8-4. Another factor to consider is "dark focus." When light levels decrease, the focusing mechanism of the eye may move toward a resting position and make the eye more myopic. These factors become important when aircrew members rely on terrain features during unaided night flights. Special corrective lenses can be prescribed to correct for night myopia.

• Dynamic acuity.
• Recovery from glare.
• Function under low illumination.
• Information processing.

8-12. Various surgical procedures are available to correct visual deficiencies; not all are listed. The procedures described above are currently the most common. AR 40-501 and AR 95-1 state that all corrective eye surgeries involving LASIK or PRK or other forms of corrective eye surgery disqualify Army aircrew members from flight duty. Aircrew members must consult their flight surgeons before undergoing these procedures.

8-17. Cone Neurology. The retina contains seven million cone cells. Each cone cell in the fovea is connected to a single nerve fiber that leads directly to the brain. This single-nerve connection of each foveal cone to the brain means that each cone generates a nerve impulse under sufficient light levels. This occurs during daylight or viewing conditions of high-intensity light exposure. Cone cells provide sharp visual acuity and the perception of color. When crew members view under low light or dark conditions, cone cells depict shades of black, gray, and white; crew members will perceive other colors if the light intensity is heightened by artificial light sources:

• Aircraft position lights.
• Anticollision lights.
• Runway lights.
• Beacon lights.
• Artificial lighting related to metropolitan areas.

8-18. Rod Neurology. There are 120 million rod cells in the retina. Rod cells have a 10-to-1, up to a 10,000-to-1, ratio of rod cells to neuron cells within the retina. Because of the large number of rod cells that are connected to each nerve fiber outside of the fovea, dim light can trigger a nerve impulse to the brain. The periphery of the retina, where the rods are concentrated, is much more sensitive to light than is the fovea. This concentration of rods is responsible for night vision (peripheral vision), which provides for silhouette recognition of objects. This is also why aircrew members’ eyes are highly sensitive to light when viewing during low ambient light or dark conditions.

• Angular size of the object increases as the distance between the object and the viewer decreases.
• Illumination (overall brightness) of ambient light increases.
• Degree of retinal adaptation increases.
• Color and contrast between the target and background increase.
• Position of the target within the visual field (visibility threshold) increases.
• The focus of the eye and length of time viewing the object increase.
• Atmospheric clarity increases. ND-15 sunglasses can aid visibility during viewing conditions of excessive light or brightness.

8-29. As aircraft speed increases, there is interference in the perception of instantaneous visual pictures. In some cases, it may take one to two seconds or longer to recognize and consciously assess a complex situation. By the time that an object is eventually perceived, it may have already been overtaken. The time that it takes to perceive an object becomes significant to the aircrew member. Perception time includes the time that it takes—

• The message indicating that an image of an object has been identified within the visual field and that image information travels from the eye to the brain to include the time it takes the brain to receive, comprehend, and identify the information.
• The eye to turn toward and focus on the unknown object.
• An individual to recognize the object and determine its importance.
• To transmit a decision to move muscles and cause the aircraft to respond to control inputs.

8-31. Dark adaptation for optimal night-vision acuity approaches its maximum level in about 30 to 45 minutes under minimal lighting conditions. If the eyes are exposed to a bright light after dark adaptation, their sensitivity is temporarily impaired. The degree of impairment depends on the intensity and duration of the exposure. Brief flashes from high-intensity, white (xenon) strobe lights, which are commonly used as anticollision lights on aircraft, have little effect on night vision. This is true because the energy pulses are of such short duration (milliseconds). Exposure to a flare or a searchlight longer than one second can seriously impair night vision. Depending on the brightness (intensity), duration of exposure, or repeated exposures, an aircrew member’s recovery time to regain complete dark adaptation could take from several minutes to the full 45 minutes or longer.

8-32. Exposure to bright sunlight also has a cumulative and adverse effect on dark adaptation. Reflective surfaces—such as sand, snow, water, or man-made structures—intensify this condition. Exposure to intense sunlight for two to five hours decreases visual sensitivity for up to five hours. In addition, the rate of dark adaptation and the degree of night visual acuity decrease. These cumulative effects may persist for several days.

