Why is Red Light Used for Night Vision?
Due to the way our eyes work, we require at least half an hour of exposure to a dimmed environment in order to completely adapt to “seeing” in the dark. That’s because the cones and rods in our eyes react chemically to the light it’s receiving, adjusting vision in correlation to how much light is available. Exposure to any bright light – even for a brief moment – could ruin that carefully crafted sensitivity, which is why the use of a dim, red light (below 650 nanometers) is paramount to retaining night vision.
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Seeing at night can be a challenge. Moving about when it’s dark without disturbing sleeping people is a skill that can be managed using red lighting. Years ago I served on a nuclear submarine and often was on watch in the Control Room when the ship was brought up to periscope depth. At night this was a critical evolution since night vision would be needed to see out through the periscope.
About 30 minutes before “going to periscope depth” the OOD (Officer of the Deck) ordered “Rig Ship for Red.” This meant that all white lighting in the entire ship would be switched to red. All panel indicators were switched from white light to red light. In the ceiling fluorescent light fixtures there were two tubes—one white and the other covered in red. Switching to the red tubes would darken all the spaces on the ship—particularly in the Control Room. Everyone in the Control Room needed time for their eyes to adapt to the darkness. The rods in our eyes quickly began to adjust to the dim light, but they needed 30-45 minutes of darkness to fully adapt so we could clearly see the indicators on the ship control panels and for the OOD on the scope to see the night world topside. Some OODs actually put on sun glasses to speed up the adaptation process.
By shifting to red illumination in the ship our eyes immediately began to adapt. This meant that when at periscope depth, the eyes of the OOD could quickly discern objects in the night as the scope broke out above the surface. It turns out there is a physiological adaptation that our eyes make when we are in a darkened environment. This is called “night vision.”
In the back of each eye is a “retina” containing rod and cone cells (photoreceptors) that are light sensitive. These cells respond to light entering through the eye’s pupil. Rods respond to low light levels. There are about 17 rods for each cone. Cones respond to color and bright light conditions. The rods and cones send electric signals to the brain through our optic nerves. The brain interprets these as vision.
The rods can enable you to see the ambient light of an overcast night with no moonlight. The cones respond to the ambient light of a night with 50% moonlight all the way to full sunlight.
How Night Vision Works
The key to seeing at night is a chemical in the rods called “rhodopsin.” In bright light the rhodopsin molecules split into two other molecules (retinal and opsin) preventing the rods from exhibiting night vision. When the lights are dimmed or turned off, the space becomes darker. The cones are no longer useful in the dim light and the rods initially don’t have enough rhodopsin for night vision, so we are temporarily night blind. However in the darkened environment, the retinal and opsin molecules immediately begin to merge creating rhodopsin once again. In a few minutes, night vision comes to us and we can see in the dimly lit environment.
Using Red Light for Night Vision
So where does the red come in? It turns out that the color red has the least effect on night vision. By adapting to the dark and then using a flashlight with a red lens, we can see well enough to walk among objects including sleeping pets and people without waking them up. A bright light flickering through a darkened room can waken others especially pets. The cones in your eyes are more sensitive to yellow and red, but the cones barely work in dim light. The rods are more sensitive to the wavelengths of blue and green light. At night, blue-green light will look brighter than red light. So red is used to enable night vision with little impact on others and particularly on what you can see.
I found a number of flashlights that produce red light. The Coleman LED Multi-Color Flashlight (about $30) provides up to 90 lumens and reaches out to 450 feet (137 meters) with a run time of 10 hours. It comes with a bright white mode, red night vision mode, and a blue sportsman mode. The Streamlight Night Com LED Flashlight ($50) contains both white and red LEDs. The three white LEDs can produce 105 lumens and remain energized for 3.5 hours. Switch to the two red LEDs and you get 2.5 lumens for up to 28 hours using two CR123A lithium batteries. In addition, a number of headlamps have a red light night mode, so you have options for creating red illumination at night..
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You can also make a red lens for an inexpensive white-only LED flashlight using red nail polish, a red felt tip marker or red cellophane (like used in flower arrangements and for gifts). Some flashlight manufacturers sell red filter attachments that fit on their products. Fenix sells a red filter attachment for their TK-Series flashlights for $6.