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Originally Posted by cardician
So why not just leave the light white all the time?
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Another reason, besides enabling night vision, is that red light is harder to detect with the human eye over long distances.
Imagine you are cruising along on the surface at night. You need to change lookouts and Barnard accidenlty cracks the hatch on a compartment just oozing with white light. Even non-carrot eating Tommies will see the white light and mention it to the Captian of the destroyer.
With the compartment illuminated with red light, even if Barnard forgets to douse the light before cracking the hatch, the chances of Bugs Bunny being able to see the light is much less.
Smart crew members always douse the light when cracking the hatch at night. But by using red light, even if Barnard is on deck, the chances of self-illuminating are reduced.
I believe I read somewhere that modern theory says that using a blue light of a specific frequency protects night vision while enabling all colours to be seen.