Signal mirror flashes at various ranges from 3"x5" glass
signal mirror with retroreflective mesh aimer, as described athttp://www.equipped.com/pho...
... . Modern signal mirrors
were developed in WWII for stranded personnel to signal rescue
planes/ships for help. Plastic, glass or metal signal mirrors are
issued to all US military pilots and US Coast Guard personnel, and are
required by international convention in life boats on ocean-going ships
and airliners. They are commonly recommended (and sometimes required) in
survival kits for hikers and boating. While the very narrow beam
reflected by the mirror lets it be seen at long range, as shown here, it
also means that they have to be pointed very accurately at the rescuer.
US Government testing in WWII established that retroreflective aimers
were greatly superior to the other types, and almost all US military
issue mirrors since WWII have had retroreflective aimers.