Night Vision Illusions
Cones are primary color vision receptors located centrally on the retina of the eye. Cones don't play a major role in night vision. A peripheral location of rods, for night time vision and/or dim light , are responsible for the basic reception of images created in night flying. The impairment of the rods by bright light can create night illusion conditions. You are susceptible to these illusions as your rods recover.
Rods are 10,000 times more sensitive to light than the cones, making them the primary receptors for night vision.
The importance of time for preparation and/or recovery for rods plays a factor in night vision.
For example, from a safety point of view, remember the limited view of the highway after facing the bright lights of an oncoming car? Sure you do. It takes a little time for your "night vision" to recover. You can recover faster if you close one eye and focus the open eye directly at the oncoming cars headlights. Closing one eye preserves some rods for immediate use when you open your one closed eye after the car passes. No bright lights - no temporary shutdown of the rods functions. Why look directly at the bright light source - it focuses on the fovea for day light vision, not the periphery of the retina reserved for night vision. This may preserve enough rods to help your vision and prevent night-time illusions.
To see an object clearly at night, you must expose the image to the functioning rods to interpret the image. This can be done by looking 5° to 10° off center of the object to be seen. The example above involves a car. You can locate a light switch in a dark room by looking a few degrees away from the switch location. This can be tried in a dim light in a darkened room. When looking directly at the light, it dims or disappears altogether. When looking slightly off center, it becomes clearer and brighter.
Cones are primary color vision receptors located centrally on the retina of the eye. Cones don't play a major role in night vision. A peripheral location of rods, for night time vision and/or dim light , are responsible for the basic reception of images created in night flying. The impairment of the rods by bright light can create night illusion conditions. You are susceptible to these illusions as your rods recover.
Rods are 10,000 times more sensitive to light than the cones, making them the primary receptors for night vision.
The importance of time for preparation and/or recovery for rods plays a factor in night vision.
For example, from a safety point of view, remember the limited view of the highway after facing the bright lights of an oncoming car? Sure you do. It takes a little time for your "night vision" to recover. You can recover faster if you close one eye and focus the open eye directly at the oncoming cars headlights. Closing one eye preserves some rods for immediate use when you open your one closed eye after the car passes. No bright lights - no temporary shutdown of the rods functions. Why look directly at the bright light source - it focuses on the fovea for day light vision, not the periphery of the retina reserved for night vision. This may preserve enough rods to help your vision and prevent night-time illusions.
To see an object clearly at night, you must expose the image to the functioning rods to interpret the image. This can be done by looking 5° to 10° off center of the object to be seen. The example above involves a car. You can locate a light switch in a dark room by looking a few degrees away from the switch location. This can be tried in a dim light in a darkened room. When looking directly at the light, it dims or disappears altogether. When looking slightly off center, it becomes clearer and brighter.
When looking directly at an object, the image is focused mainly on the fovea, where detail is best seen in bright light. At night, the ability to see an object in the center of the visual field is reduced as the cones lose much of their sensitivity and the rods become more sensitive. Looking off center can help compensate for this night blind spot. Along with the loss of sharpness (acuity) and color at night, depth perception and judgment of size may be lost if the rods are incapacitated.
While the cones adapt rapidly to changes in light intensities, the rods take much longer. Walking from bright sunlight into a dark movie theater is an example of this "dark adaptation period" experience. Older people will require more time to adjust to dark rooms than younger folks. It is embarrassing to stand in the aisle before you can find a seat. Remember that when you fly. The rods can take approximately 30 minutes, or more, to adapt to darkness. A bright light, however, can completely destroy night adaptation, leaving night vision severely compromised while the adaptation process is correcting over time.
Red flight deck lighting also helps preserve night vision, but red light severely distorts some colors and completely washes out the color red. This makes reading an aeronautical chart difficult. A dim white light or a carefully directed flashlight can enhance night reading ability. a fishing head light is one way to focus on a map and still have both hands free to maintain control of your plane.
While flying at night, keep the instrument panel and interior lights turned up no higher than necessary. This helps to see outside references more easily. If the eyes become blurry, blinking more frequently often helps.
Diet and general physical health have an impact on how well a pilot can see in the dark. Deficiencies in vitamins A and C have been shown to reduce night acuity. Other factors, such as CO poisoning, smoking, alcohol, certain drugs, and a lack of oxygen also can greatly decrease night vision.
Red flight deck lighting also helps preserve night vision, but red light severely distorts some colors and completely washes out the color red. This makes reading an aeronautical chart difficult. A dim white light or a carefully directed flashlight can enhance night reading ability. a fishing head light is one way to focus on a map and still have both hands free to maintain control of your plane.
While flying at night, keep the instrument panel and interior lights turned up no higher than necessary. This helps to see outside references more easily. If the eyes become blurry, blinking more frequently often helps.
Diet and general physical health have an impact on how well a pilot can see in the dark. Deficiencies in vitamins A and C have been shown to reduce night acuity. Other factors, such as CO poisoning, smoking, alcohol, certain drugs, and a lack of oxygen also can greatly decrease night vision.
