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#215054 - 01/16/11 07:42 PM Re: Spotting Signal Mirrors at sea from other vessels [Re: rafowell]
Teslinhiker Offline
Veteran

Registered: 12/14/09
Posts: 1379
Great posts and info Rafowell.

As much as I like signal mirrors and carry one, my AO is not the best for this. Being in the PNW, we have overcast cloud, rain etc where we will do not see any significant amount of sun for days and sometimes weeks this time of year. Depending on where we are hiking/camping, the thick tree cover over large areas here can also really hamper any change of using a mirror if you were in the situation where you were incapacitated and unable to move.

The other potential drawback with a signal mirror even when there is sun and clear skies, is being able to attract an aircraft in remote areas where there is no commercial flight corridors (there are only a few major ones around here) is also crucial. Then again if a person out on a hike, whether it be a day or multi-day hike previously left a detailed trip Itinerary or route card with a responsible person, that would give the SAR air people ample reason to search the area you are in.

That said, this is why I carry a PLB and like anything else, be it a match to light a fire, a mirror, or whistle to attract potential rescue, these are all better options then not having a way at all.
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.

John Lubbock

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#215078 - 01/16/11 09:52 PM Re: Spotting Signal Mirrors at sea from other vessels [Re: Teslinhiker]
rafowell Offline
Enthusiast

Registered: 11/29/09
Posts: 214
Loc: Southern California
Originally Posted By: Teslinhiker
Great posts and info Rafowell.
That said, this is why I carry a PLB and like anything else, be it a match to light a fire, a mirror, or whistle to attract potential rescue, these are all better options then not having a way at all.


Absolutely - a signal mirror should be a backup signaling device, not primary.

You want some form of 24/7 all-weather radio means for calling help, ideally a PLB. You will summon help much more surely and quickly that way. PLBs/EPIRBs contributed to the rescue of 1,596 people in 2009 alone, and in areas with good coverage, the cell phone is probably the first thing to try. As a licensed amateur radio operator, I also carry a 2 meter radio with builtin GPS.

Most recent signal mirror rescues occured because the primary radio system failed - malfunction, batteries, crushed in the fall, or went down with the boat.

Any visual signalling method is far from a sure thing - survivor's stories are rife with tales of ships and planes that blithely passed by flares, smoke, and signal mirror flashes. Signal mirrors also require coherent illumination, direct line of sight to the rescuer, and are easily mistaken for natural phenomena, unlike, say, orange smoke.

Mirrors, however, have an infinite shelf and battery life, are not expendable, and are extremely light and compact. After you've run out of everything else, they are the thing to try.

I focus on signal mirrors (somwhat quixotically) because I think that, while their utility is limited, there is a lot more potential there than is realized.

Where signal mirrors could be useful, all too often, people don't have a mirror, have an inferior mirror, or don't know how to use the mirror effectively.
_________________________
A signal mirror should be backup for a 24 hr, all-weather radio distress signal, such as a 406 MHz PLB (ACR PLB)

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#215092 - 01/16/11 11:10 PM Re: Spotting Signal Mirrors at sea from other vessels [Re: rafowell]
Ann Offline
Newbie

Registered: 01/04/11
Posts: 42
Loc: Western Washington
Excellent info, rafowell. Thanks for taking the time and effort to educate us. smile

I've been wondering something that maybe you know the answer to...does a larger signal mirror mean a larger reflection dot or a brighter one?

And if it's a brighter one, then how would the reflection of a larger plastic mirror, say 3"X5", compare with a smaller 2"X3" glass mirror?

I apologize if the answers are obvious.

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#215105 - 01/17/11 04:02 AM Re: Spotting Signal Mirrors at sea from other vessels [Re: Ann]
rafowell Offline
Enthusiast

Registered: 11/29/09
Posts: 214
Loc: Southern California
Originally Posted By: Ann
Excellent info, rafowell. Thanks for taking the time and effort to educate us. smile
You're welcome!
Originally Posted By: Ann
I've been wondering something that maybe you know the answer to...does a larger signal mirror mean a larger reflection dot or a brighter one?

