Laser Visibility

[elitex]

I see claims for lasers being visible 50-100 miles away on many of the websites. Here's my question: What would be seen by someone at a distance of 50+ miles when they are at the other end of the beam? How is this tested? I live against an uninhabited mountain desert area but it is a bit hard to get to it and test this as there are no real roads to it. Obviously no one should beam someone up close but what about the mountainside about 12 miles away if I were to send someonme with a camera up there?

 

[mw1111]

all of that is made up. visible is something that varies to extremes often. even a slight amount of dust can cause a lasers range to be cut to a few hundred yards visibly. a laser, in space, with no obstructions, will carry on forever (supposedly) so rating your lasers range is pure crap

 

[Rasel]

You can probably imagine this one on your own if you know about divergence and how it works...

50-100mi = 80-160km

OK rule of thumb: beam diameter(mm) = distance(m) X divergence(mRad) + initial beam diameter(mm)


(right? ;D)

 

[mw1111]

well, wouldnt the divergence happen after the air itself disperses the laserbeam?

 

[Rasel]

lol

The air doesn't really have a noticable effect on the laser's divergence. The divergence is a characteristic caused by the optics.

Air is very important to the visibility of the beam, however, since you only see it because of the light's coming into contact with particles.

Say you wanted to figure out the amount of wattage going into the viewer's eyes if it was pointed straight at them. You take the area of the larger diameter and set up a proportion:
i[sub]1[/sub]/a[sub]1[/sub] = i[sub]2[/sub]/a[sub]2[/sub]
the "i"s stand for the Irradiance of the beam, in other words the amount of power per area, and the "a"s are the areas.
i[sub]1[/sub] is the initial irradiance:
if your laser is 100mW and initial beam diameter is 2mm, then the irradiance I=100mW/pi ~ 32mW/mm[sup]2[/sup]

 

[mw1111]

well because the light is coming in contact with the air particles, i assumed that over vast distances those particles would mess up the light badly enough as to make it negligibly dim. i dont know where the divergence is in a laser though, so truthfully my comments are speculatory in nature

 

[BlueFusion]

divergence is kinda the angle of a beam. Ideally the laser beam would have a divergence of 0 meaning that both sides of the beam are parallel, and therefore the beam diameter is the same at any distance. in reality they diverge, meaning the sides of the beam get gradually further apart, and the beam diameter increases.

 

[mw1111]

so i guess it would all depend on conditions. in foggy weather the fog will disperse the laser before the divergence becomes too great, but maybe on a very clear night the divergence would scatter the light first

 

[SenKat]

Here is basic divergence :
---> the "=" sign represents the laser beam, and the "<" is where the beam converges, and the ">" is where the beam diverges.
============================><=============
Now, after the point of divergence, the beam continues to spread out - so over TOO long of a distance, and you would have a big old green spotlight ! Now - if you have the capability to try this out on your mountain, I would LOVE to see the result ! a foggy morning or evening would be best so REALLY see the beam - or simply a nice and humid night, like immediately after a rain. I think it would make for a nice picture !

 

[Cyparagon]

When a class IIIb laser's range is given in tens of miles, it's not the range you can see the beam or anything but it's the range at which a person standing inside the beam can see the laser. at 100 miles in complete darkness, they would only be able to tell if the laser is on or off and it would probably be as bright as a dim LED 100 yards away.

So 100 mile range for a 100mW green is pretty much BS.

 

[steve001]

I like senkat would also like to see photos of this type. I would suggest you use a tripod to steady your laser. To give you an idea how divergence can play a significant role in beam visibility look here http://www.lasercomms.org.uk/france.htm This is done using a laser with an output <5mw

 

[elitex]

I will try photographing it through a telescope to see what the effect will be. I did successfully beam a tall pickle-like Evergreen tree from a distance of about 2 miles. Looked cool through binoculars, though. Photos coming soon... ;D

 

[steve001]

Something else that would be interesting would be to shine the laser into the eye piece of the telescope and see how that affects the visibility/ range. Somewhere on nero_design's website he has done what you want to do on an object 5 miles distant