During the late 70s and early 80s, the Spokane Daily
Chronicle, printed a column in the Outdoors section, called “Could You
Survive”, which was written by U.S. Air Force survival instructors.
No
one sees as well at night, as they do during the day, but did you ever stop to
think about why that was, or how you preserve your night vision when you are
out in the wilderness? Well, that is
exactly what the authors of the “Could You Survive” article asked back on
Tuesday, November 3, 1981, in the Spokane Daily Chronicle.
An excerpt from “Could You Survive” published in the Spokane Daily Chronicle, November 3, 1981.
So,
what would you do? Would you pick answer
A, B ,C or D?
“Hmmm,
that’s a good question, isn’t it? But
first before we choose an answer, let’s talk about the anatomy of your eyes and
about day vision versus night vision”.
The Mark-1 Eyeball
The anatomy of the eye, an excerpt from “Seeing at Night”, by Robert A. Alkov, PhD, from Fathom, Summer 1970, page 58.
Your
eyes are like very sophisticated biological cameras, and certain
parts of your eye resemble the parts of an analog camera, such as the lens
which in a camera focuses light on to film and in a human eye focuses light
onto the retina. And your iris is like
the diaphragm of the camera and opens or closes to regulate the amount of light
entering your eye.
An excerpt from Scouting and Patrolling, MCWP 3-11.3, by the Marine Corp, Figure 5-1, showing how human eyes are like a simple camera.
Your
retina, just like the film in an old-time camera, is a light sensitive layer
made up of both cone cells and rod cells, which transmit a visual image to you
brain when stimulated by light. The
center of the retina is a called the fovea centralis and contains only
cone cells. This part of retina is about
2 degrees wide, or about the width of your thumb, and at night is what is
responsible for causing the “night blind spot” in the center of our
visual field. The rest of the retina is
composed of both cone and rod cells, with rod cells becoming more common, until
at the edges of the retina, there are only rod cells. The “optic disk” is the spot in your
eye where the optic nerves and blood vessels enter and leave your eye, and this
is the spot that is responsible for causing the “day blind spot”.
An excerpt from “Oh Say, Can You See?”, by Major Nicholas E. Barreca, MD; from the U. S. Army Aviation Digest, June 1971, page 16.
Each
cone cell contains a light and color sensitive pigment called iodopsin,
which is not sensitive to any light below the level of full moonlight, and each
single cone cell is connected by a single nerve to your brain. Cone cells are what allow us to see in daylight,
in color and in detail. Rod cells contain
a light sensitive pigment called rhodopsin, which is also called “visual
purple”, and only distinguish shades of gray, black and white. Many rod cells are connected in a series to a
single nerve, and this is what allows low light to send a visual impulse to your
brain.
An excerpt from Rotary Wing Flight, FM 1-51, by U.S. Department of the Army, page 6-1.
Day
Vision...Night Vision
There
are three types of vision, phototropic, which allows you to see during
the day, this type of vision only uses the cone cells of your retina; mesopic,
that allows you to see during dusk and dawn, and which uses both the cone and
rod cells of your retina; and scotopic, which is what allows you to see
in the dark, and that only uses the rod cells of your retina.
Night,
or dark adaptation, is the process by which your eyes adjust to seeing in low
levels of light. It involves both
physiological and chemical changes in your eyes and it takes your rod cells
about 30 to 45 minutes to produce enough rhodopsin, or “visual purple” to increase rod cell
sensitivity to light and allow your eyes to see in low light conditions. Imagine entering a dark room, at first it is
difficult to see anything. Quickly
however, your pupils dilate, or enlarge, to let in as much light as possible,
and after about 5 to 10 minutes your cone cells become adjusted to the dim
light and your eyes will be about 100 times more sensitive to light, than they
were when you first entered the darkened room.
When fully adapted to the darkness of the room, your visual sensitivity
to light is about 10,000 times higher than it was in a brightly lit area. This process happens independently in both
eyes and can be quickly lost if you are exposed to a bright, or even moderately
bright light, which is why you should close one eye before being exposed to
bright lights to prevent the total loss of your night vision.
Seeing at
night...
An excerpt from the Flight Training Handbook. Revised 1980, by the U. S. Flight Standards Service, page 194.
Scotopic
vision or, night vision, is the type of vision we experience at night during
conditions of partial moonlight or starlight.
