[PavelKirilovich]

Any light on the ocean at night is very, very visible. There is nothing darker than the ocean on a half-moon or a concealed moon night.

The human eye adjusts to darkness, as we all know. Being exposed to light "bleaches" the visual rods and causes the eye to contract. When entering a dark environment afterwards, your eye takes longer to widen and regenerate the "Visual Purple" that enables night vision.

Red light and blue light do not affect the eye as much as other colours. This is because they interfere less with the visual rods that carry the 'visual purple.' Chirascuro (I think I gimped the spelling of that) is the play of light in an environment or on an object - or both - and the control thereof. It's a principle factor in concealment.

You'll notice that you get a clearer image through binoculars or an optic with the less light that seeps in between your eye and the eyepiece lens. The same would apply to a periscope; redlight is a more neutral light and allows you to see more effectively in the darkness through an optic, hence its use in the control room at times when the periscope is in use.