an instantaneous field of view, which is oval and typically  measures 120° vertically by 150° horizontally. When both  eyes are used for viewing, the overall field of vision measures  about 120° vertically by 200° horizontally. Vision is primarily the result of light striking a photosensitive  layer, called the retina, at the back of the eye. The retina is  composed of light-sensitive cones and rods. The cones in the  eye perceive an image best when the light is bright, while the  rods work best in low light. The pattern of light that strikes  the cones and rods is transmitted as electrical impulses by the  optic nerve to the brain where these signals are interpreted  as an image. Cones are concentrated around the center of the retina. They  gradually diminish in number as the distance from the center  increases. Cones allow color perception by sensing red, blue,  and green light. Directly behind the lens, on the retina, is  a small, notched area called the fovea. This area contains  only a high concentration of cone receptors. The best vision  in daylight is obtained by looking directly at the object.  This focuses the image on the fovea, where detail is best  seen. The cones, however, do not function well in darkness,  which explains why color is not seen as vividly at night as  it is during the day. Concentrated outside the fovea area, the rods are the dim  light and night receptors. The number of rods increases as the  distance from the fovea increases. Rods sense images only  in black and white. Because the rods are not located directly  behind the pupil, they are responsible for most peripheral  vision. Images that move are perceived more easily by the  rod areas than by the cones in the fovea. If you have ever  seen something move out of the corner of your eye, it was  most likely detected by rod receptors. During daylight, objects can be perceived at a great distance  with good detail. At night, range is limited, and detail is poor.  Objects along the flight path can be more readily identified at  night, by using the proper techniques to scan the terrain. To  Good night visual acuity is needed for collision avoidance.  Night scanning, like day scanning, uses a series of short,  regularly spaced eye movements in 10° sectors. Unlike day  scanning, however, off-center viewing is used to focus objects  on the rods rather than the fovea blindspot.  [Figure 12-4]  When  looking at an object, avoid staring at it too long. If staring  at an object without moving the eyes, the retina becomes  accustomed to the light intensity and the image begins to  fade. To keep it clearly visible, new areas in the retina must  be exposed to the image. Small, circular eye movements help  eliminate the fading. Also, move the eyes more slowly from  sector to sector than during the day to prevent blurring. Diet and general physical health have an impact on how well  a person can see in the dark. Deficiencies in vitamins A and C  have been shown to reduce night acuity. Other factors, such  as carbon monoxide poisoning, smoking, alcohol, and certain  drugs can greatly decrease night vision. Lack of oxygen can  also decrease night vision as the eye requires more oxygen  per unit weight than any other part of the body. In low light, the cones lose much of their function, while  rods become more receptive. The eye sacrifices sharpness for  sensitivity. The ability to see an object directly in front of you is  reduced, and much depth perception is lost, as well as judgment  of size. The concentration of cones in the fovea can make a  night blindspot at the center of vision. How well a person sees  at night is determined by the rods in the eyes, as well as by the  amount of light allowed into the eyes. At night, the wider the  pupil is open at night, the better night vision becomes. The Eye Cones Night Vision Night Scanning Rods Once a target is detected in the peripheral  field of dark-adapted vision, aircrews  maintain continual surveillance by using the off-center vision technique. They look 10 degrees above, below, or to either side of the target, viewing it no longer than two  to three seconds at each position. Observer FOCAL POINTS X 10° 10° X X 10° X 10° Figure 12-4.  Off-center vision technique.  12-4