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From Life Magazine, November 27, 1970, page 88
This is Part One of a two part series, don’t forget to
come back next week for Part Two – Author’s Note
Everyone
has heard of signal mirrors and of survivors “flashing” the Sun to alert
rescuers to their location. But how many
people know the science behind signals mirrors?
The science of signal mirrors
It
is hard to believe, for those who have never seen it demonstrated, but “flashes”
of Sunlight from a mirror, no more than 3 inches by 4 inches (10 by 8 cm) can
be seen three to five times farther away than a crashed plane or a life raft
bobbing in the ocean can be seen!
A
signal mirror is the most basic and best all-around signaling device there is,
it is compact, light and above all else easy to use, even if you have little or
no training. And a trained person with
one mirror can easily signal in an arc up to 270o wide, and sometimes
over an arc of 360o if the Sun is high.
What is it
made of...
What
a mirror is made of determines its reflectance, which you would think would be
of primary importance in its ability to generate a “flash” that can be
seen for miles. Actually though, it is
only of secondary importance. The
difference in the brightness of flashes
from similarly sized mirrors made of different materials can only be noticed if
the flashes are made simultaneously and side by side, the brightness of the
flashes are virtually identical.
Glass
mirrors should be sealed, so that water cannot infiltrate the reflecting
surface, fogging or damaging it. Many
modern signal mirrors are made of acrylic which are lighter and less brittle
than glass and have a reflectance that is second only to glass mirrors.
Flatness...
The
characteristic that is of primary importance in a mirror’s ability to generate
a “flash” that can be seen for miles, is its flatness. A perfectly flat mirror will reflect a beam
of Sunlight in the form of a cone 0.5o in diameter as measured from
the mirror2. If the mirror is
not perfectly flat, the spread of the cone of Sunlight will be greater than 0.5o
and the beam of light will be correspondingly weaker. Strangely enough, tests conducted during the
1940s by the U.S. National Bureau of Standards determined that over 8 miles (13
km), the distance at which a survivor would be able to see an aircraft, that a
slight deviation from perfect flatness spread the reflected beam of Sunlight
and improved a signaling mirror’s ability to generate a flash that would be seen
by observers.
However,
in a pinch anything that is “...reasonably flat and shiny enough to reflect...”3can
be used for signaling. The less flat a
shiny surface is, whether it is a tin can top or a piece of folded aluminum
foil, the wider and weaker the reflected cone of light is, and the less
distance over which it will be able to be seen.
And size
matters...
The
standard United States Coast Guard signal mirror is 4 inches by 5 inches (10 by
13 cm) and the standard large mil-spec size is 3 inches by 5 inches (8 by 13 cm), both are ideal. Anything larger quickly becomes unwieldy and
difficult to use aim accurately for extended periods. However, for children, or those with smaller
hands, the USCG sized mirror can be awkward to use, especially if it is made of
heavy materials. The standard small
mil-spec size, which is 2 inches by 3 inches (5 by 8 cm), although smaller,
generally works well.
Improvised
mirrors that are less flat than purpose-built mirrors will need to be larger to
create the same strength flash. A square
mirror creates 25% more reflecting surface, and therefore a brighter flash,
than a round mirror of the same dimensions.
So,
if you have to improvise a signaling mirror, find the flatness and shiniest surface
you can, and don’t forget a square 3 inches by 5 inches (8 by 13 cm), is probably
the best. Don’t forget to come back next week and read “The Science Behind
Signal Mirror, Part Two ©”, where we will discuss the ways to aim
a signal mirror.
I
hope that you continue to enjoy The Woodsman’s Journal Online and look for me
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That
is all for now, and as always, until next time, Happy Trails!
Notes
1 Doug
Ritter, “Signaling Group”, Equipped to Survive,
2
“Signaling With Mirror”, Air Sea Rescue Bulletin, July 1944, by Richard
S. Hunter,
3
This is because the Sun is 0.5o in diameter, when observed from the
earth.
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