• Reduced ability to see crisp and clear images (visual acuity).
• Inability to distinguish colors under certain conditions.
• Reduced depth perception.
• Difficulty in seeing objects at certain distances due to the night blind spot.
• Lost dark adaptation due to exposure to light.
• Confusion (your eyes may seem to play tricks on you).
1. NORMAL BLIND SPOTS - The normal blind spot is always present, day and night. It is caused by the lack of light receptors where the optic nerve inserts into the back of the eye. The normal blind spot occurs when you use just one eye. When you close the other eye, objects 12 to 15 degrees away from where you are looking will disappear. When you uncover your eye, the objects will reappear.
2. NIGHT BLIND SPOTS AND VIEWING TECHNIQUES - When you stare at an object under starlight or lower levels of illumination, it can disappear or fade away. This is a result of the night blind spot. The night blind spot affects both eyes at the same time and occurs when using the central vision of both eyes. Consequently, when looking directly at an object, Soldiers miss larger objects as the distances increase. A hand grenade 2 meters away might not be seen; an enemy Soldier at 50 meters may be missed. An M1 tank at 300 meters can even be missed.
3. AVOIDING NIGHT BLIND SPOTS - To avoid night blind spots--
• Look to all sides of objects you are trying to find or follow.
• DO NOT STARE.
**Diamond viewing, a technique similar to the off-center vision technique taught in rifle marksmanship, is a good technique for peripheral viewing. To use the diamond viewing technique--
• Move your eyes just slightly, a few degrees, in a diamond pattern around the object you wish to see.
• Do not move your head; use your peripheral vision.
4. DARK ADAPTATION - In order for your visual system to work efficiently at night, you need to dark-adapt, or get your eyes used to seeing things under low light conditions. Dark adaptation is similar to walking into a movie theater when it’s very dark; you can’t see things at first, but as your eyes gradually adapt, you can see better. "It takes about 30 to 45 minutes to fully dark-adapt when going from a brightly lighted area into the dark. However, people dark-adapt at varying rates. People who are older, people who smoke, or people who are not in great physical shape will take longer to dark-adapt. There are three stages of dark adaptation:
• Daylight vision.
• Twilight vision.
• Night vision.
a. Daylight Vision - Daylight vision occurs under maximum lighting conditions, such as when the sun is shining or in a well-lit room. Under these conditions, Soldiers have--
• Optimal visual acuity. In daylight conditions, Soldiers use both their central and peripheral vision, which provides optimal visual acuity—20/10, 20/15, and 20/20 vision.
• Optimal color vision. Colors look most vivid under daylight conditions.
• Quickest reaction time.
b. Twilight Vision - Twilight vision occurs during many military night operations and when driving around in a car at night. It occurs at dawn and dusk, down to full moonlight; when there is artificial illumination; and when snow is on the ground at night. It can occur in the daytime with several layers of jungle canopy. Under these conditions, Soldiers have--
• Poorer visual acuity. Visual acuity can be as poor as 20/100. Under twilight conditions, optimal visual acuity is between 20/50 and 20/100.
• Poorer color vision. Colors will not be as vivid.
• Slower reaction times.
c. Night Vision - Night vision occurs under starlight, as well as on moonless and cloudy nights when there are no stars or cultural lighting. Remember, there is a night blind spot, as discussed earlier. Under these conditions, Soldiers have--
• The worst visual acuity—from 20/200 to 20/400 and possibly much worse. Soldiers can recognize silhouettes, but not details of the objects. This is why knowing the silhouettes of vehicles and critical natural and man-made objects is
• Poor color vision. Soldiers cannot see colors, only various shades of gray. The longer wavelengths of light, such as the reds and oranges, are hard to see and appear dark. Unless a dark color is bordered by two lighter colors, it becomes totally invisible. On the other hand, greens and blues appear brighter, but Soldiers may not be able to determine their color. Note: Reds are almost invisible at night. Red crosses are on white backgrounds on tents or vehicles so they can be seen more easily at night.
5. PROTECTING (BEFORE OPERATION) - Soldiers must protect their eyes before night operations so they can dark-adapt in an efficient manner. To efficiently dark-adapt, use the following suggestions:
• Don’t smoke before night operations. Not smoking four to six hours before night operations will aid in dark adaptation.
• Wear sunglasses if you plan to spend time in the sun. Without sunglasses, it will take longer to dark-adapt.
• Watch what you eat. Maintain adequate levels of Vitamin A.
• Use dim white lighting or red lighting before night operations.
6. PROTECTING (DURING OPERATION) - Once a Soldier has dark-adapted, it is important to maintain that dark adaptation. To maintain dark adaptation, use the following suggestions:
• Minimize your use of unnecessary lighting.
• Close one eye before being flashed by flares and other bright lights.
7. ILLUSION (APPARENT MOVEMENT OF LIGHT) - The illusion of movement, which a static light exhibits when stared at in the dark, is related to the loss of surrounding visual references that normally serve to stabilize visual perceptions. Consequently, very small eye movements are perceived by the brain as movement of the light. To avoid illusions of movement--
• Begin a scan pattern, and control the eye movement.
• Use large movements, and scan.
• Try to find another light, and shift your gaze back and forth between the lights.