Dark interiors at night

[JohnCarterOfMars]

Originally Posted by VikingGrandad

U-boat watch crewmen usually wore rubber goggles fitted with red glass lenses (as seen in the 1981 Das Boot movie) to help adjust their eyes to low light levels at night. Hence there was no significant need for a red light as well - and not much evidence for it even in conning towers (possibly an early war thing that became obsolete).

Thanks for that explanation.