Figure 17-19.  Off-center viewing.  If a night flight is scheduled, pilots and crew members should  wear neutral density (N-15) sunglasses or equivalent filter  lenses when exposed to bright sunlight. This precaution  increases the rate of dark adaptation at night and improves  night visual sensitivity.  Unaided night vision depends on optimum function and  sensitivity of the rods of the retina. Lack of oxygen to the rods  (hypoxia) significantly reduces their sensitivity. Sharp clear  vision (with the best being equal to 20–20 vision) requires  significant oxygen especially at night. Without supplemental  oxygen, an individual’s night vision declines measurably at  pressure altitudes above 4,000 feet. As altitude increases,  the available oxygen decreases, degrading night vision.  Compounding the problem is fatigue, which minimizes  physiological well being. Adding fatigue to high altitude  exposure is a recipe for disaster. In fact, if flying at night at  an altitude of 12,000 feet, the pilot may actually see elements  of his or her normal vision missing or not in focus. Missing  visual elements resemble the missing pixels in a digital image  while unfocused vision is dim and washed out.  For the pilot suffering the effects of hypoxic hypoxia, a simple  descent to a lower altitude may not be sufficient to reestablish  vision. For example, a climb from 8,000 feet to 12,000 feet for  30 minutes does not mean a descent to 8,000 feet will rectify  the problem. Visual acuity may not be regained for over an  hour. Thus, it is important to remember, altitude and fatigue  have a profound effect on a pilot’s ability to see.  If, during the flight, any high intensity lighting areas are  encountered, attempt to turn the aircraft away and fly in the  periphery of the lighted area. This will not expose the eyes to  such a large amount of light all at once. If possible, plan your  route to avoid direct over flight of built-up, brightly lit areas.  Flightdeck lighting should be kept as low as possible so that  the light does not monopolize night vision. After reaching  the desired flight altitude, pilots should allow time to  adjust to the flight conditions. This includes readjustment  of instrument lights and orientation to outside references.  During the adjustment period, night vision should continue  to improve until optimum night adaptation is achieved. When  it is necessary to read maps, charts, and checklists, use a dim  white light flashlight and avoid shining it in your or any other  crewmember’s eyes.  Often time, pilots have no say in how airfield operations are  handled, but listed below are some precautions that can be  taken to make night flying safer and help protect night vision.  Airfield Precautions  Flightdeck Lighting  Oxygen Supply  High Intensity Lighting  • Airfield lighting should be reduced to the lowest  usable intensity.  • Maintenance personnel should practice light discipline  with headlights and flashlights.  • Position the aircraft at a part of the airfield where the  least amount of lighting exists.  17-24  Sunglasses  ° Focal points  X  X  X X  X  Once an object is detected in the peripheral field of dark- adapted vision, continued surveillance is maintained by  use of “off-center” vision. Looking 10 right or left and  above and below the object, viewing no longer than 2 to 3 seconds at each position.  OBSERVER  10 degrees  10 degrees  10 degrees  10 degrees 