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'[EE] Two way speech light communications record se'
2006\04\28@073416 by Russell McMahon

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A must-see page for anyone with even a passing interest in free-space
optical communications.
Two way speech light communications record set using Luxeon 1 watt
LED - 104 miles , February 2005.

   http://www.bluehaze.com.au/modlight/index.htm

An utterly superb page with links to a number of optical free-space
communicator systems and a detailed account of a record of 104 miles
set in 2005 in Tasmania, Australia.

Optical receivers.
Very good.

       http://www.bluehaze.com.au/modlight/modlightrx.htm

Transmitter uses a 1 watt Luxeon LED.
Many excellent reference links.
Details given as to why LASERs are NOT the best free-space
communications drivers and why LEDs are better.
Explains how partially decohering a LASER at source can improve range
(!).

This record was for bidirectional speech communications.
The current US record using optical light is about 75% of this.
There are various other records for lesser capabilities eg 1 way
speech, tone only etc.

Ultimate world record for free space optical communications is 183
miles by US Army Signal Sergeants. Comms used Morse code and solar
mechanical heliographs with human eye as the receiver. This was
achieved in 1896 [11111]. Modern equipment should be able to better
this given an adequate path and atmospheric conditions.

Links include a monolog on Fresnel lens theory, manufacture and usage.
Superb.

       http://www.fresneltech.com/pdf/FresnelLenses.pdf

Where are my Luxeons .... ?


______________

Fresnel lenses for sale

           http://www.3dlens.com/enter.html?target=Large_Fresnel_Lens.html


       RM

_________________________


Ref:    Ken Mardle


2006\04\28@082711 by Bob Axtell

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The US military has been using this for years. This avoids clandestine
teams' having to use
a radio, which is easy to intercept. They speak over them, and their
voice is encrypted.

There are probably a few teams in Iran now, I expect...

--Bob

Russell McMahon wrote:
{Quote hidden}

2006\04\28@090927 by Russell McMahon

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> The US military has been using this for years. This avoids
> clandestine
> teams' having to use
> a radio, which is easy to intercept. They speak over them, and their
> voice is encrypted.
>
> There are probably a few teams in Iran now, I expect...

> Russell McMahon wrote:
>> A must-see page for anyone with even a passing interest in
>> free-space
>> optical communications.
>> Two way speech light communications record set using Luxeon 1 watt
>> LED - 104 miles , February 2005.
>>
>>     http://www.bluehaze.com.au/modlight/index.htm

I'm aware of free-space optical communications, but what attracted my
attention was the extreme range (except when compared to 1896
technology) and the use of a 1 Watt Luxeon for performance better than
achievable from a LASER of any sensible spec.


           RM

2006\04\28@140313 by Russell McMahon

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> The US military has been using this for years.

Manifestly, since, as I noted,  the world record was set in 1896 by
the US Army :-).

I haven't seen any heliographs in action recently. Maybe they are now
black.?


       RM

2006\04\28@143301 by David VanHorn

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>
> I haven't seen any heliographs in action recently. Maybe they are now
> black.?


Black light, of course :)

--
> Feel the power of the dark side!  Atmel AVR

2006\04\28@162552 by Peter

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On Sat, 29 Apr 2006, Russell McMahon wrote:

> I'm aware of free-space optical communications, but what attracted my
> attention was the extreme range (except when compared to 1896 technology) and
> the use of a 1 Watt Luxeon for performance better than achievable from a
> LASER of any sensible spec.

I guess anyone who tried to use a laser to make a perimeter sensing
alarm or similar could have told you this. What is more interesting is
to estimate the transmitter power used by the heliograph. A 4 inch
heliograph should get at most 8 Watts input (using 1kW/m^2 sun). And the
receiver saw this in daylight (wow, probably using night binoculars or
whatever equivalent they used at the time). Maybe they cheated a little
bit and one of them was in darkness while the other wasn't (dusk/dawn).
Also getting the mirror angle right for such a distance would be really
hard in despite of the heliograph being designed specifically for this.

Anyway it would be interesting to compare performance with todays
technology using comparable sender power. I would use a light
intensifier equipped ccd camera as receiver at night in near infrared
... (for morse or data, not voice).

Peter

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