Night Vision

 
 
 
Information
         

Q: I've heard that a red LED light will preserve my night vision. Is this true?
A:
Short answer: Yes. A dim red light will preserve your night vision.

The LONG answer:

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%.

So how does red preserve our night vision?

Let's say that it is dark outside and you are using only starlight to see by. You have been away from bright lights for 1/2 hour so your night adapted vision is working at its maximum and you see the landscape in shades of gray. You need a brighter light to walk around or see some details in the environment but still want to be able to see all the stars when you shut off your flashlight. Using the above information, we have a sneaky way to preserve our night adapted vision but still use light bright enough to keep us from tripping over a stump.

When our eyes are fully night adapted, we are using our rods. The rods cannot detect red light, but our cones can. So if we use a red light flashlight we can see what is around us using our cones. The rods in our eye can't see the red light and our night adapted vision is unaffected. Turn off the red light and you can go right back to looking at the stars in the same detail as before without having to wait another 1/2 hour for our rods to work again.

Does the brightness of the red light matter?

Actually, yes! You know how when you look at a bright landscape and then quickly look at a dark area you can sometimes still see the landscape in your vision? It looks like a bright negative image? This is called an "afterimage" and it fades over time. If you use a very bright red light your cones will see "afterimages" when you shut it off. The afterimages overlap your field of vision and will make it hard to see using your rods until the afterimages fade. Therefore you should use as dim of a red light as possible for the task at hand to preserve your night adapted vision.

But you said our rods are most sensitive to Cyan light! Can't I use a Cyan LED light to look around and still preserve my night vision?

Yes, our rods are most sensitive to 500nm (cyan/turquoise) light. Remember though, that the rods are very, very, very sensitive to intensity. A bright light of any color (except red) will ruin your night adapted vision. So cyan is actually the WORST to use when you need a brighter light to see the environment around you since this light wavelength is what the rods pick up best. Cyan light has to be kept very, very dim to keep it from ruining night adapted vision.

If you want to thoroughly ruin someone's night adapted vision, shine a bright Cyan/Turquoise at them. Since the rods are responsible for our night vision and are most sensitive to this color light, they'll immediately go "night blind" and will be unable to see in the dark at all immediately after turning off the light. Of course this will work with a bright white light too...

Here is an example of the worst possible "night vision" light you can have, and what appears to be a bit of a misunderstanding by the maker of how our night adapted vision works! Since the headlamp in the article produces a very bright cyan light, the user's rods would be completely shut down and only the cones in the eye would be used. In that case you would be better off using a white headlamp so you can see all the colors of the spectrum using your cones instead of just blue/green!

What about Green light? My relative in the military says they use Green light for night vision!

Green light is used by the military for electronic night vision equipment which is less sensitive to green light. However a green light can still have a bad effect on your night adapted vision, just like any other color of light unless it is kept very dim.

How can I tell if a light (other than red) is too bright to preserve my night adapted vision?

How can you tell how much light is too much for the rods? Easy! If you can see the color of the light, it's too bright for the rods and the cones are now doing the work. If you only detect it as "light" and you still see the world around you in shades of gray, the cones are shut down and the rods are doing the work.

 

 
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