noVision is a pilot’s most important sense to obtain reference information duringflight. Most pilots are familiar with the optical aspects of the eye. Before westart flying, we know whether we have normal uncorrected vision, whether we arefarsighted 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 glasseswith us when we fly, just as a backup. But, vision in flight is far more than a lessonin optics. Seeing involves the transmission of light energy (images) from the exteriorsurface of the cornea to the interior surface of the retina (inside the eye) and thetransference of these signals to the brain.The Anatomy of the EyeLight from an object enters the eye through the cornea and then continues throughthe pupil.The opening (dilation) and closing (constriction) of the pupil is controlled by theiris, which is the colored part of the eye. The function of the pupil is similar to thatof 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 surfaceof the retina.The retina is the inner layer of the eyeball that contains photosensitive cells calledrods 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 theretina known as the macula, that measures about 4.5 mm in diameter. The exactcenter of the macula has a very small depression called the fovea that contains conesonly. The cones are used for day or high-intensity light vision. They are involvedwith 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 about10,000 times more sensitive to light than the fovea. Rods are used for low-lightintensity or night vision and are involved with peripheral vision to detect positionreferences including objects (fixed and moving) in shades of grey, but cannot be usedto detect detail or to perceive color.••••••pqLight energy (an image) enters the eyes and is transformed by the cones and rodsinto electrical signals that are carried by the optic nerve to the posterior area of thebrain (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 SpotThe area where the optic nerve connects to the retina in the back of each eye isknown 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 spotof both eyes at the same time. On the other hand, where the field of vision of one eyeis obstructed by an object (windshield post), a visual target (another aircraft) couldfall in the blind spot of the other eye and remain undetected.The Night Blind SpotThe “Night Blind Spot” appears under conditions of low ambient illuminationdue to the absence of rods in the fovea, and involves an area 5 to 10 degrees widein the center of the visual field. Therefore, if an object is viewed directly at night, itmay go undetected or it may fade away after initial detection due to the night blindspot.•The FoveaThe fovea is the small depression located in the exact center of the macula thatcontains a high concentration of cones but no rods, and this is where our vision ismost sharp. While the normal field of vision for eacheye is about 135 degreesvertically and about 160 degrees horizontally, only thefovea has the ability to perceive and send clear, sharplyfocused visual images tothe brain. This foveal fieldof vision represents a smallconical area of only about 1degree. To fully appreciatehow small a one-degree field is, and to demonstrate foveal field, take a quarter fromyour pocket and tape it to a flat piece of glass, such as a window. Now back off 41⁄2 feet from the mounted quarter and close one eye. The area of your field of viewcovered 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 onlyabout 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 5,000feet 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 closerdistance of .7 of a mile to recognize the same aircraft. That is why when you werelearning to fly, your instructor always told you to “put your head on a swivel,” tokeep your eyes scanning the wide expanse of space in front of your aircraft.r sTypes of VisionPhotopic Vision.During daytime or high intensity artificial illumination conditions, the eyes rely on central vision (foveal cones) to perceive and interpret sharpimages and color of objects.Mesopic Vision.Occurs at dawn, dusk, or under full moonlight levels, and ischaracterized by decreasing visual acuity and color vision. Under these conditions,a combination of central (foveal cones) and peripheral (rods) vision is required tomaintain appropriate visual performance.Scotopic Vision.During nighttime, partial moonlight, or low intensity artificialillumination conditions, central vision (foveal cones) becomes ineffective to maintain visual acuity and color perception. Under these conditions, if you look directlyat an object for more than a few seconds, the image of the object fades away completely (night blind spot). Peripheral vision (off-center scanning) provides the onlymeans of seeing very dim objects in the dark.Factors Affecting VisionThe greater the object size, ambient illumination, contrast, viewing time, and atmospheric clarity, the better the visibility of such an object.During the day, objects can be identified easier at a great distance with good detailresolution. At night, the identification range of dim objects is limited and the detailresolution is poor.Surface references or the horizon may become obscured by smoke, fog, smog,haze, dust, ice particles, or other phenomena, although visibility may be above Visual Flight Rule (VFR) minimums. This is especially true at airports located adjacent••••••to large bodies of water or sparsely populated areas where few, if any, surface references are available. Lack of horizon or surface reference is common on over-waterflights, at night, and