Do you remember those movies where a submarine or some large ship is sailing at night? As the captain is staring into the darkness, searching for enemy ships or dangerous icebergs, the bridge is illuminated by a dim, ominous red light. That’s because red light is the best choice to maintain our captain’s night vision. Or is it?
As you know, our eyes can adapt to a dark environment pretty well, if we give it enough time to do so. Upon entering a dark room, we will see nothing at first, but after a few minutes shapes and objects will become visible. That’s because the rods in our retina become active, and they work very well at low light intensities. After about 30 minutes in the dark, they are working at their full strength. Unfortunately, this sensitivity comes at a price. As soon as we enter a brighter area again, the light will affect a chemical in the rods, known as rhodopsin, which stops them from working, and the cones take over again. In a few tens of seconds, all the effort put into our dark adaptation is lost.
Luckily, there is a solution. The rods inside our retina are mainly sensitive to blue and green light, and only a little bit to red light. Therefore, if we illuminate our ship’s bridge with dim red light, this will only affect the red-sensitive cones, and the rods will remain relatively unaffected. In this way, our night vision is preserved. So if our captain looks up from his charts to stare into the night, he doesn’t have to wait for half an hour before he can see the enemy ships again. But as always, that’s not the entire story. Why can’t life just be simple?
Ever since the advent of red illumination on ships, sailors started to complain about it. It becomes difficult to read and write, it increases eye-fatigue, and colors can’t be distinguished any more. Imagine having to read color-coded maps or charts when all you can see is red! Also, night vision is not strictly necessary for ships. As a military report states: “Even on a clear night when there is no moon, the brightness level of the starlit sky is an order of magnitude greater than the level required for absolute dark adaptation of a young observer”. It might even be dangerous to rely purely on night vision: detail is lost, only movement can be detected clearly, and since there are no rods in the center of the retina, no objects can be seen right in front of your eyes!
All these reasons have led to the end of red illumination on ships. Nowadays, a dim blue-green or white light is commonly used to illuminate the bridge, comparable to the dim blue light of the instruments on a car’s dashboard. This makes it much easier to read, write, and look at charts. And when dark adaptation is really necessary, for example on a dark night with a cloudy sky, it will only take a short time, provided the intensity of the illumination is low enough. Therefore, our captain can continue staring into the darkness, without having to worry about not seeing the enemy coming…