Accuracy is key in signal mirror use, and operator skill [1] and a good signal mirror aimer[2] can make a huge difference.

Flashing a signal mirror at a target doesn't guarantee you will hit it, and the brightness of flashes that hit the target is strongly dependent on how squarely you hit the target.

You can see this by comparing the number of flashes seen, and the variation in their intensity, from the eight stations flashing the camera in this video clip.

The eight stations are fairly evenly spaced in a horizontal line across the frame from #1 near the right edge to #8 near the left, using signal mirrors to flash the camera for 172 seconds. The flash visible in the starting frame is from station #2 - station #1 first appears (dimly) at 02:14.

Note that:

(a) The total number of visible bright flashes in this 172 second clip ranges wildly between stations, ranging from 7 by station #3 to 210 by station #2, with a median of 49. Even discarding the two extremes, and scaling for the evident duration of operation, the spread is a factor of three (from 33 to 98 flashes in 172 seconds).

(b) The brilliance of flashes from each station ranges over a fairly continuous range from barely visible glowing points to dazzling sunbursts with radiating rays.

I believe these variations are due to variation in operator aiming skill and possibly mirror aimer type, though there is much I don't know about this video[4].

Here's the video (for more viewing options, you can
view it directly at YouTube)



My conclusion is:

(a) Having a good signal mirror aimer, and practice, can make a huge difference in one's visibility.
For example, in our last signal mirror test at 22 miles range, the flash rate from my friend with a Coghlan's 2"x3" glass mirror was similar to the best result here, and 3.9 times the median rate here. [5]

(b) For maximum visibility, you want the mirror flash dead on target, not merely close, or merely visible - pointing accuracy is extremely important.


Here are my observations on the eight stations (which I've numbered from right to left) with ( number / start time / high flash rate period / number of flashes / comment)

#1 / 02:17 / 2:44-2:52 / 20 / a very late start - flashed at a rate that would be 98 flashes in 172 seconds
#2 / 00:00 / 00:16-00:22 / 210 / visible from the first frame - more than four times the median number of flashes
#3 / 00:07 / 00:12-00:14 / 7 / a very poor showing, even though present from almost the beginning
#4 / 00:06 / --- / 52
#5 /00:00 / --- / 40 / Many flashes, while very bright, were extremely short, hence still hard to see
#6 / 00:00 / --- / 44
#7 / 00:39 / --- / 54 / another late start - signaled at a rate equivalent to 70 flashes for the full 172 seconds
#8 / 00:02 / --- / 33

[1] For initial practice, a target that flashes back when you hit it is good, such as a reflective licence plate on a parked car, or a reflective traffic sign, at 100 yards or so. (But don't practice near traffic - blinding a driver can cause an accident).
Then you can graduate to practicing with a partner at a range of a mile or two, with the partner initially calling out hits on the cell phone or radio, then later simply silently counting flashes over a fixed period like 30 seconds to a minute.
All of these practice tasks are much easier than a real emergency, where you may be shaky with adrenaline, and the target may be moving (like a helicopter), and only visible for a short time. In addition you may be on an unstable platform (raft bobbing in the waves), with your primary hand unavailable due to injury, as well as ill and in pain.

[2] In both American and British testing during WWII, the test subjects scored roughly four times as many hits on the search plane with retro-reflective aimers than they did with the British "foresight aimer" mirrors ( a method akin to the "V-finger" method, but far more accurate).
Short report on American tests:
Page 32, column 3 of "Signaling with Mirrors",
Naval Aviation News, September 15, 1944
Download PDF of full issue here
Long report on american tests (margin clipped, alas)in
"Heliographic Signaling Mirrors", Richard S. Hunter, National Bureau of Standards,
Air-Sea Rescue (published by the US Coast Guard), March 1946 pp. 24-29, 48
Link to start of article
Testing: "Section VII: Comparative Test of Signaling Mirrors"
Link to section describing of signal mirror test
Link to section summarizing test results

[3] http://www.youtube.com/watch?v=UxG_UjthvfY
Title: espejos de señal (signal mirrors)
< literally: "mirrors of signal">
Description; arte publico en la ciudad de Guanajuato durante el festival yep
(translated): public art in the city of Guanajuato during the festival
< since the poster is from Mexico, I believe this is Guanajuato, Mexico >
Uploaded on Dec 17, 2008

[4] Of course, there could be something going on of which I'm unaware - for example, if there were four operators at station #2, rather than just one, then this could explain the higher number of flashes there. Similarly, the low number of flashes at station #3 might be because the operator was distracted (e.g., a passing policeman asked him what he was doing with that mirror.)

[5] Over my last count (from 01:10-2:03) in this video:
Link to video: "22 mile flash from 2"x3" glass signal mirror" my friend scored 59 flashes in 53 seconds, which scales to 191 flashes per the 172 seconds of the current video.
Of course, with only a 2"x3" mirror at a range of 22 miles, his flashes are much less spectacular than the close-range flashes in the current video.

The author of that video posted a nice large format still here showing two of the signal mirror flashes:
http://www.ivanabreu.net/#works/artworks/signalmirrors

(6/5/2013) The three links at the bottom of footnote 2 above are broken - use the links below (the forum won't let me fix the original post).

While I'm at it, I found some more great links for the 2008 signal mirror "performance art" above. It seems he made "foresight" style signal mirror aimers of a novel type.

I think the "black X" on the front of the mirrors is part of his aiming system - that you align the shadow "X" with the "V" edge of the foresight.

I would have used a smaller mirror aiming hole and the British "foresight" aiming method of an "aiming spoon" with a hole the same size as the mirror hole - you align the center of the mirror hole and the center of the spoon hole with the target, and then centralize the "shadow spot" cast by the mirror hole on the spoon hole. (Diagram) (Photo)

Worldwide, non-US life rafts tend to carry the British "foresight" type of signal mirror.

Here are the links for the 2008 performance art:

• High Resolution Video on Vimeo
• High Res Photo with hidden links (Hover over the bottom inside edge of the bottom left corner of the photo to see the hidden hyperlinks)
• Magazine article (p.2) showing signal mirror aimers