At night your visual acuity drops from 20/20 to 20/400 and you will only
be able to see large objects. Also, you will
be colorblind, because only your cone cells can detect color and at night only your
rod cells are used. Also, don’t forget
that at night, you will experience a two-degree wide “night blind spot” in
the center of your visual field, and you will have to rely on your peripheral
vision to see, which means looking away from what you want to see!
An example of off-center viewing, from Long-Range Surveillance Unit Operations, FM 3-55.93, Figure J-2, page J-2.
During the day looking straight at
something and using your central vision works, but not at night. At night you must use your peripheral vision, because
of the “night blind spot”, This
is called “off-center viewing”, and instead of looking directly at an
object, you must look instead out of the corner of your eye, above, below, or
to either side of what you want to see, just like in Figure J-2, above. It varies from individual to individual, but experts
recommend looking from 6o to 15o degrees away from what
you want to see. With your arm is fully
extended out in front of you, this is between the width of three fingers (about
5o) to one hand-span (or 15o), which is the distance from
the tip of your little finger to the tip of your index finger.
From “Measuring The Sky”, by Brian Ventrudo, April 19, 2009, HERE. When fulling extending your arm, your fingers can measure degrees of distance.
At
night, when using an off-center viewing, move your eyes in a series of short, systematic
hops over and around what you are looking at.
This prevents “bleaching”, which is the neutralization of the
rhodopsin in the rod cells. When you use
your rod cells, the visual purple in the individual rod cells “bleaches”
or blacks out after 4 to 10 seconds and the object you are looking at will seem
to fade and then disappear. As the visual
purple in the rod cells in one area bleaches out, you must shift your eyes slightly
to use fresh rod cells.
Examples of scanning techniques for off-center viewing at night, an excerpt from Long-Range Surveillance Unit Operations, FM 3-55.93, Figure J-1, page J-2.
And the
correct answer is...
An excerpt from “Could You Survive” published in the Spokane Daily Chronicle, November 3, 1981.
“So,
what did you choose, did you choose D? I
hope so, but if not, now you know!”
The complete “Could You Survive” article, published in the Spokane Daily Chronicle, November 3, 1981, HERE.
Don’t forget to come back next week and read “Making an Altoids® Tin
Spice Container ©”, where we will talk about spicing up that bland dehydrated
food with an easy to make and carry Altoids® tin spice container.
I
hope that you continue to enjoy The Woodsman’s Journal Online and look for me
on YouTube at BandanaMan Productions for other related videos, HERE. Don’t forget to follow me on both The
Woodsman’s Journal Online, HERE,
and subscribe to BandanaMan Productions on YouTube. If you have questions, as always, feel free
to leave a comment on either site. I
announce new articles on Facebook at Eric Reynolds, on Instagram at
bandanamanaproductions, and on VK at Eric Reynolds, so watch for me.
That
is all for now, and as always, until next time, Happy Trails!
Sources
Alkov,
Robert A., PhD; “Seeing at Night”, Fathom, Volume 2,
Summer 1970, pages 58-61, https://books.google.com/books?id=AN-8DgE5FWkC&pg=PP69&dq=night+vision+cigarette&hl=en&newbks=1&newbks_redir=0&sa=X&ved=2ahUKEwir0L39zsb5AhVeEGIAHdl9DZs4FBDoAXoECAQQAg#v=onepage&q=night%20vision%20cigarette&f=false,
accessed August 15, 2022
Barreca, Major Nicholas
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accessed August 15, 2022
Marine Corp; Scouting and Patrolling,
MCWP 3-11.3, [Department of the Navy, Headquarters United State Marine Corp,
Washing ton DC, April 17, 2000], page 5-1, https://www.marines.mil/Portals/1/Publications/MCWP%203-11.3%20%20Scouting%20and%20Patrolling.pdf,
accessed August 15, 2022,
Strauss,
Lt. Col. Samuel, “Night Sight”, Flying Safety, Volume 46, April 1990,
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accessed August 15, 2022
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accessed August 9, 2022
U. S. Department of the
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accessed August 15, 2022
U. S. Department of the
Army, Rotary Wing Flight, FM 1-51, [Headquarters Department of the Army,
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accessed August 15, 2022
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accessed November 27, 2018
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