Precision Signal Mirror Use

I've seen incorrect information on the usage of signal mirrors on the internet. Those instructions would not allow the accuracy of reflection necessary for long range signalling.

To get long range accuracy you need three (3) things:
  1. The source of brilliant light (usually the sun) needs to be on the same side of you as whom you're trying to signal (you may have to recline or lie down to accomplish this).
  2. The dot of light coming through the center or sighting hole of the mirror needs to shine on a surface that you can see in YOUR side of the mirror (such as your chest).
  3. To direct the light precisely at your target and holding the mirror as far from you as is easy, you must see your target through the center of the sight hole, and at the same time have the reflection of the dot of light also lined up with the center of the sight hole. That is, in your side of the mirror you will see a reflection of the area surrounding the dot, but the dot itself will be in the hole. Actually, if you can see where the reflected light is hitting, as you bring the image of the dot (on your side) into the hole, the reflected light spot will seem to replace it! So if you have the target in one corner of the hole, you can bring the dot's reflection around to that same edge and move just enough to put the dot right ON the target (if you could see it).
I've seen some pretty expensive "signal mirrors" that were very small and made of plastic "to resist breakage". But the amount of light reflected to the target depends on the area, the reflectivity, and the flatness of the reflective surface. Glass is harder and generally more rigid than plastic, so it is harder to scratch (scratches reduce reflectivity), and can be flatter for a given area. So a cheaper, larger glass signal mirror with a highly reflective reflecting surface can be much more effective than a small, expensive plastic one.

In fact, a pair of 12 in. mirror tiles glued back to back and parallel, with a sight hole in the reflective backings, can be effective at a considerable distance with only the light from a full moon!

3rd Edition
SS103 2012