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Answered 2011-05-14 22:56:23

Here's the short answer: to preserve the night-vision of the submarine's crew, while retaining adequate color and depth-perception abilities.

The long answer? Well, to understand that, let me take you back, just before World War I, when early submarines were starting to come into their own.

Back then, all military vessels--surface and submerged--relied on red-colored lighting for night operations. The theory, at the time, was that red-hued light only saturated the red photo-receptors of a person's eyes, thus leaving the green and blue receptors unaffected. Hence, all-important "night-vision" could be preserved better by stimulating only *one* of the three photo-receptors (humans can see red, green, and blue, and have receptors for each of these three "primary" colors). In addition, red had the advantage of a high-frequency wavelength (about 560-570 nm). When coupled with a low-powered intensity of illumination (through lower wattage, typically 4-6 W; what your standard incandescent "night-light" puts out), the light effectively "died-out"; it was hard to detect from long distances. Perfect for surface-going ships.

But, how did this make its way to the world of the submarine?

Remember, early submarines were really not much more than surface-going small frigates (some would say, using the British term, more like "corvettes") that *could* submerge under water. Back then, submarines did the majority of their fighting "above-the-beam"; from a surfaced position. That's why all those old WWI-WWII era subs have huge deck guns--you're not going to fire that thing from underwater!

Instead, the purpose of a submarine was to stealthily sneak up on an enemy contact, quickly surface, fire off it's deck guns, and submerge before it could be fired upon. (Although torpedoes could be used, they were--for the time--fraught with unreliability, were expensive, and required the boat to remain on-station until impact--due to their cable-operated guidance--thus rendering a submarine highly vulnerable to counter-attack.) Thus, the deck gun was the most favored means of primary attack.

Most submarine surface attacks were carried out at night (I don't care what's depicted in U-571, that was a completely different, highly-desperate situation). Thus, transitioning from the enclosed, controlled conditions inside the boat to the exposed, uncontrolled conditions *outside* the boat had to be done as quickly as possible. As the ship came to Battle Stations, and subsequently surfaced, the Chief of the Boat would "rig for red," dimming (or extinguishing) all work lights, and lighting the red lights, so as to prepare the men for going topside, into the night. Remember what I said, a few paragraphs ago? By saturating the men's eyes with red-colored light, their blue and green "night-vision" perception was thus unaffected, making "recovery time" once topside much quicker!

In fact, this tradition is still used by submariners, to this day. When a sub surfaces, the lights are switched to red (as accurately portrayed aboard the USS Dallas in The Hunt for Red October). As Cpt. Mancuso and his XO go up to the weather bridge, the lights switch to red--the holdover from the old submariner days!

So, what's with the funky bluish-green lights, you ask? It has to do with two developments, namely: better understanding of how the human eye works, and advancements in computer-display technologies.

First-off, the human eye. By the mid-60's, scientists had discovered that red-light saturation actually *decreased* night-vision, to some extent. It wasn't necessarily the hue of the light that affects night-vision (though that fact is still debated), but rather the intensity of illumination. See, red-hued light requires almost *twice* as much illumination to provide the same candle-power as lower-frequency (420-460 nm) blue-hued light. Thus, red-light is prone to induce more glare, and will negatively affect the cones (photo-receptors) of the eye. Blue-green light at a *lower* illumination is actually better, providing more color "information" per degree of illumination.

Which leads to the second development...

Red light kills depth and color perception. Sonar operators--who relied not only on their well-trained ears, but also their well-trained *eyes* to interpret the sonar plot)--were reporting trouble reading their spectral plots in the intense red light (coincidentally, most sonar plots are now green-and-blue, and sonar shacks are *always* rigged in blue lighting). The Navy conducted many studies and tests, and found out--sure enough--that blue light would actually be better for *modern* submariners. Since they hardly go topside, now, to fire their armaments, the whole "preserving night-vision" mechanism is unnecessary. Instead, accurate color and depth-perception, especially when translating instrument output, is paramount. Again, using The Hunt for Red October, as the Dallas goes to Battle Stations, the blue lamps are switched on!

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