The Science Behind Signal Mirrors© -- Part Two

 

 

This is Part Two of a two part series, For Part One go HERE – Author’s Note

 

Four ways of aiming a signal mirror...

Besides flatness and what the mirror is made of, the third thing that determines how effective a signal mirror is, how easy it is to aim.  The most difficult part of aiming a signal mirror is finding the correct angle, so that the mirror faces exactly halfway between the Sun and your target.  This is why a regular, or simple mirror, without a centered hole, is more difficult to use than a signal mirror with built-in aiming sight.

 

There are four ways to aim a signal mirror, the sighting surface, foresight, or W.D. Twitchell method⁴, the attached foresight method, the rearsight method, and the reflex-button or retroreflector method.

 

The first method, the sighting surface, foresight, or W.D. Twitchell method is the method most often used with simple or improvised mirrors, without a sighting hole.  The two-man method, shown in Square 1, Figure 2 above, is difficult to use, particularly in a liferaft in rough water and the signaller is more likely to be successful when using his or her fist or fingers as aforesight.  On land, where you are not in continous motion, as you are in a liferaft, it is easier to keep a foresight in line with the target. 

 

Sometimes instead of your hand, it is convenient to use a pole, a bush, or part of a tree as a foresight. 

A mistake that signallers commonly make when using their fist or fingers as a foresight is not holding the mirror close to their eye.  The signals will not be directed towards the target unless the mirror-reflected beam of Sunlight and your line of vision both start from the same area, so place the mirror just above your eye and direct the reflected Sunlight towards the foresight to correctly align the beam of light on the target.  Place the target within the “V” of your fingers or on top of your thumb or fist.  Tilt the mirror so that the reflected Sunlight sweeps across your fingers or fist.  Each time you sweep the bright spot of reflected Sunlight across your fingers you are flashing your target.

 

The second method is the attached foresight method (Square 2, Figure 2), where the mirror is equipped with an attached sighting key to use as a foresight.  The sighting key has a keyhole and so does the mirror .To use this type of mirror correctly, the signaler must align the beam of light on the target by keeping his line of sight through the mirror and keyhole aimed towards the target, while keeping the shadow of the sighting hole centered on the keyhole.  Like the two-man foresight method, this sighting method is difficult to use, particularly in a life raft in rough water.

 

The rearsight method of aiming a signal mirror (Square 3, Figure 2) can be used when there is a rearward facing mirror surrounding thesighting hole or cross, which is called an aperature, in addition to the mirror surface facing forward towards the target.  The dot of Sunlight coming through the sighting aperture is intercepted by the signaler’s face or hand and is then seen reflected in the rearward facing mirror.  The signaler aims this mirror by turning the mirror until the reflected dot or cross of Sunlight disappears from the rearward mirror into the sighting hole, while it is directed towards the target.  Aiming a mirror by the rearsight method is not a easy task, particularly from a moving surface.

 

Retroreflection is used in street signs, bicycle reflectors and is the reason why eyeshine is produced by the tapetum lucidum of cat’s eyes.  The reflex-button or retroreflector method (Square 4, Figure 2) is the simplest and easiest way to aim a signal mirror.  It uses a retroreflector layer or grid, which reflects light back to its origin instead of off at an angle, this produces a red dot or “fireball” where the “flash” is aimed.  Rotate the mirror until the “fireball” and the

target overlaps within the sighting aperture.  Because the “fireball” in the sighting aperture will not move much, move the entire mirror, and remember the “fireball” is much larger than the “flash”, so either sweep the fireball over, or center the fireball on, the target.

 

The retroreflector/reflex-button type of mirror is the easiest to aim and flash, especially when used from a life raft, and , according to a study done in the 1940’s by the National Standards Bureau, it allows the signaler to flash 35 times per minute.  The rearsight method is the next easiest to aim, according to the study, allowing the operator to flash 14 times per minute.  Aiming your signal mirror using the sighting surface, or with an improvised foresight method, only allows the operator to create less than one aimed flash per minute.

 

Even if you are using a retroreflector or rearsight type of mirror, you can still use the “V” finger method to help you aim your mirror; this will give you visible proof that your “flash” is on target, as the light flashes over your fingers.

  

Flatness, size, what the mirror is made of, and how easy a signal mirror is to aim, all determine how effective a signal mirror will be at attracting attention.  And that is the science behind signal mirrors!

 

Don’t forget to come back next week and read “Woods Shock, Don’t Lose Your Head ©”, where we will talk about being “misplaced”, woods shock and not giving into your fears.

 

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!

 

 

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 0.5^o in diameter, when observed from the earth.

 

⁴ This sighting method was developed by W. D. Twichell, while doing survey work in Texas during the 1940s.

 

“Recent Developments in Signaling Mirrors”, Air Sea Rescue Bulletin, June 1944, by Richard S. Hunter.

 

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