8-33. The retinal rods are least affected by the wavelength of a dim red light. Figure 8-11 compares rod and cone cell sensitivities. Because rods are stimulated by low ambient light levels, red lights do not significantly impair night vision if the proper techniques are used. To minimize the adverse effect of red lights on night vision, crew members should adjust the light intensity to the lowest usable level and view instruments for only a short time.

8-45. An object viewed longer than two to three seconds tends to bleach out and become one solid tone. Therefore, the object can no longer be seen. This creates a potentially unsafe operating condition. The aircrew member must be aware of the phenomenon and avoid viewing an object longer than two to three seconds. By shifting the eyes from one off-center point to another, the aircrew member can see the object in the peripheral field of vision.

8-53. Apparent Foreshortening. The true shape of an object or terrain feature appears elliptical (oval and narrowed appearance) when viewed from a distance when aircrew members are flying at both higher and lower altitudes. As the distance to the object or terrain feature decreases, the apparent perspective changes to its true shape or form. When flying at lower altitudes and viewing at greater distances, aircrew members may not view objects clearly. If the mission permits, pilots should gain altitude and decrease distance from the viewing area to compensate for this perspective. That is, once the aircraft increases in altitude and distance between the aircraft and the viewing area decreases, the viewing field widens and enlarges so that objects within that field of view become apparent. Part B of Figure 8-15 illustrates how the shape of a body of water changes when viewed at different distances while the aircraft maintains the same altitude.

8-54. Vertical Position in the Field. Objects or terrain features that are at greater distances from the observer appear higher on the horizon than do those that are closer to the observer. In part C of Figure 8-15, the higher vehicle appears to be closer to the top and is judged as being at a greater distance from the observer. Before flight, aircrew members should already be familiar with the actual sizes, heights, or altitudes of known objects or terrain features within and around the planned flight route. If the situation and time permit, aircrew members can reference published information to verify actual sizes, heights of objects, and terrain features within their flight path. In addition, the aircrew members should cross-reference their aircraft’s altitude indicator to confirm that actual aircraft altitude is adequate to safely negotiate the object or terrain feature without prematurely changing the aircraft’s heading, altitude, or attitude or a combination of these.

8-57. Known Size of Objects. The nearer an object is to the observer, the larger its retinal image. By experience, the brain learns to estimate the distance of familiar objects by the size of their retinal image. Figure 8-16 shows how this method is used. A structure projects a specific angle on the retina, based on its distance from the observer. If the angle is small, the observer judges the structure to be at a great distance. A larger angle indicates to the observer that the structure is close. To use this cue, the observer must know the actual size of the object and have prior visual experience with it. If no experience exists, aircrew members determine the distance to an object primarily by motion parallax.

8-59. Terrestrial Association. Comparison of one object, such as an airfield, with another object of known size, such as a helicopter, will help to determine the relative size and apparent distance of the object from the observer. Figure 8-17 shows that that objects ordinarily associated together are judged to be at about the same distance. For example, a helicopter that is observed near an airport is judged to be in the traffic pattern and, therefore, at about the same distance as the airfield.

8-62. Fading of Colors or Shades. Objects viewed through haze, fog, or smoke are seen less distinctly and appear to be at a greater distance than they actually are. If atmospheric transmission of light is unrestricted, an object is seen more distinctly and appears to be closer than it actually is. For example, the cargo helicopter in Figure 8-19 is larger than the observation helicopter, but because of the difference in viewing distance and size, they both project the same angle on the observer’s retina. From this cue alone, assuming the observer has no previous experience with their appearance, both helicopters appear to be the same size. However, if the cargo helicopter is known to be a larger aircraft but is seen less distinctly because of visibility restrictions, it would be judged to be farther away and larger than the observation helicopter. Another example is that aircrew members may not be able to distinguish green from red anticollision lights and the actual interval between aircraft when an additional aircraft is operating at a distance. Both lights may appear to be white, and in addition, they may even blend in with the surrounding foreground.

