Night flying is very different from day flying. The eyes function differently; references available in the day are no longer visible; there are many illusions that can affect you, and much more. Flying at night presents unique situations which, if ignored, can lead to dangerous situations. Let’s take a look at how our eyes are affected by flying at night.
Rods and Cones
Photo credit: FAA
- Light enters the eye through the cornea, travels through the lens, and falls on the retina.
- The retina contains light-sensitive cells that convert light into electrical impulses that are sent to the brain.
There are two types of light-sensitive cells, rods and cones:
- Cones – Responsible for color, detail, and far away objects
- The cones are located in the center of the retina
- Rods – Function when something is seen in the peripherals and provide vision in dim light
- The rods are located in a ring around the cones (peripherals)
Both the cones and rods are used for vision during the day but, without normal light, the process of night vision is placed almost entirely on the rods.
Rods, Cones, and Night Vision
- Cones are located in the center of the retina (the layer upon which all images are focused). There is a small pit called the fovea where almost all the light-sensing cells are cones. This is the area where most looking occurs (your center of vision)
The Rods
During daylight, objects can be seen by looking directly at them, using the fovea, but at night a scanning procedure to permit off-center viewing is more effective. The cones need light to function, without sufficient light (at night, for example) the cones are effectively a blind spot. Rods are concentrated around the cones and are used to see in dim light. Since cones are only useful with sufficient light and the rods lie outside the fovea (outside the center of vision), off-center viewing is used for night flights. When attempting to find traffic do not stare directly at it, look slightly off to the left or right to allow the rods to see the aircraft. The problem with rods is that a large amount of light overwhelms them and they take a long time to reset and adapt to the dark again (Example: Stepping out of a dark movie theatre into the daylight).
The rods can take approximately 30 minutes to fully adapt to the dark. Once fully adapted, the rods are about 10,000x more sensitive to light. After the rods have adapted to the dark, the process is reversed when exposed to light. The eyes adjust to the light in a matter of seconds. If a dark room is reentered, the 30-minute process to adapt is started again. The lack of adequate vision and other hypoxic symptoms are particularly prevalent at night when the eyes need more oxygen to produce a special night vision protein in the rods of the eyes called rhodopsin. Therefore, it is important to avoid bright lights before and during a flight. This is why red flashlights are recommended during flight, they do not disrupt the rods’ dark adaptation.
What to take away from this lesson
Night vision is based on the rods and off-center viewing is necessary. Staring directly at an object at night could result in not seeing the object at all since the cones in the center of your vision are considerably less effective without sufficient light. It is important to avoid bright lights before and during a flight to maintain adequate night vision. There are a lot of differences between daytime flying vs nighttime. Your safety during a night flight is pertinent to your understanding of how your body is responding.
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