the retina - an area that is about 10,000 times more sensitive to light than the fovea. Rods are used for low-light intensity or night vision and are involved with peripheral vision to detect position references including objects (fixed and moving) in shades of grey, but cannot be used to detect detail or to perceive color.
Light energy (an image) enters the eyes and is transformed by the cones and rods into electrical signals that are carried by the optic nerve to the posterior area of the brain (occipital lobes). This part of the brain interprets the electrical signals and creates a mental image of the actual object that was seen by the person.

The Anatomical Blind Spot

Illustration of the eye's anatomical blind spot in an aircraft cockpit, showing opposing sircraft 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.

The Night Blind Spot

Graphic demonstrating how cones and rods affect vision, in this case, night vision 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

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 1/2 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.

Visual showing normal vertical field of 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.

Graphic illustrating normal horizontal field of vision