Why was there red lighting in some spaces?
The primary purpose was to protect night vision. When going from white lights to darkness – in this case up on the bridge or looking through the periscope – the men were practically blind for 15 seconds or more. Sailors preferred not hurting themselves when going to their assignments. By going from red lights to darkness, they usually avoided the time that they were completely blind.
Red lights also reduced the time to fully adapt to the dark. Although some people claim that it takes hours to get full night vision, eyes mostly adapt to darkness in about 20 minutes if the person comes from white lights. If, however, the person comes from red lights, eyes adapt in about half that time
The lookouts in WW2 were looking for smoke on the horizon, usually the first indication of enemy ships. There may not have been a moon up yet, or maybe the sky was clouded over. They were looking for smoke in a dark, grey night. The lookouts needed the best opportunity of seeing enemy activity for the safety of the whole boat.
When getting ready for a watch on the bridge, sailors would usually be given red glasses or goggles to help preserve their night vision. This would prevent a stray flashlight or match from doing any damage to their night vision. In addition, all flashlights used in red-light spaces should have a red lens for the same reason.
Red lights also didn’t give away our location. White lights can show a beam for a great distance. If you remember the first 9/11 memorials in New York City, you know what this means. The memorials were just lights shining up to the sky from where the towers had stood. Those beams of light were visible for miles all around like pillars. Submarines didn’t want to show something like that.
Most of the boat would be rigged for red. Only maneuvering and the engine rooms would still be using white lights. The after torpedo room would be rigged for red for sleeping purposes. Crew there would have to go through the machinery spaces, where they are exposed to white lights, to get to the bridge unless they are wearing red glasses.
Reds lights in the boat at night did not cause serious difficulties. It was a bit harder to see gauges and labels in red lights, but the crew got used to that quickly. The most famous issue was that it was briefly impossible to play cards. If you have a deck in true red light, the red numbers and spots (hearts and diamonds) completely disappear. Sub sailors are quite resourceful and quickly solved the problem. They outlined the hearts and diamonds, and repeated the numbers, in black. Soon there were decks of cards printed for submarines produced by the Electric Boat Company. For an example of the solution, Google https://www.wopc.co.uk/search/query/submarine+playing+cards/ and click on option 2, Submarine Cards.
Is this a problem for you in your home, most commonly when coming out of the bathroom at night? We turn off the lights and open the door, and we see nothing for 15 seconds or so. One solution would be to install red lights in the bathroom. However, we assume that is not likely to happen. Another option is to do the “pirate thing.” (What on earth are we talking about? Is this person daft?) You have probably noticed that nearly all depictions of pirates include an eye patch. Certainly, you didn’t think that every pirate has lost an eye to injury. Pirates wore the eye patch when they were inside their lighted ship. Then, when they went up on deck, they would flip the eye patch up and out of the way. Then they could see out of the eye that had been covered while the other was still adjusting to the dark. We don’t need to use an eye patch. Just close one eye about 15 seconds before we turn out the lights. It looks strange for a bit, but it works.