Making a Signal Mirror with a Retroreflective Aimer
Channel: rafowell
Date: 2016-02-24

I show how to turn a regular mirror into a signal mirror by adding a retroreflective aimer that lets you accurately point the reflected sunbeam.  See https://flic.kr/p/2dfBivi for what the "red dot" sighting aid produced by this aimer looks like in actual use, and https://flic.kr/p/2nGXxiE if you want to understand how it works. The center of the bright yellow dot  at the core of the red halo in the photo is the center of the reflected sunbeam from the mirror (though the sunbeam is even narrower — about 1/3 the diameter of the core.  To signal something, you tilt the mirror so the very center of that bright core crosses the target. The extreme narrowness of the sun beam (1/4 degree in radius) is why you can see the sunbeam of a 2"x3" mirror at 20 miles or more — it is very concentrated — roughly 4 million candlepower.

All current issue US military signal mirrors use retroreflective aimers, though of the mesh type, not this "red-dot" type.

This type of aimer was used in the B-1 signal mirrors issued to US troops in the late 1940s, and is used by French and Italian air force pilots to the current day. You can see a photo of one of the 1940s US military signal mirrors and read about the  history here: https://flic.kr/p/qFybxT

There are two other good YouTube videos on this topic, but this one adds a number of practical tips that are not in the others. I do recommend the others, though: they are: https://youtu.be/lTy-o5Ci4-k and https://youtu.be/wh5MkBnsnfE One more practical tip from another leader: he says that running the Dremel tool at 1/3 of max rpm extends the life of the disk brush and reduces the likelihood of scorching the glass.

For the BSA Scouting peak-peak signaling event, "Operation On-Target", we put these aimers on 12"x12" mirror tile and signal 30-70 miles or more. You can read about "Operation On-Target" at https://en.wikipedia.org/wiki/Operation_On-Target .  There's a nice page on making signal mirrors, both of this type and the double-sided mirror type, including photos of the view through this red aimer in use. That page is: https://web.archive.org/web/20230401100129/http://www.bsaontarget.org/mirrors I have a specialized video on making the 12"x12" signal mirrors, here:
https://youtu.be/JYTpWYcKeUw