At (extremely) short range, larger, at long range, brighter.

Since the expected range of use of signal mirrors counts as "long range", the textbook answer is that the larger signal mirror creates a brighter reflection, not a larger one.

More precisely, the beam of a 3"x5" mirror is in fact 2" wider than that of the 2"x3" mirror in the largest dimension, at any range, but since the overall beam is 50 feet wide at 1 mile range, the extra 2 inches is negligible.

The reason is that each point on a mirror surface illuminated by a circular light source reflects a cone of light whose angular diameter is that of the light source. The overall beam is the summed effect of the cones from each point. If the mirror is perfectly flat, the cone from any point is parallel to the cones produced by any other point, and simply offset by the separation between the the two points. So, the maximum width of the beam in this case is the diameter of the cone plus the diameter of the mirror.

In the case of the sun the cone diameter is roughly 0.54 degrees, or a 50 foot spread to the mile. Since the dimensions of typical signal mirrors are small compared to 50 feet, the dimensions of the mirror have little effect on the shape or size of the cone at long range.

If the mirror is not flat, the beam can be wider than the sun cone, but no narrower.

This is all simplistic geometric optics, neglecting diffraction, interference and atmospheric effects.

Originally Posted By: Ann
And if it's a brighter one, then how would the reflection of a larger plastic mirror, say 3"X5", compare with a smaller 2"X3" glass mirror?

As a rule, a quality 3"x5" plastic mirror should be brighter than a quality 2"x3" glass mirror. A 2"x3" glass mirror has less than 40% of the useful area of the 3"x5" plastic mirror, and the reflectivity of commercial plastic survival mirrors is usually (though not always) greater than 40% that of glass.

This question is a bit tougher, since the reflective quality of both glass and plastic mirrors varies with manufacture and the angle of incidence of the light, and flatness plays a large role, too. However, good mirrors are flat enough that that effect is negligible, and good plastic mirrors are more than 40% as reflective as glass.

For example, 3rd party measurements of the reflectivity of the Starflash "Ultra" mirrors ranged from 60% to over 90% that of glass mirrors. The Starflash Ultra press release of 12/13/2007 said the Ultra was "over 90% as reflective as glass", and "now 50% more reflective", which implies their standard product was "over 60% as reflective as glass".

Although a 3"x5" version of the Starflash Ultra was announced, with a quoted weight of 2.0 oz (62.2 g), AFAIK it never shipped, and after ordering several and getting the "old-style" ones, I stopped trying.

Taking the press release at face value, though, with a reflectivity > 60% that of glass for the classic 3"x5" Starflash mirror, it should outshine a 2"x3" glass mirror.

Of course, a 3"x5" laminated glass mirror should be even brighter, and they are still being made.



Edited by rafowell (01/17/11 04:23 AM)
Edit Reason: typo
_________________________
A signal mirror should be backup for a 24 hr, all-weather radio distress signal, such as a 406 MHz PLB (ACR PLB)

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#215142 - 01/17/11 07:05 PM Re: Spotting Signal Mirrors at sea from other vessels [Re: rafowell]
Ann Offline
Newbie

Registered: 01/04/11
Posts: 42
Loc: Western Washington
Thank you, rafowell. I feel kinda spoiled to get answers like that for free. grin

Edit: Removed part of my post; I had thought I found a source for the 3"x5" Starflash Ultra but it was just for the regular Starflash.


Edited by Ann (01/17/11 07:14 PM)

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#216776 - 02/09/11 08:50 PM Re: Usefullness of a signalmirror in the mountains [Re: Tjin]
ireckon Offline
Pooh-Bah

Registered: 04/01/10
Posts: 1628
Loc: Northern California
Somewhat related, if you're stranded out there in the snow, it would be good to start a fire if you can. I'm thinking fire making skills up there would be more valuable than mirror signaling skills. The fire would perform double duty as a visual signal also. I'm not abandoning the mirror signaling, just keeping things in perspective for survival.
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If you're reading this, it's too late.

