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!)