8-64. Position of Light Source and Direction of Shadow. Every object will cast a shadow if there is a source of light. The direction in which the shadow is cast depends on the position of the light source. If the shadow is cast toward the observer, the object is closer than the light source to the observer. Figure 8-20 shows how light and shadow help determine distance.

8-67. The ambient light level is gradually reduced as cloud coverage increases. Visual acuity and contrast of terrain features are lost, possibly to complete obscurity. If this condition should occur, pilots should initiate inadvertent IMC procedures. Aircrew members must follow their local SOPs and command directives and realize that inadvertent IMC at night is one of the leading causes of Class A mishaps.

8-68. If the moon and stars cannot be seen, clouds are present. The less visible the stars and moon, the heavier the cloud coverage.

8-69. Clouds obscuring the illumination of the moon create shadows. These shadows can be detected by observing the varying levels of ambient light along the flight route.

8-70. The halo effect, which is observed around ground lights, indicates the presence of moisture and possible ground fog. As the fog and moisture increase, the intensity of the lights will decrease. This same effect is apparent during flight. As moisture increases, the light that is emitted from the aircraft is reflected back upon the aircraft. When this reflection occurs, it is possible to misjudge terrain features, man-made structures, and the actual position, heading, and altitude of other aircraft including the layout and height of the terrain below.

8-71. The presence of fog over water surfaces indicates that the temperature and dew point are equal. It also indicates that fog may soon form over ground areas.

8-79. The exposure time required to cause miosis depends on agent concentration. Miosis may occur gradually as eyes are exposed to low concentrations over a long period. On the other hand, exposure to a high concentration can cause miosis during the few seconds it takes to put on a protective mask. Repeated exposure over a period of days is cumulative.

8-80. The symptoms of miosis range from minimal to severe, depending on the dosage to the eye. Severe miosis, with the resulting reduced ability to see in low ambient light, persists for about 48 hours after onset. The pupil gradually returns to normal over several days. Full recovery may take up to 20 days. Repeated exposure within the affected time will be cumulative.

8-81. The onset of miosis is insidious because it is not always immediately painful. Miotic subjects may not recognize their condition, even when they carry out tasks requiring vision in low ambient light. After an attack by nerve agents, especially the more persistent types, commanders should assume that some loss of night vision has occurred among personnel otherwise fit for duty and consider grounding the aircrew members until they fully recover. All exposed aircrew members and aircraft-related maintenance personnel must consult medical personnel and the flight surgeon immediately after exposure.

• Taking cover.
• Getting out of the path of the laser beam.
• Using available protective gear.
• Keeping all exposed skin areas covered to prevent skin burns.

• Using countermeasures, as taught or directed by the unit commander.
• Applying evasive action.
• Scanning the battlefield with one eye or monocular optics.
• Minimizing the use of binoculars in areas where lasers may be in use.

Crew members should use hardened optical systems and built-in or clip-on filters (BLPs) and deploy smoke, if capable. FM 4-02.50(8-50) contains information regarding prevention and medical management of laser injuries.

• Understand the types of vision and the limitations of each and that visual acuity will normally be lost under low levels of illumination.
• If corrective lens are prescribed to aircrew members, they must use corrective lens (glasses) in all modes of flight to include night-aided (ANVIS, night-vision devices/goggle systems) flight.
• Be aware that it will take 30 to 45 minutes for the average individual’s eyes to reach maximum dark adaptation.
• Remember to use off-center vision when viewing objects under reduced lighting conditions.
• Use supplemental oxygen, if available, on flights (especially night flights) at or above 4,000 feet pressure altitude.
• Avoid self-imposed stress.
• Protect night vision by avoiding bright lights once dark adaptation has been achieved.
• Scan constantly when viewing objects outside the cockpit, day or night.
• Know and understand the effects of nerve agents and take protective measures against laser injury.