The use of red light for illumination.

Posted: August 2, 2011 in Numbers & Statistics, Random facts and WTFs

Remember how in Hollywood movies they always show submarines, which use red light to illuminate their control rooms?

Well, that’s not far from the truth, actually. Military submarines always use red light during night time, when it’s dark outside. But why red?

The answer lies in the physiology of the human eye.

There are two major types of color receptors on human retina: rods and cones.

Rods work only in dim light, allowing people to see in the dark (scotopic vision), while cones require bright light to function and are used for colour vision (photopic vision). In most cases, they can’t function at the same rate simultaneously (simply because colour-capturing cones need bright light to perform, while rods get screwed by excessive amount of light (the process known as bleaching)). It might take up to 30 minutes for a person to switch from photopic vision to scotopic vision.

Submarine crew has to keep both photopic vision (in order to be able to read all the control panels) and scotopic vision (in case if they have to look into a periscope during night time).

Now, lets look at the electromagnetic wave frequencies, which activate certain photorseptors on the human retina:

As you can see, 3 types of cones cover the entire visible spectrum (400-700 nm), while rods are only capable of getting turned on by the light waves, which are not longer than 640 nm. That leaves L-cones with approximately 60 nm long piece of visible spectrum, which can be handled without any overlapping with other photoreceptors.

640-700 nm electromagnetic waves appear red to us.

Night-vision rods are insensitive to red light, therefore they can not be bleached by it. So, using red light in dark conditions allows people to keep both photopic and scotopic visions without any overlaps and conflicts between these two systems. Pretty cool, eh?

Red light is also used during nighttime by pilots, tank crew members, old-school astronomers and some lab-workers, who experiment with photosensitive materials.

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