Red lights for good night vision - but why?

1:02 AM Fri 23 Sep 2011 GMT

'Good night vision from red lights in cabin or cockpit' .

Red lights in the cockpit or cabin are there to protect your night vision - that critically important sense to achieve the very best outcome when scanning for objects on the horizon at night. But have you ever wondered just why the red lights achieve this? Read on...

The back of our eye, called the 'retina' detects light and allows us to 'see'. The retina is made of of 2 types of structures, cones and rods.

The cones are responsible for our normal daytime vision. Cones detect both the wavelength (color) and intensity (brightness) of light that goes into our eyes and passes that information to our brain.

The rods are responsible for our 'night adapted vision'. Rods do not detect wavelength (no color), but are very sensitive to intensity (brightness) of light. They pass on only shades of gray to our brain. They only work at very low light intensities (dim light), are most sensitive to light at about 500nm (turquoise/cyan), and are blind to red light (around 620nm).

If you are walking around under daylight and you see the world in a kaleidoscope of colors, you are using your cones to see with. If you are walking around under starlight and the whole landscape appears as shades of gray, you are using your rods to see with.

The last thing you really need to know about the rods is that it takes some time for them to work after moving from bright lights to a dim environment. Usually it takes about 15-30 minutes for them to work at 100%. However, even a fraction of a second of bright light will cause the clock to reset and you may have to wait another 15-30 minutes for your night vision to be back to 100%.

This is why it is very important to have a red light in your cabin to switch on when you wake for your next night watch. It will prevent you arriving in the cockpit temporarily blinded.

by Lee Mylchreest

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