It is legal for Sota S2S? We have here same summits with a clear view of each other
My personal view NOT the MT view:
Light and radio signals are both EM signals differing in frequency.
For SOTA I need a radio transmitter/receiver at each end and must use an amateur band of frequencies. i.e. I generate the radio frequency signal at the summit.
For a light based QSO it would be valid if I generate the light signal at the summit. i.e. I use a laser, LED etc. and not a mirror and the sun. I need to check but the UK amateur licence says all frequencies above 3 THz are licence exempt and visible light is 370THz to 700THz so you would not be using regulated frequencies, I think that’s valid as this way it’s analagous to a pair of radios.
In the case of the Heliograph (handheld and signalling) you’d be using amateur radio talkback to cooordinate anyway so there is a loggable S2S QSO and writing up the Heliograph side gets you kudos from your peers if you manage to send a message which is received OK. 
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Interesting site… I like the 10m repeater optical link. I steer clear of repeater use normally… they’re just a bit tedious. But building them, operating them and experimenting with technologies is much more interesting.
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