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#216778 - 02/09/11 09:07 PM Re: Usefullness of a signalmirror in the mountains [Re: ireckon]
Hikin_Jim Offline
Sheriff
Pooh-Bah

Registered: 10/12/07
Posts: 1804
Loc: Southern California
Originally Posted By: ireckon
Somewhat related, if you're stranded out there in the snow, it would be good to start a fire if you can. I'm thinking fire making skills up there would be more valuable than mirror signaling skills. The fire would perform double duty as a visual signal also. I'm not abandoning the mirror signaling, just keeping things in perspective for survival.
Good point, but if you're up high, good luck on finding any wood. A signal mirror also has the advantage of speed. You're out foraging while awaiting rescue after a vehicle malfunction/plane crash/whatever, and you hear an aircraft. A signal mirror takes seconds to employ. Try getting a fire started in time. A signal mirror is a great ad hoc, moment's notice device. Fire's great, and you're absolutely right to stress it's importance, but don't underestimate the value of a good signal mirror.

HJ
_________________________
Adventures In Stoving

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#216782 - 02/09/11 10:43 PM Re: Usefullness of a signalmirror in the mountains [Re: Hikin_Jim]
MostlyHarmless Offline
Old Hand

Registered: 06/03/09
Posts: 982
Loc: Norway
Good point on the lack of firewood above the three line! Signal fires are excellent, make one if you can.

If you're high up - above the three line - in the winter you will make an excellent contrast to the white background. Assuming good visibility, anything above the three line that isn't snow will be seen. But that doesn't help much if they think you're a rock. Your challenge becomes to be as different from a rock as possible.


Colors not found in nature is good. Movement is good. Combine them is even better. Flapping a big orange rescue bag around is probably top notch.


Remove snow from the equation and being spotted becomes much harder. Same principle of movement, contrast and colors, but the background is much less uniform and much more colorful, making everything harder.


Of course, a signal mirror is excellent on a sunny day, everywhere and anywhere.


Edited by MostlyHarmless (02/09/11 10:47 PM)

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#217056 - 02/14/11 06:09 PM Re: Usefullness of a signalmirror in the mountains [Re: Tjin]
Tirec Offline
Newbie

Registered: 08/24/07
Posts: 48
Loc: Rocky Mountain West
ABSOLUTELY bring a signal mirror!

I'm in the Civil Air Patrol. The light reflecting from a mirror is significantly different from that of other surfaces. Other surfaces will make a flash as they reflect, but a light which follows the plane will attract far more attention from the aircrew and they will come back to check out the source, get a GPS fix, and send in ground teams.

On an exercise, my crew was having difficulty finding our target in the trees. The ground team target pulled out a signal mirror and were found almost immediately afterward.

In general, if you make a fist and hold it at arm's length, you will see the area which can be clearly focused on. For safety reasons, CAP aircraft will not often fly below 1000' above the ground. In hilly or mountainous terrain, that elevation will be increased.

Anything you do to make yourself more visible from the air will aid in being found. The signal mirror is great. Blaze orange stands out tremendously as soon as there's enough light to see by and also stands out more than other colors when overcast. It's not a natural color so it will also catch an aircrew's attention. A blaze orange hunting vest (3 ounces), handkerchief, hat, stocking cap, jacket, backpack, sleeping bag, etc., is also quite visible. The larger the better, but also will be heavier. If you don't particularly like orange, find items which are reversible with an orange side.

Above all, let someone know the area where you expect to be. That will greatly reduce the search area size, should you need assistance.

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#217072 - 02/14/11 10:23 PM Re: Usefullness of a signalmirror in the mountains [Re: Tjin]
Hikin_Jim Offline
Sheriff
Pooh-Bah

Registered: 10/12/07
Posts: 1804
Loc: Southern California
And a PLB doesn't hurt either -- but never depend on anything electronic.

HJ
_________________________
Adventures In Stoving

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