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How to nearly collimate white light ?


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Is there any way to have a 30cm ⌀ source of intense enough white light beam to reach a couple of hundred miles with the least divergence possible, perhaps up to a mile ⌀ ; by 'cheap' methods/lensing/whatever ?

 

A lighthouse fresnel lensing can neatly reach 20 miles; what would it take to make it 200 ?

How do airport beacons work ?

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It is very difficult to collimate non-pointlike sources. It's not that hard to create a source that is diverging enough to be seen a long way away. If you think about if you look up at night and see a plane it is a long way from you but you can see it's non-directional lights, so even a bit of mirroring can help make it brighter. Also the eye is non-linear so will pick out a quite dim light against an even darker background quite easily.

 

200 miles I'd have thought would have issues from the curvature of the earth.

 

The best way to collimate a light source, that I've found, is to use mirrors to get it coming out of half a sphere, capturing as much of the light as you can with a lens, use another lens to focus the light onto a very small aperture and then put collimation optics using the aperture as the light source. The smaller the aperture the better collimation you will get but the more light you will loose.

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Would a white laser be sufficient? Given enough power?

 

You can combine red, blue, and green lasers to produce a "white" laser. Lasers are collimated...

 

That's not a true white light source though... You can get something that acts like a laser though... super something light source, I'll try and remember what they're called when I'm more awake.

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No idea on what the OP is asking, but a related anecdote....

 

Many years ago my old man had some interest in optical comm links. Blah blah blah. We ran an experiment whereby he made a telescope out of two dime-store magnifying glasses (read: plastic, crappy optics!) and a length of PVC pipe. He put a plain ol' red LED light (powered by a 9V battery) at the focal point.... The idea was that this contraption would quasi collumate the light and allow it to be seen at a much greater distance.

 

He stood in our driveway and I started walking down the street to see how far I could see the LED. Just over a mile away the road curved.... The light was still easily visible, mind you, but we lost clean line of sight.

 

The point is that with maybe $10 worth of "optics" we made it such that the naked eye could see a garden variety LED at slightly over a mile away (probably significantly more but we couldn't prove that).

 

I imagine that with a decent narrow field of view 6" telescope on both ends (send and receive) the light would be visible for a very long ways indeed. I'd wager on the order of 10 miles given ideal viewing conditions (a scope on the receiving end would really help there!). And I'm talking about nothing more exciting than an LED as a light source!

 

Extrapolate as you will to the original question.

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No idea on what the OP is asking...
:)

 

Am not sure if white light behaves the same on lenses as monochromatic red led.

 

To avoid the curvature of the earth as impediment, imagine a modulated beam of white light hitting a cubic reflector on a geostationary satellite :rolleyes: -no clouds:rolleyes:-

With a telescope, you could 'receive' the reflection, couldn't you ?

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How far away could you see it in the same lighting conditions without the optics? A simple test to the collimation in this case would be to see how far you can step sideways before it disappeared.

Unknown. I don't remember if we checked that or not as this would have been sometime around 1998.

 

I do remember though that it was basically impossible for him to manually aim the scope such that I could see it "full time." What he took to doing was slewing the scope through a pattern so that I would see a flash of red every few seconds.

 

Also, in case it wasn't obvious in my original, that this was done at night.

 

Note that I've recently regained interest in this concept for other purposes and am toying with the concept of a pair of quality binoculars. One side of the binocs would be "send" and the other side "receive." The plan calls for a narrow pass visible filter and a carrier frequency (so that a little noise won't kill us). Don't know if/when I'll actually get a chance to try it as I've a lot of other irons in the fire, but it's something for me to ponder.


Merged post follows:

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To avoid the curvature of the earth as impediment, imagine a modulated beam of white light hitting a cubic reflector on a geostationary satellite :rolleyes: -no clouds:rolleyes:-

With a telescope, you could 'receive' the reflection, couldn't you ?

 

Sure... But you're gonna need a lot further than 200 mile range for that. :) And if you're interested in optical comm links, why the requirement for white light?

Edited by InigoMontoya
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Estimates seem to vary but it seems that you can see someone strike a match from about 10 miles away (on a clear dark night)

Even a poor job at collimating that would let you see it from further away. People use simple searchlights to shine on to the clouds to attract attention to events.

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