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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