# Signaling with a Flashlight from 18 Miles Away

#### PhotonWrangler

##### Flashaholic
Nice comparison; thanks. At 20 miles the earth curvature is 267 feet so this signaling method wouldn't work with both ends being on flat ground. I would like to see a test where they use the light to illuminate a tall object like the top of a tree, using the treetop as a periscope reflector to get the signal to the other end.

#### thermal guy

##### Flashaholic
I thought the the drop was only 8 inches a mile?

#### PhotonWrangler

##### Flashaholic
I will have to revisit some old notes on this. Back when we were designing some 5 mile long microwave paths I remember having to add around 50 feet to the path calculations to accomodate earth curvature, but I'm going on memory here and it's certainly not perfect!

#### aznsx

##### Flashlight Enthusiast
I will have to revisit some old notes on this. Back when we were designing some 5 mile long microwave paths I remember having to add around 50 feet to the path calculations to accomodate earth curvature, but I'm going on memory here and it's certainly not perfect!
You're probably remembering correctly, and that likely included some consideration for other factors (other than pure line-of-sight) as well. I don't know offhand, but you're probably both right!

#### Poppy

##### Flashaholic

A six foot man can see about 3.1 miles to the horizon.
Therefore 21 miles divided by 3 is 7 (six feet tall) for a total of 42 feet height

#### PhotonWrangler

##### Flashaholic

A six foot man can see about 3.1 miles to the horizon.
Therefore 21 miles divided by 3 is 7 (six feet tall) for a total of 42 feet height
Thanks for the sanity check, Poppy. This number seems more reasonable. The number I was origiinally thinking of included a fresnel zone clearance figure so that threw me off at first.

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#### Poppy

##### Flashaholic
Thanks for the sanity check, Poppy. This number seems more reasonable. The number I was origiinally thinking of included a fresnel zone clearance figure so that threw me off at first.
That of course is if my source is correct. Afterall, I did get it from the internet, so it must be right, right?
I figured that your numbers were based in part on the wavelength or something.

#### roostre

##### Newly Enlightened
A six foot man can see about 3.1 miles to the horizon.
Therefore 21 miles divided by 3 is 7 (six feet tall) for a total of 42 feet height

Using division followed by multiplication to ratio and calculate a height of "42 feet" would not be correct since this is not a linear relationship.

How far away is the horizon:

You would need to know both the distance between the two locations and the elevation at each location along with the topology of the terrain between the two locations (to be sure there are not any obstructions) to determine if "line of sight" is possible.

Using this online calculator:

Results from the calculator indicate that for "line of sight" (assuming there are not any obstructions) with an eyesight elevation of 6 feet, the other object would need to be at an elevation of 216 feet to be seen at a distance of 21 miles.

The calculator has a disclaimer stating the "calculator might be slightly wrong in some cases" because the "calculator doesn't account for the phenomenon of refraction. When light travels through a medium that is not perfectly uniform, such as air, it bends or refracts. For example, refraction can happen when light hits a pocket of cold air or a hot draft of rising air."

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#### aznsx

##### Flashlight Enthusiast
RF and light have a lot in common, but there are other factors that come into play in RF (MW links), which are not in play with light links (which may well be pure line-of-sight), and perhaps vice versa.

#### 3_gun

##### Enlightened
For me this just points out the need for light management in a SHTF or weather emergency. Big lumen lights are not your friend, a triple A 180L light was seen from 18 miles. Flood lights were easier to spot than throwers. It's a Bat Signal that you don't want to send. Most of my big lumen lights have an UI that gives me sub 5L outputs from start that also means run times of DAYS on a single battery. Most of my "bug out bag" lights are smallish AA & 123a lights with a tighter beam than not + most are very low "flash" styling, looking very much like a cheap "free w/purchase" give away lights. My bag(s) are good gym bag type items that look less out of place the the high speed low drag backpacks that are so often hyped in u tube videos. Having what you need & not drawing a lot of attention to yourself & your gear goes a long way in avoiding headaches. On the other hand I know my 4k/L lights are the way to go if I want/need to get attention

#### DaveTheDude

##### Enlightened
Curvature of Earth's surface notwithstanding, if I get into a pickle in the backcountry the only thing I'm likely to be signaling is an aircraft, in which case the planet's curvature will be largely irrelevant. In this I'm encouraged that the "standard" 18650 light tested in the video was visible at eighteen miles, which means that there is a pretty decent chance of a pilot detecting an emergency signal aimed at an aircraft "only" ten miles away. Of course, a Rescue Laser can easily reach 20 or 30 miles, depending on which model is being carried, but a single-purpose laser signaling tool is probably overkill for all but the most remote adventures (or misadventures, as the case may be).

As a practical matter the monster throwers shown in the video come with a significant space and weight penalty: they are prohibitively bulky and heavy for hunting or backpacking, although I can see some utility if they are carried in a vehicle, a canoe, on horseback, or perhaps in an ATV. The space and weight considerations involved in day hikes, hunts, or backpacking argue for carrying a 6P-sized light or smaller, with some variation depending on the length of the adventure and the kind of terrain being traversed. (For example, I can justify carrying a 21700-fueled light in a desert environment or an excursion in the Sierra Nevada range, where the terrain is more open, and greater power and more throw are useful.) But since rescue lasers are smaller, lighter weight, and less costly than any of the monster throwers currently available, if serious emergency signaling is something you need to plan for, I'd go with a dedicated laser (not an LEP, but a real signaling laser). As an added bonus, the lasers are visible in daylight, not just at night.

#### idleprocess

##### Flashaholic
In the simple sense, yeah, even a low-output low-intensity light source can be seen from ~18 miles away line-of-sight. When you're looking for it. And know where it's coming from. And the situation allows for some degree of distinction.

For me this just points out the need for light management in a SHTF or weather emergency.
Eh, in the case of the former and you wish to avoid the attention of ne'er-do-wells sure. In the latter you may well wish to summon attention.

Light discipline is not something that the average person much thinks about, without even going into opsec principles. I see people with flashlights carelessly blazing away with little concern for what they're lighting up - porches/doors/windows/walls of houses, portions of front yard closer to the house than the sidewalk, driveways, gaps between houses. While a saving grace is that the overwhelming majority of these folks are sporting nothing more capable than a cheap 3xAAA non-zoomie putting out well under 100 lumens not particularly prominent past ~5m, it's still bad form. Suspect these folks are a real treat when camping, snapping their flashlight right into everyone's face.

Big lumen lights are not your friend
In an actual emergency situation (be it a power outage, forced movement in darkness, major disaster) they generally represent excess - more power consumption, light output, and mass - that costs you in terms of runtime, dark adaptation, and carrying other stuff.

More generally, I personally find that high-lumen lights represent similar excess in casual use. Bigger, hotter, trash what passes for night vision in the 'burbs with continuous ambient light, and can attract unwanted annoyance attention. Sure I've got a collection of Emisars that can kick into turbo for 10-30s at a time for a what's that situation, but that's at a low duty cycle and often not used walking mutts or performing other tasks.

#### Lou Minescence

##### Flashlight Enthusiast
Thanks Lumencraft for the fun video !
I like to see these real world tests.
It's been a while since Marshall @ Going Gear took a bunch of current lights and lit up the woodline behind a large field. Keep the Vids coming

#### PhotonMaster3

##### Newly Enlightened
A green laser would be good in an emergency but not safe for this purpose otherwise obviously. I have a focusing lens for mine and I don’t even know how I’d estimate the length of the beam.