in low-visibility conditions.Excessive ambient illumination, especially from light reflected off the canopy, surfaces inside the aircraft, clouds, water, snow, and desert terrain can produce glare thatmay cause uncomfortable squinting, eye tearing, and even temporary blindness.Presence of uncorrected refractive eye disorders such as myopia (nearsightedness— impaired focusing of distant objects), hyperopia (farsightedness — impaired focusing of near objects), astigmatism (impaired focusing of objects in different meridians), or presbyopia (age-related impaired focusing of near objects).Self-imposed stresses such as self-medication, alcohol consumption (includinghangover effects), tobacco use (including withdrawal), hypoglycemia, and sleep deprivation/fatigue can seriously impair your vision.Inflight exposure to low barometric pressure without the use of supplemental oxygen (above 10,000 ft during the day and above 5,000 ft at night) can result in hypoxia, which impairs visual performance.Other factors that may have an adverse effect on visual performance include:windscreen haze, improper illumination ofthe cockpit and/or instruments, scratched and/ordirty instrumentation, useof cockpit red lighting,inadequate cockpit environmental control (temperature and humidity),inappropriate sunglassesand/or prescription glasses/contact lenses, andsustained visual workloadduring flight.FocusingThe natural ability to focus your eyes is critical to flight safety. It is important toknow that normal eyes may require several seconds to refocus when switching viewsbetween near (reading charts), intermediate (monitoring instruments), and distantobjects (looking for traffic or external visual references).Fatigue can lead to impaired visual focusing, which causes the eyes to overshootor undershoot the target, and can also affect a pilot’s ability to quickly change focusbetween near, intermediate, and distant vision. The most common symptoms of visual fatigue include blurred vision, excessive tearing, “heavy” eyelid sensation, frontalor orbital headaches, and burning, scratchy, or dry eye sensations.Distance focus, without a specific object to look at, tends to diminish ratherquickly. If you fly over water or under hazy conditions with the horizon obscured orbetween cloud layers at night, your distance focus relaxes after about 60-80 seconds.•••••4ABLE#OMPARISONOFTHE2EDUCTIONIN.IGHT6ISIONAT6ARYING!LTITUDESBETWEEN.ON3MOKERSAND3MOKERS
CARBONMONOXIDEBLOODCONTENTt uIf there is nothing specific on which to focus, your eyes revert to a relaxed intermediate focal distance (10 to 30 ft). This means that you are looking without actuallyseeing anything, which is dangerous. The answer to this phenomenon is to conditionyour eyes for distant vision. Focus on the most distant object that you can see, evenif it’s just a wing tip. Do this before you begin scanning the sky in front of you. Asyou scan, make sure you repeat this re-focusing exercise often.Dark Adaptation or Night Vision AdaptationDark adaptation is the process by which the eyes adaptfor optimal night visual acuity under conditions of lowambient illumination. The eyes require about 30 to 45minutes to fully adapt to minimal lighting conditions.The lower the starting level of illumination, the more rapidly complete dark adaptation is achieved. To minimizethe time necessary to achieve complete dark adaptationand to maintain it, you should:avoid inhaling carbon monoxide from smoking or exhaust fumesget enough Vitamin A in your dietadjust instrument and cockpit lighting to the lowestlevel possibleavoid prolonged exposure to bright lightsuse supplemental oxygen when flying at night above 5,000 ft (MSL)If dark-adapted eyes are exposed to a bright light source (searchlights, landinglights, flares, etc.) for a period in excess of 1 second, night vision is temporarilyimpaired. Exposure to aircraft anti-collision lights does not impair night vision adaptation because the intermittent flashes have a very short duration (less than 1second).Visual ScanningScanning the sky forother aircraft is a very important factor in avoidingmidair collisions, and itshould cover all areas ofthe sky visible from thecockpit. Most of us areinstinctively alert for potential head-on encounterswith another aircraft. Actually, a study of 50 midair collisions revealed thatonly 8% were head-on. However, 42% were collisions between aircraft heading inthe same direction. So, compared with opposite-direction traffic, your chances ofhaving a midair are over 5 times greater with an aircraft you are overtaking or one•••••that is overtaking you. It is necessary for you to develop and practice a techniquethat allows the efficient scanning of the surrounding airspace and the monitoring ofcockpit instrumentation as well. You can accomplish this by performing a series ofshort, regularly spaced eye movements that bring successive areas of the sky into thecentral (foveal) visual field. To scan effectively, scan from right to left or left to right.Begin scanning at the top of the visual field in front of you and then move your eyesinward toward the bottom. Use a stop-turn-stop type eye motion. The duration ofeach stop should be at least 1 second but not longer than 2 to 3 seconds.To see and identify objects under conditions of low ambient illumination, avoidlooking directly at an object for more than 2 to 3 seconds (because it will bleach out).