So just how visible are lasers from a distance?

[laze_doctor]

[...]

I know from personal experience that when I'm directly behind a beam it's more visible than when I am standing away to the side. And I've been told that it's even more bright when it's shining towards you (as opposed to shining away from you). So in theory, if the beam is not shining towards or away from you it won't be that visible from a distance? For example a beam being shined perpendicular to the viewer.

Has anyone tested this?

I've attached two images I found on the net. The first one is supposedly a 100mW from 6 miles down the road. I don't know what power the other is, but it's damn visible. And part of the beam (between the tower and the building is perpendicular to us.

So... any thoughts?

 

[Switch]

Yes you are correct. If your viewing direction is perpendicular to the beam you might not even notice it, while if you were looking down the beam at a narrow angle it would be plenty visible.It's because of the scattering in the air and particles suspended in the air.
If you are looking away it's less visible than when you're looking toward the source.This effect is the strongest when the air is cleaner as opposed to when it's really foggy for instance (Rayleigh scattering versus Mie scattering).

So if you shine a powerful RPL over the city, preferably from a high spot and shining it downward , people within a relatively narrow cone having the beam as the axis would find the beam very visible when looking toward or away from you.

The visibility in the second pic is enhanced by the camera's long exposure time.It appears that the beam is very visible in pretty bright conditions although in reality, it's darker outside and the beam is less bright.

 

[Deleted member 8382]

beware with aircrafts!

 

[laze_doctor]

@Switch thanks that's very interesting... so given that you've said the preferable thing for visibility is to shine it downwards, I guess my best bet is to always shine it directly up, or failing that out to the horizon

 

[Cyparagon]

It looks to me that laserglow image is taken with the laser just to the left. There's no way the beam would be that slim otherwise.

 

[scopeguy20]

It looks to me that laserglow image is taken with the laser just to the left. There's no way the beam would be that slim otherwise.

Yes, and I would presume as well that the second beam in the background is not the same laser, but a second high output, maybe a labby laser from one tower or the other. It just seems to be reflected. -Glenn

 

[Autarchus]

Yes, and I would presume as well that the second beam in the background is not the same laser, but a second high output, maybe a labby laser from one tower or the other. It just seems to be reflected. -Glenn

There's no reflection off the tower - the reflection is off that wire running left to right in the foreground.

 

[laze_doctor]

Hmmm I tend to agree with scopeguy20... not sure how it could be reflected off that wire. The other beam seems just too powerful to be a reflection off a wire.

What do people think about the first image? It's definitely not shining towards or away from our line of sight. Is 100mW really _that_ visible from 5 miles away?

 

[Switch]

There is no second beam.The second "beam" you see is the wire being illuminated by speckle around the initial beam as far as I can tell.
And I agree with cyparagon, it most definitely is not perpendicular.

 

[caleb]

Perhaps the second image brightness is unfortunatly related to a long exposure...

 

[laze_doctor]

Ohhhhhhh I see what you mean, it's not a beam but the wire being illuminated.

 

[bschott]

Dumb hick question, as I live in a fairly backwater community in the great wastes of North Dakota (even though it is our largest city in the state, it barely equals up to a suburb of any major metro area in the US)....

Wouldn't a person just be asking for trouble if they used their laser outside? I'm afraid of using a laser with my astronomy gear in the middle of a field 30-40 miles from anything resembling civilization, let alone firing off a laser in town. Aiming up and any aircraft that sees your laser...well you are pretty much toast in the U.S. The FAA doesn't play around even if you were not using the laser to intentionally harm anyone...and even if you were not close to a flight path.

Perhaps I am just a little paranoid but I'd think the only safe and responsible place to use your lasers would be indoors with proper safety equipment. Anything else and we are risking doing damage our hobby for a couple minutes of 'oh boy, look at that'!

 

[ElektroFreak]

^Meh.. I don't believe that it is illegal to point lasers into the sky as long as you have a variance and local authorization for powers over 5mW, and as long as you don't point them at aircraft. I use my laser for starpointing and I live in metropolitan St. Louis. I also live directly next door to a major hospital, so there are always helicopters flying around. I've never had any problem, but I've never pointed any of my lasers anywhere near any aircraft ever. As long as you don't point them even in the general direction of aircraft I don't think you'll have any problems, but I'm not a lawyer, and this is not legal advice.

 

[steve001]

green Laser > 35 mile laser shoot
http://www.laserpointerforums.com/laser_pointer_forums_3/forum/showthread.php?t=40481

 

[steve001]

Dumb hick question, as I live in a fairly backwater community in the great wastes of North Dakota (even though it is our largest city in the state, it barely equals up to a suburb of any major metro area in the US)....

Wouldn't a person just be asking for trouble if they used their laser outside? I'm afraid of using a laser with my astronomy gear in the middle of a field 30-40 miles from anything resembling civilization, let alone firing off a laser in town. Aiming up and any aircraft that sees your laser...well you are pretty much toast in the U.S. The FAA doesn't play around even if you were not using the laser to intentionally harm anyone...and even if you were not close to a flight path.

Perhaps I am just a little paranoid but I'd think the only safe and responsible place to use your lasers would be indoors with proper safety equipment. Anything else and we are risking doing damage our hobby for a couple minutes of 'oh boy, look at that'!

Believe it or not you can check for yourself . There is no federal law that states specifically it is against the law to shine a class 3b or higher laser in the sky
unless the law has been recently changed to include private citizens. The CDRH jurisdiction concerns commercial application. As for notifying the FAA that is also not required by law. It is recommended. Check the CDRH and FAA sites site for further info.

An anecdote. A few years ago a fellow that went by the moniker Gottaluvlasers who owns and operated at the time a laser show argued that it was against the law to shine a class 3b or greater laser into the the sky without a variance. This no you can't, yes you can went back and forth. I finally shut him up by going to an authority from ILDA of which he was a member as I recall. His mistake was interpreting the laws governing commercial enterprises to also govern hobbyists. If the CDRH required every hobbyist to file for a variance everytime someone wanted to shine a laser into the sky they'd be inundated with paper work to the point of impracticability.

 

[laser83]

I wouldn't play games like that for testing. As for pointing at the sky just keep it away from planes. The police have other things to worry about then lasers especially 40 miles away in the middle of nowhere. It's only a problem when some moron decides it's funny to point one at a plane or other aircraft.

Sorry, I find that too amusing... Would the crime of having a 200mW red pointer be worse or equal to having a fully automatic AK-47 in your county? :gun: I can imagine some naive hobbyist being arrested and thrown in the same cell with a professional weapons smuggler and criminal.