I constructed this "mirrorless" signal mirror for short-range signal mirror training/practice.

The fact that, at 0.7 miles, you can see both the direct flash from the plain 2"x3" scrap of plexiglas, and the "echo of an echo" flash created by the double-pane window in front of the camera, emphasizes the point that if a mirror is lacking in an emergency, a plain pane of glass can serve as an emergency signal mirror.

The flash here is about half as bright as it would have been if I hadn't taped the rear surface to prevent rear-surface reflection.

I wanted something students could pair up at 100 yards and signal each other without after-images.
It turns out even this was too bright - around here at noon on summer solstice, we still need sunglasses and taping off 2/3 of the mirror at 100 yards.



Sun flashes from signal mirrors are extremely bright – even the flash from a small unsilvered scrap of plexiglas can be seen for miles. Here I've taped the back of the plexiglas, so the only reflecting surface is the front air-plastic interface, with a (computed) reflectance of about 4% at normal incidence.

A regular 2"x3" glass signal mirror sun flash is visible to the naked eye at 24 miles on a clear day. Even if I assume the glass mirror is 100% reflective, using the inverse square law, the sun flash from the front face of this unsilvered 2"x3" plexiglass should be visible at 4.8 miles, and about 6.8 miles if I hadn't taped the back (which would allow reflections from both front and back surfaces). So - the fact that the flash is visible here should come as no surprise.

I need to use a much smaller piece, or a much longer distance, to test my computations.

To give you an idea of how bright it is - notice that when there's a white flash with the plexiglas, there's also a blue flash to my right, next to the white stake to my right? That's the flash from my plexiglass bouncing off the inner pane of the double-pane window we are filming through, then off the outer pane of the double-pane window, then back into the camera!

(With the actual mirror, in the second half of this video, you see a white flash were the blue flash was, and another blue flash - you see the flash after even more bounces!)

That's surprising but good to know!

> That's surprising but good to know!

I've seen the use of plain glass as an emergency signal mirror suggested a time or two.

{ Just to be clear <heh> - a glass signal mirror is definitely much brighter for the same area, as the 2nd half of this video demonstrates. The reflectivity of a glass mirror at normal incidence is about 11x brighter than a plain sheet of glass/plexiglass of the same area ( 90% reflectance vs. 2 surfaces x 4% per surface = 8% for the plain glass)). Since brightness falls off with distance squared (a bit faster due to atmospheric attenuation) the range is about 3.4x less for the plain glass. However, since the flash from a 2"x3" glass mirror is naked-eye detectable (admittedly, not very) at 24 miles naked eye, that leaves a lot to work with. Also, your scavenged sheet of glass may be larger than 2'x3". Finally, as quantified by the Fresnel equations the plain glass reflectance goes up at grazing angles, so the advantage of the mirror over plain glass is less at the (more problematic) high off-angles. }

Richard S. Hunter, the "father of the modern US military emergency signaling mirror"[1], said:

"Pieces of flat glass and flat plastic windshields and windows make good signaling mirrors." in:

Signaling With Mirror, July 1944, Air Sea Rescue Bulletin

and touches on this again in his 1946 article summarizing the development of US military emergency signaling mirrors in WWII:

Heliographic Signaling Mirrors, March 1946 Air Sea Rescue Bulletin

saying "Flashes of sunlight from ... or a plain piece of glass can be aimed quite easily and accurately by one on firm ground."

-R

[1] When requests came in for an improvement on the ESM/1 signal mirror that began issue in 1943, Hunter took the idea offered by Learned[2] and reduced it to practice in Feb 1944 (including sea trials), and went on to develop the "red bullseye" aimer that replaced it (and is used to this day by the French military), then the retroreflective mesh aimer featured in most US military emergency signaling mirrors since 1949.

Learned vs. Thompson

[2] Per the above, Learned first reduced his idea to practice in 1933, and from other stories, developed it while a Scoutmaster for his troop.

That's fascinating. Thanks!

And thinking it over, it's exactly what we've observed over the years -- a bright flash of light from a window as we drive by.

One more tool in the toolbox!

BTW, somehow I never realized you have a Youtube channel! Subbed!

Thanks! It is mainly signal mirror videos, and my posting rate is low.

Here's my YouTube channel: https://www.youtube.com/user/rafowell

I post most things I find about signal mirrors & heliographs online at my Facebook page: https://www.facebook.com/richard.fowell.3/

Modern signal mirror success stories, I always post here at ETS.

The racoon video was pretty cute!