 Vision is a pilot's most important sense to obtain reference information during flight. The eyes send pictures to the brain about the aircraft's position, velocity and attitude relative to the ground. Here is how the eye works. Light from an object enters the eye through the cornea and then continues through the pupil. The dilation or constriction of the pupil is controlled by the iris, which is the colored part of the eye. The pupil's function is similar to that of the aperture of a photographic camera. It controls the amount of light that enters the eye. 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 retina's function is to record an image similar to that of the film in a camera. The macula in the center of the retina have cones that are more densely distributed. In the center of the macula there is a tiny depressed spot called the fovea centralis, which contains only cones and is the area with the sharpest vision. The rods are mainly located 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 conditions or night vision. They are involved with our peripheral vision to detect position references, including objects, both fixed and moving, in shades of gray. They cannot be used to detect detail or to perceive color. Light stimulates cones and rods, which in turn initiate nervous impulses that are carried to the brain via the optic nerve. The point where the optic nerve connects to the eye is known as the optic disc. There is an absence of cones and rods in the optic disc. Therefore, the eye is completely blind in this spot, day or night. Under normal binocular vision conditions, that is when both eyes are used, 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, when the field of vision of one eye is obstructed by an object, like a windshield post, then a visual target could fall on the blind spot of the other eye and remain undetected, like another aircraft. The night blind spot appears under conditions of low ambient illumination, due to the absence of rods on 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. 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, also known as central vision, represents a small conical area of only about one degree. To fully appreciate how small a one degree field is, and to demonstrate the foveal field, take a quarter from your pocket and tape it to a flat piece of glass, such as a window. Now, back off four and a half 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. In terms of an oncoming aircraft, if you are capable of seeing an aircraft within your foveal field from 5,000 feet away, using only peripheral vision, you wouldn't detect it until you were only 500 feet away. Another example is, 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 tenths 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. The types of vision are photopic, mesopic and scotopic. Photopic vision occurs during daytime or under conditions of high-intensity artificial illumination. Mesopic vision occurs at dawn, dusk, or under full moonlight levels and is characterized by decreasing visual acuity and color vision. Scotopic vision occurs during nighttime, partial moonlight, or low-intensity artificial illumination. Also, under these conditions, if you look directly at an object for more than a few seconds, the image of the object fades away completely due to the night blind spot. Off-center scanning with peripheral vision provides the only means of seeing very dim objects in the dark. Several factors affect vision. The greater the object's size, ambient illumination, contrast, viewing time, and atmospheric clarity, the better the visibility is of such an object. During the day, objects can be identified easier at a great distance with good detailed resolution. At night, the identification range of dim objects is limited and the detailed resolution is poor. Excessive ambient illumination, especially from light reflected off the canopy, surfaces inside the aircraft, clouds, water, snow, and desert terrain can produce glare that may cause uncomfortable squinting, eye tearing, and even temporary blindness. Other impediments to clear vision include the presence of eye disorders such as nearsightedness, which is impaired focusing on distant objects, far-sightedness, which is impaired focusing on near objects, astigmatism, which is impaired focusing of objects in different meridians, and presbyopia, which is age-related impaired focusing on near objects. Self-imposed stresses such as self-medication, alcohol, tobacco use, low blood sugar, and sleep deprivation or fatigue can seriously impair your vision. Flights to altitudes above 10,000 feet during the day and above 5,000 feet at night without the use of supplemental oxygen can result in hypoxia, which impairs visual performance. Other factors that may have an adverse effect on visual performance include scratched dirty windshield, improper illumination of the cockpit or instruments, scratched dirty instrumentation, use of cockpit red lighting, inadequate cockpit environmental control such as temperature and humidity, inappropriate eyewear, sustained visual workload during flight. The natural ability to focus your eyes is critical to flight safety. It is important to know that your eyes may require several seconds to refocus when switching views between near, intermediate, and distant objects. For example, like switching views between reading charts, monitoring instruments, and looking for traffic or external visual references. Fatigue can lead to impaired visual focusing, which causes the eyes to overshoot or undershoot the target. It can also affect a pilot's ability to quickly change focus between near, intermediate, and distant vision. The most common symptoms of visual fatigue include blurred vision, excessive tearing, heavy eyelid sensation, headaches, and burning, scratchy or dry-eyed sensation. Distant focus without a specific object to look at tends to diminish rather quickly. If you fly over water or under hazy conditions with the horizon obscured or between cloud layers at night, your distance focus relaxes after about 60 to 80 seconds. If there's nothing specific on which to focus, your eyes revert to a relaxed, intermediate focal distance of 10 to 30 feet. This