Scientists call it scotopic vision, but to the layman it is known as night vision. Night vision is a very important aspect of human physiology that affects 24-hour marine operations much more than most think. Not surprisingly, it is widely misunderstood by both mariners and vessel owners alike.
The eye has two kinds of photoreceptor cells: rods and cones. Cones, which predominate in the center, give you color-sensitive, sharply detailed day vision. Rods, which predominate around the periphery of the eye, are what you use at night. They do not perceive color, only black, white and gray. Because they are more openly spaced than cones, rods don’t provide nearly as much detail but are extremely sensitive to light. When it is dark the cones are basically dead. This is why, if you stare directly at them, sighted objects seem to disappear. Only when you sweep your eyes around your field of vision and look at objects off-center—utilizing the rods—can you pick things out.
The time varies from person to person, but it usually takes at least 20 to 30 minutes for your eyes to adapt adequately to darkness and another 20 to 30 minutes before they are optimized. However, a blast from a cigarette lighter, flipping on the overhead lights, or turning on a white chart table light to check the tide book will have you starting over from scratch.
The use of colored lighting is strongly recommended. Red light has long been considered ideal for night use because it doesn’t adversely affect night vision. However, this isn’t exactly true. Any visible light, no matter the color, will have a negative effect on night vision. It is just that the rods are least sensitive to the red spectrum.
