Optical communication in daylight

Earlier, I noticed the the reflection of the sun from my watch face on a distant wall. This got me thinking of heliographs. These were used to send Morse code messages in sunny countries. I think ranges were remarkable.  See https://en.wikipedia.org/wiki/Heliograph. I seem to recall ranges were up to 80km. The best DX (line of site) was 295km.  Basically, the sun's reflection was obstructed to send CW.

My mind turned to QRSS beaconing. For this to be possible it would be necessary to divert the beam when the QRSS was not on. One possibility would be to have a loudspeaker with a mirrored sheet (kitchen foil?) over it with a tone keyed.  It would also be necessary to have a receiver that was optimised for daylight.  I have no idea how good this would be. 

I guess the same idea could be used for speech. The mirror would be deformed as speech was said. I seem to recall there was a 19th century patent for this called the Photophone. For speech I guess one could simply speak against the metal foil mirror.

I would imagine an optimised system could be used for over-the-horizon (NLOS) tests. Sadly, my health these days means I can only dream about this. It may already have been tried.

Optical DXing at around 460THz

Until I read the webpage below I didn't know that the record for optical morse DX goes back to 1896 and was an astounding distance of 183 miles using a Heliograph between Colorado and Utah in the USA. This link about the heliograph makes fascinating reading. The heliograph is essentially a keyed mirror that reflects sunlight and uses it to send morse code over long distances.  Modern versions of heliograph mirrors are available on eBay for emergency communications. See for example eBay item 250681260168. A good article about heliographs is on Wikipedia, see http://en.wikipedia.org/wiki/Heliograph.

Modern amateur optical DXing uses either lasers or high intensity LEDs. There is a piece about this on p51-52 of the Feb 2011 RSGB RadCom in Sam Jewell's "GHz Bands" column with further links including http://www.lasercomms.org.uk/index.htm which looks like a useful resource.  There was also a funny article in the RSGB Bulletin of April 1962 called "Getting Going on Bottom Band".