When changing from a brightly lit place (like inside a lit sub) to a dark place (like the bridge of a U-boat at night) the human eye takes several minutes to fully aclimate to the dark - to get peak night vision. Try it at night. Sit in your room with the light on, and then turn the light off. At first you can see hardly anything. After a few seconds a few details appear as your iris adjusts and fully opens. After a few minutes the photochemicals in your retina actually become more sensitive to low light levels, and you can see things you couldn\'t see at all at first. This photochemical change gives the peak night vision needed, and it takes several minutes to build up - and can be destroyed by only a few seconds of exposure to bright light. It is a hangover from caveman days - good to see at night, but doesn\'t blind us once the sun comes up.

The trick is that a U-boat crew needs to have this peak night vision from the instant they step up to the bridge - it\'s too much of a risk to need several minutes for your lookouts to see an enemy destroyer or aircraft.

Red reduces adaptation to night vision less than any other color, blue the most. This has to do with the reaction of the light sensing elements on the retina - not sure if has to do with wavelength, a photochemical thing, or what, but it is a longe stablished fact that exposure to a dim red light will allow you to adapt to night vision while white light will destroy it.

By wearing the goggles (and using red lights in the conning tower) the crewmen could adapt to low light levels before surfacing at night. This allowed them to adapt to night vision while still functioning in the conning tower (blindfolding the lookouts prior to surfacing would work too, but the Captain and officers might not want to be blindfolded while they conned the sub!). The red interior lighting also allowed the captain or other crew to go topside to take a look, and jump back down to consult with sonar or TDC operators, etc, while on the surface without the interior lights blinding their night vision.

So just prior to a night surfacing, and while on the surface, the conning tower was lit with red lights and the crew wore red goggles before going topside. Once topside they took off the goggles, but the red lights remained in the conning tower. More remote parts of the sub, far removed from the in and out activity of the bridge crew (like the torpedo room or engine room) remained with white lights.

Pilots, when they fly at night, also use red flashlights and the flight instruments are lit with red lights for the same reasons.