Red night lighting is among the few electrical components that I put in the "very important" category for even the cheapest of cruising boats. Why red, and why is it so critical?

You likely recall from high school biology that the human retina contains four types of light-sensing cells: "rod" cells, and three kinds of "cone" cells. The rods cover a wide field of vision and are sensitive in low light; the cones kick in when the light gets brighter to provide us with detailed colour vision, mainly in the centre of the visual field. Each type of cell responds to a different range of wavelengths; when all four are combined, they cover the entire visible spectrum.

If we view something under white light, all four types of cells respond at roughly equal levels.  Monochromatic light also triggers all four types, but not equally- at 420 nm (a deep blue-violet), your vision is coming primarily from the short-wavelength cones, while at 600 nm (red) most of the response is due to the long-wavelength cones.

Rod cells barely respond at all to red light of 600 nm or longer. It is therefore possible to make a red light bright enough to read your charts and pilotage notes using your long-wavelength cone cells alone, allowing your rod cells to remain in their sensitive, dark-adapted state. In other words, deep red light allows you to read and work without disrupting your night vision.

For this effect to work, you need a better red than you'll get by putting cellophane over a flashlight- that would let enough shorter wavelengths through to trigger your rod cells and disrupt their dark-adapted state. Several layers of dark red cellophane over a standard bulb will work, but leaves you with a very dim light. Photographic lighting gels are a bit more expensive and noticeably superior. By far the best option, though, is red LEDs with wavelengths of 600 to 650 nm. They're very cheap, very power-efficient, and as close to monochromatic as you can get.

Every work space that might be used at night should have red lights. Indeed, I would argue that on a well-designed cruising boat, you should be able to sail from dusk to dawn without ever turning on a white light. Save the whites for when you're at anchor or when you need extra light to cook dinner in twilight; if you'll be relying on your eyes in the dark, red is where it's at.

Like many things aboard ship, the best way to do this is to plan it out before the boat's even built. If you think carefully about where people will walk, where they'll look and what tasks they'll do at each location, it's possible to come up with lighting designs that are considerably more useful and more efficient than simply sticking a few flood lamps on the deck beams. And I'm seriously considering a master "Red/White" switch for a couple of the boats I'm working on, making it easier to ensure that you don't accidentally dazzle yourself by turning on the wrong light.