producing what appears to be a series of light fl ashes at  regular intervals. These flashes may be one or two different  colors that are used to identify various types of landing areas.  For example: eyes correctly and know their limitations, night vision can  be improved significantly. The human eye is constructed so  that day vision is different from night vision. Therefore, it is  important to understand the eye’s construction and how the  eye is affected by darkness. Innumerable light-sensitive nerves called cones and rods are  located at the back of the eye or retina, a layer upon which all  images are focused. These nerves connect to the cells of the  optic nerve, which transmits messages directly to the brain.  The cones are located in the center of the retina, and the rods  are concentrated in a ring around the cones.  [Figure 12-6] Beacons producing red flashes indicate obstructions or areas  considered hazardous to aerial navigation. Steady burning  red lights are used to mark obstructions on or near airports  and sometimes to supplement flashing lights on en route  obstructions. High intensity flashing white lights are used to  mark some supporting structures of overhead transmission  lines that stretch across rivers, chasms, and gorges. These  high intensity lights are also used to identify tall structures,  such as chimneys and towers. The function of the cones is to detect color, details, and  faraway objects. The rods function when something is seen  out of the corner of the eye or peripheral vision. They detect  objects, particularly those that are moving, but do not give  detail or color—only shades of gray. Both the cones and the  rods are used for vision during daylight. Although there is not a clear-cut division of function, the rods  make night vision possible. The rods and cones function in  daylight and in moonlight; in the absence of normal light,  the process of night vision is almost entirely a function of  the rods. As a result of technological advancements in aviation,  runway lighting systems have become quite sophisticated  to accommodate takeoffs and landings in various weather  conditions. However, the pilot whose flying is limited to VFR  needs to be concerned only with the following basic lighting  of runways and taxiways. The basic runway lighting system consists of two straight  parallel lines of runway-edge lights defining the lateral limits of  the runway. These lights are aviation white, although aviation  yellow may be substituted for a distance of 2,000 feet from  the far end of the runway to indicate a caution zone. At some  airports, the intensity of the runway-edge lights can be adjusted  to satisfy the individual needs of the pilot. The length limits  of the runway are defined by straight lines of lights across the  runway ends. At some airports, the runway threshold lights are  aviation green, and the runway end lights are aviation red. At many airports, the taxiways are also lighted. A taxiway-  edge lighting system consists of blue lights that outline  the usable limits of taxi paths. See the Pilot’s Handbook  of Aeronautical Knowledge for additional information on  airport lighting.  Generally, most pilots are poorly informed about night vision.  Human eyes never function as effectively at night as the eyes  of nocturnal animals, but if humans learn how to use their  The eye’s adaptation to darkness is another important aspect  of night vision. When a dark room is entered, it is diffi cult to  see anything until the eyes become adjusted to the darkness.  In the adaptation process, the pupils of the eyes fi rst enlarge  to receive as much of the available light as possible. After  approximately 5 to 10 minutes, the cones become adjusted to  the dim light and the eyes become 100 times more sensitive  to light than they were before the dark room was entered.  About 30 minutes is needed for the rods to become adjusted  to darkness; when they do adjust, they are about 100,000  times more sensitive to light than in the lighted area. After  the adaptation process is complete, much more can be seen,  especially if the eyes are used correctly. The fact that the rods are distributed in a band around the  cones and do not lie directly behind the pupils makes off- center viewing (looking to one side of an object) important  during night flight. During daylight, an object can be seen  best by looking directly at it, but at night a scanning procedure  to permit off-center viewing of the object is more effective.  Therefore, the pilot should consciously practice this scanning  procedure to improve night vision. • Lighted civilian land airports—alternating white and  green • Lighted civilian water airports—alternating white and  yellow • Lighted military airports—alternating white and  green, but are differentiated from civil airports by  dual peaked (two quick) white flashes, then green  Night Vision 12-5