# Seeing a laser pointer from 16km away

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1. Nov 21, 2014

### Hal

A friend and I would like to try communicating (Morse code signaling) with off the shelf laser pointers from two hill tops (~350m above sea level) separated by about 16km. It would be nice to do this without binoculars but I’m not sure if that is possible with readily available off the shelf laser pointers. 5mW lasers are easiest and cheapest to acquire, however, I can go higher since this is not regulated where we will be.

My basic question is: what wavelength and power laser pointer would work over this distance while trying to contain the cost? Our main concern is the haze due to pollution and humidity which is the norm in the place that we want to try this but we would do it on a day where we could at least see the outline of the hills from 16km – though, perhaps, not individual features.

I understand that the human eye is most sensitive to green (555nm) but suspect red lasers (650nm) would propagate through atmosphere better due to less Rayleigh scattering (but I have not been able to verify for this laser application).

As for the power side of the question, obviously the more the better but then cost starts to become a factor. From what I’ve seen red lasers are cheaper due to their simplicity so I could get a more powerful one for less. I'd like to do this with the least power for any chosen wavelength, though.

In trying to find out what sort of attenuation I'd be up against, I googled for phrases like “atmospheric effects on propagation of laser light” but didn’t turn up much for the wavelengths of standard laser pointers. Most of the literature was about communication equipment that seems to be set around 785nm and 1550nm. I tried to read some of those papers to see if I could glean anything but I haven’t looked at any real physics in 20 years so it was difficult to parse.

Any suggestions would be greatly appreciated.

2. Nov 21, 2014

### f95toli

I don't think this can be done. The maximum distance in typical conditions will be something like a few km regardless of the setup.
The maximum range for commercial systems is something like a few hundred meters. although that can probably be extended to perhaps a km or so if your data-rate is low enough.

3. Nov 21, 2014

### davenn

hi and welcome to the forums

fellow amateur radio friends of mine both in Australia and in the UK have experimented with varying success with lasers over reasonable distances
They ALWAYS used electronic detectors for the receivers. Visually seeing a well collimated laser over 16km shouldn't really be problem, but detecting switching on and off accurately doing your Morse code would be difficult unless it was transmitted very slowly.

Interestingly, my fellow hams discovered that the collimated beam of a laser turned out to be a disadvantage and they had much better success over longer distances with a normal light globe that was well focussed
Cant remember the exact reasons offhand, I would have to search through emails of ~ 3 yrs ago

cheers
Dave

4. Nov 21, 2014

### davenn

no, much better that that as I have indicated in my previous post

5. Nov 21, 2014

### Staff: Mentor

How many mW is the laser they bounce off the moon? There's a corner reflector there for this purpose. Double the distance ....

6. Dec 7, 2014

### Rob Rushworth

Evening.
I'm not a physicist, but have been looking into this topic for a while for a school project.

First, here's a link to a site claiming 12.7 miles on a 0.5mw, $3, red laser pointer - not only sight, but a voice communication link. http://www.earthsignals.com/Collins/0036/index.htm And then here's another claiming 172kms with a$3 red laser pointer - also a voice communication link.
http://modulatedlight.org/optical_comms/revisiting_the_107_mile_path.html

Apologies to site moderators if the links flag up and burn your time - they're good links.

I'll be sending Morse across the Mekong near Phnom Penh shortly using standard red laser pointers; aiming those lasers will be the biggest problem, I think. After that we'll be trying longer distances perhaps between hilltops, there's plenty of scope for that here.

Cheers
Rob

7. Dec 7, 2014

### Danger

I haven't looked into that, but the most "noticeable" colour is yellow/orange. That's why traffic signs and the like are made with it. You can't go by the eye itself exclusively, as much as how the brain reacts to the input from the eye.
Again, this is not something that I have any experience with. Just as an educated(?) guess, though, maybe it's because no single interference factor such as humidity will affect all present wavelengths whereas a laser has vulnerability due to using only one frequency?

8. Dec 7, 2014

### davenn

From memory, it was the coherence of the laser light that was the problem

9. Dec 7, 2014

### Danger

Ah... hmmm...
Maybe mismatched polarity at the receiver? (Just grasping at straws here...)

10. Dec 18, 2014

### Rob Rushworth

Got the aiming device 'finished'. it's untested on a tripod as yet, but works well sat on a 2nd-floor, flat, outdoor wall-top and aiming at distant objects - trees, etc.
There's a small backlash movement when moving the laser dot horizontally, and the crosshairs formed by moving the dot left-right-up-down against a drawn target at 5metres are slightly tilted.. maybe 2 degrees clockwise, but that's an adjustment for tomorrow.
Cheap mikes and springs, kitchen chopping boards, aluminium angle, pen body, allen key, drinking straw.
Inch adjustment at hundreds of meters.

Last edited: Dec 18, 2014
11. Dec 18, 2014

### davenn

from memory they found the coherent light source had greater atmospheric attenuation

12. Dec 18, 2014

### DaveC426913

What the heck IS that thing??

I'm starting to get the idea. You use the left calipers to make fine adjustments on the horizontal axis.

But I can't figure the foreground calipers. They push on the blue stick. I can guess that they must adjust the Y-axis, perhaps by pushing apart the top bard from the bottom, but I don't see how.

13. Dec 18, 2014

### Danger

That, unfortunately, is far past my ability to reply sensibly. Even if I knew more about the subject, I wouldn't know which component of the atmosphere was responsible. If it was water vapour, there's no way to predict it and therefore compensate for it given your "kitchen table" equipment (which I love, by the bye, since I build stuff the same way). Same for regular thermal "wavering". If those are the culprits, you might need some kind of "intelligent optics" as are used in serious astronomy to probe the environment and send correction signals to the mechanical elements. That is probably beyond your reach financially, but one never knows.

14. Dec 18, 2014

### Drakkith

Staff Emeritus
Was the light output the same, or did the lightglobe put out more light than the laser?

15. Dec 19, 2014

### Rob Rushworth

Evening, Dave.

My first concern when the idea of pointing a laser so far was raised was accurately gaining control of the pointing of the laser, and doing so in a way such that any movement was as repeatable as possible. I'm pretty sure that I won't see reflection of the beam back to the sending point, so spotters will be radioing or on the phone - waiting for a red flash from my location. I'll narrow a series of sweeps down until I can nail a position.

The calipers are micrometers (mikes) - they're used to measure size. These have measurement lines marked at 0.01mm, but can quite accurately measure in 0.0025mm increments - judged between the lines.
I use the mikes to move the laser pointer in both the horizontal and vertical planes.
As you can imagine. a movement of 0.0025mm at the mike gives good control over placement of the laser at a distance.
You've got the horizontal movement already - one of the two smaller-sized white blocks set in a wedge shaped V moves towards the other - pushed by the mike and repelled by the spring-steel of the bulldog clip riveted to the back - pivoting around the vertically placed aluminium 90 degree angled channel.

The vertical movement comes from an allen (hex) key placed upward through a hole/slot in the top plate. The long arm of the key points upward through the plate and the short arm of the key points toward the back of the device under the top plate.
When the mike pushes the allen key, the key tilts, and thus the key's short arm acts as a lever that pushes up the top plate. There's a lot of friction between mike and key, so I placed a blue biro shaft/body over the key - this is oiled inside and rotates around the key as the mike turns, vastly smoothing movement . Look closely inside the blue pen in the bottom photo and you can see the allen key.
Under the key and on the contact point with the bottom plate is fastened a large metal washer. A cropped drinking straw covers the short arm of the allen key here to remove some friction when contacting the washer, and the washer is also oiled.