The attached 7 page article is both the most detailed article, and describes the most thorough signal mirror testing, that I've seen.

It describes the evolution of signal mirror sighting methods to the use of retroreflective aimers, and the test results that caused the US Navy to switch from the "rearsight" method (still common in lifeboat mirrors to this day) to retroreflective aimers.

I obtained this through the kind offices of the US Coast Guard historian.

A complementary overview of signal mirror development across this period (with lots of color photos) may be found here:

http://www.usmilitariaforum.com/forums/index.php?showtopic=2412

Wow..that was a great bit of history.i remember back in the 50's when as a kid i would see bins of those still in the wrap at the local surplus store.

Great article. Thanks for posting.

Yes, according to some 1946 Congressional testimony, over 1,000,000 were ordered in WWII, so I imagine there was quite a surplus glut after the war. I think I've even seen a 1950's survival book suggest picking one up.

Here's an ad in the Sept 1946 Popular Mechanics (top right corner) advertising surplus GE "cross-in-glass" signal mirrors for $1.
http://books.google.com/books?id=seADAAAAMBAJ&pg=PA279
( You can zoom in with the "magnifying glass" icon)

$1 back then corresponds to $10.62 today, according to http://research.stlouisfed.com/fred2/data/CPIAUCNS.txt

There was quite a bit of surplus after the war, not just of signal mirrors, but of nearly everything else. It was still so common fifteen years after the war, that my first sleeping bag and tent, were milsurp, among many other items.

a dollar was $10 in those days? it's been so long i forget just how much my allowance was and what i spent it on.if they were a $1=$10 that why i did not buy one but just looked!

Those were the days when, if you saw a penny lying in the street, you actually picked it up. It had reasonable value.

I still pick up pennies.

I figure that it takes about a second to pick up a penny. A penny a second is $36 an hour. A much more than reasonable wage for the task of picking up pennies.

A mirror and a whistle are useful addition to any kit.

There was a remarkable surge of effort in the United States to perfect and deploy signaling mirrors that began in September 1942 when the U.S. Coast Guard, Office of Strategic Services, and the National Inventor's Council approached the National Bureau of Standards for assistance in designing a practical signaling mirror for life rafts.

(see the March 1946 article attached at the start of this thread)

What set this off?

Two 1944 books about the US Coast Guard both suggest it was a single dramatic event, using the phrases:

"... That gave us the idea of adding mirrors" and
"... a few of them can be traced to individual occurrences"

Signal mirrors were not unknown - they were already standard issue in Navy life rafts.

For example, in the British Signal mirror test report H/Eq/249 from the Marine Aircraft Experimental Establishment, St. Helensburgh "Heliograph mirrors for emergency dinghies", issued on Oct 9, 1942 (after the NBS was approached, and before Rickenbacker went down) amongst the 13 mirrors the British tested in May and August 1942 was test mirror "G":

"Standard U.S. dinghy equipment No sighting hole" which the report says was a 4 inch chrome-plated brass mirror.

The article on pages 44-45 of the Feb 1, 1943 issue of Naval Aviation News ( a service magazine sent to Navy pilots) reinforces that:

"Life rafts in service and under procurement are equipped with a chromium plated steel mirror for use as a signalling device."
which at the time was "provided with a 1/8" hole near the edge", and is depicted as a round mirror on page 45.

What instigated the fresh interest? Some accounts point to the massive publicity associated with the Rickenbacker search and rescue, but Rickenbacker went down in mid-October 1942, a month after the National Bureau of Standards project began.

Richard S. Hunter of the National Bureau of Standards alludes to prior events in his March 1946 article: "A number of reports were published, at about the time of the Rickenbacker search, of survivors from ship sinkings and downed aircraft who had successfully improvised methods of reflecting flashes of sunlight toward searchers from tin-can bottoms or other shiny objects."

====================================================

This account from the US Coast Guard Official History suggests a single incident was the cause:

From:
Marine Inspection , December 7, 1941 - July 18, 1944 (Volume 13 of the Coast Guard at War Official History Series).

http://www.uscg.mil/History/articles/CGWar_13_Marine_Inspection.pdf


THE COAST GUARD AT WAR DEC 7, 1941 - JULY 18, 1944 MARINE INSPECTION VOLUME XIII
PREPARED IN THE STATISTICAL DIVISION U.S. COAST GUARD HEADQUARTERS JULY 31, 1944

SIGNALING MIRRORS

Occasionally a survivor would have an idea for adding something new to the lifeboat equipment. "There's one case where survivors told us how they signaled to a passing ship by tearing off the lid of a tin can and catching the reflection of the sun on its surface," the Captain related. "That gave us the idea of adding mirrors to the lifeboat equipment, along with the parachute flares and other signaling devices." New safety measures adopted by the Coast Guard are based squarely upon the experience and need of survivors, and after consultation with maritime unions and others concerned.

=================================================
This other 1944 book also suggests a single occurence:

http://www.archive.org/stream/firstfleetthesto011061mbp/firstfleetthesto011061mbp_djvu.txt

Full text of "First Fleet The Story Of The U S Coast Guard At War"

The Story of the U. S. Coast Guard at War-

REG INGRAHAM

INTRODUCTION BY SECRETARY OF THE NAVY FRANK KNOX

THE BOBBS-MERRILL COMPANY

Publishers
INDIANAPOLIS NEW YORK

pp. 260-261

"The majority of the Coast Guard's wartime safety measures are based on careful analysis of the experiences and recommendations of a large number of survivors, but a few of them can be traced to individual occurrences.

One group of seamen, for instance, came ashore after a protracted period in a lifeboat and it was discovered they had been overlooked repeatedly by patrol planes.

"We could see the PBY's [Navy flying boats] go by," they related, "but we were unable to attract their attention."

Finally one of them hit on an idea. Taking the bottom of a tin ration can, he fastened it to the blade of an oar and used it to flash the sun's rays back at the planes. This crude heliograph proved effective, too, and soon thereafter the Coast Guard made it mandatory for all lifeboats to be equipped with polished steel signaling mirrors. They are somewhat more elaborate, of course, than the piece of tin can fastened to an oar blade, and each mirror is accompanied by simple instructions for sighting it so as to give the maximum chance of the signal being seen. "

========================================
The text above was also verified by searching each paragraph to verify it as being present in the Google Books online "snippet view" version here:

http://books.google.com/books?id=zRnTAAAAMAAJ

Title First fleet: the story of the U.S. coast guard at war
Author Reg Ingraham
Publisher Bobbs-Merrill, 1944
Original from Indiana University
Digitized Mar 17, 2009
Length 309 pages

Here's a recently posted series photos of a World War II 4"x5" tempered glass ESM/1 emergency signaling mirror
and the instructions on the box.

Click on any photo for a larger view. Find the "full size" link on the resulting page to see an even larger view.











( For more photos of old emergency signaling mirrors, there's my photo gallery, too.

http://www.richard-fowell.fotopic.net/c1663439.html

Similarly, there's a "full size" button
if you want to see photos even larger than the ones resulting from clicking on the thumbnail photo.)