means that you are looking without actually seeing anything, which is dangerous. The answer to this phenomenon is to condition your eyes for distant vision. Focus on the most distant object that you can see, even if it's just a wing tip. Do this before you begin scanning the sky in front of you. As you scan, make sure you repeat this refocusing exercise often. Dark adaptation is the process by which the eyes adapt for optimal night visual acuity under conditions of low ambient illumination. The eyes require about 30 to 40 minutes to fully adapt to minimal lighting conditions. The lower the starting level of illumination, the more rapidly complete dark adaptation is achieved. To minimize the time necessary to achieve complete dark adaptation and to maintain it, you should avoid inhaling carbon monoxide from tobacco smoking or exhaust fumes. Adjust instrument and cockpit lighting to the lowest level possible. Avoid prolonged exposure to bright lights. Use supplemental oxygen when flying at night above 5,000 feet MSL. If dark adapted eyes are exposed to a bright light source, like search lights, landing lights, flares or laser lights, for a period in excess of one second, night vision is temporarily impaired. A study of 50 mid-air collisions revealed that only 8% were head-on. However, 42% were collisions between aircraft heading in the same direction. So compared with opposite direction traffic, your chances of having a mid-air collision are over 5 times greater with an aircraft you are overtaking or one that is overtaking you. Scanning the sky for other aircraft is a very important factor in avoiding mid-air collisions. It should cover all areas of the sky visible from the cockpit and monitoring cockpit instrumentation as well. You can accomplish this by performing a series of short, regularly spaced eye movements that bring successive areas of the sky into the central or foveal visual field. Each movement should not exceed 10 degrees and each area should be observed for at least one second to enable detection. To see and identify objects under conditions of low ambient illumination, avoid looking directly at an object for more than 2 to 3 seconds because it will disappear. Instead, use the off-center viewing that consists of searching movements of the eyes 10 degrees above, below or to either side to locate an object. By switching your eyes from one off-center point to another every 2 to 3 seconds, you will continue to detect the object. Pilots should practice this off-center scanning technique to improve safety during night flights. Because vision is so important, monocular vision and contact lenses are two areas of concern with regard to medical certification. A pilot with one eye, monocular vision or with effective visual acuity equivalent to monocular vision may be considered for any class of medical certification. This is done through the special issuance procedure of Part 67, Code of Federal Air Regulations. Effective visual acuity means the best corrected distant visual acuity in the poor eye is not better than 2200. The use of contact lenses has been permitted to satisfy the distant visual acuity requirements for a Civil Airman Medical Certificate since 1976. However, monovision contact lenses, a technique of fitting patients who require reading glasses with one contact lens for distant vision and the other lens for near vision are not acceptable for piloting an aircraft. If you notice any change in your visual capabilities, bring it to the attention of your Aviation Medical Examiner or AME. If you use corrective glasses or contacts, carry an extra pair with you when you fly. Learn about your own visual strengths and weaknesses. Changes in vision may occur imperceptibly or very rapidly. Periodically self-check your range of visual acuity by trying to see details at near, intermediate and distant points. In review, vision is a pilot's most important sense to obtain reference information during flight. Vision works by having light from an object enter the eye through the cornea. The pupil controls the amount of light that enters the eye. The lens focuses light on the surface of the retina. The visual process occurs as the retina records an image similar to that of the film in a camera. Light stimulates cones and rods which in turn initiate nervous impulses that are carried to the brain via the optic nerve. The point where the optic nerve connects to the eye is known as the optic disc. The night blind spot appears under conditions of low ambient illumination due to the absence of rods on the phobia. Normal field of vision for each eye is about 135 degrees vertically and about 160 degrees horizontally. This foveal field of vision represents a small conical area of only about one degree. The types of vision are photopic vision occurs during daytime, mesopic vision occurs at dawn, dusk or under full moonlight level. Skatopic vision occurs during nighttime. The factors that affect vision are object size, ambient illumination, contrast, viewing time and atmospheric clarity. Impediments to clear vision include nearsightedness, impaired focusing of distant objects, far sightedness, impaired focusing of near objects, astigmatism, impaired focusing of objects in different meridians and presbyopia which is age-related impaired focusing on near objects. Outside stressors that can affect vision are self-imposed stresses such as self-medication, alcohol, tobacco use, low blood sugar and sleep deprivation or fatigue. Flights to altitudes above 10,000 feet during the day and above 5,000 feet at night without the use of supplemental oxygen. Dark adaptation. It is the process by which the eyes adapt for optimal night visual acuity under conditions of low ambient illumination. To achieve complete dark adaptation you should avoid inhaling carbon monoxide, adjust cockpit and instrument lighting, avoid prolonged exposure to bright lights, use supplemental oxygen when flying above 5,000 feet at night, monocular vision and monovision contact lenses. Because vision is so important these are two areas of concern with regard to medical certification. If you notice any change in your visual capabilities bring it to the attention of your Aviation Medical Examiner or AME.