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, and small eye movements to keep the object in sight. By switching your eyes from one off-center pointto another every 2 to 3 seconds, you will continue todetect the object in the peripheral field of vision. Thereason for using off-centerviewing has to do with thelocation of rods in the periphery of the retina fornight or low-intensity nightvision (peripheral), and theirabsence in the center of theretina (fovea). Pilots shouldpractice this off-center scanning technique to improvesafety during night flights.A Word about Monocular VisionA pilot with one eye (monocular), or with effective visual acuity equivalent tomonocular (i.e. best corrected distant visual acuity in the poorer eye is no better than20/200), may be considered for medical certification, any class, through the specialissuance procedures of Part 67 (14CFR67.401) if:A 6-month period has elapsed to allow for adaptation to monocularity; duringthe adaptation period to monovision, an individual may experience hazy vision andoccasional loss of balance.A complete evaluation by an eye specialist, as reported on FAA Form 8500-7,Report of Eye Evaluation, reveals no pathology of either eye that could affect thestability of the findings.Uncorrected distant visual acuity in the better eye is 20/200 or better and is corrected to 20/20 or better by lenses of no greater power than ±3.5 diopters sphericalequivalent.The applicant passes an FAA medical flight test.••••Medical Facts for PilotsPublication:AM-400-98/2 (revised 8/02)Written by: Melchor J. Antuñano, M.D.Prepared by:Federal Aviation AdministrationCivil Aerospace Medical InstituteAerospace Medical Education DivisionTo request copies of this brochure and others listed below, contactFAA Civil Aerospace Medical InstituteShipping Clerk, AAM-400P.O. Box 25082Oklahoma City, OK 73125(405) 954-4831Other Pilot Safety Brochures AvailableNumber TitleAM-400-94/2 Alcohol and Flying: A Deadly CombinationAM-400-95/2 Altitude Decompression SicknessOK05-0270 Carbon Monoxide: A Deadly ThreatAM-400-03/2 Deep Vein Thrombosis and TravelAM-400-98/3 Hearing and Noise in AviationAM-400-91/1 Hypoxia: The Higher You Fly, the Less Air...AM-400-97/1 Introduction to Human Factors in AviationOK05-0005 Medications and FlyingAM-400-01/1 Physiological Training Courses for Civil Aviation PilotsAM-400-91/2 Seat Belts and Shoulder HarnessesAM-400-95/1 Smoke!AM-400-00/1 Spatial Disorientation: Visual IllusionsAM-400-03/1 Spatial Disorientation: Why You Shouldn’t Fly By the Seat of Your PantsAM-400-05/1 Sunglasses for Pilots: Beyond the ImageTo view these pilot and passenger safety brochures,visit the Federal Aviation Administration’s Web Sitewww.faa.gov/pilots/safety/pilotsafetybrochures/Physiological Training Classes for PilotsIf you are interested in taking a one-day aviation physiological training coursewith altitude chamber and vertigo demonstrations or a one-day survival course, learnabout how to sign up for these courses that are offered at 14 locations across the U.S.by visiting this FAA Web site:www.faa.gov/pilots/training/airman_education/aerospace_physiology/index.cfmvA Word about Contact LensesUse of contact lenses has been permitted to satisfy the distant visual acuity requirements for a civil airman medical certificate since 1976. However,monovisioncontact lenses, a technique of fitting older patients who require reading glasseswith one contact lens for distant vision and the other lens for near vision, ARENOT ACCEPTABLE for piloting an aircraft.The use of a contact lens in one eye for distant visual acuity and a lens in the othereye for near visual acuity is not acceptable because this procedure makes the pilotalternate his/her vision; that is, a person uses one eye at a time, suppressing the other,and consequently impairs binocular vision and depth perception. Since this is not apermanent condition for either eye in such persons, there is no adaptation, such asoccurs with permanent monocularity. Monovision lenses, therefore, should NOT beused by pilots while flying an aircraft.The Eyes Have ItAs a pilot, you are responsible to make sure your vision is equal to the task of flying—that you have good near, intermediate, and distant visual acuity because:Distant vision is required for VFR operations including take-off, attitude control,navigation, and landingDistant vision is especially important in avoiding midair collisionsNear vision is required for checking charts, maps, frequency settings, etc.Near and intermediate vision are required for checking aircraft instrumentsLearn about your own visual strengths and weaknesses. Changes in vision mayoccur imperceptibly or very rapidly. Periodically self-check your range of visual acuity by trying to see details at near, intermediate, and distant points. If you notice anychange in your visual capabilities, bring it to the attention of your Aviation MedicalExaminer (AME). And, if you use corrective glasses or contacts, carry an extra pairwith you when you fly. Always remember: Vision is a pilot’s most important sense.KEY POINTSThe sharpest distant focus is only within a one-degree cone.Outside of a 10° cone, visual acuity drops 90%.Scan the entire horizon, not just the sky in front of your aircraft.You are 5 times more likely to have a midair collision with an aircraft flying in thesame direction than with one flying in the opposite direction.Avoid self-imposed stresses such as self-medication, alcohol consumption, smoking, hypoglycemia, sleep deprivation, and fatigue.Do not use monovision contact lenses while you are flying an aircraft.Use supplemental oxygen during night flights above 5,000 ft MSL.Any pilot can experience visual illusions. Always rely on your instruments to confirm your visual perceptions during flight.••••••••••••