Types of Vision
. During daytime or high intensity artificial illumination conditions, the eyes rely on central vision (foveal cones) to perceive and interpret sharp images and color of objects. Mesopic Vision
. Occurs at dawn, dusk, or under full moonlight levels, and is characterized by decreasing visual acuity and color vision. Under these conditions, a combination of central (foveal cones) and peripheral (rods) vision is required to maintain appropriate visual performance. Scotopic Vision
. During nighttime, partial moonlight, or low intensity artificial illumination conditions, central vision (foveal cones) becomes ineffective to maintain visual acuity and color perception. Under these conditions, if you look directly at 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 only means of seeing very dim objects in the dark.
Factors Affecting Vision
The 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 detail resolution. At night, the identification range of dim objects is limited and the detail resolution 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-water flights, 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 that may 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 (including hangover 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 of the cockpit and/or instruments;
- Scratched and/or dirty instrumentation;
- Use of cockpit red lighting;
- Inadequate cockpit environmental control (temperature and humidity):
- Inappropriate sunglasses and/or prescription glasses/contact lenses;
- Sustained visual workload during flight.
The natural ability to focus your eyes is critical to flight safety. It is important to know that normal eyes may require several seconds to refocus when switching views between near (reading charts), intermediate (monitoring instruments), and distant objects (looking for traffic or external visual references). Fatigue can lead to impaired visual focusing, which causes the eyes to overshoot or undershoot the target, and 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, frontal or orbital headaches, and burning, scratchy, or dry eye sensations. Distance 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 60-80 seconds. If 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 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 re-focusing exercise often.
Dark Adaptation or Night Vision Adaptation
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 45 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 smoking or exhaust fumes;
- get enough Vitamin A in your diet;
- 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 ft (MSL).
If dark-adapted eyes are exposed to a bright light source (searchlights, landing lights, flares, etc.) for a period in excess of 1 second, night vision is temporarily impaired. Exposure to aircraft anti-collision lights does not impair night vision adaptation because the intermittent flashes have a very short duration (less than 1 second). We continue this discussion about keeping a pilots eyes safe in our next article about pilot sunglasses.
Until next time keep your wings straight and level Hersch! JetAviator7
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