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
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.