You are walking down the trail in the woods at night. There is a partial moon so it is not as dark as it could be. You focus on the trail ahead of you and the path is very difficult to see. Yet off to the side of the trail things are much more clear and distinct. In another situation you are in a field in mid-August with a group of other Scouts anticipating the meteor shower light show that has been predicted. You look into the night sky and see several meteor traces but they are not where you were looking but off to the side. What can explain these happenings?
In order to understand why these situations occur we need to learn some basics about the human eye. Light comes into the eye and the lens focuses the image on the retina at the back of the eye. The retina contains two kinds of light sensitive cells- rods and cones. These rods and cones are pretty evenly distributed throughout the retina with one exception. The area directly opposite the pupil, the fovea, contains no rods- only cones. Why is this important? Cone cells are much better at seeing color and rods are much better in sensing movement and seeing in low light situations. When you look at the trail straight ahead of you or you look at the sky directly in front of you for meteors, that part of the eye is looking for color. Your peripheral vision (your vision off to the side of directly in front of you) sees movement and gives better images in low light situations.
How can you improve your night vision capabilities? Although rods and cones play a large part in determining what we can see at night, there are at least two other major factors- rhodopsin and the intensity of the light source. Rhodopsin (sometimes referred to as visual purple) is the pigment in the eye that allows low light vision. It is very sensitive to light and “photobleaches” when exposed to normal light. So after exposure to normal light it takes a while for the eye to “dark adapt”- become able to see in low light situations. Times for this dark adaption varys from one person the next but generally in about 10 minutes you have regained about 10% of your night vision and after about 30 to 45 minutes you will have regained about 80%. Regaining 100% of your night vision could take a long time but most of the time 80% is enough to function normally.
Intensity is the second factor. The more intense the light source the stronger the effect on the rhodopsin. Very low level light is much more forgiving than a higher intensity light. The color of the light is also important and there is a lot of discussion about this. For years (including the time I flew in the Navy) red light was the only acceptable night light. There are now some studies that indicate that in some situations a blue-green light is better. Other studies recommend a combination of red and blue-green. Finally some studies recommend a very low level white light. While there is some debate over which color light is best in a given situation, all agree that the lowest intensity of the light is best.
What are some things that you can do to improve your night vision?
- Get plenty of Vitamin A in your diet- lettuce, carrots and green leafy vegetables are a good source of Vitamin A
- Don’t smoke- this is very bad for night vision
- Keep in good general health- some prescription drugs can effect night vision
What are some tips for navigating on a dark night?
- Prior to leaving a lighted area squint your eyes to reduce the light and the intensity. Covering or closing one eye can also be effective.
- Try to allow 30 minutes in the dark before you need to perform any tasks requiring more than minimal perception.
- Although there appears to be no scientific proof that this technique works, some groups recommend closing both eyes and applying slight pressure to both eyes with the palms of your hands for 5 to 10 seconds. When you release the pressure and open the eyes, you see white for a short period of time and then “improved” vision.
- After you have adapted to the dark, don’t look at any light source. The rhodopsin “bleaches” quickly and you will have to dark adapt again.
- Use your peripheral vision to see the trail in front of you.
- If it is very dark and forward vision is very minimal, slide your feet along the path to keep from tripping. Also hold your arms out in front of you but crossed at the wrist. This technique will help to keep you from walking into a tree.
Being in the woods at night is a fun and exciting time. Leave the flashlight in the tent and spend some time in the dark. Once you get used to it you may find that it is much more fun without the light.