Night Vision

Mobility Forum, Jul/Aug 2005 by Meintel, Julie

Do you ever wonder why it always takes several minutes for your eyes to adjust when you go from bright light to dimmer light, such as going from being outside on a sunny day to inside a building or inside your house? It's really amazing when you think about it - the human eye is only about an inch wide, an inch deep and .9 inches tall, yet it can detect almost innumerable degrees of brightness as well as a huge range of colors and shades. It is our window to the world.

As aviators, vision is especially important to us. We all know that in order to be accepted to UPT, or Undergraduate Pilot Training, we need to have 20/20 vision. In our jobs, as pilots, engineers, navigators, loadmasters or boom operators, we must be able to read small instruments, examine small parts, and read and fill in small forms, among other things. It is very important to be able to look around and make sure that the instruments are reading accurately or that tiedown chains are fastened properly and securely. This is a matter of knowing what to look for, yes, but it's also a matter of sharp, keen vision.

Let's take a look at how our eyes work, and how they work at night in the aviation environment.

In simple terms, there are three main components to the range that our eyes have. The first is the pupil - it contracts and expands to adjust to the amount of ambient light; basically, it can physically block too much light from entering the eye. The rods and the cones, which are very important to how we see, comprise the second component and are found in the retina. The third is a chemical called rhodopsin - it is found in the rods and is the key to night vision. The rods use rhodopsin to absorb photons and perceive light.

Here's a short course on the basic anatomy of the eye. The front portion of the eye is essentially a lens system. It is comprised of the cornea, which is clear and through which light must pass, and the lens. The lens' purpose is mainly to focus light onto the retina, which holds receptor cells, also known as rods and cones. Rods and cones work together to discriminate color, light and fineness of detail and then send those signals to the brain, which then interprets those signals as sight. Rods are mostly located in the outer perimeter of the retina. They cannot discriminate fine details, nor differentiate between colors. However, they have a very high sensitivity to light. Conversely, cones do not have a great deal of light sensitivity, but they are capable of detecting colors and shades, and they allow a much greater resolution of detail than the rods. This is how they work together.

One common misunderstanding is that rods only work during the night and cones only work during the day. While it may seem only logical, given the way rods and cones operate, it is not actually true. Both receptors work over a very wide range of light levels and, at some levels in the middle of the range, they work at the same time. There is a kind of transition between bright light vision and dim light vision, which is twilight or dusk, vision in this zone is called mesopic vision. Neither the rods nor the cones are at their best in this range but they both contribute to visual perception.

This dual-receptor system of vision allows us to see well over a huge range of ambient light levels. The human eye can adjust over a billion times over the range between the dimmest illumination and the brightest.

How does any of this relate to night vision and aviation? The effect of lower light levels can be quite drastic on your vision. First, you may lose a good deal of your visual acuity. People and objects can only be seen if they are darker or lighter than their surroundings, and can only be discriminated by slight changes in contrast. Even if you have 20/20 vision, you cannot be sure of being able to see that well when the light level drops below that of deep twilight. Night visual acuity is largely a function of small subtle differences between people and objects and their backgrounds.

Adapting to lower light levels is called, fittingly, dark adaptation. It is an independent process that each eye goes through as it adjusts to lower illumination. This process varies in how long it takes, and rods and cones achieve their maximum ability at different times. Cones take about 5-7 minutes to reach their maximum capability, while rods may require as much as 30-45 minutes of total darkness to attain their highest level of sensitivity. Generally, dark adaptation is approximately 80% complete within 30 minutes, but it can take several hours to reach total dark adaptation.

If by chance you should see a bright flash of light after your eyes have done all that work to become acclimated to the dark, you will likely end up going through the whole process again. Dark adaptation can be lost in just a few seconds, but there is something you can do to minimize the effects of "flash blindness." Always protect one eye by closing it or covering it if you are exposed to bright light after achieving dark adaptation. Remember, adaptation is an independent process and if one eye is exposed to bright light, you can still protect the other one. When operating in dark conditions, it's best to use the lowest light setting for cockpit lighting and avoid looking at exhaust flames, strobes, searchlights or other sources of bright light that will seriously degrade your night vision for a period of time.