eli5: Why do submarine/ships/helicopters switch to red light under stealth mode ?


Haven’t seen in real life but nearly all movies show switching to red light under stealth mode (of course also during emergency mode). Shouldn;t red light be visible from a long distance ?

In: 4

It’s not really for stealth, it’s for night vision. Red lights don’t blind you like white lights do.

Red or blue light is used by the military to help soldiers adjust their eyes for night time.

It takes 10-20 minutes for your eyes to adjust to darkness, but only moments to adjust to bright light.

So when you are working at night blue or red light is used to prevent soldiers from breaking their night vision.

On a submarine this is used for several reasons.

One it helps with the circadian rhythm. Since the sub doesn’t see day or night out the window having a lower light level on the ship during night time helps the crews with their sleep cycles.

During combat having lower light in the ship makes it easier to see instrument panels and doesn’t interfere with the periscope.

It also helps in case the lights on the ship get knocked out, if the sailors eyes are adjusted to the dark, they won’t poking around blindly in the dark under minimal light trying to fix the ship.

One the surface: red light doesn’t carry as far as while light does, so low-powered red lights are used at night for illumination when keeping things dark is of paramount importance (keep in mind things are generally lit just enough to keep you from tripping over stuff). It also has the benefit of not affecting your night vision as much as white light does, as well.

Undergound/underwater/in enclosed spaces: red lights are used sometimes in these conditions to allow screens and instrument panels to be as visible as possible without glare from white lights on the screen. It’s easier on the eyes when looking at the screens and panels.

Several other commenters have already mentioned that red light preserves your night vision (which is the main reason it’s used), but I don’t think anyone’s really explained why it does that:

There are two types of cells in your eye that you use to see, which are called rods and cones. Cones are really good at sensing colors, which makes them great in most situations, but they don’t work unless there’s enough light. Rods are the exact opposite – they can only really see in black and white, but they’re *incredibly* sensitive to light, so they’re great for night vision.

The only problem with having both types of cells is that we spend most of our time in brightly lit areas, and because rod cells are so sensitive, we’d be blind if they were always working. To fix this, your rod cells only activate when they’re given a special protein called rhodopsin that’s constantly being created by your eyes.

The reason rhodopsin is special is because light destroys it, which means that it never reaches most of the rod cells when you’re somewhere bright. When it gets dark, it’s able to start activating rod cells, but because your eyes don’t create it very quickly, it takes around 20 minutes to fully build up and turn all the cells on.

This is where red light comes in. Unlike cone cells, rod cells (and rhodopsin) only respond to certain colors of light. Red isn’t one of them, so it makes your rod cells activate even though you can still see with your cones, and you don’t have to wait if it gets dark. This is why ships use red lights when they’re in combat – everyone can see what they need to, but if something bad happens they won’t be stumbling around in the dark.