Hitting a target with a signal mirror flash at long range is a skill that requires some practice.

For initial, short range practice, a good target is a retroreflective target that will "light up" when illuminated. This can be a strip of retroreflective tape, a bicycle reflector, reflective traffic sign, or the license plate of a parked car. (Keeping always in mind that you don't want to flash anyone without their consent, particularly if they are operating a motor vehicle - signal mirror flashes are blindingly bright at close range.)

However, these sorts of targets reflect in a cone that is several degrees in diameter, so they don't work well beyond a few hundred yards. In an actual survival scenario, you want to be able to attract attention from people miles away - you'd like to practice those conditions.

An optical corner cube, on the other hand, reflects in a cone that is 1/50 of a degree in diameter or less, so it will work just fine over a range of 0.25-3 miles. ( I wouldn't use it at less than 300 yards - it is too much like looking into the sun. Even at 0.25 mile, I'd use sunglasses, and check to make sure I"m not getting afterimages.)

Here's a video I took of how a 1.5" optical corner cube prism looked at 2.9 miles ( I couldn't find a spot 3 miles away). The cube was visible even when I used a small, low reflectance 2"x3" stainless steel mirror at that range, but I don't remember which mirror I used for this video.
(For fancier video options like HD and full screen, view this at the YouTube site directly here: http://www.youtube.com/watch?v=OdD7un8lE1w )




Frankly, I don't expect many people will be that keen on this to buy a corner cube for this purpose. But, for the few of you who are survival instructors, it might be worth it. You are unlikely to want to pay the price an optical supply house will want for a corner cube ( hundreds of dollars), but the corner cube prisms available on Ebay for about $40 should be just fine.

A few things to keep in mind:

- While the corner cube will return the flash to the signal mirror operator, bystanders won't see the return unless they are careful or lucky. The return beam is exceedingly narrow. To give you an idea - the classic demo of a corner cube is to, at night, walk 100 yards away from it, and, holding a flashlight flush against your right temple, light up the corner cube. Now, by closing each eye in turn, you can verify that you can see it with your right eye, but not with your left (my nephews thought this was cool). If someone other than the operator wants to see the return flash, I'd recommend they position themselves so that the line from their eye to the cube is within about 1/2 inch from the edge of the signal mirror.

- For mirrors with retroreflective "fireball" aimers, the signal mirror operator will likely need to be within 0.5 miles in order for the corner cube return to be bright enough to be visible through the "fireball". At longer ranges, you can do what I did here - use a video camera on "record", and hold the mirror so the edge touches the side of the camera lens barrel.

The fact that the "fireball" will hide the corner cube return from the operator at long range is actually a good thing - it makes for more realistic practice. In a real survival situation, you won't know if you are hitting the target or not at long range, so for advanced training, you don't want that crutch. You can use the camera to record how well you were doing, and play it back afterwards.

Of course, if you want to check to make sure you know where the cube is first (a problem I had in this case - at 3 miles, one bush looks a lot like the bush next to it), you can just look under the edge of the mirror and scan for it using the "finger-vee" method until it lights up, and you can determine where it is with respect to local features. At that point, you can start practicing with the retroreflective aimer and video camera.


====== Why corner cubes are so bright ============

The reason that the corner cube works at such long ranges is that its response to a point light source is to return a nearly parallel cylindrical beam of light that is twice the diameter of the prism aperture, whose center passes through the point light source. If you are looking through the center of a signal mirror that is illuminating a corner cube of diameter "R", every point on your signal mirror that is within a radius "R" of the center will have an optical path out to
the cube and directly back into your eye - the apparent brightness of the optical cube return to you is pretty much* the same as the apparent brightness that a signal mirror of radius R would have for an observer at the location of the corner cube.

* It will be dimmer by:

• the reflection losses of the triple reflection inside the corner cube
• the transmission losses throught the cube glass
• the divergence angle of the corner cube

Fascinating post - thank you. I'm on to learn more about corner cube prisms, which I never knew about.

Very informative, thank you for posting.